Handbook of Porous Silicon [2 ed.] 9783319713793, 9783319713816

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Leigh Canham Editor

Handbook of Porous Silicon Second Edition

Handbook of Porous Silicon

Leigh Canham Editor

Handbook of Porous Silicon Second Edition

With 438 Figures and 183 Tables

Editor Leigh Canham School of Physics and Astronomy University of Birmingham Birmingham, Worcestershire, UK

ISBN 978-3-319-71379-3 ISBN 978-3-319-71381-6 (eBook) ISBN 978-3-319-71380-9 (print and electronic bundle) https://doi.org/10.1007/978-3-319-71381-6 Library of Congress Control Number: 2018936744 1st edition: # Springer International Publishing Switzerland 2014 # Springer International Publishing AG, part of Springer Nature 2018 This work is subject to copyright. All rights are reserved by the Publisher, whether the whole or part of the material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilms or in any other physical way, and transmission or information storage and retrieval, electronic adaptation, computer software, or by similar or dissimilar methodology now known or hereafter developed. The use of general descriptive names, registered names, trademarks, service marks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. The publisher, the authors and the editors are safe to assume that the advice and information in this book are believed to be true and accurate at the date of publication. Neither the publisher nor the authors or the editors give a warranty, express or implied, with respect to the material contained herein or for any errors or omissions that may have been made. The publisher remains neutral with regard to jurisdictional claims in published maps and institutional affiliations. Printed on acid-free paper This Springer imprint is published by the registered company Springer International Publishing AG part of Springer Nature. The registered company address is: Gewerbestrasse 11, 6330 Cham, Switzerland

Preface to Second Edition

Materials (matter that is often useful and sometimes influential) underpin so much of our lives and culture; from cuisine to transport to medicine to economics to art to fashion. Materials abound and we have invented uses for all types of materials, be they metals, ceramics, polymers or foodstuffs, textiles or explosives. The king of semiconductor materials, silicon, has not only been very useful but also incredibly influential. Individual material influence on a global scale depends not only on usefulness but also popularity and need of that material function(s), material availability, and cost. Many new materials with very useful properties are developed every year, but if their production is difficult, unreliable, costly, or not sustainable, their influence on society will be limited. Solid pure silicon currently dominates electronics and solar production of energy. Humans love communicating and we also need energy. Silicon is a very earth abundant element and, among semiconductors, relatively cheap and nontoxic. Unlike state-of-the-art healthcare, which is increasingly the domain of only the wealthy, electronic technology costs have plummeted enough to be accessible for the vast majority. While average life expectancy has got worse in many countries, communication and access to information has improved dramatically for the poor. By 2019, estimates suggest over five billion people (67% of the global population) will carry around silicon-based electronics – a mobile phone. Solar power is the most abundant available renewable energy source. Harvesting just a few percent of the achievable levels would supply enough energy for all humans today. In 2010, the estimated global power consumption was 17.5 TW while the lowest estimates for achievable global solar power are 400 TW. The costs of silicon-based solar panels have also plummeted. They now make up 50 nm). Recently, chemical conversion of porous or solid silica using magnesium vapor has received much attention for applications that require inexpensive mesoporous silicon in powder form. Very few techniques are currently available for creating wholly microporous silicon with pore size below 2 nm. Keywords

Porous silicon · Fabrication · Formation routes · Nanoporous · Macroporous · Mesoporous · Microporous

L. Canham (*) School of Physics and Astronomy, University of Birmingham, Birmingham, Worcestershire, UK e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_1

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Introduction Porous silicon, solid silicon with voids therein, can be generated by diverse means. The complexity and variety of porous materials has also led to useful and broadly accepted IUPAC (International Union of Pure and Applied Chemistry) recommendations for categorizing their structure (Rouquerol et al. 1994; Thommes et al. 2015) which we adopt in this handbook (Table 1). “Macroporous silicon” thus has voids predominantly greater than 50 nm diameter; “Mesoporous silicon” 2–50 nm; and “microporous silicon” less than 2 nm in size. The increasingly popular term “nanoporous silicon” embraces the above three categories of pores, but with an upper limit of 100 nm diameter. Although “top-down” techniques utilizing electrochemical etching techniques have dominated the academic literature over the last 50 years, from 1960 to 2010, there have since been many other routes demonstrated: both “top-down” routes from solid silicon and “bottom-up” routes from silicon atoms and silicon-based molecules. The purpose of this review is to capture for the reader, in one brief document, all those fabrication techniques the author is aware of, and highlight their potential applicability, depending on desired structures, targeted application area, and acceptable levels of cost. In the following chapters of this part of the handbook, eight of these techniques and nine physical forms (ultrathin films, multilayers, superlattices, membranes, needles, nanoparticles, nanowires, suspensions, and nanocomposites) are then chosen to be reviewed in detail. There are also reviews surveying fabrication options for the three types of porous silicon (macroporous, mesoporous, and microporous) as classified by pore size.

Schematic Route Map Figure 1 illustrates the traditional route whereby porous silicon is created from solid silicon, which itself is derived from solid silica. A number of techniques such as anodization (see handbook chapter ▶ “Porous Silicon Formation by Anodization”), vapor etching (chapter ▶ “Porous Silicon Formation by HNO3/HF Vapor Etching”), glancing angle deposition, lithographic etching, and photoetching (chapter ▶ “Porous Silicon Formation by Photoetching”) are suitable for Si wafer-based processing. Others can be used on both wafer and powder silicon feedstocks, such Table 1 Nomenclature for categories of pores in silicon

Term Nanopore Macropore Mesopore Micropore

Pore diameter (nm) Up to 100 nm Greater than 50 nm 2–50 nm Less than 2 nm

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SOLID SILICA

SOLID SILICON

SILICON WAFERS SILICON MICROPARTICLES

SILICON NANOPARTICLES

SILICON ATOMS

POROUS SILICON Fig. 1 Routes to porous silicon via solid silicon

as stain etching (handbook chapter ▶ “Porous Silicon Formation by Stain Etching”), galvanic etching (chapter ▶ “Porous Silicon Formation by Galvanic Etching”), and the very popular MACE technique (chapters ▶ “Porous Silicon Formation by Metal Nanoparticle-Assisted Etching”and ▶ “MACE Silicon Nanostructures”). Most of these techniques create highly directional porosity and therefore properties that can be highly anisotropic (see handbook chapters ▶ “Electrical Transport in Porous Silicon,” ▶ “Mechanical Properties of Porous Silicon,” and ▶ “Optical Birefringence of Porous Silicon”). Until quite recently, etching of highly porous structures from solid silicon was reliant on acidic fluoride chemistry; however, alkali-based etches have now been shown to be at least capable of macropore generation under restricted conditions. Porosifying controlled areas of a silicon wafer enables porous silicon to be integrated with silicon circuitry or MEMS devices within chip-based products. Although porous silicon particles (microparticles and nanoparticles) can be derived from anodized wafers (see handbook chapters ▶ “Milling of Porous Silicon Microparticles,” ▶ “Porous Silicon Nanoparticles,” and ▶ “Photoluminescent Nanoparticle Derivatization Via Porous Silicon”), this route is only viable for low-volume high-value product areas, as in some medical therapy applications (see handbook chapters ▶ “Drug Delivery with Porous Silicon,” ▶ “Chemotherapy with Porous Silicon,” ▶ “Immunotherapy with Porous Silicon,” ▶ “Theranostic Imaging with Porous Silicon,” and ▶ “Wound Management Using Porous Silicon”). If highly porous structures are required at high volumes, etching techniques will typically have to remove large quantities of solid silicon as waste, unless recycled. For lower-value, high-volume porous silicon products that are not silicon chipbased (see handbook chapters ▶ “Porous Silicon and Functional Foods”and

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POROUS SILICA SILICA NANOPARTICLES SILICON –BASED MOLECULES

POROUS SILICON

Fig. 2 Routes to porous silicon using chemical conversion

▶ “Porous Silicon for Oral Hygiene and Cosmetics”), there is therefore increasing interest in fabrication routes that utilize existing highly porous feedstocks or siliconbased molecules that are themselves waste products from solid silicon manufacturing. These increasingly use chemical conversion of, for example, silica, silane, or silicon tetrachloride (see Fig. 2). The chemical conversion can be promoted thermally, mechanically, or electrochemically. Here the morphology of porosity can reflect that of the starting solid feedstocks (see handbook chapter ▶ “Porous Silicon Formation by Porous Silica Reduction”) or how the silicon nanoparticles are assembled into a porous body via sintering (see handbook chapter ▶ “Porous Silicon Formation by Mechanical Means”).

Specific Fabrication Techniques Table 2 illustrates the variety of processes (currently more than 40) now available to create different forms of porous silicon, arranged in approximately the chronological order they have been introduced. Historically, it was high levels of mesopores (see handbook chapter on ▶ “Mesoporous Silicon”) that were created first via anodization (1) and stain etching (2) of electronic-grade crystalline silicon. Depending on wafer resistivity and anodization conditions, it was subsequently shown that both macropores (see chapter ▶ “Macroporous Silicon”) and micropores (see chapter ▶ “Microporous Silicon”) could also be realized via the anodization route. In the 1990s, a multitude of different techniques for creating mesoporous luminescent silicon were identified. All the etching techniques tend to create “open” porosity where pores are accessible from the external surfaces of the structure. Specific techniques to create “closed” porosity include melt gasification (Nakahata and Nakajima 2004) and milling/sintering (Jakubowicz et al. 2007). Mesopores created in silicon via wet etching tend to nucleate and propagate with varied size and spatial arrangements (Smith and Collins 1992; Korotcenkov and Cho 2010). Attempts to

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Table 2 A multitude of routes to form porous silicon. The techniques highlighted in black are reviewed in detail in this part of the handbook Fabrication technique Anodization Stain etching Anodization Anodization

Class of technique Etching (wet) Etching (wet) Etching (wet) Etching (wet)

Class of porosity Mesoporous Mesoporous Macroporous Microporous

Spark erosion

Etching (dry)

Mesoporous

Photoetching

Etching (wet)

Mesoporous

Laser ablation Hydrothermal etching Metal ion-assisted chemical etching (MACE) Galvanic etching Plasma deposition Vapor etching

Thermal Etching (wet) Etching (wet)

Mesoporous Mesoporous Mesoporous

Etching (wet) Deposition Etching (wet)

Mesoporous Mesoporous Mesoporous

Laser-induced plasma

Etching (dry)

Macroporous

Glancing angle deposition

Deposition

Mesoporous

Melt gasification

Thermal

Macroporous

Plasma hydrogenation Dealloying

Deposition Etching (wet)

Mesoporous Mesoporous

Templated plasma etch Laser-induced silane decomposition Magnesiothermic reduction of silica

Etching (dry) Deposition Conversion reaction

Mechanochemical reduction Milling/sintering

Conversion reaction Mechanical

Macroporous Mesoporous Mesoporous Microporous Mesoporous

Sodiothermic reduction of silica gel and templated sintering DRIE-UV lithography

Conversion reaction Etching (dry)

Macroporous

Femtosecond laser ablation

Thermal

Mesoporous

Ultrathin film annealing Anodization (alkali)

Thermal Etching (wet)

Mesoporous Macroporous

Macroporous Macroporous

Early paper on technique Uhlir (1956) Archer (1960) Theunissen (1972) Canham and Groszek (1992) Hummel and Chang (1992) Noguchi and Suemune (1993) Savin et al. (1996) Chen et al. (1996) Dimova-Malinovska et al. (1997) Ashruff et al. (1999) Kalkan et al. (2000) Saadoun et al. (2002) Kabashin and Meunier (2002) Beydaghan et al. (2004) Nakahata and Nakajima (2004) Abdi et al. (2005) Fukatani et al. (2005) Tian et al. (2005) Voigt et al. (2005) Bao et al. (2007) Zheng et al. (2007) Jacubowicz et al. (2007) Kim et al. (2008) Woldering et al. (2008) Mahmood et al. (2009) Fang et al. (2010) Abburi et al. (2010) (continued)

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Table 2 (continued) Fabrication technique Electrodeposition Ultrasonic etching Carbothermal reduction of silica Sacrificial template Sodiothermic reduction of silica Magnetron sputtering Micromachining and wet etching Platinum NP-assisted etching (PaCE) Ion implantation Templated silicon tetrachloride reduction Rochow reaction-based etching Dealloying in a metallic melt Magnesium silicide decomposition Ion track template etch Focused electron beam induced etching Regenerative electroless etching (ReEtching)

Class of technique Deposition

Class of porosity Mesoporous

Etching (wet) Conversion reaction Deposition Conversion reaction Deposition

Mesoporous Macroporous

Early paper on technique Krishnamurthy et al. (2011) Skorb et al. (2012) Yang et al. (2012)

Mesoporous Mesoporous

Huang et al. (2013) Wang et al. (2013)

Mesoporous

Etching (wet) Etching (wet)

Macroporous Mesoporous

Godhino et al. (2013) Deng et al. (2013) Li et al. (2013)

Irradiation

Macroporous

Conversion reaction Etching (dry) Conversion reaction Conversion reactions Etching (wet)

Mesoporous

Stepanov et al. (2013) Dai et al. (2014)

Macroporous Macroporous

Zhang et al. (2014) Wada et al. (2014)

Mesoporous

Liang et al. (2015)

Mesoporous

Kaniukov et al. (2016) Peer et al. (2016)

Etching (dry)

Macroporous

Etching (wet)

Mesoporous

Kolasinski et al. (2017)

lower the dispersity of both their size distribution and spacing have continued over the last decade (see handbook chapter ▶ “Colloidal Lithography” and for example, Smith et al. 2016). The most popular conversion reaction is currently the magnesiothermic reduction of porous silica, as introduced by Sandhage and coworkers in 2007 (Bao et al. 2007). This has been utilized with both synthetic silicas and biogenic silicas extracted from plants (see handbook chapter ▶ “Porous Silicon Formation by Porous Silica Reduction”). The major challenge in scalability for mesoporous silicon via this route is control of the strong exothermic nature of the reaction to avoid sintering. Indeed, carbothermal reduction (Yang et al. 2012) requires much higher temperatures and is more amenable to macroporous silicon fabrication. Sodiothermic reduction (Wang et al. 2013) can be carried out at very low temperatures but is probably less scalable because of the high cost and reactive nature of sodium metal. Similar restrictions are also applicable to the recent study using NaK alloy (Dai et al. 2014). Aluminothermic

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reduction (Zheng et al. 2007) looks much more attractive in this regard since aluminum is a very inexpensive metal. Note that there are currently very few techniques to make wholly microporous silicon (see handbook chapter ▶ “Microporous Silicon“) where the average pore diameter is under 2 nm. For virtually all top-down techniques, the porous silicon created is polycrystalline. For some bottom-up techniques such as sputtering/ dealloying (Fukatani et al. 2005), electrodeposition (Krishnamurthy et al. 2011), or sodiothermic reduction (Wang et al. 2013), it is reported to be amorphous. Choice of fabrication technique for both mesoporous and macroporous silicon is very much dictated by application area, which in turn has differing requirements on porosity levels, pore morphology, skeleton purity, physical form, cost, and volume (see handbook chapter ▶ “Porous Silicon Application Survey”).

References Abburi M, Bostrom T, Olefjord I (2010) Electrochemical texturing of multicrystalline silicon wafers in alkaline solutions. In: Proceedings of the 24th European photovoltaic solar energy conference, Hamburg, pp 1779–1783 Abdi Y, Derakhshandeh J, Hashemi P, Mohajerzadeh S, Karbassian F, Nayeri F, Arzi E, Robertson MD, Radamson H (2005) Light emitting nano-porous silicon structures fabricated using a plasma hydrogenation technique. Mater Sci Eng B124-125:483–487 Archer RJ (1960) Stain films on silicon. J Phys Chem Solids 14:104–110 Ashruf CMA, French PJ, Bressers PMMC, Kelly JJ et al (1999) Galvanic porous silicon formation without external contacts. Sens Actuat A 74:118–122 Bao Z, Weatherspoon MR, Shian S, Cai Y, Graham PD, Allan SM, Ahmad G, Dickerson MB, Church BC, Kang Z, Abernathy HW III, Summers CJ, Liu M, Sandhage KH (2007) Chemical reduction of three-dimensional silica micro-assemblies into microporous silicon replicas. Nat Lett 446:172 Beydaghyan G, Kaminska K, Brown T, Robbie K (2004) Enhanced birefringence in vacuum evaporated silicon thin films. Appl Opt 43(28):5343–5349 Canham LT, Groszek AJ (1992) Characterization of microporous silicon by flow calorimetry comparison with a hydrophobic silica molecular sieve. J Appl Phys 72:1558 Chen Q, Zhou G, Zhu J, Fan C, Li X-G, Zhang Y (1996) Ultraviolet light emission from porous silicon hydrothermally prepared. Phys Lett A 224:133–136 Dai F, Zai J, Yi R, Gordin ML, Sohn H, Wang D (2014) Bottom-up synthesis of high surface area mesoporous crystalline silicon and evaluation of its hydrogen evolution performance. Nat Commun 5:3605 Deng T, Chen J, Wu CN, Liu ZW (2013) Fabrication of inverted pyramid silicon nanopore arrays with three step wet etching. ECS J Solid State Sci Technol 2(11):419–422 Dimova-Malinovska D, Sendova-Vassileva M, Tzenov N, Kamenova M (1997) Preparation of thin porous silicon layers by stain etching. Thin Solid Films 297:285–290 Fang DZ, Striemer CC, Gaborski TR, JL MG, Fauchet PM (2010) Methods for controlling the pore properties of ultra-thin nanocrystalline silicon membranes. J Phys Cond Mater 22:454134 Fukatani K, Ishida Y, Aiba T, Miyata H, Den T (2005) Characterization of nanoporous Si thin films obtained by al-Si phase separation. Appl Phys Lett 87:253112 Godhino V, Caballero-Hernandez J, Jamon D, Rojas TC, Schierholz R, Garcia-Lopez J, Ferrer FJ, Fernandez A (2013) A new bottom-up methodology to produce silicon layers with a closed porosity nanostructure and reduced refractive index. Nanotechnology 24:275604 Huang X, Gonzalo-Rodriguez R, Rich R, Gryczynski Z, Coffer JL (2013) Fabrication and size dependent properties of porous silicon nanotube arrays. Chem Commun 49(51):5760–5762

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Hummel RE, Chang S-S (1992) Novel technique for preparing porous silicon. Appl Phys Lett 61(16):1965–1967 Jakubowicz J, Smardz K, Smardz L (2007) Characterisation of porous silicon prepared by powder technology. Physica E38:139–143 Kabashin AV, Meunier M (2002) Fabrication of photoluminescent Si-based layers by air optical breakdown near the silicon surface. Appl Surf Sci 186:578–582 Kalkan AK, Bae S, Li H, Hayes DJ, Fosash SJ (2000) Nanocrystalline Si thin films with arrayed void-column network deposited by high density plasma. J Appl Phys 88(1):555–561 Kaniukov EY, Ustarroz J, Yakimchuk DV, Petrova M, Terryn H, Sivakov V, Petrov AV (2016) Tunable nanoporous silicon oxide templates by swift heavy ion tracks technology. Nanotechnology 27(11):115305 Kim H, Han B, Choo J, Cho J (2008) Three-dimensional porous silicon particles for use in high performance lithium secondary batteries. Angew Chem Int Ed 47:10151–10154 Kolasinski KW, Gimbar NJ, Yu H, Aindow M, Makila E, Salonen J (2017) Regenerative electroless etching of silicon. Angew Chem Int Ed 56:624–627 Korotcenkov G, Cho BK (2010) Silicon porosification : state of the art. Crit Rev Solid State Mater Sci 35(3):153–260 Krishnamurthy A, Rasmussen DH, Suni II (2011) Galvanic deposition of nanoporous Si onto 6061 A1 alloy from aqueous HF. J Electrochem Soc 158(2):D68–D71 Li X, Xiao Y, Yan C, Song JW, Talvev V, Schweizer SL, Pielkieska K, Sprafke A, Lee JH, Wehrspoon RB (2013) Fast electroless fabrication of uniform mesoporous silicon layers. Electrochim Acta 94:57–61 Liang J, Li X, Hou Z, Qian Y (2015) Nanoporous silicon prepared through air oxidation demagnesiation of Mg2Si and its lithium ion batteries property. Chem Commun 51(33):7230 Liebes-Peer Y, Bandalo V, Sokmen U, Tornow M, Ashkenasy N (2016) Fabrication of nanopores in multilayered silicon-based membranes using focused electron beam induced etching with XeF2 gas. Microchim Acta 183:987–994 Mahmood AS, Sivakumar M, Venkatakrishnan K, Tan B (2009) Enhancement in optical absorption of silicon fibrous nanostructure produced using femtosecond laser ablation. Appl Phys Lett 95:034107 Nakahata T, Nakajima H (2004) Fabrication of lotus-type porous silicon by unidirectional solidification in hydrogen. Mater Sci Eng A 384:373 Noguchi N, Suemune I (1993) Luminescent porous silicon synthesized by visible light irradiation. Appl Phys Lett 62:1429–1431 Rouquerol J et al (1994) Recommendations for the characterization of porous solids. Pure Appl Chem 66(8):1739–1758 Sadadoun M, Mliki N, Kaabi H, Daoudi K, Bessais B, Ezzaouia H, Bennaceur R (2002) Vapouretching-based porous silicon: a new approach. Thin Solid Films 405:29–34 Savin DP et al (1996) Properties of laser ablated porous silicon. Appl Phys Lett 69(20):3048–3050 Skorb EV, Andreeva DB, Mohwald H (2012) Generation of a porous luminescent structure through ultrasonically induced pathways of silicon modification. Angew Chem Int Ed Engl 51(21):5138–5142 Smith RL, Collins SD (1992) Porous silicon formation mechanisms. Appl Phys Rev 71:R1–R22 Smith BD, Patil JJ, Ferralis N, Grossman JC (2016) Catalyst self-assembly for scalable patterning of sub 10nm ultrahigh aspect ratio nanopores in silicon. ACS Appl Mater Interfaces 8(12):8043–9049 Stepanov AL, Trifonov AA, Osin YN, Valeev VF, Nuzhdin VI (2013) Fabrication of nanoporous silicon by ag + ion implantation. Nanosci Nanoeng 1(3):134–138 Theunissen MJJ (1972) Etch channel formation during anodic dissolution of n-type silicon in aqueous hydrofluoric acid. J Electrochem Soc 119:351–360 Thommes M, Kaneko K, Neimark AV, Olivier JP, Rodriguez-Reinoso F, Rouquerol J, Sing KSW (2015) Physisorption of gases with special reference to the evaluation of surface area and pore size distribution (IUPAC technical report). Pure Appl Chem 87(9–10):1051–1069

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Tian L, Ram KB, Ahmad I, Menon L, Holtz M (2005) Optical properties of a nanoporous array in silicon. J Appl Phys 97:026101 Uhlir A (1956) Electrolytic shaping of germanium and silicon. Bell Syst Tech J 35:333–347 Voigt F, Bruggemann R, Unold T, Huisken F, Bauer GH (2005) Porous thin films grown from sizeselected silicon nanocrystals. Mater Sci Eng 25(5–8):584–589 Wada T, Ichitsubo T, Yubuta K, Segawa H, Yoshida H, Kato H (2014) Bulk nanoporous silicon negative electrode with extremely high cyclability for lithium ion batteries prepared using a top-down process. Nano Lett 14:4505–4510 Wang JF, Wang KX, Du FH, Guo XX, Jiang YM, Chen JS (2013) Amorphous silicon with high specific surface area prepared by a sodiothermic reduction method for supercapacitors. Chem Commun 49:5007–5009 Woldering LA, Tjerkstra RW, Jansen HV, Setija ID, Vos WL (2008) Periodic arrays of deep nanopores made in silicon with reactive ion etching and deep UV lithography. Nanotechnology 19:145304 Yang X, Zhang P, Shi C, Wen Z (2012) Porous graphite/silicon micro-sphere prepared by in-situ carbothermal reduction and spray drying for lithium ion batteries. ECS Solid Lett 1(2):M5–M7 Zhang Z, Wang Y, Ren W, Tan Q, Chen Y, Li H, Zhong Z, Su F (2014) Scalable synthesis of interconnected porous silicon/carbon composites by the Rochow reaction as high performance anodes of lithium ion batteries. Angew Chem Int Ed Eng 53(20):5165–5169 Zheng Y, Yang J, Wang J, NuLi Y (2007) Nano-porous Si/C composites for anode material of lithium ion batteries. Electrochim Acta 52:5863–5867

Porous Silicon Formation by Anodization Armando Loni

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anodization of Silicon Wafers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anodization Cells . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rinsing and Drying of Porous Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Layer Uniformity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nonaqueous Electrolytes . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrolyte Additives: Surfactants, Oxidizers, and Others . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Tunable Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Process Variability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrode Materials . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Health and Safety . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The key aspects of porous silicon manufactured by anodization are reviewed, with the following subjects being addressed: anodization of different wafer types, wafer cell design, post-anodization handling requirements (rinsing/drying/storage), parameters affecting layer uniformity, the use of nonaqueous electrolytes and electrolyte additives (surfactants, oxidizers, and other types), methods for tuning porosity, process control and natural variability, different electrode materials, and the requirements for maintaining health and safety. Keywords

Alcohol-based surfactant · Anodizsation · Anodizsation cells · Chemical oxidizser · Degree of anodizsation · Double electrochemical cell · Electrode A. Loni (*) Ledbury, Herefordshire, UK e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_2

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material · Health and safety · Non-aqueous electrolytes · Porosity · Rinsing · Silicon wafer · Surfactant

Introduction While the vast majority of published research on porous silicon is based upon layers produced by “bench-top” processing of wafer pieces and small-diameter wafers, this historical work underpins more recent developments in equipment and processing methodologies for commercial applications, where scaled-up manufacture, waferscale integration, and batch-to-batch reproducibility are key. Three comprehensive books on porous silicon have been published, wherein detailed information can be found related to silicon anodization (Canham 1997; Lehman 2002; Sailor 2012a). The topics covered include dissolution chemistries and the dependences of porosity, pore morphology, and pore size distribution on various parameters (e.g., wafer type/doping, electrolyte composition, current density, time); additionally, different types of electrochemical cells are discussed (Lehman 2002; Sailor 2012a), as well as some of the more practical aspects related to anodization (Sailor 2012a; e.g., wafer preparation, equipment and instrumentation, health and safety). The reader is referred to these references for essential background reading.

Anodization of Silicon Wafers The porosification of the surface of a silicon wafer is generally referred to as “anodization” and occurs when the wafer is anodically biased in a fluoride-based electrolyte solution. The most commonly used electrolyte component is hydrofluoric acid (HF), with ammonium fluoride (Kuhl et al. 1998) being less common. The degree of anodization is defined by the layer formation rate and porosity and, together with pore morphology, depends on wafer type and resistivity, the applied current density and time, and the electrolyte composition (HF concentration, with or without additives). Secondary parameters include electrolyte temperature and pH. The surface of a silicon wafer, as received from the manufacturer, will always be covered with a native oxide film. The oxide layer will be removed when the wafer is immersed in HF, although the cleanliness of the underlying surface can influence the anodization process and certain applications may therefore require wafer pre-cleaning prior to anodization (Sailor 2012b). A heavily doped wafer (p+,++, n+,++) can be readily anodized in a variety of HF-based electrolytes to form mesoporous silicon. A lightly doped wafer (p
, n
, and majority carrier concentration 90%) would normally exhibit crazing and pore collapse on air exposure (with loss of pore volume), unless kept wet before utilizing supercritical drying (Canham et al. 1994a). Chapter ▶ “Drying Techniques Applied to Porous Silicon” of this handbook focuses on such issues. In practice, it can be extremely difficult to completely remove residual solution from within micro-/mesopores due to inherently strong capillary forces – hence, the importance of repeated rinsing and washing, at the very least, to minimize the concentration of any toxic component.

Layer Uniformity The physical properties of a porous silicon sample can vary depending on, for example, the degree of lateral variation in layer thickness and/or porosity, the interface roughness between porous layers or at the layer/substrate boundary, and any vertical porosity gradation. As mentioned previously, careful design of the anodization equipment can minimize most sources of inhomogeneity. A uniform current presented across the whole area of the exposed wafer surface is essential, and for this reason, it is best to incorporate a counter electrode of a size (and shape) that is similar to the wafer. Additionally, electrical contact to the wafer must be uniform across the whole of the rear surface, especially important for low conductivity wafers. The use of a large graphite backing/guard plate has been shown to improve uniformity across the wafer, to some extent (Hossain et al. 2002). If light assistance is required during anodization, then electrode shadowing must be minimized on the front face of the wafer (e.g., by using a mesh or spiral arrangement). Electrical fringe effects can affect the anodization uniformity due to different relative current densities in comparison with the center of a wafer. If the wafer is partially immersed in the electrolyte, for example, the meniscus region will form with a higher current density, as will the thin edge/perimeter of the wafer that is in contact with the electrolyte; protecting such areas with a suitably defined electrolyteresistant mask can improve uniformity. If the wafer is patterned on the surface, the current density and charge flow differ at the mask edges and undercutting can result (Guendouz et al. 2000).

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The existence of chemical concentration gradients and the accumulation of hydrogen gas bubbles at the surface of the wafer (and also forming within the porous region itself), as well as the occurrence of a nonuniform temperature distribution caused by heating, are all problematic when anodizing in a static (non-flowing) electrolyte. While ultrasonic agitation can be used to liberate gas bubbles during anodization (Takai and Itoh 1986; Liu et al. 2003), a continuous (laminar) flow of electrolyte is recommended as a simpler means of flushing the hydrogen away from the wafer surface (eventually to be vented to the outside ambient), thus improving layer uniformity. A continuous flow also helps to dissipate heat buildup in the electrolyte and can minimize chemical concentration gradients within the electrolyte and at the pore fronts, both of which will affect the anodization process and properties of the resulting layers. For porous silicon multilayer structures (see chapter ▶ “Porous Silicon Multilayers and Superlattices” of this handbook), improved interface uniformity has been achieved by incorporating a short period of zero bias between individual layers, thereby allowing the hydrogen to diffuse from within the pores and the fluoride to be replenished at the pore tips (Takai and Itoh 1986). The “pulsed etching” (Billat et al. 1997) and “stop-etching” (Hou et al. 1996) techniques are based on a similar principle, with the latter (Khokhlov 2008) reportedly narrowing the pore size distribution for layers produced with high current density (although the role of static chemical leaching during the relatively longer periods under zero bias was not addressed) and the former finding use in the anodization of p–n junctions patterned on silicon wafers (McGinnis et al. 1999).

Nonaqueous Electrolytes Aside from the more common aqueous HF-based electrolytes, nonaqueous organic electrolytes in combination with a suitable fluoride source have been used, primarily for the production of macroporous layers both on silicon wafers (Propst and Kohl 1994; Rieger and Kohl 1995; Ponomarev and Levy-Clement 1998, 2000; Flake et al. 1999; Thakur et al. 2012a) and in freestanding form (Thakur et al. 2012b). Examples include acetonitrile, propylene carbonate, and dimethylformamide, with anhydrous HF (up to 2 M), tetrafluoroborate, and lithium fluoroborate being used as fluoride sources. The inclusion of a supporting electrolyte, such as tetrabutylammonium perchlorate (up to 0.25 M), has been shown to offer additional flexibility with regard to the resulting pore morphologies attained (Ponomarev and Levy-Clement 2000). In contrast to aqueous electrolytes, nonaqueous electrolytes facilitate silicon dissolution without hydrogen evolution.

Electrolyte Additives: Surfactants, Oxidizers, and Others The addition of a surfactant (wetting agent) to the electrolyte helps to prevent evolving hydrogen bubbles from “sticking” to the porous silicon surface. Alcohol (e.g., methanol, ethanol) is the most common, with, among others, formic acid

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(Baranov et al. 2000), acetic acid (Baranov et al. 2000; Semai et al. 2009), and sodium laurel sulfate (Ogata et al. 2000); commercially available surfactants such as NCW-1001 (Ogata et al. 2000), Mirasol (Lehman and Foll 1990), Triton X-100 (Chao et al. 2000), and DECON (Kordas et al. 2001) have also been used. While alcohol-based surfactants are used in substantial quantities (>15% v/v), the cationic or anionic surfactants (Chao et al. 2000; Sotgiu et al. 1997) are used in concentrations as low as 10
4 M (typically 5% v/v) and sometimes in combination with alcohol. An alcohol-based surfactant will instigate chemical leaching of the porous silicon layer during anodization (discussed below), particularly so with long anodization times and high alcohol content, and this can result in a porosity gradient within the layer (more porous at the surface). Without surfactant, however, an anodized layer can be nonuniform in thickness, with substantial interface roughness at the porous silicon substrate (Halimaoui 1993) or between different layers. Adding hydrochloric acid to the electrolyte changes the pH and can lead to favorable changes in the properties of the anodized material such as, for example, enhanced and stable photoluminescence (Zangooie et al. 1998; Belogorokhov and Belogorokhova 1999). Various types of chemical oxidizer have been incorporated into HF-based electrolytes. CrO3 has been used (Foll et al. 2000; Christophersen et al. 2000; Ouyang et al. 2005) to produce macroporous silicon, while potassium permanganate (KMnO4) also acts to increase pore size (Ogata et al. 2000; Harraz et al. 2008). The inclusion of hydrogen peroxide (H2O2) has been shown to produce layers with monohydride passivation (Yamani et al. 1997), although its role in the formation of layers with wider pores has been the subject of greater interest (Ogata et al. 2000; Ge et al. 2010). Reduced interface roughness, previously observed (Setzu et al. 1998; Servidori et al. 2000) in samples anodized at low temperature and attributed to an increased electrolyte viscosity, has been replicated (for some conditions) by adding a small fraction of glycerol to the electrolyte (Kan et al. 2005). In situ functionalization of porous silicon during anodization has been achieved by incorporating the HF-compatible organic molecule 1-heptyne (C7H12) in the electrolyte, in concentrations up to 0.9 M (Mattei and Valentini 2003).

Tunable Porosity If an alcohol surfactant is incorporated in the electrolyte, then the porosity of a layer will depend on the dilution used, higher porosities being obtained with higher alcohol content. As mentioned previously, the in situ chemical leaching of a layer (with or without electrical bias) in HF-alcohol electrolytes can be used to good effect to increase porosity (Halimaoui 1994; Herino et al. 1987) and pore size (Herino et al. 1987; Herino 1997). Porosity can be more easily “tuned” through choice of the applied current density and anodization time (Herino et al. 1987; Herino 1997), and this is the basis of forming complex multilayer or graded structures. When comparing different anodization processes, the current densities (assuming the wafer areas are the same)

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Table 1 Layer parameter dependency on current density and time Current density (mA/cm2) 33 99 99

Anodization time (min) 90 30 60

Porosity (%) 66 78 86

Depth (μm) 161 125 234

Surface area (m2/ g) 296 434 495

Pore volume (ml/g) 0.759 1.435 2.267

Average pore diameter (nm) 10 10 18

Detached mesoporous silicon layers produced with 600 p-type wafers, 0.005–0.02 Ω cm, using 1:1 40% HF:methanol electrolyte; porosity determined gravimetrically; surface area and pore volume/ diameter determined by nitrogen gas adsorption – author’s data

and anodization times should be chosen such that the overall charge flow is comparable for each process – this facilitates “like-for-like” comparison of the resulting properties. If an alcohol surfactant is present, however, it can be difficult to completely preclude the effects of chemical leaching (Herino et al. 1987), and this should be considered when making process comparisons. From Table 1, it can be tentatively surmised that, for a fixed charge flow, the porosity difference between 33 mA/cm2 (90 min) and 99 mA/cm2 (30 min) is primarily due to the enhanced electrochemical dissolution of the layer at the higher current density; when the current density is fixed (99 mA/cm2), the higher porosity for the 60 min process is primarily due to chemical leaching of the layer. As expected, the different porosities are accompanied by variations in surface area, pore volume, and average pore diameter. Porosity can also be tuned by applying a magnetic field during anodization. A magnetic field applied perpendicular to the wafer surface, with the electrolyte temperature maintained at 0  C, can regulate the supply of holes, the resulting porosity being dependent on the magnetic field strength (Nakagawa et al. 1996).

Process Variability When comparing individual wafers or layers produced sequentially, or under different anodization conditions, the condition of the electrolyte must be considered, as this leads to process variation. The accumulation of silicon in the electrolyte from each dissolution process (e.g., in the form of H2SiF6), combined with the steady depletion of fluorine, is evinced by a gradual increase in electrolyte conductivity and pH; this natural evolution of electrolyte composition with continuous use is particularly important for highthroughput production, where associated variations within a fixed volume of electrolyte include a gradual reduction in layer thickness and an increase in porosity (reduced yield). Indeed, very high porosities can be achieved with a “well-used” electrolyte. The point that defines the end of the useful life of the electrolyte depends on the highest porosity or yielded weight of porous silicon that is acceptable for the intended product or application. In order to maintain the desired blend, the electrolyte

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can be replenished by auto-dosing (with fresh HF and/or surfactant) on a continual basis; this requires a feedback control loop whereby some indicator of the condition of the electrolyte is continuously monitored, e.g., HF concentration (Nehmann et al. 2012). However, the working volume of the equipment and the need to dispose of large quantities of waste electrolyte in a safe manner both limit the extent to which auto-dosing can be used. When a number of wafers are anodized simultaneously, the amount of electrical power supplied to the electrolyte is much greater than that required for a single wafer; this leads to electrolyte heating, which, depending on current density, anodization time, and wafer batch size, can be significant. The chemical dissolution process is temperature dependent (Garman et al. 2001; Balagurov et al. 2006), while the evaporation rate of any volatile surfactant will also increase at elevated temperature. It has been shown, also, that photoluminescence wavelength and intensity and layer crystallinity are dependent on the electrolyte temperature used for anodization (Ono et al. 1993). The control of electrolyte temperature during anodization is therefore important in many respects. Any heat generated during anodization will be dissipated to some extent by continuously pumping the electrolyte within the equipment, although the use of a cooling coil (e.g., in the electrolyte reservoir) or integrated chilling bath can be more effective.

Electrode Materials Platinum metal is commonly used as an electrode material, due to its relatively high chemical resistivity. It has been shown (Pourbaiz et al. 1959; Llopis and Sancho 1961; Kodera et al. 2007), however, that anodically biased platinum slowly corrodes in the presence of acidic solutions. While the corrosion rate is extremely small, if the same counter electrode is used continuously, a buildup of platinum (and its oxide) occurs in the electrolyte; this is deposited within the equipment, as evinced by a dark film lining the inside of the components; deposition also occurs within the porous silicon layers (typically 80 min in concentrated HF solutions by a 3 μm-thick AZ 6130 photoresist film. The morphological, electrical, and mechanical effects of etched poly-Si structures have been extensively investigated by Stoldt and co-workers (Miller et al. 2005, 2007, 2008; Becker et al. 2010a, b, 2011). Micromachined p-type polysilicon in contact with a Au layer demonstrated heterogeneous cracking or porosity across the poly-Si surface as a result of etching. This resulted in greatly increased electrical resistance and decreased the characteristic frequency of mechanical resonators.

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Because the mechanical properties of MEMS depend strongly on their structure, changes that occur during post-processing are critical for determining device characteristics and whether they perform as designed. Miller et al. (2007) measured the decrease in the frequency of mechanical resonance that occurred as a function of immersion time in HF for microcantilevers as well as “comb drives” in contact with Au. Time-dependent variation was also observed in the modulus and hardness measured during indentation testing. They observed that grain delineation accompanied the formation of a nanoscale porous layer in the near surface region of the poly-Si. The por-Si layer exhibited decreased stiffness, which changed the effective thickness of the beams. Galvanic etching can greatly influence the material properties, design, performance, lifetime, tribology, manufacture, and required operating environment of microscale and nanoscale devices. Morphology, resistive probe, surface wetting, and electrochemical characterization of single- and polycrystalline Si subjected to galvanic corrosion in concentrated 48 wt% HF, 23 wt% HF diluted with water, and a 20:1 solution of 23 wt% HF with Triton X-100 were performed by Miller et al. (2008) The measured current density of micromachined Si was compared with (100) wafers using polarization characterization, identifying the por-Si formation regime. Porosity ranged from 20% to 47% but in some cases was as high as 70%. The porosity generally increased as the surface area ratio of Au to Si increased. Chronopotentiometry, resistive probe, and microtensile characterizations were used to identify regimes of rapid initiation, subsequent steady-state corrosion, and the final catastrophic failure of the microtensile specimens. Corrosion current depended exponentially on the amount of metal present. This implied that the corrosion rate was limited by the surface area of the metal cathode. Addition of surfactant led to higher current densities and more uniform layers. Subsequently, this group (Becker et al. 2010b) used focused ion beam (FIB) milling of microscale silicon-on-insulator (SOI) devices to determine the depth uniformity of the galvanically formed por-Si as a function of the geometry of the device. They developed a finite element method simulation to model the galvanic corrosion process. The model reproduced the current-limited condition resulting from the finite surface area of metal relative to Si and predicted the uniform etch rate across the device for surfactant-enhanced HF solutions as observed after FIB milling. Becker et al. (2010a) have optimized the process to produce thick, high surface area por-Si films. Films up to ~150 μm thick with specific surface area ~700 m2 g 1 and pore diameters ~3 nm were fabricated. Subsequently, this group optimized their Galvanic process for even greater specific area (Becker et al. 2011). A 170-nm-thick Pt layer was sputtered on the back of the Si to form the cathode, and an H2O2/HF/ ethanol solution acted as the etchant. The H2O2 concentration was 2.4 vol% of 30% H2O2 in water. The HF to ethanol ratio was either 20:1 or 3:1. The surface area ratio of Pt to Si was ~5. In the 20:1 solution, films with specific surface areas of ~840 m2 g 1 and porosities of 65–67% were obtained. Somewhat higher specific areas (890–910 m2 g 1) and porosities (79–83%) were observed for the 3:1 solution.

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An important feature of the extremely high surface area films is that they retain their H-termination. The oxidation of both Si and H to form SiO2 (or suboxides because of incomplete combustion) and H2O is extremely exothermic. Filling the pores with an oxidant such as perchlorate, nitrate, S, or O2 creates a potentially explosive source of energy (Clement et al. 2005; Kovalev et al. 2001). Nanostructured composites outperform conventional powder-based composites as energetic materials (Zhou et al. 2014). Becker et al. (2015) have probed the ignition of patterned microfabricated wires with high-speed video methods. Piekiel and Morris (2015) have explored the minimum spacing between galvanically etched por-Si combustion zones that can be independently ignited. Combustion depends on surface area and porosity as well as oxidant packing (Piekiel et al. 2015; Abraham et al. 2016; Parimi et al. 2014a, b; Ohkura et al. 2013; Churaman et al. 2015). Application of por-Si as an energetic material has recently been reviewed (du Plessis 2014). Galvanic cells can be formed on Si powders just as they can for c-Si and poly-Si. Nielsen et al. (2007) used galvanic displacement to deposit Pt on Si powder grains, which were dispersed in an aqueous mixture of HF and H2PtCl6 for 15 min. The grains were then removed and dispersed in a 1:3:2 mixture of HF/H2O2/methanol to affect galvanic etching. They used this procedure to produce dispersions of photoluminescent Si nanoparticles in the size range of 3–6 nm by subsequent sonication of the grains in isopropanol. Only a thin porous layer was produced, and the core of the originally 80 μm diameter particles remained unaffected. Nanoparticles could be generated in a similar fashion by substituting a Si wafer for the Si powder. Nakamura et al. investigated both Ag/por-Si (Nakamura et al. 2010) and Pt/por-Si (Nakamura et al. 2011) composite powders. Metallurgical grade Si powder was added to the 18% HF, and then AgNO3 was added to the mixture held at 30  C. Nanometer-sized Ag particles were deposited singly or as aggregates on the Si powder surface. Higher concentrations of AgNO3 resulted in decreased Si volume and increased Ag layer thickness. The powders were photoluminescent. The PL intensity was weaker but more stable than that of conventionally stain-etched por-Si powders. Pt layers were deposited on a Si surface together with the formation of a porous structure. It was found that the oxidation state of Pt layers strongly depended on the conditions for the preparation of Pt/PSi composite powders. Galvanic cell formation and Si etching are also strongly linked to metal deposition on Si surfaces (Kolasinski 2014, 2015, 2016; Ogata and Kobayashi 2006; Allongue and Maroun 2006; Gorostiza et al. 2000, 2003; da Rosa et al. 2008; Kolasinski et al. 2015). Electron transfer from the Si valence band to metal ions deposited from HF solutions leads to etching in the vicinity of deposited metal. Applications to micro- and nanoscale devices in fields ranging from electronic devices to chemical sensors including schemes developed for the metallization and nanopatterning of semiconductor substrates with high selectivity and with optimal interfacial properties have been discussed by Carraro et al. (2007). To understand the rate as well as pH and concentration dependence of deposition, it is essential to apply

Porous Silicon Formation by Galvanic Etching

a

F-

Si3N4

33

b Si3N4

PS

holes Si Pt H+

H2O2 + 2H+ + 2e-→2H2O

Fig. 1 (a) Illustration of galvanic etching with a backside Pt layer and frontside Si3N4 mask to facilitate patterned porous silicon formation. (b) Scanning electron microscope cross section of porous layer bordering the Si3N4 mask. (Reprinted with permission from Becker et al. Nano Lett. 2011, 11 803–807. Copyright 2011 American Chemical Society)

mixed-potential theory (Gorostiza et al. 2000, 2003; da Rosa et al. 2008). The morphology of the deposit is of great interest in metal-assisted etching (Huang et al. 2011; Peng et al. 2003). Large-area nanostructured noble-metal films of Ag, Pt, and Au can be deposited (Song et al. 2005) with various morphologies on Si. The morphology of Ag films, which is different from that of Pt and Au, depends sensitively on the deposition conditions. Metal nanoparticles adhere weakly to Si, and their size and spatial distribution can be affected by drying conditions (Ogata and Kobayashi 2006; Kolasinski et al. 2015; Carraro et al. 2007; Magagnin et al. 2002). The roughness of Cu films was found to decrease with increasing HF to CuSO4 concentration ratio. Concurrently, the Si surface became less oxidized and lateral connectivity between Cu nuclei increased (da Rosa et al. 2008). Au nanostructures, clusters, or dendrites with leaflike or branch-like structure can be prepared by adjusting the temperature, concentration of HAuCl4, ultrasonic agitation, and addition of ethanol (Wang et al. 2006) (Fig. 1).

Conclusion Galvanic and metal-assisted etching follows the same electrochemistry. The differences between them are related to the initial structure of the metal electrocatalyst that is responsible for making the electrochemical etching spontaneous. While studies of galvanic etching are dwarfed by the number devoted to metal-assisted etching, it will continue to be an essential process to understand for any application that involves the formation of a metal/silicon interface that is subsequently exposed to HF, such as in the fabrication of MEMS devices. Thick uniform films with surface areas as high as 910 m2 g 1 have been achieved with optimized galvanic etching (Becker et al. 2011) and have significant potential as on-chip energetic material.

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Kolasinski KW, Barclay WB, Sun Y, Aindow M (2015) The stoichiometry of metal assisted etching of Si in V2O5 + HF and HOOH + HF solutions. Electrochim Acta 158:219–228 Kovalev D, Timoshenko VY, Künzner N, Gross E, Koch F (2001) Strong explosive interaction of hydrogenated porous silicon with oxygen at cryogenic temperatures. Phys Rev Lett 87:068301 Lehmann V (2002) Electrochemistry of silicon: instrumentation, science, materials and applications. Wiley-VCH, Weinheim Li XL (2012) Metal assisted chemical etching for high aspect ratio nanostructures: a review of characteristics and applications in photovoltaics. Curr Opin Solid State Mater Sci 16:71–81 Li X, Bohn PW (2000) Metal-assisted chemical etching in HF/H2O2 produces porous silicon. Appl Phys Lett 77:2572–2574 Liu YF, Xie J, Zhao H, Luo W, Yang JL, An J, Yang FH (2012) An effective approach for restraining electrochemical corrosion of polycrystalline silicon caused by an HF-based solution and its application for mass production of MEMS devices. J Micromech Microeng 22:035003 Macuk AL, Timpe SJ (2013) Effect of galvanic corrosion-induced roughness on sidewall adhesion in polycrystalline silicon microelectromechanical systems. J Microelectromech Sys 22:259–261 Magagnin L, Maboudian R, Carraro C (2002) Gold deposition by galvanic displacement on semiconductor surfaces: effect of substrate on adhesion. J Phys Chem B 106:401–407 Meltzer S, Mandler D (1995) Study of silicon etching in HBr solutions using a scanning electrochemical microscope. J Chem Soc Faraday Trans 91:1019–1024 Miller DC, Gall K, Stoldt CR (2005) Galvanic corrosion of miniaturized polysilicon structures morphological, electrical, and mechanical effects. Electrochem Solid State Lett 8:G223–G226 Miller DC, Hughes WL, Wang ZL, Gall K, Stoldt CR (2007) Mechanical effects of galvanic corrosion on structural polysilicon. J Microelectromech Syst 16:87–101 Miller DC, Becker CR, Stoldt CR (2008) Relation between morphology, etch rate, surface wetting, and electrochemical characteristics for micromachined silicon subject to galvanic corrosion. J Electrochem Soc 155:F253–F265 Muhlstein CL, Stach EA, Ritchie RO (2002) A reaction-layer mechanism for the delayed failure of micron-scale polycrystalline silicon structural films subjected to high-cycle fatigue loading. Acta Mater 50:3579–3595 Nakamura T, Hosoya N, Tiwari BP, Adachi S (2010) Properties of silver/porous-silicon nanocomposite powders prepared by metal assisted electroless chemical etching. J Appl Phys 108:104315 Nakamura T, Tiwari BP, Adachi S (2011) Direct synthesis and enhanced catalytic activities of platinum and porous-silicon composites by metal-assisted chemical etching. Jpn J Appl Phys 50:081301 Nielsen D, Abuhassan L, Alchihabi M, Al-Muhanna A, Host J, Nayfeh MH (2007) Current-less anodization of intrinsic silicon powder grains: formation of fluorescent Si nanoparticles. J Appl Phys 101:114302 Noguchi N, Suemune I (1993) Luminescent porous silicon synthesized by visible light irradiation. Appl Phys Lett 62:1429–1431 Noguchi N, Suemune I (1994) Selective formation of luminescent porous silicon by photosynthesis. J Appl Phys 75:4765–4767 Ogata YH, Kobayashi K (2006) Electrochemical metal deposition on silicon. Curr Opin Solid State Mater Sci 10:163–172 Ohkura Y, Weisse JM, Cai LL, Zheng XL (2013) Flash ignition of freestanding porous silicon films: effects of film thickness and porosity. Nano Lett 13:5528–5533 Parimi VS, Tadigadapa SA, Yetter RA (2014) Reactive wave propagation mechanisms in energetic porous silicon composites. Combust Sci Technol 187:249–268 Parimi VS, Tadigadapa SA, Yetter RA (2014) Effect of substrate doping on microstructure and reactivity of porous silicon. Chem Phys Lett 609:129–133 Peng K-Q, Yan Y-J, Gao S-P, Zhu J (2002) Synthesis of large-area silicon nanowire arrays via selfassembling nanoelectrochemistry. Adv Mater 14:1164–1167

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Peng K-Q, Yan Y-J, Gao S-P, Zhu J (2003) Dendrite-assisted growth of silicon nanowires in electroless metal deposition. Adv Func Mater 13:127–132 Piekiel NW, Morris CJ (2015) Small-scale, self-propagating combustion realized with on-chip porous silicon. ACS Appl Mater Interfaces 7:9889–9897 Piekiel NW, Morris CJ, Churaman WA, Cunningham ME, Lunking DM, Currano LJ (2015) Combustion and material characterization of highly tunable on-chip energetic porous silicon. Propellants Explos Pyrotechnics 40:16–26 Pierron ON, Macdonald DD, Muhlstein CL (2005) Galvanic effects in Si-based microelectromechanical systems: thick oxide formation and its implications for fatigue reliability. Appl Phys Lett 86:211919 Quint SB, Pacholski C (2009) A chemical route to sub-wavelength hole arrays in metallic films. J Mater Chem 19:5906–5908 Song YY, Gao ZD, Kelly JJ, Xia XH (2005) Galvanic deposition of nanostructured noble-metal films on silicon. Electrochem Solid State Lett 8:C148–C150 Splinter A, Stürmann J, Benecke W (2001) Novel porous silicon formation technology using internal current generation. Mater Sci Eng C 15:109–112 Splinter A, Sturmann J, Benecke W (2001) New porous silicon formation technology using internal current generation with galvanic elements. Sens Actuators A 92:394–399 Sun NN, Chen JM, Jiang C, Zhang YJ, Shi F (2012) Enhanced wet-chemical etching to prepare patterned silicon mask with controlled depths by combining photolithography with galvanic reaction. Ind Eng Chem Res 51:793–799 Turner DR (1960) On the mechanism of chemically etching germanium and silicon. J Electrochem Soc 107:810–816 Wang CH, Sun DC, Xia XH (2006) One-step formation of nanostructured gold layers via a galvanic exchange reaction for surface enhancement Raman scattering. Nanotechnology 17:651–657 Xia XH, Ashruf CMA, French PJ, Kelly JJ (2000) Galvanic cell formation in silicon/metal contacts: the effect on silicon surface morphology. Chem Mater 12:1671–1678 Zhou X, Torabi M, Lu J, Shen RQ, Zhang KL (2014) Nanostructured energetic composites: synthesis, ignition/combustion modeling, and applications. ACS Appl Mater Interfaces 6: 3058–3074

Porous Silicon Formation by Stain Etching Kurt W. Kolasinski

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Etchant Composition . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Rational Formulation of Stain Etchants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Practical Advice . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

40 42 49 49 50

Abstract

Electroless etching of silicon engendered by hole injection from solution phase strong oxidants in the absence of metal films or metal nanoparticles is reviewed. This type of electroless etching to form porous silicon is commonly known as stain etching. This updated and expanded review encompasses stain etching of crystalline wafers as well as silicon powders. Results for the most important porous silicon-forming oxidants (NO3
, VO2+ and Fe3+) as well as all other oxidants that cause etching with or without significant porous silicon formation are discussed. Emerging diverse application areas for stain-etched silicon include drug and gene delivery, bioimaging, photovoltaics, and lithium batteries. Keywords

Porous silicon · Electroless etching · Stain etching · Thin films · Porous materials · Nanoparticles · Porous powders · Hydrofluoric acid · Vanadium pentoxide · Nitric acid · Ferric ion

K. W. Kolasinski (*) Department of Chemistry, West Chester University, West Chester, PA, USA e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_4

39

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K. W. Kolasinski

Introduction The topic of this updated review is the electroless etching of silicon (wafer or powder, Fig. 1) by a fluoride solution in which the reaction is initiated by hole injection from a sufficiently strong oxidant. Spontaneous electroless etching is commonly called stain etching when it leads to the formation of a porous silicon film (por-Si). Fuller and Ditzenberger (1957), Turner (1960), and Archer (1960) were unaware of the porous nature of the films they formed but named the process after the colored stains that appeared on their wafer substrates. Because some of the most effective oxidants contain a metal ion, it has sometimes also been denoted metal-assisted etching. This term is, however, ambiguous and easily confused with a related catalytic etching process in which a metal nanoparticle acts to catalyze hole injection from an oxidant. I will call the latter catalytic etching process metal-assisted etching (MAE). Oxidants that are capable of stain etching (spontaneous electroless etching) can also be used in conjunction with metal catalyst particles (Kolasinski 2016; Kolasinski et al. 2015; Cao et al. 2016; Bai et al. 2012; Megouda et al. 2009, 2013). Interestingly, as reviewed elsewhere in this volume, the most frequently used oxidant for metalassisted etching (HOOH) has such poor hole injection kinetics (Kooij et al. 1998; Gondek et al. 2014) that it does not lead to significant por-Si formation in the absence of a metal catalyst. Sapelkin and coworkers have recently demonstrated a variant of stain etching for Ge (Karatutlu et al. 2015). Stain etching is spontaneous, but what differentiates it from chemical etching is that free charge transfer is involved in stain etching. Hydroxide exhibits both chemical and electrochemical etching pathways (Allongue et al. 1993a, b). Etching in acidic fluoride is exclusively electrochemical (Kolasinski 2008), with an extremely low etch rate in the absence of a bias, light, or dissolved oxygen (Kolasinski 2003).

Fig. 1 (a) Four Si(100) substrates stain etched for different times to produce homogeneous smooth films of different depths, hence different colors caused by thin film interference in the porous silicon layers. (b) Green to orange photoluminescence under UV excitation during electroless etching of metallurgical grade Si powder in a V2O5 + HF solution

Porous Silicon Formation by Stain Etching

41

Chemical etching can be used to produce nearly perfectly flat and hydrogenterminated Si surfaces (Burrows et al. 1988; Chabal et al. 1993; Hines et al. 2012). Chemical etching is initiated by the action of OH
. Over 30 years after the first reports of stain etching, it was demonstrated that it could produce photoluminescent por-Si films (Sarathy et al. 1992; Fathauer et al. 1992). A sketchy early report of electroluminescence in a stain layer was attributed by Gee (1960) to amorphous Si. However, the film was likely a por-Si layer created by a 0.1% HNO3 + 49% HF solution. It is in some respects unfortunate that Turner discovered stain etching by using HNO3 as the oxidant. The HNO3 + HF system has the advantage that the solution composition can be varied to cover both an electropolishing regime, in which very high etch rates are observed accompanied by the formation of flat surfaces, as well as a por-Si formation regime. Unfortunately, the reduction of NO3
and the myriad of other nitrogen-containing species that are formed as byproducts are extremely complex. Only recently have Acker and coworkers been able to establish the role of these various species in the electropolishing regime (Acker et al. 2012; Hoffmann et al. 2011; Steinert et al. 2005, 2006, 2008; Weinreich et al. 2007; Steinert et al. 2007; Jadzinsky et al. 2007; Henssge and Acker 2007; Acker and Henssge 2007). This complexity (i.e., high sensitivity to initial composition, temperature, extent of reaction, various dissolved gases, age of solution, etc.) also leads to irreproducibility with respect to the formation of photoluminescent por-Si films in HNO3 + HF solutions. The formation of thick films as well as homogeneous films is rather difficult with this system. One application of stain etching in HF + HNO3 solutions has been in the production of “black silicon,” that is, broadband antireflection coatings, which are particularly useful in solar cell applications (Menna et al. 1995; Liu et al. 2014; Otto et al. 2015). Control of the porosity profile, and therefore refractive index profile, can be particularly useful for tuning the reflectivity of por-Si films (Schirone et al. 1997, 2000; Striemer and Fauchet 2002). A stain-etched por-Si layer can also increase the carrier lifetime (Khezami et al. 2016) in a photovoltaic device. Stain etchants can be used not only on crystalline Si but also multicrystalline Si solar cells to reduce reflectivity (Gonzalez-Diaz et al. 2009; Bilyalov et al. 2003), and treatment with HF + HNO3 is the most widely used technique to texturize multicrystalline silicon wafers for solar cell production (Meinel et al. 2014). Similar texturing can be achieved with HF + Fe(NO3)3 solutions (Zhang et al. 2013). Fe(NO3)3 is a particularly aggressive oxidant since both the Fe3+ and NO3
ions can inject holes into the silicon valence band (Dudley and Kolasinski 2009a). Porous Si layers have long been used in micromachining (Steiner and Lang 1995), and etching of Si in HF + HNO3 or NaNO2 solutions has also found uses in these applications (Melnikov et al. 2008; Yamamura and Mitani 2008). Etching through a mask with HF + HNO3 has been used to produce an ordered array of microlens molds. These studies have also lead to the development of a numerical model to simulate the isotropic wet etching of silicon (Baranski and Albero 2011). Stain-etched layers have been doped with Er3+ and Yb3+ to produce layers that may help to extend and enhance the wavelength response of Si solar cells (Diaz-Herrera et al. 2009, 2011). Porous Si films can also be transformed into superhydrophobic surfaces (Liu et al. 2009).

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This review attempts to be comprehensive regarding etching with HF + oxidants other than NO3
solutions. One motivation for this is that some of these solutions produce por-Si in a much more reliable, reproducible, and controllable manner than NO3
solutions. A particularly promising recent application of stain etching is the production of powders and nanocrystals. Loni et al. (2011) have demonstrated that Fe3+ can be used to create high-surface-area por-Si from low-cost metallurgical grade Si. Such powders and nanoparticles are particularly interesting for drug delivery and bioimaging applications (Loni et al. 2015, 2016; Batchelor et al. 2012; Cheng et al. 2014). Their structure has been investigated by Chadwick and coworkers (2012, 2013) as well as Ge et al. (2014). Similarly, Sato and coworkers have used HNO3 + acetic acid + HF solutions (with added methanol and ultrasonic agitation) to produce Si nanocrystals with variable visible photoluminescence (Sato et al. 2009a, b). Hao et al. have used HNO3 + ethanol + HF solutions to porosify ultrafine Si powders (Hao et al. 2014). Swihart and coworkers have used laser pyrolysis of silane to produce Si nanoparticles that were subsequently etched in HNO3 + HF to produce photoluminescent Si nanoparticles (Liu and Swihart 2014; Hua et al. 2006, 2005; Li et al. 2003, 2004). Boukherroub and coworkers have applied stain etching to the formation of a variety of Si nanostructures (Megouda et al. 2009, 2013; Pan et al. 2013a, b, c, d; Ayat et al. 2014). They have created a new family of luminescent and stable siliconbased nanoparticles in which decyl-capped Si nanocrystals are loaded into micelles (Pan et al. 2013b). Si wafers were etched in V2O5 + HF solutions then sonicated and hydrosilylated to produce the decyl-capped Si nanoparticles. Alternatively, these nanoparticles can be trapped in 80 nm lipid nanocapsules (Pan et al. 2013a). The resulting aggregates are highly luminescent in both cases. Mughal et al. (2014) used sonication of por-Si layers produced on wafers by stain etching with Fe3+ to generate Si nanoparticles. Ge et al. (2014) etched metallurgical powder with Fe(NO3)3 + HF and studied the reversibility of the lithiation. The results demonstrate that por-Si powders are promising candidates for use as anodes in high energy density rechargeable Li ion batteries. A particularly ingenious application of electroless etching is the formation of porous particles in which Si is encapsulated within a C coating (Park et al. 2013; Xu et al. 2014). Such particles have demonstrated favorable characteristics for reversible Li storage and discharge.

Etchant Composition Fluoride in a stain etchant is usually supplied by HF, although solutions with strong fluorine containing acids such as HBF4 and HSbF6 have also been reported (Parbukov et al. 2001; Ünal et al. 2001; Sapelkin et al. 2006). NH4HF2 can be used to replace HF as long as the solution is acidified (Mills and Kolasinski 2004; Nahidi and Kolasinski 2006). An unusual example is that of CeF4 dissolved in concentrated H2SO4 (Kolasinski et al. 2012; Dudley and Kolasinski 2009b). In this case, the oxidant and fluoride are supplied by the same species. This stain etchant

Porous Silicon Formation by Stain Etching

43

leads to extremely uniform and highly luminescent por-Si layers but is limited to very slow etch rates because of the low solubility of CeF4. By far the most commonly used oxidant is HNO3, either to etch Si uniformly (Yamamura and Mitani 2008; Robbins and Schwartz 1959, 1960; 1961, 1976; Robbins 1962; Jenkins 1977; Kooij et al. 1999; Kulkarni and Erk 2000; Svetovoy et al. 2006; Bauhuber et al. 2013) or to form por-Si (Turner 1960; Archer 1960; Beckmann 1965; Beale et al. 1986; Shih et al. 1992; McCord et al. 1992; Kidder et al. 1992; Dubbelday et al. 1993; Shih et al. 1993; Steckl et al. 1994; Chandler-Henderson et al. 1994; Liu et al. 1994; Kalem and Rosenbauer 1995; Anaple et al. 1995; Amato 1995; Schoisswohl et al. 1995; Jones et al. 1995; Di Francia et al. 1995; Di Francia and Citarella 1995; Winton et al. 1996, 1997; Velasco 2003; Guerrero-Lemus et al. 2003; González-Díaz et al. 2006; Liu et al. 2007; Zeng et al. 2005; Melnichenko et al. 2005; Luchenko et al. 2007; Balaguer and Matveeva 2010; Mogoda et al. 2011; Lippold et al. 2011, 2012; Terheiden et al. 2011). Other sources of nitrogen oxo ions such as NaNO2 (Archer 1960; Melnikov et al. 2008; Kelly et al. 1994; Campbell et al. 1995; Vázsonyi et al. 2001; Abramof et al. 2006; Abramof et al. 2007), NO2 (Archer 1960; Yoshioka 1969), NOBF4 (Kelly et al. 1994; Safi et al. 2002), or NOHSO4 (Patzig et al. 2007) have been used, but these are essentially running off of the same chemistry. A variation on standard HNO3-based stain etching was demonstrated by Chen et al. (1996, 2001; Li et al. 1999). They applied a hydrothermal reaction to produce por-Si. In their first report, they dissolved 0.3 mol L
1 LiF in 10.0 mol L
1 HNO3 then heated this solution with the Si substrate to 140  C inside of a stainless steel vessel for 2 h. The resulting 70% porous material was heavily oxidized, photoluminescent, and 10 μm thick. Subsequently, they switched to 0.3 mol L
1 Fe(NO3)3 in 40 wt% HF heated to 142  C for 45 min. This treatment leads to the incorporation of iron into the por-Si. Anisotropic etching has been reported in solutions composed of H2SiF6(aq) + HNO3(aq) (Famini et al. 2006). Acker and coworkers (2012), Hoffmann et al. (2011), Steinert et al. (2005), (2006), (2007), (2008), Weinreich et al. (2007), Jadzinsky et al. (2007), Henssge and Acker (2007), and Acker and Henssge (2007) have significantly advanced our understanding of silicon etching in HF + (nitrate, nitrite and related solution phase species) solutions. The temperature and composition of the solutions are critical. A number of different N-containing species form in solution. The concentrations of these as well as the interactions of etch products with etching are found to be temperature dependent. Both N(III) species and dissolved gases play roles in the mechanism. Denoting (3NO+ • NO3
) as [N4O62+], they found a linear relationship between etch rate and [N4O62+] concentration and concluded that NO+ is a reactive species in the rate-limiting step. High concentrations of this intermediate led to uniform etching (electropolishing). The electropolishing regime can be exploited to produce high aspect ratio tranches with smooth surfaces. When run in the regime that etches trenches rather than producing por-Si, etching with HF + HNO3 (with or without the addition of acetic acid) is often referred to as HNA chemistry. Bauhuber et al. (2013) have investigated this in detail and have shown that the addition of H2SO4 and H3PO4 to HNO3 + HF can be particularly advantageous.

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K. W. Kolasinski

The rate of stain etching is enhanced by the presence of defects in the silicon. This characteristic has been used to create a hybrid amorphous porous/defect-followedmesoporous structure (Woo et al. 2012a, b). Gerischer and Lübke (1988) found that oxidants with an electrochemical potential more positive than the potential that corresponds to the Si valence band maximum can inject holes into the valence band. These included MnO4
, Br2, IrCl62
, and I2. However, since these experiments were carried out in 1 M NH4F solutions, no por-Si was formed. Corrosion was noted for all these oxidants but, curiously, not for Fe3+. As detailed in Table 1 below, a number of other oxidants have been tried or may still be capable of producing stain etchants. Table 2 lists examples in which electroless etching has been used to produce por-Si powders. Of the alternatives listed in Table 1, Fe3+, supplied by FeCl3•6H2O, and VO2+, supplied by V2O5 are by far the most interesting for producing photoluminescent nanocrystalline thick films. The former is used at a concentration of roughly 1 M, the latter closer to 0.1 M (Kolasinski et al. 2010, 2012; Dudley and Kolasinski 2009b). Reproducible por-Si formation is possible with both oxidants when concentrated HF (aq) (49 wt% = 29 mol L
1 in HF) is used as the acidic fluoride source. Lowering the concentration of fluoride lowers the etch rate. The etch rate is linearly proportional to the concentration of either Fe3+ or VO2+; however, unlike HNO3, there is no threshold concentration below which stain etching does not occur (Kolasinski and Barclay 2013a). The kinetics of solutions made from V2O5 possess the distinct advantage that both the oxidized V(V) species and the reduced V(IV) species can be probed quantitatively by UV/Vis absorption spectroscopy (Kolasinski et al. 2015; Kolasinski and Barclay 2013a, b). Thus it was shown that unlike nitrate-based systems, the reduction of VO2+ to VO2+ is quantitative and there are no side reactions during stain etching. The formation of por-Si films in Fe3+ + HF solutions under illumination with a Xe lamp has also been studied (Xu and Adachi 2007). Kolasinski and Barclay (2013a) have demonstrated that the stoichiometry of stain etching is distinct from that of anodic por-Si formation. This occurs because the injection of a conduction band electron in the current doubling step is used to reduce H+ to H2 in anodic etching, whereas this electron is consumed by the oxidant in stain etching. Therefore, two moles of oxidant are required to etch one mole of Si (twice as much as expected), but much less H2 is formed (with concomitantly reduced bubble formation and improved film homogeneity). Specifically for the vanadium system, the etching reaction is SiðsÞ þ 6HFðaqÞ þ VO2 þ ðaqÞ ! H2 SiF6 ðaqÞ þ H2 ðgÞ þ VO2þ ðaqÞ þ H2 OðlÞ þ eCB


(1)

where eCB
denotes an electron injected into the Si conduction band by an adsorbing F
ion (the so-called doubling electron). The counterreaction to consume the doubling electron is
þ 2þ VOþ 2 ðaqÞ þ 2H ðaqÞ þ eCB ! VO ðaqÞ þ H2 OðlÞ;

(2)

Porous Silicon Formation by Stain Etching

45

Table 1 Oxidants that may be or have been proven to be capable of initiating etching by hole injection into silicon. E values taken from Haynes (2010) Oxidant I2

E /V 0.5355

Etch Yes

por-Si No

Comments Destroys por-Si

UO2+ Fe3+

0.612 0.771

Yes

Yes

Not yet attempted Uniform, thick, visible PL

Yes

Yes

Hydrothermal etching

Fe (NO3)3 Ru (CN)63
IrCl62


0.86 0.8665

Yes

Yes

NO3


0.957

Yes

Yes

Reference Gerischer and Lübke (1988) and Kolasinski and Gogola (2012) Cao et al. (2016), Loni et al. (2011), Nahidi and Kolasinski (2006), Kolasinski et al. (2012), Dudley and Kolasinski (2009b), Kolasinski et al. (2010), and Kolasinski (2010) Famini et al. (2006) and Tian et al. (2015)

Not yet attempted Slow etch rate, no visible PL Irreproducible, visible PL

Kolasinski et al. (2012) Turner (1960), Archer (1960), Khezami et al. (2016), Sapelkin et al. (2006), Beckmann (1965), Beale et al. (1986), Shih et al. (1992), McCord et al. (1992), Kidder et al. (1992), Dubbelday et al. (1993), Shih et al. (1993), Steckl et al. (1994), Chandler-Henderson et al. (1994), Liu et al. (1994), Kalem and Rosenbauer (1995), Anaple et al. (1995), Amato (1995), Schoisswohl et al. (1995), Jones et al. (1995), Di Francia et al. (1995), Di Francia and Citarella (1995), Winton et al. (1996), (1997), Velasco (2003), Guerrero-Lemus et al. (2003), González-Díaz et al. (2006), Liu et al. (2007), Zeng et al. (2005), Melnichenko et al. (2005), Luchenko et al. (2007), Balaguer and Matveeva (2010), Mogoda et al. (2011), Lippold et al. (2011), Lippold et al. (2012), Terheiden et al. (2011), Carter et al. (2015), Khalifa et al. (2012), (continued)

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K. W. Kolasinski

Table 1 (continued) Oxidant

E /V

Etch

por-Si

Comments

NO3


0.957

Yes

Yes

Hydrothermal etching

VO2+

0.991

Yes

Yes

Rapid, uniform, thick layers, visible PL

Br2

1.0873

Yes

No

Destroys por-Si

ClO4


1.189

No

No

IO3


1.195

Yes

Yes

Cr2O72
HCrO4
(CrO3)

1.36 1.350

Yes

Yes

Minimal etching maybe Byproducts destroy por-Si Patchy, irreproducible

Cl2 ClO3
BrO3


1.3583 1.47 1.482

Yes Yes Yes

No No No

Destroys por-Si Destroys por-Si Destroys por-Si

Reference Rustamov et al. (2014), Hajji et al. (2013), Khan et al. (2014), Greil et al. (2012), Rustamov et al. (2013), Halim et al. (2014a), Toyoda et al. (2000), Sampath et al. (2016), Zarroug et al. (2014), and Pavlikov et al. (2012) Chen et al. (1996), (2001), Li et al. (1999), Chen et al. (1997) Pan et al. (2013d), Ayat et al. (2014), Kolasinski et al. (2012), Dudley and Kolasinski (2009b), Kolasinski et al. (2010), Kolasinski and Barclay (2013a), (b), Kolasinski (2010), Kolasinski and Yadlovskiy (2011), and Kolasinski and Gogola (2011) Kolasinski and Gogola (2012), Meltzer and Mandler (1995), Bressers et al. (1996), and Zhang et al. (2006) Dudley and Kolasinski (2009a) Kolasinski and Gogola (2012) and Badawy et al. (2015) Fathauer et al. (1992), Nahidi and Kolasinski (2006), Jenkins (1977), Beale et al. (1986), Kelly et al. (1994), Safi et al. (2002), van den Meerakker and van Vegchel (1989a), (b) Gabouze et al. (2003), Badawy et al. (2015), Secco d’Aragona (1972), Hoshino and Adachi (2007), Xu et al. (2013), Schimmel (1979), Heimann (1984), Hadjersi (2007), and Hadjersi et al. (2005a) Kolasinski and Gogola (2012) Kolasinski and Gogola (2012) Nahm et al. (1997), Kolasinski and Gogola (2012), Badawy et al. (2015), and Seo et al. (1993) (continued)

Porous Silicon Formation by Stain Etching

47

Table 1 (continued) Oxidant MnO4


E /V 1.507

Etch Yes

por-Si Yes

Comments Patchy and irreproducible

Ce4+

1.72

Yes

Yes

H2O2

1.776

Yes

No

Dissolved in conc H2SO4, visible PL only after a few days, uniform, slow Significant porosification only with metal catalyst

Co3+ S2O82


1.92 2.01

Yes

No

Not yet attempted Nanowire formation with Ag catalysis

Reference Bai et al. (2012), Nahidi and Kolasinski (2006), Mogoda et al. (2011), Kelly et al. (1994), Nahm et al. (1997), and Hadjersi (2007) Kolasinski et al. (2012), Dudley and Kolasinski (2009b), and Kolasinski (2010) Huang et al. (2011), Li (2012), Ashruf et al. (1999), Li and Bohn (2000), Rao et al. (2007), Nielsen et al. (2007), Eom et al. (2002), Tomioka et al. (2007), and Ljungberg et al. (1996) Megouda et al. (2009), Kolasinski and Gogola (2012), Hadjersi (2007), and Hadjersi et al. (2005b)

thus the overall reaction is SiðsÞ þ 6HFðaqÞ þ 2VO2 þ ðaqÞ ! SiF6 2
ðaqÞ þ H2 ðgÞ þ 2VO2þ ðaqÞ þ 2H2 OðlÞ:

(3)

Several candidates for por-Si formation lead to poor results, such as HCrO4
, MnO4
, and S2O82
. The cation that accompanies these oxidants (usually Na+, K+, or NH4+) can precipitate in the form of a hexafluorosilicate and interfere with film formation (Koker et al. 2002). For example, Hoshino and Adachi produced photoluminescent films with 5–7 wt% K2Cr2O7 in 50% HF solution; however, they also observed the deposition of K2SiF6 as did Mogoda et al. (2011). Usually, HF + CrO3 solutions lead to uniform etching or only roughening of the Si surface (Nahidi and Kolasinski 2006; Kooij et al. 1999; Kelly et al. 1994; van den Meerakker and van Vegchel 1989a, b; Nahm et al. 1997; Gabouze et al. 2003). There are several reports of the appearance of thin (Fathauer et al. 1992; Beale et al. 1986) or patchy porous layers (Kelly et al. 1994) as well. Just as for H2O2, the kinetics of hole injection from ClO4
are so slow as to preclude significant etching (Dudley and Kolasinski 2009a, b). This suggests that the presence of a metal catalyst might lead to enhanced etching for ClO4
just as it does for H2O2; however, this has yet to be demonstrated. Oxidants that have yet to be attempted include Co3+, Ru(CN)63
, and UO2+. Co3+ complexes are explosive (Smirnov et al. 2004). In comparison, the expense of Ru(CN)63
and the potential radioactivity of UO2+ seem appealing. Metals such as Cu, Ag, Au, Rh, Pd, Pt, Hg, and Tl that will plate out onto Si

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Table 2 Examples of electroless etching being used for the production of porous silicon powder or nanoparticles Oxidant Fe3+ Fe3+

Fe(NO3)3 Fe3+

VO2+ VO2++H2O2 VO2+

HNO3

Comments Etching of metallurgical grade Si powder Etching of wafer with subsequent sonication to create powder/ nanoparticles Etching of metallurgical grade Si powder Fe3C coated Si particles used to form Si nanoparticles encapsulated within porous C coating Etching of metallurgical grade Si powder Regenerative etching of metallurgical grade Si powder Etching of wafer with subsequent sonication to create powder/ nanoparticles Etching of metallurgical grade Si powder

HNO3

Etching of chemical grade Si powder

HNO3

Etching of ultrafine Si powder

HNO3

Etching of wafer with subsequent sonication to create powder/ nanoparticles

Reference Loni et al. (2011) Mughal et al. (2014)

Ge et al. (2014) Park et al. (2013) and Xu et al. (2014)

Kolasinski et al. (2017) Kolasinski et al. (2017) Pan et al. (2013a), (b), (c), (d)

Chadwick et al. (2012), (2013), Litvinenko et al. (2010), Limaye et al. (2007), Koynov et al. (2011), and Polisski et al. (2008) Sato et al. (2009a), (b), Khalifa et al. (2012), (2013), Halim et al. (2014b), Nakamura and Adachi (2012), Nakamura et al. (2010), Ensafi et al. (2014a), (b), Chirvony et al. (2007), and Le and Jeong (2014) Hao et al. (2014), Liu and Swihart (2014), Hua et al. (2005), (2006), Li et al. (2003), (2004) Choi et al. (2007)

(Ogata and Kobayashi 2006; Allongue and Maroun 2006) have not been included in Table 1. Metals such as these that spontaneously plate out onto Si are capable of catalyzing metal-assisted etching (Kolasinski 2005; Huang et al. 2011; Li 2012). The halogens and halogenates are extremely reactive with Si when mixed with HF solutions (Kolasinski and Gogola 2012). In particular, the iodate ion IO3
produces extremely high etch rates when ethanol is added to the etchant to avoid the precipitation of I2. However, the halogens are able to react thermally with Si and do so in a manner that destroys por-Si films. On the other hand, Xu and Adachi (Xu and Adachi 2006) investigated etching in KIO3 + HF solutions. While they observed no por-Si formation in the dark, with illumination at 600 nm from a Xe lamp, they reported the formation of a photoluminescent film.

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The addition of surfactants to stain etchants has occasionally been attempted. Sometimes this resulted in rather thin films (Vázsonyi et al. 2001). Acetic acid (Robbins and Schwartz 1960; Jenkins 1977) or ethanol can be used in this respect. CAUTION: ethanol reacts with the oxidants, particularly HNO3. Such solutions should never be placed in a closed container. Whereas ethanol addition is commonly used during anodic formation of por-Si to reduce the deleterious effects of bubbles, its use in stain etching is rather limited. The apparent reduction of bubble formation and sticking to the substrate is more closely related to a strong reduction in the etch rate, rather than a superlative surfactant action (Kolasinski et al. 2010).

Rational Formulation of Stain Etchants Through experiments on a broad range of oxidants and a better understanding of the fundamentals of the charge transfer process, Kolasinski developed a set of guidelines for the formulation of effective stain etchants (Nahidi and Kolasinski 2006; Kolasinski et al. 2010, 2012; Dudley and Kolasinski 2009b; Kolasinski and Gogola 2012; Kolasinski 2005, 2010, 2014): • The oxidant must be able to inject holes into the Si valence band at an appreciable rate; thus its electrochemical potential should be more positive than approximately +0.7 V. • The fluoride solution must be acidic to avoid OH
catalyzed etching. • Oxide formation needs to be slow or nonexistent. • Sufficiently high fluoride concentration compared to the oxidant concentration avoids electropolishing caused by the buildup of oxide. • The oxidant and all products must be soluble. Not only must the oxidant be soluble so as to collide with the surface and inject holes, but also metals or other products should not plate out on the surface. • Film homogeneity is enhanced if the oxidant’s half-reaction does not evolve gas. • The net etching reaction from hole injection to Si atom removal (including the reactions of any byproducts) has to be sufficiently anisotropic (attacking all kinds of sites but only at the bottom of the pore) to support pore nucleation and propagation.

Practical Advice • Cleanliness, especially regarding removal of organic hydrophobic contaminants, is essential for the production of homogeneous film on Si wafers. Teflon beakers and crystals have to be degreased before etching. Ultrasonication in acetone then ethanol then water is usually sufficient. This is not a concern for electroless etching of powders because the highly defective surfaces are much more reactive. • Keep cleaned wafers in distilled water and AVOID LANGMUIR-BLODGETT FILM DEPOSITION. Do not ruin a perfectly good clean crystal by pulling it

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through the surface film that naturally forms on water exposed to the atmosphere. Before removing a wafer from water, always break the surface layer by agitating the top of the water with forceps. The semiconductor industry does this routinely by using overflowing baths. • Get a tank of argon and use it liberally. Samples should be dried in streaming Ar. Argon is heavier than air and, therefore, preferable to N2 because it forms a buffer layer above solutions that helps avoid Langmuir film formation. Bubbling Ar through etchants not only sparges them of dissolved O2 and CO2 but also stirs the etchant and helps remove H2 when large-scale etching is performed. Filling the headspace in HF bottles and distilled water containers with Ar helps to reduce dissolved O2. Covering etchants, particularly Fe3+-based etchants, with a buffer layer of Ar helps to avoid precipitation for long etch times. • For uniform thick films, slow etching using VO2+ or Fe3+ with low H2 production is preferred. Critical point drying is a must to avoid cracking of thick (several μm) films. • V2O5 dissolves in concentrated HF(aq) but not water. To make diluted solutions, always dissolve first in concentrated HF then add water (Fig. 1).

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Porous Silicon Formation by Metal Nanoparticle-Assisted Etching Claude Lévy-Clément

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 1-Step Metal-Assisted Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 2-Step Metal-Assisted Etching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Reviews . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

62 62 64 70 71 71

Abstract

Essential aspects in the fabrication of porous silicon and nanostructures by metalassisted particle etching are presented. Basic processes using 1-step or 2-step method are described as well as mechanism of metal-assisted chemical etching. Influence of various parameters such as nature of metal, temperature, etching solution composition, intrinsic properties of silicon substrate on the morphology of porous silicon, or nanostructures is discussed. Applications of silicon nanostructures obtained by metal-assisted etching are briefly introduced, showing the promising potential of this etching method whose main properties are simplicity, low cost, easy process control, reproducibility, and reliability for fabrication of silicon nanostructures including silicon nanowires. Keywords

Electroless method deposition (EMD) · Metal particles · Metal-assisted etching (MAE) · Applications

C. Lévy-Clément (*) Institut de Chimie et des Matériaux Paris-Est, CNRS, Thiais, France e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_5

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Introduction Metal-assisted chemical etching (MAE or MACE) is based on a localized oxidation and dissolution of silicon in HF in the presence of an oxidizing agent, whereas the metal (generally noble metal) catalytically enhances the etching process (DimovaMalinovska et al. 1997; Li and Bohn 2000; Xia et al. 2000; Harada et al. 2001; Chattopadhyay et al. 2002; Peng et al. 2002; Hadjersi et al. 2004; Qiu et al. 2005; Tsujino and Matsumura 2005a, b; Yae et al. 2003). The Si underneath the metal is etched much faster than that without metal coverage. As a result the metal sinks into the Si substrate generating pores into Si substrate or Si nanowires (SiNWs) corresponding to the un-etched Si between pores (chapter ▶ “MACE Silicon Nanostructures” on SiNWs). The metal, especially noble metals, may be dissolved in HF (1-step MAE method) or deposited on silicon, as nanoparticles or thin films, before the etching process in HF solution containing an oxidizing agent (2-step MAE method).

1-Step Metal-Assisted Etching The metal deposition and etching are carried out in the same chemical solution (Fig. 1). A typical solution is 0.02 M AgNO3 + 4.6 M HF. This method leads to the formation of porous Si (Qiu et al. 2005; Peng et al. 2003; Peng and Zhu 2004) or pillar-like or craterlike microstructures (Peng et al. 2003) or arrays of silicon nanowires (SiNWs) standing vertically on the Si substrate (Peng et al. 2002, 2003, 2004, 2005a, 2006a; Peng and Zhu 2003, 2004; Cheng et al. 2008; Benoit et al. 2008). Metal ions are dissolved in HF. When the redox potential of the ions is more positive than the valence band of Si, a galvanic reaction occurs in which the ions are reduced to metal as particles, dendrites, and film, while the Si is oxidized and dissolved in HF (case of noble metals) following the reaction: 

4 Mþ ðaqÞ þ Si0 ðsÞ þ 6F ! 4 MðsÞ þ SiF6 2 ðaqÞ

(1)

The formation of dendrites in the case of Ag and Au is at the origin of the formation of SiNWs. AgNO3 (Peng et al. 2002, 2003, 2004, 2006a; Peng and Zhu 2004; Cheng et al. 2008; Benoit et al. 2008; Smith et al. 2013), KAuCl4 or HAuCl4 (Peng et al. 2002; Qiu et al. 2005; Peng and Zhu 2003, 2004), and K2PtCl6 or H2PtCl6 (Peng et al. 2003) have been used, as well as other nitrate metal salts (Cu, Ni, Mn, Fe, Co, Cr, Mg) (Peng et al. 2002, 2003; Table 1). It is a very easy method to In situ metal particle deposition Si Substrate

+ Si localized dissolution

Fig. 1 Schematic of 1-step metal-assisted etching

Si nanopores or nanowires + Metal nanoparticles

5M

0.01 0.2 M

0.2 M

0.08–0.15 M

K2PtCl6

CuNO3

NiNO3 MnNO3 FeNO3 CoNO3 CrNO3 Mg (NO3)2

Note: M stands for Molarity.

12 M

12 M 5M

0.02

KAuCl4

HF M 4.5–5

5M 5M 5M NH4F 5M 5M

Metal ion M 0.02 M

0.015 M 0.02 M Idem Idem

Catalyst AgNO3

140 60 min

50

50 10–30 min 50 50

50 120 15

Temp  C/ etching time 50 20–60 min

Au dendrites Continuous Pt grain films Continuous Cu grain films No metal deposition

Ag dendrites Idem Idem Continuous Ag grain film Au nanowhiskers

Metal deposit Ag dendrites

Porous Si consisting of μm pillars, cones, craters pits nanostructures

Disordered shallow pits

SiNWs Disordered shallow pits

Disordered shallow pits

Si morphology SiNWs 10–30 μm long 30–300 nm wide Regular porous Disordered porous μm stalagmites Uniformly etched

Table 1 Various morphologies produced on single crystal Si by the 1-step metal-assisted etching method

Peng et al. (2002, 2003)

Peng et al. (2003)

Peng et al. (2003)

Peng and Zhu (2003, 2004) Peng et al. (2003)

Qiu et al. (2005)

Peng et al. (2003) Peng and Zhu (2004) Peng et al. (2003) Peng et al. (2003)

References Peng et al. (2002, 2003, 2006a) and Cheng et al. (2008)

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produce Si nanostructures especially nanowires, but there is little possibility to control their dimension and homogeneity as well for the Si microstructures.

2-Step Metal-Assisted Etching The metal particles are deposited on the Si surface generally by electroless metal deposition (EMD) or chemical vapor deposition or sputtering, prior to the etching in the HF solution in the presence of an oxidizing agent. During the etching, dissolution of Si underneath the metal particles is strongly enhanced, and pores are formed while the particles sink into the Si pores (Li and Bohn 2000; Harada et al. 2001; Chattopadhyay et al. 2002; Tsujino and Matsumura 2005a, b; Yae et al. 2003; Fig. 2). The etching can be done in various HF solutions containing an oxidizing agent, typically H2O2 (Li and Bohn 2000; Tsujino and Matsumura 2005a, b, 2006a, 2007; Peng et al. 2008; Huang et al. 2007; Chartier et al. 2008; Lee et al. 2008; Megouda et al. 2009a). Other oxidizing agents such as Fe(NO3)3 (Peng et al. 2005b, 2006b), Mg(NO3)2 (Peng et al. 2006b), Na2S2O8 (Hadjersi et al. 2004, 2005a; Douani et al. 2008; Hadjersi 2007), KMnO4 (Hadjersi et al. 2004, 2005a; Douani et al. 2008), K2Cr2O7 (Douani et al. 2008; Hadjersi et al. 2005b; Waheed et al. 2010), KBrO3 or KIO3 (Waheed et al. 2010), Co(NO3)2 (Megouda et al. 2009b), molecular O2 dissolved in H2O (Yae et al. 2003, 2005, 2006, 2008a, b, 2010a, 2012; Masayuki et al. 2011), and electrical holes, h+ by anodization (Zhao et al. 2007; Chouroua et al. 2010; Huang et al. 2010a), are also used. The deposited metals under the form of nanoparticles or colloidal particles or patterned thin film are most generally noble metals such as Ag (Hadjersi et al. 2004, 2005b; Tsujino and Matsumura 2005a, 2006a, 2007; Peng et al. 2005b, 2006b, 2008; Huang et al. 2007; Chartier et al. 2008; Lee et al. 2008; Douani et al. 2008; Hadjersi 2007; Waheed et al. 2010; Yae et al. 2005; Yang et al. 2008; Asoh et al. 2007a), Au (Li and Bohn 2000; Qiu et al. 2005; Peng et al. 2008; Lee et al. 2008; Megouda et al. 2009a; Yae et al. 2005; Zhao et al. 2007; Bauer et al. 2010), Pt (Li and Bohn 2000; Xia et al. 2000; Chattopadhyay et al. 2002; Tsujino and Matsumura 2005a, b; 2006a; Lee et al. 2008; Yae et al. 2005, 2006, 2012), Pd (Hadjersi et al. 2004; Tsujino and Matsumura 2005a; Waheed et al. 2010; Yae et al. 2005, 2008a, b; 2010a, 2012; Masayuki et al. 2011), Pd-Pt (Li and Bohn 2000; Asoh et al. 2008, 2009), or Rh (Yae et al. 2012), but other metals such as Al (Dimova-Malinovska et al. 1997), Bi (Megouda et al. 2009b), Cu (Tsujino and Matsumura 2005a; Peng et al. 2008; Mitsugi and Nagai 2004), Ni (Zhao et al. 2007), and Fe can also be used. Selective Dissolution of silicium

Metal deposition Si Substrate

Metal Nanoparticles

Fig. 2 Schematic of 2-step metal-assisted etching

Pore formation

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The chemical or electrochemical reactions occur preferentially near the noble metal. By analogy to electrochemical formation of porous Si, the role of the cathode is attributed to the metal and that of the anode to the silicon underneath the metal particle. The oxidant is reduced at the metal particle with production of electrical holes (Li and Bohn 2000; Peng et al. 2008; Tsujino and Matsumura 2007; Chartier et al. 2008). As an example when the oxidizing agent is H2O2, the reaction at the cathode is the following: H2 O2 þ 2Hþ ! 2H2 O þ 2hþ

(2)

At the interface between Si and the metal particle, the holes, h+, are injected into the valence band of Si, which is dissolved as SiF62. Depending on H2O2 concentration, the localized Si dissolution occurs as either a two-hole or four-hole process with direct dissolution of Si in a divalent or tetravalent state, respectively. H2 evolution occurs during metal-assisted etching and has been first attributed to H+ proton reduction (Li and Bohn 2000), but then latter it has been attributed to an anode reaction due to the strong similitude between stain etching in HF/HNO3 and metal-assisted etching (Chartier et al. 2008). A mixed reaction composed of divalent and tetravalent dissolution for the dissolution of Si in MAE, which takes also into account the H2 evolution, is proposed for the anodic reaction: Si þ 6HF þ nhþ ! H2 SiF6 þ nHþ þ




 4  n =2 H2 "

(3)

The overall reaction being Si þ 6HF þ n=2H2 O2 ! H2 SiF6 þ nH2 O þ




 4  n =2 H2 "

(4)

The morphology of the Si nanostructures depends on various etching conditions such as the metal used (chemical nature and initial morphology), etching solution composition, etching time, temperature, and Si properties (crystallography, orientation doping density, conductivity type) (Table 2). The main results using metalassisted etching are summarized: – The Si dissolution rate increases from Ag, Au, Pt, Pd, to Rh in the presence of HF/H2O2 or HF/O2 (Yae et al. 2012; Asoh et al. 2009). – The morphology of metal-assisted etched structures is defined by the shape of the metal catalyst. Well-separated metal particles usually result in well-defined pores (Tsujino and Matsumura 2005a, b; Chartier et al. 2008; Bauer et al. 2010). In the case of etching solutions also used in stain etching, it is possible that porous Si forms in the regions without noble metal. – When the metal particles (20–200 nm) are randomly distributed on Si, a disordered network of pores is formed into Si surface in the case of Ag, Au, and Pt, after etching in HF/H2O2 (Tsujino and Matsumura 2005a), HF/O2 (Yae et al. 2005) and HF/Fe(NO)3 (Peng et al. 2006b). Metal particles are observed at the bottom of the pores. Various porous Si structures are found depending on the

Idem

Idem

H2O2 5 M

Au

7.9 M ρ = 61%

10.8 M ρ = 77%

Idem H2O2 3.3 M

Idem

H2O2 0.9 M 0.6% H2O2

HF 28.9 M ρ = 91% 28.9 M ρ = 97% 10%

Ag colloïdal particles Pt EMD Au ϕ = 3 nm, 6 mm pitch, Au/Pd (3–20 nm) Pt

Ag EMD

Pt

Particle Ag EMD 30–100 nm

Oxidizing agent H2O2 2.72 M

5 min

30 s

30 s

50 30 min

Temp  C/ etching time 25 30 min 30 min

n+Si, p-Si p+-Si p-Si

p-Si, 7–14 p+-Si (100)

(100) (111) p-Si, (100) (113), (110) 7–14 Ω cm (100)

Si type resistivity Ω cm p-Si (100)

Idem, etching depth = 10 nm Columnar structures, 1 μm deep Macropores ϕ = 100 nm Similar to Ref. (Li and Bohn 2000), but bigger structures

Narrow straight cylindrical nanoholes 35 μm long Tortuous channels Nanopores ϕ = 30 nm, ⊥ Si surface, interconnected, etching depth = 350 nm

μm spherical cavities + 3 μm thick porous Si Cylindrical holes inclined to Si surface SiNWs vertical SiNWs slanted to Si surface

Nanostructure Pores 5–40 μm long, covered with microporous Si

Table 2 Examples of various Si morphologies obtained by 2-step metal-assisted etching, the symbol ϕ is used for diameter

Chattopadhyay et al. (2002)

Li and Bohn (2000)

Peng et al. (2008)

References Tsujino and Matsumura (2005a)

66 C. Lévy-Clément

Fe(NO)3 0.2 M

K2Cr2O7, 0.05 M KMnO4 0.05 M Na2S2O8 0.05 M Co(NO3)2 0.03 M

Ag EMD Pt EMD

Ag (20 nm) Vacuum evaporation

Bi (20 nm) Vacuum evaporation

O2 (5 ppm)

Pt electrodeposited 0.05–0.2 μm

22.5 M

22.5 M

5M

7.3 M

50 15 min 60 min

10 min 30 min 30 min

25 24 H 50 50 min

p-Si (100) 100 Ω cm

n- and p-Si 1 Ω cm p-Si (111) p-(100) 3–6 Ω cm n-Si (100) 1.6 Ω cm

Etch pits ϕ = 9 μm contain pillars and porous Si

Etch pits 100 nm deep

Winding macropores ϕ = 0.36 μm, covered with microporous Si Megouda et al. (2009b)

Douani et al. (2008)

Cylindrical macropores ϕ = 0.4 and 2 μm covered with microporous Si Etch pits ϕ = 0.15 μm

Peng et al. (2006b)

Yae et al. (2005)

SiNWs array Separated deep pores

Macropores ϕ = 1 μm covered with microporous Si

Porous Silicon Formation by Metal Nanoparticle-Assisted Etching 67

Fig. 3 (continued)

68 C. Lévy-Clément

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nature of the metal and etching conditions (Fig. 3). The pore diameter is the same as that of Ag or Au particles, whereas for Pt the pores are cone shaped of micrometer size and lined with mesoporous Si (Tsujino and Matsumura 2005a, b). For Pd a polishing occurs when the etching is done in 7.3 M HF/molecular O2 (Yae et al. 2005, 2010a). When the distances between noble metals in the case of Ag and Au are small, the structures evolve from pores into wall-like or wirelike structures vertically standing with diameter between 100 and 200 nm and several microns long (Peng et al. 2005b, 2006b, 2008). – The etching solution composition has a strong influence on the metal-assisted etched Si morphology. In the case of the HF/H2O2 solution, a unified notation with ρ = [HF]/([HF] + [H2O2]) allows to compare the conditions and resulting morphology published in the literature (Chartier et al. 2008). Different regimes of Si dissolution are distinguished leading to the formation of cylindrical pores for low oxidizing agent concentration (ρ > 70%), to cone-shaped pores with walls lined with porous Si for medium oxidizing agent concentration (20% < ρ < 70%), and to craters (ρ < 20%) and polishing for high H2O2 concentration (ρ < 7%) (Chartier et al. 2008). For 20% < ρ < 100% a duplex structure is formed at the surface of the Si wafer. The top layer is micro-mesoporous, and once dissolved in NaOH, a macrotexturized surface (from pores to craters) is observed. At ρ  80% the maximum etching rate is reached and n equals 3 in the overall reaction 4 meaning that the etching process occurs as a mixed 2 and 4 electrons. Switching from cylindrical pores to helical is accomplished by changing the solution concentration. The generation of helical holes is attributed to the microscopically difference in etching rate of silicon on a Pt particle (Tsujino and Matsumura 2005b).

ä Fig. 3 SEM images (plan view and cross section). (a) Ag particles deposited by EMD in 5  104 M AgNO3 + 0.14 M HF during 5 min (Chartier et al. 2008); (b) porous Si, Ag deposited on (Li et al. 2012) p-type Si etched in HF/H2O2 (ρ = 80%) for 30 s (Chartier et al. 2008); (c) straight and curved cylindrical pores with Ag particles at their bottom, same as (b) at higher magnification; (d) coneshaped macropores lined with microporous Si, Ag deposited on p-Si etched in HF/H2O2 (ρ = 20%) for 20 s (Chartier et al. 2008); (e) array of SiNWs, Ag deposited on Si etched in HF/H2O2 (ρ = 70%) for 5 min (Lévy-Clément and Chartier 2013); (f) typical helical pore, Au deposited on Si etched in HF/H2O2 (ρ = 97%) for 5 min (from Tsujino and Matsumura 2005b, with permission); (g) cone-shaped pore with Au aggregates at the bottom, Au EMD deposited on Si etched in HF/H2O2 (ρ = 97%) for 1 h (from Lee et al. 2008, with permission); (h) macroporous Si, Ag deposited on Si etched in HF/H2O2 (ρ = 70%) for 15 min, after dissolution of microporous Si in diluted KOH. Si etched thickness is 8 μm (Lévy-Clément and Chartier 2013); (i) cone-shaped pores with a Pt particle at the bottom, Pt deposited on Si etched in HF/H2O2 (ρ = 70%) for 15 min (LévyClément and Chartier 2013); (j) ordered array of macropores with uniform diameter, Pt-Pd-coated Si etched in HF/H2O2 for 1 min; polystyrene sphere honeycomb mask periodicity is 3 μm (from Asoh et al. 2009, with permission); (k) array of SiNWs (60 nm ϕ and 4.5 μm long), 40 nm Ag film deposited with polystyrene sphere mask (ϕ = 100 nm) on Si etched in HF/H2O2 (ρ = 91%) for 10 min (Lévy-Clément and Chartier 2013); (l) porous Si, Ag deposited on Si etched in HF/Na2S2O8 (22.5 M/0.15 M) for 10 min (from Hadjersi 2007, with permission)

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– When Ag, Au, and Pd/Pt metal particles are deposited in ordered arrays through a mask as a patterned thin film (~20–40 nm thick), quasi-ordered silicon micro-/ nanostructures (pores) are formed in silicon, whatever is the kind of deposited noble metal (Huang et al. 2007; Yae et al. 2010a; Asoh et al. 2007a, b, c, 2008, 2009; Bauer et al. 2010; Peng et al. 2007; Ono et al. 2007, 2009; Lévy-Clément et al. 2011; Boarino et al. 2011; Pacholski 2011; Geng et al. 2011; Hildreth et al. 2009, 2013; Rykaczewski et al. 2011; Scheeler et al. 2012; Geyer et al. 2012, 2013; Güder et al. 2013; Wang et al. 2013a). This observation is the basis of the fabrication of a rich variety of ordered Si structures with different length scales, as topographic lines, concentric rings, square arrays, etc., can be created at a microand nanoscale after etching the silicon and subsequent removal of the mask. This method is widely used to form regular arrays of macropores and SiNWs with controllable diameter, length, and density (Huang et al. 2007; Peng et al. 2007). – In situ formation of Au or Ag nanoparticles occurs by adding an Au colloidal solution (Branz et al. 2009; Li et al. 2012) or AgNO3 (Lu and Barron 2013) in the HF/H2O2/H2O solution allowing to transform the 2-step method directly in a 1-step metal-assisted etching. The H2O2 plays a dual role in reducing the Au3+ and Ag+ ions into nanoparticles onto Si wafer and facilitating the Si etching. Mesopores of 20–100 nm diameter and 500 nm long are obtained when adding a sonication process (Lu and Barron 2013). A high concentration of H2O2 in the solution leads to electropolishing and the formation of ultrathin Si wafer (Bai et al. 2013). The advantages of using metal-assisted etching are numerous. The method is easy to handle and is suitable for batch fabrication of porous Si devices. Porous Si layers can be formed on highly resistive Si. Compared to stain-etched layers, those obtained by metal-assisted etching have better uniformity and much higher thickness. MAE can be used to make high surface-to-volume ratio structures, especially when associated with a lithography method to deposit patterned metal catalyst. Control of the orientation of Si nanostructures (e.g., pores, nanowires) relative to the substrate is achieved. There is no obvious limitation on the size of features fabricated by MAE, and fabrication of straight and well-defined pores or wires with diameters as small as 5 nm or as large as 1 μm can be obtained.

Applications Metal-assisted etching is frequently used to prepare photoluminescent porous Si (Dimova-Malinovska et al. 1997; Li and Bohn 2000; Harada et al. 2001; Chattopadhyay et al. 2002; Hadjersi et al. 2004, 2005b, c; Hadjersi 2007; Megouda et al. 2009b; Zhao et al. 2007; Gorostiza et al. 1999; Chattopadhyay and Bohn 2006; Hadjersi and Gabouze 2007; Lipinski et al. 2009). MAE displays little crystallographic dependence and can be performed on crystalline or multicrystalline Si substrates, and the various Si morphologies and nano-microstructures obtained are promising for photovoltaic applications in several areas: antireflective coating (Yae et al. 2003, 2005, 2006; Peng et al. 2005a, b; Benoit et al. 2008; Lu and Barron 2013;

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Tsujino and Matsumura 2006b; Chaoui et al. 2008; Nishioka et al. 2008, 2009; Srivastava et al. 2010; Cao et al. 2011; Kim et al. 2011, 2012; Geng et al. 2012; Wang et al. 2013b; Li et al. 2013a), texturization for multicrystalline wafers (Tsujino and Matsumura 2006a; Waheed et al. 2010; Branz et al. 2009; Li et al. 2012; Wan et al. 2008; Koynov et al. 2006, 2007; Lipiński 2008; Bastide et al. 2009; Yuan et al. 2009; Lin et al. 2010; Toor et al. 2011; Oh et al. 2012; Srivastava et al. 2012; Tang et al. 2013; Shi et al. 2013a, b; Hsu et al. 2012), porous emitter (Hadjersi and Gabouze 2008; Li et al. 2013b), advanced solar cells based on cylindrical macropores (Peng et al. 2010) or Si nanorods or nanowires (Garnett and Yang 2008), and layer detachment technique to prepare solar cells based on low-quality substrates or on ultrathin Si layers (Shiu et al. 2011; Lin et al. 2012). Silicon nanostructures obtained by MAE has been successfully used in lithium rechargeable batteries (Ripenbein et al. 2010; McSweeney et al. 2011; Liu et al. 2011), nanocapacitors (Chang et al. 2010), diffusion membrane applications (Cruz et al. 2005; Chen et al. 2011), formation of nanostructured adhesive metal film on porous Si surface (Yae et al. 2010b, 2011a, b), production of Si powders (Loni et al. 2011), porous Si nanowires with photocatalytic properties (Qu et al. 2009, 2010), and biosensing chips (Xiao et al. 2013). The catalytic properties of Pt have been used to develop a slicing method showing the possibility to produce Si wafers from an ingot (Salem et al. 2010) or to form through a hole in Si (Sugita et al. 2011). MAE combined with patterning of metal catalyst thin film deposition is widely used to form regularly organized nanostructures on Si (Yae et al. 2010a; Asoh et al. 2007a, b, c, 2008, 2009; Bauer et al. 2010; Peng et al. 2007; Ono et al. 2007, 2009; Pacholski 2011; Scheeler et al. 2012; Chattopadhyay and Bohn 2004; Hung et al. 2010; Lee et al. 2011) or arrays of vertically standing Si nanowires with controlled diameter and controlled distances between them (Huang et al. 2007, 2010b; Peng et al. 2007; Lévy-Clément et al. 2011; McSweeney et al. 2011; Qu et al. 2009, 2010; Zhang et al. 2008; Chang et al. 2009; de Boor et al. 2010).

Reviews Huang et al. recently did a comprehensive and systematic review of metal-assisted chemical etching mechanism and process parameters (Huang et al. 2011). The review by Ono is focused on combination of patterning methods of metal deposition and metal-assisted etching (Ono and Asoh 2012), whereas the review by Li is related to high aspect ratio structures with an emphasis on photovoltaic applications (Li 2012). Other papers contain a mini-review on metal-assisted etching (Kolasinski 2005; Qiu and Chu 2008; Korotcenkov and Cho 2010).

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Porous Silicon Formation by Photoetching Sadao Adachi

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoetching Setup . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . n-Si/Electrolyte Interface and Photoetching Reaction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . PS Layers Formed by Photoetching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoetching of Silicon Powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

80 80 80 82 85 86 86

Abstract

This updated literature review concerns the photoetching technique of preparing photoluminescent mesoporous silicon films using hydrofluoric acid-based electrolytes, alkaline electrolytes, and aqueous alkali salt solutions. The photoetching mechanisms and types of porous silicon layers created are discussed. The benefits of using an incoherent light source and specific oxidizing agents are highlighted. The technique is particularly useful for creating thin porous regions in n-type Si wafers, SOI wafers, micromachined wafers, or those that contain electronic circuitry. Photoetching has also recently been developed for nanostructuring inexpensive silicon powder feedstocks. Keywords

Electron affinity · Photoetching · Photoluminescence · Porous silicon (PS) · Redox potential

S. Adachi (*) Division of Electronics and Informatics, Faculty of Science and Technology, Gunma University, Gunma, Kiryu-shi, Japan e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_6

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Introduction Visible photoluminescence (PL) from porous silicon (PS) observed at room temperature has inspired sustained research into its potential application in Si-based optoelectronic devices and its theoretical basis (Canham 1990). This property is reviewed in the handbook chapter “Photoluminescence of Porous Silicon.” Most PS layers are prepared by anodic etching on p-type Si substrates, a technique in which metal is often deposited on the rear surface of the Si substrate in order for it to be used as an ohmic back contact (see handbook chapter ▶ “Porous Silicon Formation by Anodization”). However, the requirement for a back contact electrode is a limitation of this method; for example, it is difficult to form a PS layer on a siliconon-insulator (SOI) structure or on Si integrated circuits. A photoetching method, on the other hand, requires no electrodes and allows the formation of a visible luminescence layer on not only single-crystalline Si substrates but also SOI structures.

Photoetching Setup An experimental setup used for the formation of PS by photoetching is shown in Fig. 1 (Xu and Adachi 2006). The sample surface is illuminated by a Xe lamp through an optical filter that blocks wavelengths shorter than 600 nm. The use of an optical filter is to block the heat rays from the Xe lamp. A laser, a W lamp, or another light source may be used instead of a Xe lamp. The use of an incoherent light source such as a Xe or W lamp enables the formation of a large and homogeneous PS layer. Typically, an n-type Si wafer is immersed in an etchant solution of HF. The addition of an oxidant (e.g., H2O2 or I2) to the HF solution results in the stable formation of PS layers in a short time period.

n-Si/Electrolyte Interface and Photoetching Reaction Figure 2 shows the energy band diagrams for n-Si electrodes in pure HF (pH = 2.3) and HF/oxidant solutions without and with light illumination (Xu and Adachi 2006). The electron affinity (χs) of Si is
4.05 eV. At zero pH, the redox coupling is defined as the normal hydrogen electrode with a potential of
4.5 eV with respect to vacuum. This potential shifts toward more positive values with the increase in pH (+0.059 eV/pH). Thus, the electron energy of the pure HF solution with respect to vacuum is
4.36 eV (χl). The Fermi levels (E F and E F,redox) on both sides of the nSi/electrolyte interface are brought to the same energy level by a transfer of electrons from the Si substrate to the electrolyte (Fig. 2a). The half reaction for the oxidizing agent KIO3 is þ

IO
3 þ 6H þ 6e ¼ I þ 3H2 O

ðEo ¼ 1:085 eVÞ

where e
represents the electron and Eo is the standard reduction potential with respect to the standard hydrogen electrode. The redox potential (Eabs) with respect to

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Fig. 1 Experimental setup used for porous silicon formation by photoetching in an HF/oxidant solution

Fig. 2 Energy band diagram for n-Si immersed in pure HF solution (a, b) and those in HF/KIO3 solution (c, d). In (b), porous silicon (PS) is formed stably on the back side in opposition to the illuminated surface. In (d), PS is formed only on the illuminated surface

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vacuum for the HF/KIO3 redox system is then given by Eabs =
4.5
Eo =
5.6 eV (Fig. 2c). It is to be noted that the larger the Eo value is in the positive (negative) scale, the stronger is the oxidation (reduction) agent (Adachi and Kubota 2007; Xu and Adachi 2007; Tomioka et al. 2007). The absorption of photons results in the generation of electron-hole pairs. The holes at the n-Si/electrolyte interface can participate in PS formation. In the case of the pure HF solution (Fig. 2b), the photoexcited holes are hard to drift toward the surface by the very small downward band bending or possibly by the almost-flat band. Thus, efficient PS formation cannot be expected in pure HF solution. When the Si wafer is dipped in the HF/oxidant solution (Fig. 2d), on the other hand, many photoexcited holes move toward the n-Si/electrolyte interface at the front surface, resulting in the formation of PS with good reproducibility (Xu and Adachi 2006, 2007; Adachi and Kubota 2007; Tomioka et al. 2007). Reproducibility has been observed to be problematic in the formation of PS by photoetching, as with stain etching (see handbook chapter “▶ Porous Silicon Formation by Stain Etching”). In an extreme case, no PS layer was formed on the front surface, although surprisingly PS was formed on the surface of the sample that was not exposed to illumination (i.e., on the back surface) (Andersen et al. 1995). The effectiveness of surface cleaning by sulfuric peroxide mixture (SPM) treatment or by KOH etching before PS formation has been reported in Tomioka et al. (2007) and Andersen et al. (1995). The photo-illuminated n-Si/aqueous NH4F interface has been shown to form a hydrogenated amorphous Si overlayer which builds up progressively as photoetching proceeds with disproportionation of Si2+ species in solution (Peter et al. 1989). It is known that a galvanic cell is formed when a p-type Si is contacted with a noble metal in a HF/oxidant solution (Kobayashi and Adachi 2010). This galvanic cell leads to metalassisted etching of p-Si, resulting in the formation of Si nanowire arrays. PS layers prepared by two routes, metal-assisted etching and laser-induced etching, have been studied by comparing surface morphologies using scanning electron microscopy (Kobayashi and Adachi 2010; Saxena et al. 2015). A PL peak at ~1.8 – 2.0 eV corresponding to red emission at room temperature was observed from such p-Si samples. The fact suggests that the PS layers can be formed not only on the laseretched surfaces but also on the Si nanowire arrays formed by metal-assisted electroless etching. In p-Si prepared by laser etching, wider pores with some variation in pore size as compared to metal-assisted etching technique were observed because a HeNe laser having Gaussian profile of intensity was used for porosification (Saxena et al. 2015).

PS Layers Formed by Photoetching A summary of PS formation by photoetching is presented in Tables 1 and 2 (Noguchi and Suemune 1993; Zhang et al. 1993; Cheah and Choy 1994; Andersen et al. 1995; Jones et al. 1996; Kolasinski et al. 2000; Yamamoto and Takai 2000, 2001; Mavi et al. 2001, 2006; Koker et al. 2002; Marotti et al. 2003; Zheng et al. 2005; Tomioka and Adachi 2005; Adachi and Tomioka 2005; Cho et al. 2006; Xu and

Gas and solid-state lasers

HeNe laser

HeNe laser

Nd:YAG laser (1064 nm), Ar laser (514 nm)

HeNe laser

40% HF

2HF:1HNO3:4H2O

48% HF

6HF:1H2O2

100HF: (17–250) H2O2

40% HF

HF:(K+, Cs+, or Rb+), etc.

n

n (4.5–6.4)

n (35–45)

n (0.22–0.38, 35–45) n (10)

n (4.5–10.4)

Gas, dye, and solid-state lasers

Ar laser, Xe lamp (465–780 nm) HeNe laser

n (2), p (2)

n (5–8)

W lamp (undispersed)

Light source HeNe laser, Xe lamp

Anhydrous and hydrous HF 32% HF

Solution 50% HF

n (0.4–0.7)

Type (Ω cm) n (0.01–15)

~2.0–2.1

~1.9–2.0

~1.9–2.0

~1.8–1.95

~1.8–2.3

~1.9–2.2

~1.7–2.0

~1.7–1.9

~2.0

PL peak energy (eV) ~1.8

Table 1 Photoetching for porous silicon formation in acidic solutions

A two-peak (1.91 and 2.02 eV) structure in the PL spectrum (Ar laser). A single PL peak at ~2.0 eV (Nd:YAG laser) Hexafluorosilicate-coated PS exhibiting blue-shifted PL emission

Comments No PS formation when excited at λ = 300–400 nm. No PS formation on p-Si PS is formed only on the metal-backed Si substrates. PS layer thickness: ~300–500 nm PL peak energy depends on photoetching wavelength PS is easily formed on the back surface of the sample PS is formed on both n-Si and p-Si. PL peak energy depends on excitation (PL) wavelength The shorter the photoetching wavelength, the higher the PL peak energy. The higher the photoetching laser power, the higher the PL peak energy The shorter the photoetching wavelength, the higher the PL peak energy Blue luminescence (420 nm) after dipping in 1C2H5OH:1H2O for 148 h PL intensity is shown to strongly depend on etching solution composition and time

(continued)

Koker et al. (2002)

Mavi et al. (2001)

Kolasinski et al. (2000) Yamamoto and Takai (2000) Yamamoto and Takai (2001)

Jones et al. (1996)

References Noguchi and Suemune (1993) Zhang et al. (1993) Cheah and Choy (1994) Andersen et al. (1995)

Porous Silicon Formation by Photoetching 83

Xe lamp (40 wt%) hydrofluoric acid was used that generates the smallest pores. Although the more recent literature contains many examples where authors refer to fabrication of “microporous” silicon, an examination of their fabrication or characterization data reveals this is somewhat a misnomer, according to the IUPAC criteria above. In extreme cases, even macropores (>50 nm diameter) have been referred to as “micropores.” Such studies are not referenced here although “microporous silicon” appears in their titles. In this updated review, we first examine the literature for strong evidence of micropores (2 nm width) by high-resolution transmission electron microscopy (see handbook chapter ▶ “Microscopy of Porous Silicon”) and the absence of any hysteresis in a type I isotherm of nitrogen adsorption and desorption in a material of very high surface area (see handbook chapter ▶ “Gas Adsorption Analysis of Porous Silicon”) Although the first Uhlir studies (1956) included work with 24–48% HF, it is not clear whether or not they processed p-silicon with the most concentrated electrolyte.

Microporous Silicon

151

Fig. 1 (a) TEM image and inset diffraction pattern from microporous silicon fabricated by anodization of p-wafers in concentrated (50%) aqueous hydrofluoric acid

Fig. 2 (a) Typical isotherm from mesoporous silicon with hysteresis (b) isotherm from microporous silicon fabricated by anodization of p-wafers in concentrated (50%) aqueous hydrofluoric acid (Canham and Groszek 1992)

Watanabe and Sakai were probably therefore the first to create highly microporous silicon, albeit without realizing it (Watanabe and Sakai 1971). Other workers who conducted early studies on the properties of layers anodized in p-wafers using highly concentrated HF included Unagami (1980) and Koshida et al. (1985) who correctly referred to a “microporous structure” (Koshida et al. 1986). The 1988 review of Bomchil et al. (1988) clarified the trends in pore-size distribution with anodization parameters for a range of mesoporous layers. They indicated the first gas adsorption evidence for some pores being below 2 nm in specific cases, but commented that the technique was no longer accurate in this pore-size regime. Table 1 summarizes more

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Table 1 Estimates of micropore (60% (mesopores) >90% (macropores in composites) >30% (mesopores) >60% (macropores) >50% (mesoporous wire array) >30% (macropores)

>50% (mesopores) >50% (mesopores)

Study examples Canham et al. (1994), Gelloz et al. (2005) and Chen et al. (2012) Yonehara et al. (1994) and Terheiden et al. (2011) Muller et al. (2000) and Nava et al. (2009)

Boarino et al. (1999) and Nassiopoulu and Kaltsas (2000) Salonen et al. (2005), Canham (2007), and Chiappini et al. (2010) Coffer et al. (2005) and Sun et al. (2007)

Cho (2010), Wu et al. (2012) and Ge et al. (2012)

Tang et al. (2010) and de Boor et al. (2012)

Qu et al. (2010) Deloiuse and Miller (2004)

References Boarino L, Monticone E, Amato G, Lerondel G, Steni R, Benedetto G, Rossi AM, Lacquanti V, Spagnolo R, Lysenko V, Dittmar A (1999) Design and fabrication of metal bolometers on high porosity silicon layers. Microelectron J 30(11):1149–1154

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Thermal Properties of Porous Silicon Nobuyoshi Koshida

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermal Constants of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Related Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

300 300 303 305

Abstract

Mesoporous silicon has tunable thermal properties that can be radically different from those of nonporous silicon. This updated review collates published data on the major thermal constants from varied techniques of measurement. Thermal properties of porous silicon are compared to those of other materials, including solid silicon. The combination of very low thermal conductivity and heat capacity of highly porous silicon has led to a range of potential applications such as thermal isolation of microdevices, chip-based ultrasound emission, thermoelectrics, and photothermal therapy. Keywords

3ω method · Heat capacity · Phonon transport · Photothermal method · Porous silicon · Thermal conductivity · Thermal diffusivity · Thermal effusivity · Thermal penetrating depth

N. Koshida (*) Graduate School of Engineering, Tokyo University of Agriculture and Technology, Koganei, Tokyo, Japan e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_20

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Introduction The major thermal constants of solids are the thermal conductivity α [W/(m
K)] and the heat capacity per unit volume, C[J/(m3K)]. Related thermal parameters are the thermal diffusivity D = α/C[m2/s] and the thermal effusivity pffiffiffiffiffiffi  e ¼ αС Ws1=2 =ðm2 KÞ . As a dynamic factor, the thermal penetration length L pffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi [m], given by 2α=Сω, is important from a viewpoint of the practical use, where ω is the angular frequency of a thermal oscillation at the solid surface. The thermal properties of single crystalline silicon (c-Si) are characterized by a very high α (148 W/(m
K)) and a relatively low C(1.66 MJ/(m3K)) (Brozel 1999). In addition, a significantly small thermal expansion coefficient and a sufficiently high melting point are compatible with the device applications.

Thermal Constants of Porous Silicon In porous silicon (PS), α is dramatically decreased as suggested from thermal flow measurements (Drost et al. 1995). For characterizing the thermal parameter in nanostructures like PS, two major ways have been employed to generate a probing signal of the thermal fluctuation: electrical input and noncontact optical incidence. The most useful method in the former case is so-called the 3ω method (Cahill 1990), in which the third-harmonic component of the alternating thermal oscillation with a certain frequency is selectively detected by a phase-sensitive measurement. The combination of steady-state electrothermal flow measurement and finite element simulation can easily evaluate the PS thermal conductivity (Siegert et al. 2012). In the case of the 3ω measurements for PS (Gesele et al. 1997; Kihara et al. 2005; Lucklum et al. 2014), the test sample is composed of a patterned thin metal film, a PS layer, and c-Si substrate. A narrow thin metal wire (e.g., 25 μm and 3 mm in width and length, respectively) was formed on the PS layer by photolithographic patterning of a deposited thin metal film and used as both a heater and thermometer. An oscillating electrical current of frequency ω is supplied to the metal wire using a signal generator. The produced Joule heating causes a surface temperature oscillation ΔT with a frequency of 2ω. This temperature fluctuation produces a small change in the resistance of the metal wire with a certain thermal coefficient. Consequently, the terminal voltage generated in the metal wire includes the third-harmonic component of 3ω. Thus, the information about the surface temperature oscillation can be obtained from the detection of a small voltage signal. The actual experimental system uses a bridge circuit and a lock-in amplifier, and the ΔT values are measured as a function of frequency. The theoretical value of ΔT is given by the solution of the diffusion equation for a radial heat flow from

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the surface electrode. At low frequencies in which the thermal penetration depth is much larger than the PS layer thickness, ΔT is the summation of two components: one from the PS layer, ΔTPS, and the other from the c-Si substrate, ΔTS. The ΔTPS value depends on the thermal conductivity of the PS layer and the experimental parameters. Since the ΔTS value is obtained from the known thermal parameters of c-Si, α value of the PS layer can be determined from the analysis of the measured ΔT. At high frequencies, on the other hand, the thermal penetration depth is smaller than the PS layer thickness, and then the contribution of the substrate to ΔT is negligible. Under this situation, the experimental ΔT data simply relate to the D value that is given as α/C. So the C value of the PS layer can be deduced from α measured at low frequencies. Owing to the insensitivity to errors from blackbody radiation, this method makes it possible to determine the thermal constants more precisely rather than the method based on simple thermal flow measurements. By employing the 3ω method, a strong anisotropic thermal conductivity of nano-PS was detected between in-plane and cross-plane thermal conductivities (Kim and Murphy 2015). Another method is based on the noncontact optical probing of PS samples. One approach utilizes the optical absorption and the subsequent photoinduced acoustic effect (McDonald and Wetsel 1978; Almond and Patel 1996). When the photoacoustic emission amplitude and its phase are measured for the sample as a function of chopping frequency, a kink appears in the frequency response at which the thermal diffusion length is equal to the sample thickness. From this critical frequency, the e and α values of different PS samples were determined in ambient air at room temperature (Cruz-Orea et al. 1996; Calderon et al. 1997; Benedetto et al. 1997; Obraztsov et al. 1997; Bernini et al. 1999; Shen and Toyoda 2003; Lishchuk et al. 2015). To detect the thermal wave propagation based on the optical methods, micro-Raman spectroscopy (Lysenko et al. 1999) and transient heating by a laser pump pulse (Bernini et al. 2001) have also been conducted. By using self-standing PS membranes (Maccagnani et al. 1999), the accuracy of these measurements was improved (Bernini et al. 2005; Wolf and Brendel 2006). The periodic “microporous” freestanding PS thin film is also useful for measuring the in-plane thermal conductivity by a steady-state method (Song and Chen 2004). The in-plane thermal and electrical conductivities of the macroporous Si membranes with pore size of 5–15 μm were investigated simultaneously by using selfheating and periodically laser-heating methods (Hagino et al. 2015). The effect of thermal oxidation treatments on thermal properties of meso-PS layers was analyzed by the use of photothermal deflection technique (Hlel et al. 2015). A significant reduction of the thermal conductivity of PS after irradiation of highenergy heavy ions (238U and 130Xe) was detected by scanning thermal microscopy (Massoud et al. 2014). It has been reported by means of the pulsed photothermal method that in addition to the porosity effect, the presence of Si–H and Si–O bonds in meso-PS participates to the decrease in the thermal conductivity (Melhem et al. 2015).

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Recently, the characteristic phonon transport in nanostructures was analyzed theoretically by molecular dynamic simulations (Fang and Pilon 2011) in relation to the scaling laws for the thermal conductivity in nanodot array (Fang et al. 2012), the hydrodynamic approach (Alvarez et al. 2010), the anisotropy and junction effects in nanoporous structures (Guo and Huang 2014), Monte Carlo simulations of phonon transport (Wolf et al. 2014; Jean et al. 2014), atomistic simulations both for macro- and nano-PS systems (Dettori et al. 2015), and the pore interface separation (Cartoixà et al. 2016). Simple analytical expressions allowing an estimation of the reduction in the thermal conduction were provided by investigating the influence of porosity and pore size for application to thermoelectric conversion (Tarkhanyan and Niarchos 2014). The experimental fact that room temperature thermal conductivities in periodically holey silicon are well below the dimensional limitation was discussed from the viewpoint of incoherent scattering of phonons at the surfaces of the pores and in the neck region (Ma et al. 2014). Extreme reduction of the thermal conductivity of c-Si by a factor up to 10,000 times at room temperature due to enhanced phonon localization was suggested in a nanoscale threedimensional Si phononic crystal with spherical pores (Yang et al. 2014). Based on analyses of the effect of the presence of nanopores on the thermal conductivity of silicon by a thermodynamic formulation coupled to the effective medium approximation (Machrafi and Lebon 2015), the rectifying thermal coefficient at low temperatures was calculated for various bulk–porous silicon configurations (Machrafi et al. 2016). Calculation of heat transport in nano-PS by using mean free pathdependent Boltzmann transport, equation provided the relative contribution of optical phonons to the thermal conductivity as a function of temperature (Romano et al. 2016). Figure 1 shows α  C mapping at room temperature of various materials. In metals, the contribution of high-density free electrons to α is dominant over that of lattice vibrations, while the situation is reversed in semiconductors. In c-Si with purely covalent bonds, particularly, the lattice contribution is enhanced due to the completely neutral lattice point. This exceptionally high thermal conductivity of c-Si disappears in PS, and then the C value is significantly decreased. For the porosity of 40% and 70%, typical experimental α values are 1.2 and 0.2 W/(m
K), respectively (Gesele et al. 1997). The corresponding C values fall within 0.4–0.2 MJ/(m3K) (Kihara et al. 2005). As a consequence, the thermal effusivity of PS is over 30 times lower than that of c-Si. The measured thermal diffusivity D of a 50%-porosity PS sample at room temperature is ~0.01 cm2/s (Bernini et al. 2005). This value, consistent with the result of a theoretical analysis, is two orders of magnitude lower than that of c-Si. When oxidized, both α and C values tend to become higher than those of as-prepared PS (Kihara et al. 2005). In the PS samples with an extremely high porosity (80–90%), α values fall to 0.05–0.03 W/(m
K) (Gesele et al. 1997) that is over three orders of magnitude lower than that in c-Si. Typical experimental values of these thermal properties are listed in Table 1.

Thermal Conductivity a (W/mK)

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Metals (Al, Cu, Ag, W, Au)

c - Si

100

Semiconductors 10 Oxidized PS

Quartz glass

1

Water

Polyethylene PS 0.1

Rubber Air

0.01

0

1

2

3

4

Heat Capacity per unit Volume C (MJ/m3K)

Fig. 1 Plots of the typical measured thermal conductivity of PS and other materials versus their corresponding heat capacity per unit volume. The most important determining factor for the thermal parameters of PS is the porosity. An extremely big contrast is induced in the thermal properties between PS and c-Si, especially in the high-porosity case

Related Applications Coexistence of low α and low C in PS is a very attractive feature as the thermal insulating material (see handbook chapter ▶ “Thermal Isolation with Porous Silicon”). Most conventional materials do not meet this requirement: Though silica, polymers, and rubbers have relatively low thermal conductivities, it is difficult to lower their C values. The exceptional one is silica aerogel powder and its mixture with carbon black whose α and C values are significantly reduced, especially at low temperatures due to its extremely low density (Fricke et al. 1989; Rettelbach et al. 1995). The abovementioned specific thermal property underpins many potential application areas: thermal isolation (Lysenko et al. 2002), thermally induced broadband sound emission (Shinoda et al. 1999; Koshida et al. 2013; Okabe et al. 2013), thermoelectric conversion (Lee et al. 2008a; Zhang et al. 2015), photothermal therapy (Lee et al. 2008b, 2012), local heating leading to blackbody radiation (Costa et al. 1998), the use in Si micro-cooling devices (Valalaki and Nassiopoulou 2013; Valalaki and Nassiopoulou 2014), and sintering (Kovivchak and Davletkil’deev 2009; Timoshenko et al. 2000; Koyama and Fauchet 2000). These applications and processing issues are reviewed in the handbook chapters ▶ “Porous Silicon Acoustic Devices,” ▶ “Porous Silicon for Microdevices and Microsystems,” ▶ “Thermal Isolation with Porous Silicon,” ▶ “Porous Silicon in Photodynamic and Photothermal Therapy,” and ▶ “Sintering of Porous Silicon.”

0.87 (oxidized) 1.86 0.72 1.68

8.7 5.1 0.23–0.37 12.3 (in-plane)

48 66 30 52 (periodic) 57 48 65 Nanowires

Optical

56

Thermal microscopy Electrical (3ω)

64 71 79 89 55 Oxidized 60 80 62 74 50

Thermal conductivity [W/(m
K)] 1.2 4.2 0.04–0.27 (20–300 K) 0.04 (5–20 K) 1.7 (after 91-MeV Xe ion irradiation) 0.20 0.14 0.05 0.03 1.1 1.9 1.5 0.025 0.9 0.3 0.9

Porosity [%] 40 20 63

Method Thermal Flow Electrothermal

Kihara et al. (2005)

0.18 0.65 – – –

Hlel et al. (2015) Lishchuk et al. (2015) Zhang et al. (2015)

–

Fang and Pilon (2011) Hagino et al. (2015)

Wolf and Brendel (2006)

Bernini et al. (2005)

0.5 (Estimated from thermal diffusivity) 0.96 0.45 – 1.2 (Estimated from thermal diffusivity) – –

Lucklum et al. (2014) Obraztsov et al. (1997) Lysenko et al. (1999)

Gesele et al. (1997)

References Drost et al. (1995) Siegert et al. (2012) Valalaki and Nassiopoulou (2013) Valalaki and Nassiopoulou (2014) Massoud et al. (2014)

–

–

Volumetric heat capacity [MJ/(m3K)] – – –

Table 1 Techniques for quantifying the thermal constants of PS and the respective experimental values

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Mechanical Properties of Porous Silicon Leigh Canham

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Young’s Modulus . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Yield Strength . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hardness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fracture Toughness . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anisotropy of Mechanical Behavior . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Properties of Nanoscale Solid Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Relevance of Mechanical Properties to Processing, Characterization, and Emerging Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

310 310 311 313 314 314 315 315 316 316

Abstract

Introducing nanoscale porosity into silicon dramatically lowers its stiffness and hardness in a tunable manner over a wide range. In this updated review, available data is collated on Young’s modulus and Vickers hardness as a function of porosity, layer morphology, and surface chemistry. There is little quantitative data on fracture toughness and strength, but theoretical work predicts that optimized nanocomposites could be very mechanically durable. The exceptional plasticity recorded for individual silicon nanowires is yet to be demonstrated in mesoporous silicon. A number of application areas are highlighted that rely heavily on the mechanical properties of porous silicon.

L. Canham (*) School of Physics and Astronomy, University of Birmingham, Birmingham, Worcestershire, UK e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_21

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Keywords

Porous silicon · Mechanical properties · Youngs modulus · Hardness · Fracture toughness · Yield strength

Introduction The remarkable mechanical properties of silicon underpin its role in MEMS (Peterson 1982). The single crystal pure material is relatively hard, very brittle, but exceptionally strong, with a yield strength that exceeds that of stainless steel. When we porosify a material, we can dramatically change mechanical properties, often in a tunable manner (Gibson and Ashby 1997; Rice 1998). The mechanical properties of mesoporous silicon have so far received nothing like the intense scrutiny that its structural, luminescent, thermal, and optical properties have (see, e.g., the handbook chapters ▶ “Microscopy of Porous Silicon”, ▶ “Photoluminescence of Porous Silicon,” ▶ “Thermal Properties of Porous Silicon,” and ▶ “Refractive Index of Porous Silicon”). Following the very first studies (Barla et al. 1984; Drory et al. 1990), there have been continued but infrequent assessments (Da Fonesca et al. 1995; Fang et al. 2008; Rahmoun et al. 2009; Aliev et al. 2011). Nonetheless, there is a growing realization that robust porous silicon device and system performance will ultimately rely on achieving the appropriate mechanical properties, in conjunction with the novel functionality achieved through nanostructuring. In this short review, we survey the literature data available on yield strength, Young’s modulus, hardness, and fracture toughness. Comparisons are made with bulk nonporous silicon and occasionally other materials. Emerging application areas where the mechanical properties have primary importance are mentioned. Finally, we identify specific mechanical properties where data is scarce and comment on perceived valuable areas for further research.

Young’s Modulus The Young’s modulus of a material, sometimes called the modulus of elasticity, quantifies its stiffness. Nonporous crystalline silicon has a value around 160 GPa, higher than, for example, the metals aluminum, titanium, and copper, but much lower than ceramics such as silicon carbide and diamond. There are now a number of studies conducted on Young’s modulus as a function of porosity in silicon, and the agreement between differing measurement techniques is generally good for similar material (see Fig. 1 from Magoariec and Danescu 2009).

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140 X-Ray diffraction Acoustic measurements Nanoindentation Brillouin scattering Theoretical prediction

Young modulus (GPa)

120

100

80

60

40

20

0

0

0.1

0.2

0.3

0.4

0.5 0.6 Porosity

0.7

0.8

0.9

1

Fig. 1 Tunability of Young’s modulus of mesoporous silicon (Magoariec and Danescu 2009) (Reproduced courtesy of Alexandre Danescu)

Table 1 lists typical values for different types of porous silicon. The higher the porosity, for similar pore size distributions and skeleton morphology, the lower the Young’s modulus. At the highest levels of mesoporosity (with small pore size) studied, it can be reduced to values around 1 GPa, even lower than that of polymers like nylon and polystyrene. Porous amorphous silicon has reported Young’s modulus values that are lower than those of porous crystalline silicon at similar porosity (Jiang et al. 2016). For primarily macroporous silicon, the reduction in stiffness versus porosity is likely to be much less dramatic (Wang et al. 2011). Recent calculations also predict that Young’s modulus of mesoporous silicon (and indeed other mechanical properties) is strongly dependent on pore geometry (Winter et al. 2017).

Yield Strength The yield strength of a material is the maximum stress it can withstand while being stretched or compressed without breaking. Silicon has very high yield strengths (7 GPa for single crystal and 1.5 GPa for polycrystalline). There is virtually no quantitative data available to date on either the tensile or compressive strength of

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Table 1 Young’s modulus of porous silicon layers over a range of porosities (34–90%), pore size (meso-, macro), and silicon substrate types (p , p+, n+) Porous Si Meso- (p+) 36% 60% 80% 90% Meso- (p+) 70%

Young’s modulus Value (GPa) 50.9 18.8 5.5 0.87 1.5

Acoustic microscopy Nanoindentation

Meso- (p ) 80%

1.25

Meso- (p+) 79%

10.4

Nanoindentation

Meso- (n ) varied

5.5–52

Nanoindentation

Meso- (p+, p ) varied

3–27

Nanoindentation

Meso-(p+): effects of thermal treatments

Calculated

Macro- 34% Macro- 48% Meso- cylindrical pores Meso- ellipsoidal pores

Significant increases upon thermal oxidation and carbonization 72.1 45.2 58.7

Method Nanoindentation

Strain analysis

Calculated

References Bellet et al. (1996)

Populaire et al. (2003) Doghmane et al. (2006) Fang et al. (2009) Oisten et al. (2009) Charitidis et al. (2011) Salonen et al. (2015) Wang et al. (2011) Winter et al. (2017)

25.0

porous silicon as a function of porosity. This is a valuable area for much further research. One study (Klyshko et al. 2008) created a “meniscus-shaped” porous silicon structure in order to investigate tensile strength. A nonlinear decrease of strength with increasing porosity was observed. A mesoporous layer (p + Si) of 50% porosity, for example, was reported to have less than 30% of the strength of bulk Si, but the control tensile strength was not given and so the accuracy of the experimental approach is difficult to verify. Another study on porous silicon derived from n-type wafers (Wang et al. 2011) gave remarkably low compression strengths (in range 0.25–0.35 GPa) at an estimated 34% porosity. Here there are concerns from the microscopy data that the layers underwent significant degradation during air drying (see handbook chapter ▶ “Drying Techniques Applied to Porous Silicon”) and wet surface modification treatments, prior to mechanical testing. Finally, a detailed study of mesoporous films generated by galvanic effects (see handbook chapter ▶ “Porous Silicon Formation by Galvanic Etching”) also reports a

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dramatic reduction in tensile strength as a function of etch time for micromachined polycrystalline silicon (Miller et al. 2008). Porosity, estimated from the morphology of layers, was in range 20–70%. They commented that nanoscale porosity alone (measured pore diameters were 5–15 nm) should change Young’s modulus, but not strength, and attributed decreased strength to cracks from etched as-fabricated defects and grain boundaries. Indeed, for corroded single crystal structures, the tensile strength was decreased or increased, depending on the etchant used. Mechanical failure analysis led to strength changes being rationalized according to either a fracture mechanics or stress concentration approach, depending on the surface wetting characteristics of the etch chemistry.

Hardness The hardness of a material quantifies its resistance to permanent shape changes induced by applied mechanical forces such as friction or indentation by a sharp object. Scratch resistance is often measured on the Moh 1–10 scale and indentation hardness on the Vickers scale. Single crystal nonporous silicon has a Moh scale hardness of 6 (for comparison, diamond is 10, quartz is 7, calcium carbonate is 3, talc is 1). It has a Vickers hardness of 11.5 GPa. There have now been a few studies of the indentation hardness of porous silicon, with typical values for different types of porous silicon listed in Table 2. Clearly, like Young’s modulus, hardness decreases rapidly with increasing porosity. Polymer-porous silicon composites (see handbook chapters ▶ “Porous Silicon Polymer Composites” and ▶ “Porous Silicon and Templating”) are likely to have very low hardness but potentially high fracture toughness (see next section). Table 2 Indentation hardness of porous silicon layers over a range of porosities (30–84%) and morphologies and the effects of impregnation and oxidation Study objective Porosity dependence of hardness

Porous Si Meso- (p+)

Effects of polymer impregnation

Meso- (p+) and heavily oxidized

Effects of oxidation Morphology dependence of hardness

Meso- (p+)

Effects of thermal carbonization

Meso- (p+)

Meso- (p + vs. p )

Vickers hardness value (GPa) 30% 8.8 60% 5.8 84% 0.75 Dramatically lower in polymer nanocomposite 0.05–0.15 60–79% Significant increases upon 300  C oxidation p+ 1.4–2.4

p

0.2–0.3 Significant increase from 0.9–1.0 to 1.9–2.3 upon carbonization

References Duttagupta et al. (1997) Ni et al. (2005) Fang et al. (2009) Charitidis et al. (2011)

Salonen et al. (2015)

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Fracture Toughness Fracture toughness quantifies the resistance of a material to fracture when it develops a crack. Ceramics like alumina and silicon carbide have a low fracture toughness (3–5 MPa.m1/2) and are brittle; ductile metals in contrast have high values (e.g., 15–50 MPa.m1/2). Single crystal silicon has an orientation-dependent fracture toughness typically in the range 0.6–1.3 MPa.m1/2 (Chen and Leipold 1980; Tanaka et al. 2006; Jaya et al. 2015). Thus, like silica, it is a very brittle material. Very little data exists on the fracture toughness of porous silicon. However, two studies suggest that optimized nanocomposites (see hand book chapter ▶ “Porous Silicon Polymer Composites”) could exhibit much higher toughness than nonporous silicon. An experimental study reported that the inclusion of poly-phenylene vinylene polymers into oxidized mesoporous silicon could yield toughness values more than five times higher than that of their bulk silicon control (Ni et al. 2005). The maximum value reported for a polymer-impregnated mesoporous film on a wafer was 4.2 MPa.m1/2. A particularly interesting theoretical study (Garcia et al. 2010) proposed that the nanoscale porosity of silica in diatoms was responsible for their remarkable toughness. Their calculations for idealized mesoporous silicon predict a toughness that is extremely high for skeleton thicknesses in range 1–2 nm and specific morphologies. This should be contrasted with the incredibly low fracture toughness of ultrahigh porosity silica aerogels, which can have measured values as low as 0.0008 MPa.m1/2 (Phalippou et al. 1989). Polymer reinforcement of silica aerogels has been extensively investigated as a means to improve strength and toughness (Randall et al. 2011). Such a strategy, applied to highly porous silicon, deserves attention. The Garcia 2010 theoretical study was recently extended (Winter et al. 2017) to include varying porosities, skeleton sizes, and pore geometries. Toughness was found to be uncoupled from Young’s modulus and strength: with appropriate design a stronger and tougher structure may also be more flexible.

Anisotropy of Mechanical Behavior The directionality of electrochemical etching can give rise to strong spatial anisotropy in the mechanical properties of mesoporous silicon (Populaire et al. 2003), as found for other properties (see handbook chapters ▶ “Optical Birefringence of Porous Silicon” and ▶ “Electrical Transport in Porous Silicon”). In a moderately oxidized 70% porosity layer generated from p + Si substrates, for example, Young’s modulus was estimated to be 0.44 GPa parallel to the predominant mesopore direction, but 1.5 GPa perpendicular to it (Populaire et al. 2003). We would expect less spatial mechanical anisotropy in mesoporous structures from p-substrates, which have a more sponge-like morphology (Charitidis et al. 2011). In sintered porous silicon (Martini et al. 2012), much of this mechanical anisotropy is also lost.

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Nonetheless, this is another important area for future research since many of the particulate porous silicon forms being developed could have anisotropic porosity and hence mechanical properties.

Mechanical Properties of Nanoscale Solid Silicon It is worth mentioning some specific mechanical phenomena reported for nanoscale solid silicon, since these are the building blocks of porous silicon and clearly have relevance. Silicon nanowires (see delRio et al. 2016; Zhang et al. 2016 for example) start to have significant changes in elastic modulus below about 100 nm diameter (Sohn et al. 2010). At diameters below 60 nm, they show significant plastic deformation before fracture, through the emission of dislocations, amorphization, and necking (Han et al. 2007). Indeed, exceptional plasticity has been recorded for stretched model bridge structures with elongations of up to 20 times the original length prior to fracture (Ishida et al. 2011). Solid silicon nanospheres also have higher hardness than microspheres (Gerberich et al. 2003). Finally, it has been demonstrated with silicon nanobeams that mechanical properties are sensitive to silicon surface chemistry: derivatized structures showed higher strength and durability than hydride-terminated ones (Alan et al. 2006). Some data exists on the effects of thermal oxidation for porous silicon (see Table 2), but none on the effects of surface derivatization on mechanical properties known to improve the stability of numerous other properties (see handbook chapters ▶ “Silicon–Carbon Bond Formation on Porous Silicon” and ▶ “Chemical Reactivity and Surface Chemistry of Porous Silicon”). This would also be worth investigation.

Relevance of Mechanical Properties to Processing, Characterization, and Emerging Applications High porosity in silicon, while bestowing remarkable properties, can have various implications and repercussions as a result of its modified mechanical properties. Specific constraints are discussed in the introductory chapters ▶ “Characterization Techniques and Challenges with Porous Silicon” and ▶ “Processing Techniques and Process Flows with Porous Silicon.” Even at the formation stage, it is the mechanical strength that limits the level of nanostructuring achievable via wet etching techniques and air drying. The capillary forces that accompany liquid condensation and evaporation can introduce both reversible and irreverssible deformations (Gor et al. 2015; Rolley et al. 2017). Table 3 gives a number of diverse applications, reviewed in detail elsewhere in this handbook, where the mechanical properties are either the primary factor in determining material suitability or play an important supporting role in desired functionality.

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Table 3 High relevance of mechanical properties of porous silicon to selected applications Application area Acoustics Drug delivery Lithium batteries Solar cells Biofiltration Tissue engineering Oral hygiene

Aspect Multilayer acoustic reflectors Maximizing drug payload Cycling stability of anode Layer transfer Filtration membranes Use as a biodegradable scaffold Use as a gentle toothpaste abrasive

Challenge High reflectivity in thin structures Capillary forces during drug loading Accommodating lithiationinduced volume expansions Facile fracture with low interface roughness Differential pressures across membrane Changing mechanical properties during biodegradation Remove pellicle without abrading tooth enamel

Desired mechanical properties Wide tunability of acoustic velocity Sufficient strength to avoid matrix collapse Ductility and high fracture toughness Low strength to facilitate fracture High strength yet high permeability Matched to those of tissue being replaced Low hardness but high strength

Concluding Remarks This updated review has highlighted that there is significant data on the stiffness and hardness of mesoporous silicon, but more data is still needed on strength and toughness. Remarkable mechanical properties have been seen with individual silicon nanowires, such as exceptional plasticity. Clearly, the mechanical properties of porous silicon deserve more scrutiny and optimization, especially for important application areas such as lithium batteries, where porosity engineering and morphology optimization is critical to avoid silicon anode fracture during cycling (Xiao et al. 2015). Many of the strategies used to improve the strength and toughness of silica aerogels are relevant to highly porous silicon and, in particular, silicon aerocrystals (Canham et al. 1994).

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Phonon Frequencies in Porous Silicon G. Todd Andrews

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Measurement of Phonon Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Influence of Processing on Phonon Frequencies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Quantities and Material Properties Derived from Phonon Frequencies . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

320 320 321 324 325 326

Abstract

An overview of the measurement, nature, and utility of phonon frequencies in porous silicon systems ranging from thin films to nanoparticles is presented. Brillouin and Raman spectroscopy are briefly described, and studies in which these techniques have been successfully applied to phonon frequency measurement in porous silicon are comprehensively listed. The results of studies on the responses of phonon frequencies to different material attributes, chemical treatments, and environmental conditions are summarized. Material properties that have been derived from phonon frequencies are noted. Keywords

Porous silicon · Thin films · Multilayers · Superlattices · Nanoparticles · Phonon frequency · Brillouin spectroscopy · Raman spectroscopy · Elastic constants · Acoustic wave velocities · Acoustic phonons · Optic phonons · Fractal · Thermal conductivity

G. T. Andrews (*) Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John’s, NL, Canada e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_104

319

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Introduction Knowledge of the frequencies of phonons that propagate within a material system allows useful information on its elastic, structural, and vibrational properties to be extracted and is key to the development of new devices that rely on phonon engineering. For example, measurement of acoustic phonon frequencies in porous silicon superlattices led to a detailed understanding of phonon dispersion in these systems and showed that such structures could function as phononic crystals, the acoustic analogs of photonic crystals (Parsons and Andrews 2012). In fact, there has been a profusion of studies of phonon frequencies in porous silicon-based systems, and this review attempts to catalogue and concisely capture the salient aspects of these works. A short description of the main measurement methods and the different structures and systems studied is first presented. This is followed by a survey of studies on the effects of material characteristics and various forms of processing on phonon frequencies in porous silicon. Also highlighted are quantities and material properties that have been derived from phonon frequencies. It should be noted that this review focuses on studies of phonon frequencies in microporous and mesoporous silicon because there has been comparatively little such work done on the macroporous variety.

Measurement of Phonon Frequencies The techniques primarily used to measure phonon frequencies in porous silicon are Raman and Brillouin spectroscopy; for reviews see Anderson (1971), Hayes and Loudon (1978), and Sandercock (1982). In these closely related and complementary inelastic laser light scattering techniques, the frequency of the incident light is shifted due to interaction with phonons in the target material. The thermal acoustic phonons probed in Brillouin scattering experiments typically have frequencies in the GHz range, while THz-frequency optic phonons are probed by Raman spectroscopy. Figure 1 shows representative Raman and Brillouin spectra of porous silicon films. Phonon frequencies are obtained directly by measurement of the frequency shifts of the spectral peaks from the incident light frequency (at an assigned frequency shift of 0 GHz or, equivalently, 0 cm 1, as commonly used in Raman spectroscopy experiments). Both optic and acoustic phonon frequencies in microporous and mesoporous silicon are, in general, lower than the corresponding phonon frequencies in bulk crystalline silicon. The versatility of Brillouin and Raman spectroscopy for measurement of phonon frequencies in porous silicon is evident from the variety of structures that have been successfully probed using these techniques. These include supported and freestanding single-layer films, multilayers, and nanoparticles (see Table 1).

Phonon Frequencies in Porous Silicon

321

b

LA

R

Brillouin intensity (arb. units)

TA

INTENSITY [a.u.]

a p+ - type Si

650

550 500

450 80%

350

35%

65% 440

750

460

4 500 520 480 RAMAN SHIFT [cm–1]

540

8 10 20 30 Frequency shift (GHz)

40

Fig. 1 (a) Raman spectra of porous silicon films with porosities of 35%, 65%, and 80% prepared on heavily doped p-type (111) substrates. The narrow Raman line at 520.5 cm 1 is that of crystalline silicon (Reprinted from Zuk et al. (1997), with permission from Elsevier. http://dx.doi.org/10.1016/ S0040-6090(96)09532-6). (b) Brillouin spectra of 70% porosity, 3.0 μm thick H-terminated porous silicon at various angles of incidence. R Rayleigh mode, TA transverse acoustic mode, LA longitudinal acoustic mode (Reprinted with permission from (Fan et al. 2002a) Copyright 2002 by the American Physical Society. http://dx.doi.org/10.1103/PhysRevB.65.165330)

Influence of Processing on Phonon Frequencies Phonon frequencies in porous silicon have been shown to be sensitive to fabrication parameters and a variety of chemical and physical treatments, many of which result in modification of the film microstructure. In particular, there have been numerous Raman scattering studies of the response of optic phonon (primarily longitudinal optic (LO)) frequencies to various forms of processing. A sampling of these is provided in Table 2. Corresponding studies on the response of acoustic phonon frequencies are fewer in number, but the effects of morphological differences (Andrews et al. 2007) and chemical modification (Andrews and Clouter 2001, Fan et al. 2001, 2002b, 2003) have been explored. Figures 2 and 3 show examples of the effects of these on acoustic phonon frequencies. The former figure shows Brillouin spectra of three different porous silicon films with similar porosities. The differences in the frequencies of the probed acoustic phonons (as indicated by the peak positions) are due to the well-known morphological differences in these films

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Table 1 Listing of Brillouin and Raman scattering studies of porous silicon films, multilayers, and nanoparticles

Technique Raman spectroscopy

Brillouin spectroscopy

Application to porous silicon Single-layer Single-layer films: films: supported freestanding Abidi et al. Abidi et al. (2010) (2009, Andrianov et al. 2010) (1999) Andrews et al. Bolotov (2009) (1997) Feng et al. (1993) Fuchs et al. Guha et al. (1996, (1993) 1997) Gregora et al. Kanemitsu et al. (1993, 1994) (1993) Korsunska et al. Kosovic et al. (2005) (2014) Manotas et al. Papadimitriou (2001) et al. (1998a, b, Mariotto et al. 1999) Pusep et al. (1995) (2009) QuirogaTanino et al. Gonzalez et al. (1996) (2014) Wang et al. Rahim et al. (2000) (2011) Xu et al. (1998, Roy et al. 1999, 2000) (1994) Zeman et al. Yang et al. (1995, 1996) (1994) Zhang et al. Zhang et al. (1992) (1992) Zuk and Kulik (1997) Andrews et al. Parsons et al. (1996, 2004) (2007) Andrews and Parsons (2007) Clouter (2001) Beghi et al. (1997) Fan et al. (2001, 2002a, b, 2003) Lockwood et al. (1999)

Multilayered films and superlattices Cho et al. (2000) Gonchar et al. (2011) Kuzik et al. (1999) Mamichev et al. (2011) Manotas et al. (1999) Sirleto et al. (2005)

Nanoparticles Amonkosolpan et al. (2012) Guha et al. (1994, 1995) Kale and Solanki (2010) Perrone Donnorso et al. (2012) Zhao et al. (1993)

Andrews et al. (2007) Parsons and Andrews (2009, 2012) Polomska and Andrews (2009) Polomska-Harlick and Andrews (2012)

(Andrews et al. 2007). The latter figure shows the frequencies of two acoustic phonon modes (Rayleigh surface and longitudinal bulk) in porous silicon films passivated with different organic molecules and those for H-passivated porous silicon (Fan et al. 2002b). It is clear that the frequencies of both modes are dependent on the type of passivating molecule. In a related study, a monotonic decrease in the

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323

Table 2 Summary of responses of optic phonon (primarily longitudinal optic) frequencies in porous silicon to various preparation parameters and post-anodization processing Process/ treatment Preanodization ion irradiation Postanodization ion irradiation Pressure

Effect on phonon frequencies Increase with increasing dose

Study examples Pavesi et al. (1994)

Small/no change with irradiation relative to unirradiated porous silicon

Jacobsohn et al. (2005), Prabakaran et al. (2004), Ushakov et al. (1997), Zhao et al. (2007) Papadimitriou et al. (1998a, b), Zeman et al. (1995, 1996), Zhao et al. (1993) Agulló-Rueda et al. (1998), Andrews et al. (1997), Korsunska et al. (2005), Zhang et al. (1992) Deb et al. (1997), Khajehpour et al. (2011), Khoi et al. (1999), Manotas et al. (2000a), Obraztsova et al. (1993), Semjonow et al. (1993) Feng et al. (1993, 1994)

Increase with increasing pressure

Laser wavelength

Decrease with decreasing wavelength

Laser power

Decrease with increasing laser power

Low temperature Wetting – isopropanol or ethanol Stress

Decrease with decreasing temperature

Drying – evaporation of water HF concentration in electrolyte

Overall decrease as sample dries but anomaly at intermediate time

H+ concentration in electrolyte Etching current density Etching time Aqueous versus

Increase with wetting relative to porous silicon with no liquid in pores

Ferrara et al. (2007, 2008a, b)

Decrease (increase) with tensile (compressive) stress relative to unstressed porous silicon

Andrianov et al. (1999), Churaman et al. (2010), Huang et al. (2009), Kompan et al. (1996), Li et al. (2010), Lysenko et al. (2009), Manotas et al. (2000b, 2001), Marty et al. (2006), Miu et al. (2011), Papadimitriou et al. (1999), Sun and Miyasato (1995) Amato et al. (1996)

Independent of HF concentration for higher concentrations. Decrease with decreasing HF concentration for intermediate concentration range Independent of H+ concentration over the range investigated

Sasaki and Kitahara (1994)

Decrease with increasing current density

Guo et al. (2012), He and Li (2005), Patel et al. (2002), Torchynska et al. (2002) Asli et al. (2011), Prabakaran et al. (2005), Amirhoseiny et al. (2012) Ben Younes et al. (2003)

Decrease in frequency with increase in etching time Peak at 480 cm 1 for sample etched in aqueous electrolyte (peak not

Xu et al. (2000)

(continued)

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Table 2 (continued) Process/ treatment ethanoic electrolyte Exposure to F2 and H2O vapor

Effect on phonon frequencies

Study examples

present in spectra of sample etched in ethanoic electrolyte) Nearly independent of exposure time

Wadayama et al. (1998)

(a)

L

p–

T

p+

Intensity (arb. units)

3000

2000

T R

1000

R

U R

0

0

n+

10 20 Frequency Shift (GHz)

Fig. 2 Anti-Stokes Brillouin spectra of porous silicon films (porosity 0.60) fabricated from p , p+, and n+ crystalline silicon. The incident angle was 60 . The R, U, T, and L peaks correspond to the Rayleigh surface mode, a mode of unknown origin, and transverse and longitudinal bulk acoustic modes, respectively (Reprinted from Andrews et al. (2007) # 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. http://dx.doi.org/10.1002/pssa.200674344)

frequencies of the Rayleigh surface and bulk longitudinal acoustic phonons with increased exposure time was observed for porous silicon films oxidized in ambient air (Fan et al. 2003).

Quantities and Material Properties Derived from Phonon Frequencies Several material properties and related quantities may be determined from phonon frequencies. In the case of porous silicon, acoustic phonon frequencies have been used to calculate surface and bulk acoustic wave velocities from which elastic constants such as Young’s modulus may be deduced (Andrews et al. 1996, 2004, 2007; see also the chapter ▶ “Acoustic Characterization of Porous Silicon” in this handbook). Optic phonon frequencies have been used to determine the thermal conductivity of porous silicon films (Périchon et al. 1999, 2001) and, when coupled with a phonon confinement model, can provide estimates of crystallite size and shape (Abidi et al. 2010; Chattopadhyay et al. 2002; Cho et al. 1998; Gregora et al. 1993,

Phonon Frequencies in Porous Silicon

a 32 30

325

b 6.0 COOEt COOH

COOH

Frequency (GHz)

Frequency (GHz)

5.6 28

26

C H

24

COOEt 5.2

C

H

4.8

OC

OC 22 4.4 20

Samples

Samples

Fig. 3 (a) Frequencies of the longitudinal acoustic mode in 3 μm thick porous silicon films passivated with different organic molecules. The full (open) circles denote data for 70% (~75%) porosity samples. (b) Frequencies of the Rayleigh mode in 70% porosity, 3 μm thick porous silicon films passivated with different molecules. COOEt ethyl undecylenate, COOH undecylenic acid, C 1-decene, H hydrogen, OC decyl aldehyde (Reprinted from Fan et al. (2002b) #IOP Publishing Reproduced with permission. All rights reserved. http://dx.doi.org/10.1088/0268-1242/17/7/310)

1994; Kanemitsu et al. 1993; Kosovic et al. 2014; Lysenko et al. 2000; Mariotto et al. 1995; Ribeiro et al. 1997; Schoisswohl et al. 1995; Sui et al. 1992; Zuk and Kulik 1997; Zuk et al. 1997; see also the chapter ▶ “Raman Spectroscopy of Porous Silicon” in this handbook). Phonon frequencies have also provided insight into the fractal nature of porous silicon (Roy et al. 1995; Wesolowski 2002). Recent Brillouin spectroscopy work has focused on measurement of acoustic phonon frequencies in porous silicon superlattices to reveal the phononic band structure (Parsons and Andrews 2009, 2012) and to test the ability of an effective elastic medium model to predict the elastic constants of these unconventional superlattices (Polomska and Andrews 2009; Polomska-Harlick and Andrews 2012).

Summary Acoustic and optic phonon frequencies in several forms of porous silicon have been successfully measured using the inelastic laser light scattering techniques of Brillouin and Raman spectroscopy. Phonon frequencies in porous silicon films were found to be sensitive to film morphology and to various chemical and physical treatments. A number of physical properties including thermal conductivity, phonon velocities, elastic moduli, and crystallite size and shape have been determined from knowledge of these frequencies.

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Mesopore Diffusion Within Porous Silicon Jörg Kärger and Rustem Valiullin

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diffusion in the Capillary-Condensed Liquid . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Diffusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Diffusion as a Function of Phase State . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Macroscopic Transport . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In applications such as sensing and drug delivery, the performance of mesoporous silicon (PSi) may be controlled by the rate of diffusive propagation of the confined molecules. The pulsed field gradient technique of nuclear magnetic resonance provides the most direct access to molecular diffusion. The different factors determining the diffusivities in PSi are the focus of this updated review. In particular, diffusivities in liquid state are shown to be most strongly affected by mesoscale disorder. Atomistic disorder is shown to control surface diffusion in applications in which PSi is brought into contact with gas phases at low vapor pressures. Correlations between the compositions of phases coexisting within the pore space, namely, liquid and gaseous, and liquid and solid ones, respectively, are briefly discussed.

J. Kärger (*) Department of Physics, University of Leipzig, Leipzig, Germany e-mail: [email protected] R. Valiullin Felix Bloch Institute for Solid State Physics, University of Leipzig, Leipzig, Germany e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_22

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Keywords

Diffusion · Pulsed field gradient NMR · Confinements · Phase state-transport correlations

Introduction Molecular transport is an important process playing a key role in various applications exploiting mesoporous silicon, such as gas sensing or biomedicine (Schechter et al. 1995; Kovalev and Fujii 2005; Prestidge et al. 2007) In these particular cases, the substances of interest (chemicals, rare elements, pharmaceuticals, biomolecules) have to be transported into or out of inner pore spaces within the PSi particles. The effective transport rate may, hence, determine the time lag of, e.g., the optical response and the intensity of this response after having brought the PSi into contact with the gas atmosphere containing the chemicals to be sensed. Similarly, molecular transport can strongly affect the release rate of pharmaceuticals from the PSi material to which they are confined upon getting into contact with a particular environment in the human body (see handbook chapter ▶ “Drug Delivery with Porous Silicon”). Three recent reviews provide a comprehensive overview of diffusion characterization and phenomena in nanoporous materials (Kärger and Valiullin 2013; Huber 2015; Kärger and Ruthven 2016). Among the methods of studying molecular diffusion in porous media, and in porous silicon in particular (see Table 1), the pulsed field gradient technique of NMR spectroscopy (PFG NMR) is of particular relevance for elucidating transport in mesoporous solids (Kärger and Valiullin 2013). As a noninvasive method, it allows the observation of molecular migration without interfering with the internal processes. The technique is based on the creation of an initial nuclear coherence and following its loss due to the molecular displacements in an applied magnetic field gradient. Thus, the characteristics of molecular motion under equilibrium conditions may be followed in a broad timescale, from milliseconds to seconds. In the most relevant cases, however, transport in porous materials occurs under (quasi)equilibrium conditions, rendering the diffusive dynamics being the decisive mode for molecular transport. Exactly for these reasons, this mini-review will mostly focus on diffusion studies using PFG NMR applied to highly anisotropic mesoporous silicon with tubular pore morphology prepared from anodization of p + silicon wafers (Lehmann et al. 2000). Such anisotropy is evident, for example, in its optical constants (see handbook chapter ▶ “Optical Birefringence of Porous Silicon”), electrical current flow (see chapter ▶ “Electrical Transport in Porous Silicon”), and mechanical behavior (see chapter ▶ “Mechanical Properties of Porous Silicon”). Complementary to PFG NMR is quasi-elastic neutron scattering (QENS) (Jobic and Theodorou 2007), which has a unique potential to monitor molecular displacements over the range of nanometers. However, among the various features, brought about by the procedure of PSi fabrication, it is in particular the intrinsic mesoscalic disorder which plays a crucial role in determining the material properties. These inhomogeneities may lead to additional transport resistances which remain unobservable

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Table 1 A survey of the experimental method applied to study molecular transport in porous silicon Method PFG NMR

Neutron scattering Gas permeation

Permeability measurements

Optical interferometry, refractometry, and microimaging

Liquid or adsorbed phases (study ref. numbers) Valiullin et al. (2004, 2005a); Valiullin and Khokhlov (2006); Kondrashova et al. (2011); Puibasset et al. (2016); Kondrashova et al. (2017) Hofmann et al. (2012); Kusmin et al. (2010)

Ileri et al. (2012); Desai et al. (1999); Velleman et al. (2010); de Smet et al. (2002) Mares and Weiss (2011); Acquaroli et al. (2011); Anglin et al. (2004); Wu and Sailor (2013); Chung et al. (2014); Zhao et al. (2016)

Gas phase (study ref. numbers) Valiullin et al. (2004)

Measuring principle Equilibrium conditions

Gaburro et al. (2001); Lysenko et al. (2004); Gruener and Huber (2008); Kavalenka et al. (2012); Li et al. (2013)

Nonequilibrium conditions

Lauerer et al. (2015)

with molecular displacements in the nanometer range as accessible by QENS but are effective for the long-range displacements followed by PFG NMR.

Diffusion in the Capillary-Condensed Liquid Under the condition of spatial confinement, two mechanisms may give rise to a slowing down of the rate of molecular rearrangements. These are (i) surface interaction of the confined species with the PSi pore walls and (ii) purely geometric effects accounted for by the tortuosity factor. Their relative contributions can most easily be separated by probing molecular diffusivities on length scales shorter and much longer than the pore size. Recently, the former type of measurement has been performed by using QENS with n-hexane as guest molecule within PSi with an average pore diameter of 6 nm (Hofmann et al. 2012). Following molecular displacements over a length scale of up to 0.65 nm, the diffusivities were obtained to be smaller than the bulk diffusivities by not more than 30%. This rather moderate slowing down can be easily attributed to the existence of a less mobile layer of nhexane near the pore walls or to a limited range of wave-vectors used in the scattering experiments (see Kondrashova et al. 2017).

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Table 2 Self-diffusivities D|| (10 10 m2/s) in channel direction of different liquids in mesoporous silicon of different average pore diameters measured using PFG NMR at 25  C. The last column shows the bulk diffusivities Average pore size Water Nitrobenzene Cyclohexane n-Eicosane

5.6 nm 5.2 1.2 3.2 0.26

6.6 nm – 2.5 6.2 0.76

8.0 nm – 3.3 7.6 0.95

10.4 nm 3.8 9.0 1.2

Bulk 23.0 6.9 14.2 –

A totally different picture emerges from measurements by using PFG NMR. In this case, the molecular trajectories are traced on a length scale above hundreds of nanometers, notably exceeding the pore dimension. The data of Table 2 show that the diffusivities in PSi with comparable pore diameters, as considered in the QENS study, measured in the direction parallel to the channel axes are found to be few times slower than the bulk diffusivity. This fact indicates that, in addition to moderate surface effects, molecular diffusion over long distances is strongly affected by heterogeneities in the pore structure. The origin of these effects of heterogeneity may be quite different. First of all, it is well established that mesoporous silicon possesses a substantial degree of disorder, which is associated with the atomistic disorder of the pore walls and a strong variation of its pore diameter along the pore axis (Wallacher et al. 2004; Guegan et al. 2006; Naumov et al. 2008a, 2009; Dvoyashkin et al. 2008; Cerclier et al. 2012). This variation may give rise to constrictions which act as additional transport resistances, blocking long-range diffusion in axial direction (Petersen 1958; Burada et al. 2009; Berezhkovskii et al. 2007). Such an explanation would additionally be supported by the strong decrease in the effective diffusivities observed with decreasing average pore diameter. Further on, the occurrence of side channels, accompanied by the formation of dead-end pores and intersections between the individual channels and, hence, an increase in tortuosity, may also lead to decreasing long-range diffusivities (Cussler 2009). Indeed, the emergence of such intersections with increasing etching current and, therefore, porosity for the materials quoted in this study has experimentally been proven by investigating diffusion anisotropy in PSi (Naumov et al. 2008a). Thus, the ratio D||/D⊥ between the diffusivities measured in the directions parallel and perpendicular to the channel axes was found to be >104 in PSi with a pore diameter of 6 nm, showing that any significant contribution of channel intersections to overall transport could be ruled out, while in PSi with channels broadened to 9 nm, this ratio was found to drop to 50. This indicates that mass transfer perpendicular to the channel direction is now notably facilitated which testifies the existence of a substantial number of intersections between the different channels.

Surface Diffusion In contrast with the situation with capillary condensation so far considered, under the conditions of low-pressure atmospheres in contact with PSi, surface diffusion can become the key mechanism of mass transfer. Upon hitting the PSi surface, gas

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Fig. 1 Arrhenius plot of surface diffusivities for n-heptane in porous silicon with an average pore diameter of 5.6 nm for different surface coverages c (Dvoyashkin et al. 2009)

molecules may get physisorbed. If the heat of desorption exceeds substantially the activation energy required for molecular hops along the surface, molecular translation in the channel direction will mainly proceed by such hops. This process is referred to as surface diffusion (Gomer 1990). Its different aspects have thoroughly been addressed experimentally using atom microscopy and optics and using computer simulation approaches (Renisch et al. 1999; Ala-Nissila et al. 2002). The experimental techniques mentioned are limited, however, to operate with flat surfaces. It has recently been demonstrated that, benefitting from the option of accommodating up to 1018 nuclear spins on the large inner surface of mesoporous silicon, PFG NMR may serve as an excellent tool for probing surface diffusion in PSi (Valiullin et al. 2005a; Dvoyashkin et al. 2009). In striking contrast to adatom diffusion on flat metal surfaces, the surface diffusivity of organic molecules in PSi was found to increase with increasing surface coverage. Moreover, it was found that the apparent activation energy for diffusion decreases with increasing surface loading (see Fig. 1). These two facts are to be associated with strong surface heterogeneities in geometry and/or chemical nature. These heterogeneities give rise to a complex landscape in the site energies and, correspondingly, a notable distribution in the residence (or hopping) times. At a given surface coverage, molecules will therefore tend to occupy vacant sites with the lowest site energies available (corresponding to the longest residence times). This phenomenon may be used to tune molecular transport through porous silicon by surface modification (see handbook chapter ▶ “Silicon–Carbon Bond Formation on Porous Silicon”) (Velleman et al. 2010).

Diffusion as a Function of Phase State With emerging potential applications of nanostructured porous silicon as, e.g., optical switches (Barthelemy et al. 2007) or drug delivery containers (Prestidge et al. 2007), the knowledge of correlations between diffusive transport of confined

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Fig. 2 Effective diffusivities of cyclohexane in PSi with an average pore diameter of 10 nm as a function of the relative gas pressure z at room temperature (Valiullin et al. 2005b)

guests and their phase state is of practical relevance. The occurrence of such correlations has recently been demonstrated for both gas-liquid and solid-liquid coexistences of fluids in the pore spaces of mesoporous silicon (Dvoyashkin et al. 2008; Valiullin et al. 2004; Kondrashova et al. 2011). Due to strong confinement effects and intrinsic geometric disorder, the phase state, namely, the phase composition, of confined fluids is typically found to be smeared out over a sufficiently broad range of the external conditions. Because transport mechanisms in different phases can be different, like in gaseous and liquid phases, or one phase can form impermeable regions for molecular transport, like the crystalline phase formed upon freezing, under otherwise identical conditions, transport can also be most significantly affected by a change in the phase composition. Understanding their relationships may thus become most useful in optimizing the relevant processes in potential applications. For gas-liquid equilibria in pores, transport may include combinations of surface diffusion, diffusion in multilayers adsorbed on the pore walls, diffusion in capillarycondensed phase, and Knudsen diffusion in the gaseous phase. At very low external gas pressures, corresponding to submonolayer surface coverages, surface diffusion is typically the rate-determining transport mechanism for most organic liquids at moderate and low temperatures. Upon further increase of the external gas pressure, multilayers of guest molecules are formed on the pore walls. Due to the increased density of the gaseous phase and a weaker guest–guest interaction (compared with the guest–pore wall interaction), the desorption-adsorption events, intermittent with “Knudsen flights” in the pore interior, may substantially contribute to the overall mass transfer, as demonstrated by Fig. 2, and cannot be neglected anymore. Notably for inert gases, which are weakly adsorbed on the pore walls, Knudsen diffusion can be the major transport mode. In this regime, the diffusivities are proportional to one third of the product of the average thermal velocity and average pore diameter (Pollard and Present 1948). It has recently been discussed that the irregular pore

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Fig. 3 Effective diffusivities of nitrobenzene in mesoporous silicon with an intentionally created random variation of the pore diameter from 6 to 12 nm measured upon warming (Kondrashova et al. 2011)

shape in PSi may also affect the resulting effective diffusivities in the Knudsen regime (Gruener and Huber 2008). At even higher vapor pressures, the molecular pathways are further complicated by the inclusion of diffusive transport through domains of capillary-condensed liquid. Notably, these domains are formed at gas pressures well below the saturation pressure, exemplifying confinement effects upon phase transitions. The impact of these domains on the diffusion properties of the fluid is twofold. First of all, their presence decreases the relative volume available for the gaseous phase, typically leading to a corresponding decrease of the effective diffusivity. In addition, there is experimental evidence that geometrical features of the spatial distribution of these domains do also play a decisive role. Indeed, in line with adsorption hysteresis, hysteresis is as well observed in the diffusivities, exhibiting quite different scenarios for the processes of emptying and filling of the pore space (Naumov et al. 2008b). Figure 3 shows effective diffusivities of guest molecules measured at different temperatures, corresponding to different compositions of the frozen and liquid phases in the pores. The measurements have been performed starting from low temperatures, at which the liquid was in the frozen state. At these temperatures, diffusion takes place along a nonfrozen surface film of a few monolayers between the pore wall and the frozen core. Upon heating, a progressive melting of the frozen domains, starting from the pores with smallest dimensions as predicted by the Gibbs-Thomson equation (Jackson and McKenna 1990), does occur. Thus, more and more free space becomes available for diffusion, leading to increasing effective diffusivities with an apparent activation energy notably different from that of the bulk liquid. It is worth mentioning that different distributions of the frozen domains, which are attained during either freezing or melting, may result in different diffusivities at identical phase compositions due to different tortuosities (Dvoyashkin et al. 2008).

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Macroscopic Transport With the microscopic diffusivities as accessed using PFG NMR, the exchange rates between the molecular ensembles confined to PSi particles and surrounding them in the environment can be easily modeled (Carslaw and Jaeger 1946). To do this, the size of PSi particles, their shape, and the internal pore space anisotropy have to be known. Thus, for mesoporous silicon with channel-like pore geometry and a thickness L of the porous film, the average lifetime τ of molecules within the porous film may be found as τ = L2/12D, where D is the intraparticle diffusivity. If the relevant structural parameters are not known, the average exchange rates may be assessed experimentally using PFG NMR or with the option provided by IR microimaging (Lauerer et al. 2015). The respective approach is known as tracer desorption technique (Kärger et al. 2012). Its application can be based on the difference in the diffusivities in the pores and in the surrounding space, allowing to identify the relative fraction p of molecules in the overall NMR signal which, during a fixed observation time t, have never left the intrapore space (Valiullin et al. 2009). This quantity is directly relevant for drug delivery and reflects the release rates of pharmaceuticals from the encapsulating body. Depending on physicochemical conditions, including size of molecular species, solvent properties, surface chemistry, etc., they may vary from inverse seconds, for small molecules, to inverse hours, for larger polymeric species. The option of PSi microstructuring and surface modification may be used to effectively tune the release rates by affecting the intrapore diffusion (Velleman et al. 2010; Kondrashova et al. 2011; Wu and Sailor 2013).

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Lauerer A, Zeigermann P, Lenzner J, Chmelik C, Thommes M, Valiullin R, Kärger J (2015) Micro imaging of liquid-vapor phase transition in nano-channels. Microporous Mesoporous Mater 214:143–148 Lehmann V, Stengl R, Luigart A (2000) On the morphology and the electrochemical formation mechanism of mesoporous silicon. Mater Sci Eng B 69:11–22 Li MD, Hu M, Liu QL, Ma SY, Sun P (2013) Microstructure characterization and NO2-sensing properties of porous silicon with intermediate pore size. Appl Surf Sci 268:188–194 Lysenko V, Vitiello J, Remaki B, Barbier D (2004) Gas permeability of porous silicon nanostructures. Phys Rev E 70(1):017301 Mares JW, Weiss SM (2011) Diffusion dynamics of small molecules from mesoporous silicon films by real-time optical interferometry. Appl Optics 50(27):5329–5337 Naumov S, Khokhlov A, Valiullin R, Kärger J, Monson PA (2008a) Understanding capillary condensation and hysteresis in porous silicon: network effects within independent pores. Phys Rev E 78(6):060601–060604 Naumov S, Valiullin R, Monson PA, Kärger J (2008b) Probing memory effects in confined fluids via diffusion measurements. Langmuir 24(13):6429–6432 Naumov S, Valiullin R, Kärger J, Monson PA (2009) Understanding adsorption and desorption processes in mesoporous materials with independent disordered channels. Phys Rev E 80(3): 031607 Petersen EE (1958) Diffusion in a pore of varying cross section. AICHE J 4(3):343–345 Pollard WG, Present RD (1948) On gaseous self-diffusion in long capillary tubes. Phys Rev 73(7): 762–774 Prestidge CA, Barnes TJ, Lau CH, Barnett C, Loni A, Canham L (2007) Mesoporous silicon: a platform for the delivery of therapeutics. Expert Opin Drug Deliv 4(2):101–110 Puibasset J, Porion P, Grosman A, Rolley E (2016) Structure and permeability of porous silicon investigated by self-diffusion NMR measurements of ethanol and heptane. Oil Gas Sci Techn 71 (4):54 Renisch S, Schuster R, Wintterlin J, Ertl G (1999) Dynamics of adatom motion under the influence of mutual interactions: O/Ru(0001). Phys Rev Lett 82(19):3839–3842 Schechter I, Benchorin M, Kux A (1995) Gas-sensing properties of porous silicon. Anal Chem 67(20):3727–3732 Valiullin R, Khokhlov A (2006) Orientational ordering of linear n-alkanes in silicon nanotubes. Phys Rev E 73(5):051604–051605 Valiullin R, Kortunov P, Kärger J, Timoshenko V (2004) Concentration-dependent self-diffusion of liquids in nanopores: a nuclear magnetic resonance study. J Chem Phys 120(24):11804–11814 Valiullin R, Kortunov P, Kärger J, Timoshenko V (2005a) Surface self-diffusion of organic molecules adsorbed in porous silicon. J Phys Chem B 109:5746–5752 Valiullin R, Kortunov P, Kärger J, Timoshenko V (2005b) Concentration-dependent self-diffusion of adsorbates in mesoporous materials. Magn Reson Imaging 23:209–213 Valiullin R, Kärger J, Gläser R (2009) Correlating phase behaviour and diffusion in mesopores: perspectives revealed by pulsed field gradient NMR. Phys Chem Chem Phys 11(16):2833–2853 Velleman L, Shearer CJ, Ellis AV, Losic D, Voelcker NH, Shapter JG (2010) Fabrication of selfsupporting porous silicon membranes and tuning transport properties by surface functionalization. Nanoscale 2(9):1756–1761 Wallacher D, Kunzner N, Kovalev D, Knorr N, Knorr K (2004) Capillary condensation in linear mesopores of different shape. Phys Rev Lett 92(19):195704 Wu CC, Sailor MJ (2013) Selective functionalization of the internal and the external surfaces of mesoporous silicon by liquid masking. ACS Nano 7(4):3158–3167 Zhao Y, Gaur G, Retterer ST, Labinis PE, Weiss SM (2016) Flow-through porous silicon membranes for real time label-free biosensing. Anal Chem 88(22):10940–10948

Refractive Index of Porous Silicon Honglae Sohn

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Data Evaluation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fresh Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidized Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Multilayer Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

342 342 345 348 350 351

Abstract

The optical properties of porous silicon layer described as a mixture of air, silicon, and silicon dioxide are determined by the thickness, porosity, refractive index, and the shape and size of pores. Refractive index of porous silicon is reviewed in this chapter. Full theoretical solutions can be provided by different effective medium approximation methods such as Maxwell-Garnett’s, Looyenga’s, or Bruggeman’s. In the case of semiempirical approaches, the refractive indices are measured using spectroscopic ellipsometry, and then, the model parameters, such as the layer thickness and calculated effective dielectric function, are adjusted to fit the spectra. Various methods based on optical transmission and reflection measurements are used to calculate the refractive index data for both fresh and oxidized porous silicon using the envelope, Maxwell-Garnett, and Goodman method. The coherent transfer matrix technique and Heavens theory were used to determine the complex refractive index of porous silicon for the reflectance of an absorbing layer on an absorbing substrate. An ab initio quantum mechanical study of the effects of oxidation process in porous silicon using an interconnected supercell structure and its complex refractive index was compared H. Sohn (*) Department of Chemistry, Chosun University, Gwangju, Republic of Korea e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_25

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with experimental data obtained from spectroscopic ellipsometry. The spectroscopic ellipsometry evaluation of a porous silicon multilayer was reported for the preparation of porous silicon multilayer stacks. The effects of oxidation on the Bragg reflector parameters and the variations in the refractive index and thickness after oxidation were reported. Keywords

Refractive index · Spectroscopic ellipsometry · Fresh porous silicon · Oxidized porous silicon · Meso-porous

Introduction Porous silicon materials are described as a mixture of air, silicon, and, in some cases, silicon dioxide. The optical properties of a porous silicon layer are determined by the thickness, porosity, refractive index, and the shape and size of pores and are obtained from both experimental- and model-based approaches. Porous silicon is a very attractive material for refractive index fabrication because of the ease in changing its refractive index. Many studies have been made on one- and two-dimensional refractive index lattice structures. The refractive index is a complex function of wavelength, i.e., ñ(λ) = n(λ)
ik(λ), where k is the extinction coefficient and determines how light waves propagate inside a material (Jackson 1975). The square of the refractive index is the dielectric function: e(ω) = ñ(ω)2, which contributes to Maxwell’s equations. The refractive index depends on the propagation of light as well as the reflected and transmitted fractions of incident waves on an interface. The Fresnel coefficients for reflection and transmission are given by the refractive indices of the two adjacent materials. For many applications of porous silicon in optics or optoelectronics, it is necessary to know the exact refractive index (see, e.g., chapters ▶ “Porous Silicon Photonic Crystals”, ▶ “Porous Silicon Optical Waveguides,” ▶ “Porous Silicon Diffraction Gratings,” ▶ “Porous Silicon Optical Biosensors”). Herein, the methods to obtain the refractive index from various types of porous silicon are discussed.

Data Evaluation Porous silicon can be specified as an effective medium, whose optical properties depend on the relative volumes of silicon and pore-filling medium. Full theoretical solutions can be provided by different effective medium approximation methods such as Maxwell-Garnett’s, Looyenga’s, or Bruggeman’s (Arrand 1997). Effective medium theory describes the effective refractive index, ñeff, of porous silicon as a function of the complex refractive index of silicon, ñSi, and that of the pore-filling material, ñair = 1, for air. The porosity P and the topology of the porous structure will also affect ñeff (Theiss et al. 1995).

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The most frequently used effective medium models are the Bruggeman approximation (Bruggeman 1935), the Looyenga formula (Looyenga 1965), and the Maxwell-Garnett mixing rule (Garnett 1904). The Bruggeman approximation is given by P

1
n~2eff n 2eff n~2Si -~ þ ð 1
P Þ ¼0 1
2~ n 2eff n 2eff n~2Si þ 2~

(1)

It is adequate for irregularly shaped particles with low porosities and shows good agreement with experimental data for as-etched meso-porous silicon (Snow et al. 1999; Pap et al. 2006). The Looyenga model is suitable for high porosities (Zettner et al. 1998) and is given by 2

2=3 n 3eff þ P n~eff ¼ ð1
PÞ~

(2)

The Maxwell-Garnett formula describes isolated silicon particles embedded in a vacuum matrix and is given by 1
n~2eff n~2Si
1 þ ð 1
P Þ ¼0 2 þ n~2eff n~2Si þ 2

(3)

Both Bruggeman and Looyenga exhibit an increased refractive index compared to the Maxwell-Garnett formula, due to the interconnection of the silicon network (Theiss 1997; Theiss and Hilbrich 1997). For a porosity P of 30% and a wavelength between 300 and 1,500 nm, the calculation deviates only 3% from the Bruggeman formula and 10% from the Looyenga model, whereas the deviation from the Maxwell-Garnett model is 74% (Wolf et al. 2008). Figure 1 shows the refractive index of bulk silicon. The application of different effective medium theories leads to different results. Figure 2 shows the comparison 8 7

Refractive index

Fig. 1 Refractive index of bulk silicon (solid line: real part; dashed: imaginary part) (Data from Aspnes and Studna 1983)

6 5 4 3 2 1 0 1

2

3

4

Energy [eV]

5

6

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H. Sohn

Fig. 2 Refractive index (real part) versus porosity, given by some effective medium approaches (Data from Theiss and Hilbrich 1997)

3.5 Mixing rule Lower bound (Maxwell Garnett) Looyenga Bruggeman Upper bound

Refr. index

3.0 2.5 2.0 1.5 1.0 0

20

40 60 Porosity [%]

80

100

of the dependency of refractive index on porosity, determined by different effective medium approaches. A value of 3.4 for the infrared bulk value has been taken for the solid pore wall phase. The refractive index of porous silicon is expected to be lower than that of silicon, as porous silicon is a two-phase composite, being a mixture of air and solid phase (Theiss and Hilbrich 1997). The optical properties are determined experimentally from the transmission or reflection spectra using, for example, the envelope method or the Fresnel’s equation (Manifacier et al. 1976; Laghla and Sched 1997). The traditional method to determine the refractive index of porous silicon takes into account that the pore size is much smaller than the wavelength of visible light. Porous silicon behaves then as a continuous material. However, porous silicon is more complex due to a large distribution of pores sizes. A simple method to determine the refractive index of a material is to observe the interference fringes. The refractive index can be calculated directly from the frequency difference of neighboring interference maxima of minima, when the layer thickness d is determined independently. However, these procedures are limited when the materials have strong optical absorption or scattering effects. In the case of semiempirical approaches, the refractive indices are measured using spectroscopic ellipsometry, and then, the model parameters, such as the layer thickness and calculated effective dielectric function, are adjusted to fit the spectra (Theiss 1997; Theiss and Hilbrich 1997; Jellison and Modine 1994). The optical parameters using the transfer matrix method were varied to find the best fit to the measured spectra (He and Cada 1992). Equation 4 can be used to measure the real part of the refractive index of porous silicon through the interference fringes of the transmittance (T ), or reflectance (R), spectra (Theiss et al. 1995; Bruggeman 1935). n¼

1 2dð1=λ2
1=λ1 Þ

(4)

λ1 and λ2 are the wavelength values of two successive maxima or minima of the spectra, and the thickness (d ) is measured independently. The use of Eq. 4 has

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345

several restrictions; it can only be applied in the infrared region where n is approximately constant and neglects absorption. It cannot be used from the visible to UV region where strong absorption exists. For an absorbing and slightly absorbing thin film, the refractive index is a complex function and may be obtained by spectroscopic ellipsometry from its optical transmittance and reflectance spectra (Bisi et al. 2000). Table 1 lists the information for the type of porous silicon structure, porosity, method and model, and spectral range.

Fresh Porous Silicon Fresh etched porous silicon is described as a mixture of air and silicon. Various methods based on optical transmission and reflection measurements are used to calculate the refractive index data for porous silicon layers. The refractive index can be determined by multiple-angle-of-incidence ellipsometry. Ellipsometry allows very accurate determination of the optical constants. The multiple-angle-of-incidence ellipsometry study was investigated to evaluate the refraction indices, as well as the porous silicon layer thickness values for samples of different porosities, from 23% to 62% (Krzyżanowska et al. 1999). From the ellipsometric measurements of the incidence angle with the Fresnel complex refection coefficients, the transmitted light intensity increases with increased porosity, and the refractive index decreases. The refractive index in the violet spectral range, where strong absorption exists, is obtained from the direct measurement of the change in phase of the transmitted wave (Strashnikova 2002). The optical parameters from reflection measurements on freestanding samples indicated that optical scattering at the air-porous silicon interface is the principal reason for the loss of transmitted light intensity and for the inaccurate results obtained by the envelope method. The porous silicon showed poor transparency at shorter wavelengths (λ < 800 nm), due to strong optical absorption in silicon. Therefore, the experimentally determined refractive index is always larger than that predicted by Bruggeman’s method. The mean of the relative difference between experimental and theoretical values is 9.1% with a standard deviation of 4.4% (Pap et al. 2006). The effective refractive indices of silicon-supported and freestanding porous silicon layers were evaluated from the transmission spectra by applying the Goodman method (Khardani et al. 2007). Good agreement between theory and experiment was found in the case of silicon-supported meso-porous silicon for all porosities (Fig. 3). However, for freestanding meso-porous silicon (Fig. 4), the Bruggeman effective medium theory fits the experimental results only for porosities ranging between 50% and 70% (Khardani et al. 2007). The coherent transfer matrix technique was used to calculate the specular reflection and transmission of porous silicon. In the wavelength range from 500 to 1500 nm, the refractive index of porous silicon obtained using effective medium theory and Mie scattering is 19–35% below the value for bulk silicon. Except for the Maxwell-Garnett formula, the measured values agree with the predictions of the

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Table 1 Information on the refractive index for various types of porous silicon Porous silicon structure Single layer

Porosity (%) 23–62

Data information Refractive index versus porosity

Single layer

75

Freestanding

40–60

Single layer, freestanding Freestanding

30–80

Refractive index versus wavelength Refractive index versus wavelength Refractive index versus porosity Refractive index versus wavelength Refractive index versus wavelength Refractive index versus porosity

40

Single layer, freestanding

60–87

Single layer

15–50

Oxidized

34–63

Oxidized

10–80

Oxidized

40

DBR

35–90

DBR

35–90

DBR

35–90

Gradient

63

Refractive index versus porosity and oxidation degree Refractive index versus porosity Refractive index versus wavelength Refractive index versus applied current Refractive index versus wavelength and applied current Refractive index versus wavelength and applied current Refractive index versus wavelength and applied current

Method/ model Fresnel, least-squares fit Direct measurement Envelope/ Bruggeman Goodman/ Bruggeman KM theory/ Lambertian

Spectral range (nm) 632.8

References Krzyżanowska et al. (1999)

500–770

Strashnikova (2002)

700–1700

Pap et al. (2006)

500–1500

Khardani et al. (2007) Wolf et al. (2008)

Heavens

300–2,500

Arenas et al. (2010)

Semianalytical/ Bruggeman Bruggeman least-squares fit

632.8

Bazaru et al. (2010)

632.8

Astrova et al. (1999), Astrova and Tolmachev (2000) Pan et al. (2005)

250–850

Cisneros et al. (2007)

Bruggeman

400–800

Volk et al. (2003)

Genetic algorithm

400–800

Torres-Costa et al. (2004)

Fresnel

400–900

Nava et al. (2009)

KramersKronig analysis

400–850

Shokrollahi et al. (2012)

Serialparallel model Fresnel, ab initio

Refractive Index of Porous Silicon

2.4 2.2 Refractive index

Fig. 3 Refractive index versus porosity for nanoporous silicon obtained from experiment (○) and the prediction of the effective medium theory for nanoporous silicon (●) (Data from Khardani et al. 2007)

347

2.0 1.8 1.6 1.4 1.2 30

60 50 Porosity (%)

70

80

3.0 2.8 2.6 Refractive index

Fig. 4 Refractive index versus porosity for freestanding meso-porous silicon obtained from experiment (□) and the prediction of the effective medium theory (■) (Data from Khardani et al. 2007)

40

2.4 2.2 2.0 1.8 1.6 1.4 1.2 30

40

60 50 Porosity (%)

70

80

effective medium models for 500 < λ 56% remains porous even after 100% oxidation of the silicon, and those with lower Pin become poreless at a particular value of oxidation degree and form another two-component system: Si-SiO2. As oxidation proceeds, the refractive index and optical thickness of the film decreases gradually, whereas the film thickness increases (Astrova and Tolmachev 2000). The dependence of the relative thickness of the porous silicon layer d/d0 on the degree of oxidation of the silicon base layer is reported (Astrova et al. 1999; Astrova and Tolmachev 2000). The refractive index and thickness, as well as the optical constants of the photoluminescent porous silicon, were determined by measuring the dependence on the angle of incidence of the reflected light intensity measured for three wavelengths of the visible spectral region (Makara et al. 1999). The relative thickness of the porous silicon layer, d/d0 = 1.33, on the degree of oxidation of the silicon base layer is due to the 98% oxidation of the silicon base layer in columnar structure of

60%

70%

349

pin=

90% 80%

Refractive Index of Porous Silicon

100

Oxidation degree of silicon carcass s, %

pin = 50% 80

60

40%

Si + SiO2

30%

40

20% 20

0 1.0

1.5

2.0

2.5

3.0

Refractive index n

Fig. 5 Calculated curves characterizing the relation between the refractive index of oxidized porous silicon and the degree of oxidation of the silicon base layer, λ = 632.8 nm. The Si + SiO2 curve corresponds to a film in which no voids are left after oxidation; Pin is the initial porosity of the unoxidized film (Data from Astrova et al. 1999)

100

80

Porosity p, %

Fig. 6 The porosity versus the refractive index of oxidized porous silicon, calculated for various initial porosities pin of the unoxidized film, λ = 632.8 nm. The dot-dashed curves representing Si + V (porous silicon) and SiO2 + V (porous oxide) bound the region of values having physical significance; Pin is determined from the point of intersection of the p-n curve with the Si + V curve (Data from Astrova and Tolmachev 2000)

Si + V

60

40

20 SiO2 + V

0 1.0

1.5

2.0

2.5

Refractive index n

3.0

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H. Sohn

porous silicon (Astrova and Tolmachev 2000). A serial-parallel model for the porosity and oxidation dependence of dielectrics for porous silicon has thus been reported. Predictions agree with experimental results, which enable us to measure the extent of oxidation (Pan et al. 2005). An ab initio quantum mechanical study of the effects of oxidation process in porous silicon using an interconnected supercell structure and its complex refractive index was also reported and compared with experimental data obtained from spectroscopic ellipsometry (Cisneros et al. 2007).

Multilayer Porous Silicon There has been growing interest in the development of efficient control for the preparation of porous silicon multilayer stacks (see chapter ▶ “Porous Silicon Multilayers and Superlattices”). As porous silicon porosity is a function of the current density, different refractive indexes of porous silicon layers can be built up, one after another, on a silicon substrate in the vertical direction by alternating the applied current densities during the electrochemical etching. The refractive index contrast between the dielectric materials plays a crucial role on the photonic properties. The quality of porous silicon multilayers in photonics can be improved if the refractive index contrast between layers is increased. The bandgap widening of porous silicon due to quantum confinement effects leads to a decrease of the extinction coefficient k, making porous silicon transparent in the infrared region. Berger et al. proved the feasibility of fabrication of high reflectivity porous silicon mirrors, Bragg reflectors, and rugate filters (Berger et al. 1997). The Bragg reflector is characterized by its central wavelength λ0 (at normal incidence) and by the reflection bandwidth which is determined mainly by the index contrast. Interference filters in the IR have been demonstrated with good spectral behavior due to the low absorption of porous silicon in this wavelength range. To achieve the same degree of performance in the visible range, the layers of higher porosity are required to further reduce the value of k, thus blue-shifting the absorption edge. The spectroscopic ellipsometry evaluation of a porous silicon multilayer was reported for the process design and control for the preparation of porous silicon multilayer stacks (Volk et al. 2003). A genetic algorithm has been used to precisely determine the complex refractive index and thickness of thin films from their reflectance spectra in the visible wavelength range (Torres-Costa et al. 2004). An experimental study for the effect of the applied current density and the electrolyte composition on the refractive index contrast of porous silicon multilayers produced at low current was reported for increasing the refractive index contrast in a multilayer in the visible range (Nava et al. 2009). In Fig. 7, the refractive index at 600 nm and the porosity are shown as a function of the current density. There is a sharp drop in the porosity for current densities below 5 mA/cm that allows to increase the refractive index. However, the estimated porosities are relatively larger than the expected values for the corresponding refractive indexes, probably due to inhomogeneities in the border of the samples (Nava et al. 2009).

Refractive Index of Porous Silicon

351

100 90 3.0

70 2.5

60 50

2.0

40

hps

Porosity (%)

80

30 1.5

20 10 0

5

10

15

J

30 (mA/cm2) 20

25

35

40

45

1.0

Fig. 7 Porosity and refractive index (at 600 nm) of porous silicon versus current density. The samples produced with the electrolyte composition of hydrofluoric acid, ethanol, and glycerin in a ratio of 3:7:1 give a refractive index contrast around 1.3/2.8 at 600 nm (Data from Nava et al. 2009)

The effects of oxidation on the Bragg reflector parameters and the variations in the refractive index and thickness after oxidation were investigated. An oxidized porous silicon Bragg reflector, centered at 1,550 nm, was formed by anticipating the shift of central wavelength during oxidation. As the refractive index values of porous silicon layers decrease strongly after oxidation, the index contrast decreases. The reflection bandwidth also decreases after oxidation of the optical structure (Charrier et al. 2012). The refractive index for a gradient-porosity porous silicon which is equal to the effective index of a multilayer is also investigated (Shokrollahi et al. 2012).

References Arenas MC, Hu H, Nava R, Del Río JA (2010) Determination of the complex refractive index of porous silicon layers on crystalline silicon substrates. Int J Mod Phys B 24:4835–4850 Arrand HF (1997) Optical waveguides and components based on porous silicon. University of Nottingham, Nottingham, pp 63–66 Aspnes DE, Studna AA (1983) Dielectric functions and optical parameters of Si, Ge, GaP, GaAs, GaSb, InP, InAs, and InSb from 1.5 to 6.0 eV. Phys Rev B 27:985–1009 Astrova EV, Tolmachev VA (2000) Effective refractive index and composition of oxidized porous silicon films. Mater Sci Eng B 69-70:142–148 Astrova EV, Voronkov VB, Remenyuk AD, Shuman VB, Tolmachev VA (1999) Variation of the parameters and composition of thin films of porous silicon as a result of oxidation: ellipsometric studies. Semiconductors 33:1149–1155 Bazaru T, Vlad VI, Petris A, Miu M (2010) Optical linear and third-order nonlinear properties of nano-porous Si. J Optoelectron Adv M 12:43–47 Berger MG, Arens-Fisher R, Thönissen M, Krüger M, Billat S, Lüth H, Hilbrich S, Theiss W, Grosse P (1997) Dielectric filters made of PS: advanced performance by oxidation and new layer structures. Thin Solid Films 297:237–240 Bisi O, Ossicini S, Pavesi L (2000) Porous silicon: a quantum sponge structure for silicon based optoelectronics. Surf Sci Rep 38:1–126

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Bruggeman DAG (1935) Dielectric constant and conductivity of mixtures of isotropic materials. Ann Phys 24:636–679 Charrier J, Pirasteh P, Boucher YG, Gadonna M (2012) Bragg reflector formed on oxidised porous silicon. Micro Nano Lett 7:105–108 Cisneros R, Ramírez C, Wang C (2007) Ellipsometry and ab initio approaches to the refractive index of porous silicon. J Phys Condens Matter 19:395010 Garnett JCM (1904) Colours in metal glasses and in metallic films. Philos Trans R Soc Lond Ser A 203:385–420 He J, Cada M (1992) Combined distributed feedback and Fabry-Perot structures with a phasematching layer for optical bistable devices. Appl Phys Lett 61:2150–2152 Jackson JD (1975) Classical electrodynamics. Wiley, New York Jellison GE Jr, Modine FA (1994) Optical functions of silicon at elevated temperatures. J Appl Phys 76:3758–3761 Khardani M, Bouaïcha M, Bessaïs B (2007) Bruggeman effective medium approach for modelling optical properties of porous silicon: comparison with experiment. Phys Status Solidi C 4:1986–1990 Krzyżanowska H, Kulik M, Żuk J (1999) Ellipsometric study of refractive index anisotropy in porous silicon. J Lumin 80:183–186 Laghla Y, Sched E (1997) Optical study of undoped, B or P-doped polysilicon. Thin Solid Films 306:67–73 Looyenga H (1965) Dielectric constants of heterogeneous mixtures. Physica 31:401–406 Makara VA, Odarych VA, Vakulenko OV, Dacenko OI (1999) Ellipsometric studies of porous silicon. Thin Solid Films 342:230–237 Manifacier JC, Gasiot J, Fillard JP (1976) A simple method for the determination of the optical constants n, k and the thickness of a weakly absorbing thin film. J Phys E 9:1002–1004 Nava R, de la Mora MB, Tagüeña-Martínez J, del Río JA (2009) Refractive index contrast in porous silicon multilayers. Phys Status Solidi C 6:1721–1724 Pan LK, Sun CQ, Li CM (2005) Estimating the extent of surface oxidation by measuring the porosity dependent dielectrics of oxygenated porous silicon. Appl Surf Sci 240:19–23 Pap AE, Kordás K, Vähäkangas J, Uusimäki A, Leppävuori S, Pilon L, Szatmári S (2006) Optical properties of porous silicon. Part III: comparison of experimental and theoretical results. Opt Mater 28:506–513 Shokrollahi A, Zare M, Mortezaali A, Ramezani Sani S (2012) Analysis of optical properties of porous silicon nanostructure single and gradient-porosity layers for optical applications. J Appl Phys 112:053506 Snow PA, Squire EK, Russell PSJ, Canham LT (1999) Vapor sensing using the optical properties of porous silicon Bragg mirrors. J Appl Phys 86:1781–1784 Strashnikova MI (2002) On measurements of the refractive index dispersion in porous silicon. Opt Spectrosc 93:132–135 Theiss W (1997) Optical properties of porous silicon. Surf Sci Rep 29:91–192 Theiss W, Hilbrich S (1997) Refractive index of porous silicon. In: Canham L (ed) Properties of porous silicon. Institution of Engineering and Technology, London, pp 223–228 Theiss W, Henkel S, Arntzen M (1995) Connecting microscopic and macroscopic properties of porous media: choosing appropriate effective medium concepts. Thin Solid Films 255:177–180 Torres-Costa V, Martín-Palma RJ, Martínez-Duart JM (2004) Optical constants of porous silicon films and multilayers determined by genetic algorithms. J Appl Phys 96:4197–4203 Volk J, Fried M, Polgár O, Bársony I (2003) Optimisation of porous silicon based passive optical elements by means of spectroscopic ellipsometry. Phys Status Solidi A 197:208–211 Wolf A, Terheiden B, Brendel R (2008) Light scattering and diffuse light propagation in sintered porous silicon. J Appl Phys 104:033106 Zettner J, Thönissen M, Hierl T, Brendel R, Schulz M (1998) Novel porous silicon backside light reflector for thin silicon solar cells. Prog Photovolt 6:423–432

Optical Birefringence of Porous Silicon Minoru Fujii and Joachim Diener

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Structural and Refractive Index Anisotropy of (110) Porous Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phase-Sensitive Detection of Adsorbed Atoms and Molecules . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Polarizing Elements Made from (110) Porous Si . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidized (110) Porous Si (Porous Silica) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Electrochemically etched porous silicon can exhibit pronounced optical anisotropy even though bulk silicon is basically optically isotropic. The origin of this effect, the most significant parameters, and potential applications in sensing and micro-optic devices are reviewed. Keywords

Oxidized porous silicon · Phase-sensitive detection · Polarizing elements · Refractive index anisotropy of porous silicon · Rugate filter

M. Fujii Department of Electrical and Electronic Engineering, Graduate School of Engineering, Kobe University, Kobe, Japan e-mail: [email protected] J. Diener (*) Physik-Department, Technische Universität München, Garching, Germany e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_26

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Introduction Bulk Si having a high symmetric diamond-type crystal structure is basically optically isotropic. An alignment of pores, in porous Si preferentially in [100] directions, induces optical anisotropy. However, even with porous Si having structural sizes smaller than the wavelength of light, the related effects are easily observable in preferred geometries. For porous Si prepared from a (100)-oriented Si wafer, pores and the nanocrystals forming the nanowire skeleton are preferentially aligned/elongated normal to the wafer surface. Anisotropy of the refractive index, i.e., birefringence, appears when light is incident upon a cleaved edge of the porous Si layer (Kovalev et al. 1995; Mihalcescu et al. 1997), while it is optically symmetric when light is incident perpendicular to the surface. By changing the orientation of the etched Si wafer, an in-plane anisotropy of the refractive index can be realized, for instance, by producing porous Si from lower symmetry Si surfaces, e.g., (110) and (211) surfaces (Kovalev et al. 2000; Sarbey et al. 2000; Künzner et al. 2001, 2005; Kuznetsova et al. 2002; Koyama 2004; Golovan’ et al. 2007). In this chapter, we will start with the microscopic structure of (110) porous Si and the relation between the degree of the refractive index anisotropy and the most significant preparation parameters, known to be crucial for the structural morphology of porous Si. Potential applications of birefringent porous Si like phase-sensitive detection of dielectric media (especially adsorbed atoms and molecules) or polarizing elements made from (110) porous Si are discussed. Finally, we turn to birefringent porous silica made by the oxidation of birefringent porous Si, since a controlled change of the refractive index of the nanowire skeleton offers an additional degree of freedom.

Structural and Refractive Index Anisotropy of (110) Porous Si Figure 1a shows a cross-sectional TEM image of (110) porous Si observed from the   direction (Ishikura et al. 2008). The direction of the etching current is from the ½112 top to the bottom. Because of preferential growth and consequently orientation of pores to [100] directions, the preferential direction of the remaining Si skeleton nanowires deviate from that of the etching current. Figure 1b shows a plan-view TEM image of (110) porous Si. The projection of the Si nanowires on the (110)   direction (Shichi et al. surface appears as their in-plane alignment along the ½110 2012). This preferential alignment is the origin of the in-plane refractive index anisotropy. Figure 2a shows a polar plot of the refractive index value of (110) mesoporous Si as a function of the polarization direction of the incident light at 633 nm. In Fig. 2b, refractive indices with the polarization direction of incident light parallel    to the ½110 n½110   and [001] (n[001]) directions, respectively, and the difference Δn ¼ n½110    n½001 are shown as a function of the etching current density (Ishikura et al. 2008; Nishida et al. 2010). Δn rises with increasing etching current density, indicating an overall morphological change with a more pronounced dielectric

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shape/alignment effect at higher current densities. Δn also depends strongly on typical sizes of the nanowire skeleton and the wavelength of light, as summarized in Fig. 2c (Künzner et al. 2001; Diener et al. 2007). This anisotropy of the refractive index of (110) porous Si can be calculated and described within the framework of several different models (Kuznetsova et al. 2002; Bonder and Wang 2005, 2006;

356 Fig. 3 (a) sketch of the crystallographic orientation of (110) porous Si and the polarization directions of the incident and transmitted light (Shichi et al. 2011). (b) Intensity ratio of the linearly polarized components of light transmitted through a (110) porous Si

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Kochergin et al. 2004, 2005; Ruda and Shik 2011) and is basically understood. Consistently the structural anisotropy of (110) porous Si also affects the dielectric properties in general, like the optical absorption coefficient and nonlinear susceptibility (Zabotnov et al. 2005; Timoshenko et al. 2003; Soboleva et al. 2005; Efimova et al. 2007a, b).

Phase-Sensitive Detection of Adsorbed Atoms and Molecules When linearly polarized light, whose polarization direction is rotated 45 from the   and [001]), is normally incident to (110) porous Si, the relative optical axes (½110 phase (δ) of the electric field vectors on the optical axes shifts 2πΔnd/λ, where d is the thickness of the layer and λ is the wavelength (see Fig. 3a) (Shichi et al. 2011; Gross et al. 2001; Kovalev et al. 2001). Figure 3b shows the ratio of the intensities of transmitted light being polarized parallel and perpendicular to the polarization direction of incident light (Gross et al. 2001; Kovalev et al. 2001). The maxima (minima) correspond to odd (even) orders of π phase shift. The phase shift is very sensitive to the refractive index of the dielectric medium in the pores (Gross et al. 2001; Kovalev et al. 2001; Diener et al. 2003; Künzner et al. 2003). The differential character of this method allows monitoring of adsorptive condensation in nanopores and adsorbate detection thresholds as low as 1010 g (Gross et al. 2001). This compares favorably with several other methods including calorimetry, gravimetry, and volumetry, which can be employed to study volume fractions of condensed

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fluids, but with a sensitivity of the order of 106 g (Gross et al. 2001). Thus, monitoring this phase shift is a promising method for high-sensitivity optical detection of dielectric media, especially atoms and molecules incorporating the pores in normal incidence. Phase-sensitive optical detection is of course also possible in (100) porous Si, however, when it is tilted from the direction of incident light (Alvarez et al. 2012; Liu et al. 2002; Beom-hoan et al. 2003; Oton et al. 2003; Álvarez et al. 2011).

Polarizing Elements Made from (110) Porous Si The simplest polarizing elements made from (110) porous Si are wave plates (retarders), which control the polarization state of light (Fig. 4a) (Kovalev et al. 2000). Extending to (110) porous Si multilayers polarization-sensitive Bragg reflectors (Diener et al. 2001, 2002, 2003; Ghulinyan et al. 2003), microcavities (Diener et al. 2002, 2004; Saarinen et al. 2008) and polarizers (Diener et al. 2002, 2004; Saarinen et al. 2008) can be realized by the modulation of the porosity and thus the overall refractive index in depth directions. If the etching current is modulated gradually, a so-called rugate filter characterized by a continuous sinusoidal refractive index variation in the direction perpendicular to the film plane can be achieved. Rugate filters provide advantages of suppressed higher-order harmonics and side lobes if a proper apodization is applied to the index profile and index-matching layers are added on the surfaces. Figure 4b shows an example of refractive index profiles and transmittance spectra of a polarization-sensitive rugate filter. The wavelength of the stop bands depends on the polarization direction (Ishikura et al. 2008). All these multilayers can be realized with a high degree of freedom in a well-controlled manner.

Oxidized (110) Porous Si (Porous Silica) So far we addressed the specifics of the etching process of porous Si and basic, mainly geometric, issues relevant to the topic. A potential limitation of the system at visible wavelengths emerges from the dielectric properties of the material forming the nanowire skeleton, i.e., self-absorption. However, oxidation of porous Si results in porous silica, which can have transparency to the deep ultraviolet region. Figures 5a, b compare plan-view TEM images of (110) porous Si and the corresponding (110) porous silica after oxidation at 900  C (Shichi et al. 2011). Due to the increase of the skeleton volume by oxidation, pores become smaller. However, structural anisotropy is still evident in the TEM image (Shichi et al. 2011; Golovan et al. 2006). Figure 5c shows Δn as a function of oxidation temperature. Above 800  C, Si nanowire skeletons are completely oxidized, and thus, Δn becomes more than one order of magnitude smaller than that of porous Si. Birefringence disappears when porous Si is oxidized around 1,000  C. Δn of porous silica is small but sufficient for true-zero-order wave plates in the deep ultraviolet region (Shichi et al. 2011, 2013).

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Summary Self-orienting and self-limiting specifics of the etching process of porous Si, combined with straightforward geometrical approaches, opens a unique way of threedimensional nanostructuring of silicon by wet chemistry. It is possible to gain control of the structural and consequently dielectric properties of Si both parallel and perpendicular to the wafer surface. In that sense, nanostructuring, here anisotropic

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Fig. 5 (a) Plan-view TEM images of (b) as-prepared and oxidized (900  C) (110) porous Si. Insets are electron diffraction patterns (Shichi et al. 2011). (c) Oxidation temperature dependence of Δn of porous Si (@800 nm) and porous silica (@400 nm) (Shichi et al. 2011)

dielectric nanostructuring, of semiconductors is an alternative approach to create artificial materials. In this case, a Si-based material being sensitive to the polarization state of light, with bulk Si being basically optically isotropic, could be realized. With structural sizes smaller than the wavelength of light, a “quasiuniform” distribution/ alignment/elongation of pores and nanocrystals (for porous silica, the dielectric material) forming the nano-skeleton is sufficient for optical applications. It is a system which is rather easy to handle and to control. Moreover, it can serve as basis for other novel metamaterials, for instance, by controlled manipulation of the nano-skeleton of (110) porous Si by oxidation.

References Álvarez J et al (2011) Birefringent porous silicon membranes for optical sensing. Opt Express 19(27):26106–26116 Alvarez J et al (2012) Phase-sensitive detection for optical sensing with porous silicon. Photon J IEEE 4(3):986–995

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Beom-hoan O et al (2003) Vapor sensor realized in an ultracompact polarization interferometer built of a freestanding porous-silicon form birefringent film. Photon Technol Lett IEEE 15(6):834–836 Bonder Y, Wang C (2005) Ab initio study of birefringent porous silicon. Phys Stat Sol (a) 202 (8):1552–1556 Bonder Y, Wang C (2006) A first-principles model of birefringent porous silicon. J Appl Phys 100 (4):044319-5 Diener J et al (2001) Dichroic Bragg reflectors based on birefringent porous silicon. Appl Phys Lett 78(24):3887–3889 Diener J et al (2002) Dichroic behavior of multilayer structures based on anisotropically nanostructured silicon. J Appl Phys 91(10):6704–6709 Diener J et al (2003) Fine tuning of the dichroic behavior of Bragg reflectors based on anisotropically nanostructured silicon. Phys Stat Sol (a) 197(2):582–585 Diener J et al (2004) Planar silicon-based light polarizers. Opt Lett 29(2):195–197 Diener J et al (2007) The birefringence level of anisotropically nanostructured silicon. Phys Stat Sol (c) 4(6):1996–2000 Efimova AI et al (2007a) Effect of form anisotropy of silicon nanocrystals on birefringence and dichroism in porous silicon. Phys Stat Sol (c) 4(6):1991–1995 Efimova AI et al (2007b) Birefringence and anisotropic optical absorption in porous silicon. J Exp Theor Phys 105(3):599–609 Ghulinyan M et al (2003) Free-standing porous silicon single and multiple optical cavities. J Appl Phys 93(12):9724–9729 Golovan LA et al (2006) Form birefringence of oxidized porous silicon. Appl Phys Lett 88(24):241113-3 Golovan’ LA, Kashkarov PK, Timoshenko VY (2007) Form birefringence in porous semiconductors and dielectrics: a review. Crystallogr Rep 52(4):672–685 Gross E et al (2001) Highly sensitive recognition element based on birefringent porous silicon layers. J Appl Phys 90(7):3529–3532 Ishikura N et al (2008) Dichroic rugate filters based on birefringent porous silicon. Opt Express 16(20):15531–15539 Kochergin V, Christophersen M, Föll H (2004) Effective medium approach for calculations of optical anisotropy in porous materials. Appl Phys B 79(6):731–739 Kochergin V, Christophersen M, Föll H (2005) Surface plasmon enhancement of an optical anisotropy in porous silicon/metal composite. Appl Phys B 80(1):81–87 Kovalev D et al (1995) Porous Si anisotropy from photoluminescence polarization. Appl Phys Lett 67(11):1585–1587 Kovalev D et al (2000) Anisotropically nanostructured silicon as an efficient optical retarder. Phys Stat Sol (a) 180(2):r8–r11 Kovalev D et al (2001) Strong in-plane birefringence of spatially nanostructured silicon. Appl Phys Lett 78(7):916–918 Koyama H (2004) In-plane refractive-index anisotropy in porous silicon layers induced by polarized illumination during electrochemical etching. J Appl Phys 96(7):3716–3720 Künzner N et al (2001) Giant birefringence in anisotropically nanostructured silicon. Opt Lett 26(16):1265–1267 Künzner N et al (2003) Capillary condensation monitored in birefringent porous silicon layers. J Appl Phys 94(8):4913–4917 Künzner N et al (2005) Form birefringence of anisotropically nanostructured silicon. Phys Rev B 71 (19):195304 Kuznetsova LP et al (2002) Study of birefringence in porous silicon layers by IR Fourier spectroscopy. Phys Sol Stat 44(5):811–815 Liu R et al (2002) Novel porous silicon vapor sensor based on polarization interferometry. Sens Actuators B 87(1):58–62

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Mihalcescu I, Lerondel G, Romestain R (1997) Porous silicon anisotropy investigated by guided light. Thin Solid Films 297(1–2):245–249 Nishida K et al (2010) Temperature dependence of optical anisotropy of birefringent porous silicon. Appl Phys Lett 96(24):243102–243103 Oton CJ et al (2003) Scattering rings as a tool for birefringence measurements in porous silicon. J Appl Phys 94(10):6334–6340 Ruda HE, Shik A (2011) Optical properties of anisotropic porous semiconductors. Appl Phys Lett 99(21):213111 Saarinen JJ et al (2008) Reflectance analysis of a multilayer one-dimensional porous silicon structure: theory and experiment. J Appl Phys 104(1):013103–013107 Sarbey OG et al (2000) Birefringence of porous silicon. Phys Sol Stat 42(7):1240–1241 Shichi S, Fujii M, Hayashi S (2011) Ultraviolet true zero-order wave plate made of birefringent porous silica. Opt Lett 36(19):3951–3953 Shichi S et al (2012) Three-dimensional structure of (110) porous silicon with in-plane optical birefringence. J Appl Phys 111(8):084303–084306 Shichi S et al (2013) Porous silica true zero-order wave plate in the deep ultraviolet range. Opt Commun 287:137–139 Soboleva IV et al (2005) Second- and third-harmonic generation in birefringent photonic crystals and microcavities based on anisotropic porous silicon. Appl Phys Lett 87(24):241110–241113 Timoshenko VY et al (2003) Anisotropy of optical absorption in birefringent porous silicon. Phys Rev B 67(11):113405 Zabotnov SV et al (2005) Modification of cubic susceptibility tensor in birefringent porous silicon. Phys Stat Sol (a) 202(8):1673–1677

Nonlinear Optical Properties of Porous Silicon Leonid A. Golovan

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Nonlinear Optical Susceptibility of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effective Refractive Index and Effective Nonlinear Susceptibility of Porous Silicon . . . . . Harmonic Generation and Other Frequency Mixing Processes . . . . . . . . . . . . . . . . . . . . . . . . . . . . Light Self-Action . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control of the Nonlinear-Optical Process Efficiency . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phase Matching in Birefringent Mesoporous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Harmonic Generation in Porous-Silicon Multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Control of PBG . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

In this chapter, the nonlinear-optical properties of porous silicon and poroussilicon-based structures are discussed in close connection with their linear optical properties. Variations of the nonlinear susceptibility tensor for porous silicon in comparison with bulk silicon are discussed with special interest to the effect of optical anisotropy caused by the pore orientation. Phase matching for second- and third-harmonic generation in birefringent porous silicon and photonic crystals is also covered. Incoherent nonlinear-optical processes such as nonlinear refraction, absorption, and optical bleaching, as well as different mechanisms responsible for them, are reviewed. Finally, the possibility to employ these effects for developing photonic devices as optical gates, logical elements, and waveguides are discussed.

L. A. Golovan (*) Physics Department, M.V. Lomonosov Moscow State University, Moscow, Russian Federation e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_139

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Keywords

Birefringence · Nonlinear-optical susceptibility · Harmonic generation · Phase matching · Light self-action

Introduction Over 50 years of development nonlinear optics has become a significant part of optics and electrodynamics; at the present time, it is one of the bases of modern photonics. The nonlinear-optical effects are responsible for wave mixing and frequency conversion, including optical harmonic generation, ultrashort laser-pulse generation, and optical switching (see, e.g., classical textbooks by Shen 1984 and Boyd 2003). The nonlinear response of medium to powerful optical waves results in generation of nonlinear polarization wave at frequency ω in the medium. For the cases where the nonlinear polarization is generated by two or three waves (including degenerative cases), amplitudes of the nonlinear polarization wave PNL are connected with the amplitudes of interacting waves E by equations: ð2Þ

PNL, i ðωÞ ¼ χ ijk ðω; ω1 , ω2 ÞE1, j E2, k , ð3Þ

PNL, i ðωÞ ¼ χ ijkl ðω; ω1 , ω2 , ω3 ÞE1, j E2, k E3, l ,

(1)

correspondingly, where χ (2) and χ (3) are nonlinear susceptibility tensors of the second and third orders, correspondingly, ω1, ω2, ω3 are frequencies of interacting waves, with ω being their combination, subscripts i, j, k, l denote Cartesian coordinates. Usually, the nonlinear susceptibility tensor symmetry is determined by the symmetry of the system (symmetry inside the elementary cell and the formed nanostructure). The origin of the nonlinearities can vary from electron cloud deformation and carrier generation to thermal effect. Many semiconductors and nanostructures based on them seem to be very attractive for formation of the nonlinear-optical devices due to their high nonlinear-optical response, transmittance in certain spectral regions, well-developed technologies, and other factors. It is worth noting that nonlinear-optical properties of nanocomposites significantly differ from those of their constituents (Sipe and Boyd 2002). Nonlinearoptical techniques are often used for studying nanosystems, e.g., for detecting carrier dynamics in them (Lienau and Elsaesser 2008; Fedorov et al. 2011). Nowadays, silicon becomes an important material for photonics which is why nonlinear-optical properties of silicon-based devices are of great importance (Faist 2005; Leuthold et al. 2010). Detailed information on nonlinear-optical properties of crystalline silicon and some silicon-based nanostructures can be found in a book by Aktsipetrov et al. (2016). Porous silicon (por-Si) attracts a great interest for nonlinear optics (Golovan and Timoshenko 2013). It is a good model object for the study of the nanocomposite. Some nonlinear-optical effects found in this material make us consider it as a promising object for photonics. It is worth mentioning huge versatility of the structural and optical properties of the por-Si films and, therefore, great number of various optical applications (see Table 1).

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Table 1 Properties of por-Si-based structures, nonlinear-optical effects in them, and their possible applications Por-Si properties Field enhancement in mesoporous silicon Optical anisotropy Refractive indices depending on porosity. Ability to fabricate photonic crystals and waveguides Well-developed surface of por-Si Biocompatibility of Si Luminescence of por-Si

Nonlinear-optical effect Increase of harmonic generation efficiency. Phasematched optical processes

Possible application New spectral line source

Effective light self-action (nonlinear refraction and nonlinear absorption) Coherent anti-stokes Raman scattering Absorption saturation Two-photon absorption Two-photon photoluminescence Coherent anti-stokes Raman scattering

Optical switching Optical sensing

Visualization of biological objects Theranostics with two-photon (infrared) excitation

In this chapter, we will discuss in detail the nonlinear-optical effects in por-Si, which could be the basis for various photonic and sensor applications. First, modification of the effective nonlinear optical susceptibility caused by the pore formation and treatment of the silicon surface should be discussed. Then, we consider the ways to control the efficiency of the coherent nonlinear-optical processes such as the second- and third-harmonic (SH, TH) generation by means of optical anisotropy and formation of photonic crystals. These processes strongly depend on phase relation between the interacting waves, the so-called phase matching, when the velocities of the generated wave and the nonlinear-polarization wave coincide. Another group of nonlinear-optical processes in por-Si are the incoherent ones. The light self-action effects, including nonlinear (two-photon) absorption, optical bleaching (absorption saturation), and nonlinear refraction (laser-induced variation of the refraction index) belong to this group; they are determined by the laser beam intensity only and do not depend on the wave phases. These processes are the basis of optical switching (e.g., in multilayers) and certainly should be taken into account for developing waveguides. Special attention will be paid to the difference of the nonlinear-optical properties in micro- (pore diameter d < 2 nm), meso-(2 nm < d < 50 nm), and macroporous silicon (d > 50 nm).

Nonlinear Optical Susceptibility of Porous Silicon Effective Refractive Index and Effective Nonlinear Susceptibility of Porous Silicon Typical sizes of porous silicon constituents (pores and silicon nanocrystals) are much less than optical wavelengths, thus, at wavelength scale por-Si can be considered as homogeneous medium characterized by its effective dielectric function (effective-

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medium approximation, EMA). The effective dielectric function «eff is often determined by the obvious relation hDi ¼ «eff  hEi,

(2)

where 〈D〉 and 〈E〉 are electric displacement and electric field, correspondingly, averaged over a volume V with the sizes exceeding the sizes of structural inhomogeneities. In mesoporous silicon (mesopor-Si) the pores have a preferred growth direction, namely 〈100〉 crystallographic axis (Lehmann et al. 2000). This ordered pore structure results in optical anisotropy as it was for the first time predicted by Ghiner and Surdutovich in 1994 and then found in experiments (Mihalcescu et al. 1997; Kompan et al. 2000; Künzner et al. 2001). In particular, the mesopor-Si formed in (110) substrate has properties of a negative crystal with the optical axis lying along [001] direction for the samples formed at p++-Si (Künzner et al. 2001; Kuznetsova et al. 2002; Shichi et al. 2012). Formally, the ordered pore formation in c-Si can be considered as the symmetry lowering. Thus, in mesoporous Si formed on p++ (110) Si substrate symmetry we obtain 4/mmm symmetry instead of typical c-Si m3 m one (Zheltikov 2001). The birefringence depends strongly on the porosity; in mesopor-Si formed by etching of p++-Si the increase in the porosity is accompanied by a decrease in the effective refractive indices both for ordinary and extraordinary waves (no and ne, respectively) and an increase in the birefringence value Δn = no – ne (Golovan et al. 2007). The value of Δn can be tuned by filling of the pores with dielectric media (Kovalev et al. 2001). Birefringence in mesopor-Si is so strong that it is retained after its thermal oxidation; the maximal birefringence of oxidized mesopor-Si twice exceeds that of crystalline quartz. Structural measurement data evidences oxidized mesopor-Si is an amorphous medium, thus, the observed optical anisotropy is explained by the oriented pores only (Golovan et al. 2006). To describe optically anisotropic nanocomposites the effective-medium models can be used. Suppose the mesopor-Si is formed by the pores and nanocrystals, which have form of spheroids (ellipsoids of revolution), the local film E inside the ellipsoid of dielectric function « is connected to the field outside E0 by the relation  1 e1, ii  eeff , ii Ei ¼ 1 þ Li E0i ¼ L i E0i , eeff , ii

(3)

where i denotes the Cartesian component parallel or perpendicular to the axis of revolution, L i is the local field factor, and Li is the depolarization factor (Kittel 1953), different for the field parallel and perpendicular to the rotation axis (denoted as L|| and L⊥, respectively); the L factor values are determined by the ratio ξ = a/b of the lengths of the polar (a) and equatorial (b) semiaxes (Osborn 1945). The cases ξ < 1, ξ > 1, and ξ = 1 correspond to oblate and prolate spheroids and a sphere, respectively (in the latter case, L|| = L⊥ = 1/3). The L|| and L⊥ values are connected with a relation:

Nonlinear Optical Properties of Porous Silicon

Ljj þ 2L⊥ ¼ 1:

367

(4)

The Bruggeman model was found to be good for description of optical properties of mesopor-Si (Theiß 1997; Kovalev et al. 1999; Bisi et al. 2000; Golovan et al. 2007). It gives a relation between eeff and the dielectric functions of crystalline silicon (c-Si) e1 and the medium filling the pores e2 (e2 = 1 for void pores): ð1  p Þ

e1  eeff e2  eeff
þp
¼ 0, eeff þ L e1  eeff eeff þ L e2  eeff

(5)

where p is porosity. It is worth noting that the microporous silicon (micropor-Si), however, requires taking into account variation of the band gap due to the quantumsize effects (Kovalev et al. 1999). On the other hand, if the sizes of the pores and nanoclusters are comparable with the optical wavelength, as in the case of macroporSi, the effective-medium model is hardly applied. The dispersion of the mesopor-Si refractive indices for ordinary and extraordinary waves calculated from (5) demonstrates a good agreement with the experimental data. The further model improvement taking into account the particle size effect on the local field due to the dynamic particle depolarization is also possible (Golovan et al. 2009). Analogously to effective refractive index for description of the linear optical properties of nanocomposite, we can characterize nonlinear-optical properties by means of effective nonlinear susceptibility. In electrostatic EMA, if the nanocomposite consists of particles of the same form where only the nonlinear constituent ð3Þ and the dipole response are absent, nonlinear effective cubic susceptibility χ eff of this medium is given by expression (Boyd et al. 1996; Sipe and Boyd 2002): ð3Þ

ð3Þ

χ eff , ijkl ¼ f L i ðωÞχ ijkl ðω; ω1 , ω2 , ω3 ÞL j ðω1 ÞL k ðω2 ÞL l ðω3 Þ,

(6)

where f is the nonlinear component volume fraction. This approach predicts a decrease of the nanocomposite susceptibility unless nanoparticle’s dielectric function is negative (Boyd et al. 1996).

Harmonic Generation and Other Frequency Mixing Processes Crystalline Si possesses central symmetry and, as a result, the most effective dipole SH response is prohibited, and the SH in this material is generated through surface dipole or bulk quadrupole mechanism and its efficiency is rather low (Shen 1984; Aktsipetrov et al. 2016). Por-Si inherits the central symmetry of c-Si and possesses the same SH generation mechanisms. Experiments evidence that in micropor-Si SH generation efficiency falls drastically in comparison with c-Si (Golovan et al. 1994). Detailed study (Falasconi et al. 2001) had revealed quadratic susceptibility of micropor-Si two orders of magnitude less than one in c-Si (Fig. 1). Despite welldeveloped surface of the micropor-Si, the surface SH response is not increased due to almost complete compensation of the SH waves with opposite phases generated at

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103 102 101 100 2 10

p-Si 103 Pump Intensity (a.u.)

104

SHG Intensity (a.u.)

SHG Intensity (a.u.)

SHG Intensity (a.u.)

a

103 102 101 c-Si

100 2 10

103 Pump Intensity (a.u.)

104

SHG Intensity (a.u.)

b

c-Si

p-Si

0

2

4

6 8 Position of Spot (mm)

10

12

14

Fig. 1 SH generation efficiency at 45 incidence as a function of the laser spot position on the sample (for position below 7 mm the spot is on por-Si surface) for different input and output polarizations: (a) both input and output are p-polarized and (b) input is p-polarized and output is spolarized. Insets demonstrate the SH signals for por-Si and c-Si, straight lines give the quadratic dependences (Falasconi et al. 2001) (Reprinted with permission from Falasconi M, Andreani LC, Malvezzi AM, Patrini M, Mulloni V, Pavesi L (2001) Surf Sci 481(1–3):105–112 @ 2001, Elsevier)

different point at the pore. It is worth noting that all relatively high SH signals from micropor-Si are due to surface states (Lo and Lue 1993), in particular occurred as a result of vacuum heating (Cavanagh et al. 1995). Experiments on sum-frequency generation in por-Si in liquids evidence sensitivity to the surface chemical composition and possibility to study reactions in real time (Kolasinski et al. 2007). Similar to the SH generation, micropor-Si demonstrate the fall of the efficiency of the TH generation in comparison with c-Si (Golovan et al. 2003), which agrees with Eq. (6). Indeed, the cubic susceptibility of micropor-Si found in transmission geometry measurement taking into account phase mismatch and Fresnel factors the free micropor-Si film was χ(3) = 0.51012 e.s.u. (Kanemitsu et al. 1995), which is two-orders-of-magnitude less than one for c-Si (Bloembergen et al. 1969). Thus, the experimental observation supports the electrostatic EMA predictions both for SH and TH. The situation, however, drastically changes in mesopor-Si (the pore sizes are of tens nanometers). The experiments on confocal SH microscopy in mesopor-Si revealed the SH signal rise with porosity increase (Maidykovski et al. 2011). The SH signal is higher for the field perpendicular to the pore than for the field along

Nonlinear Optical Properties of Porous Silicon 1.8

Third-harmonic intensity (arb. units)

Fig. 2 Polarization dependences of the TH signal for mesoporous-Si layers of different porosities grown on p++-Si substrate and the third harmonic from the substrate: (a) s-polarized third harmonic and (b) p-polarized third harmonic. Inset in the bottom panel is a sketch of the experimental geometry (Golovan et al. 2003) (Reprinted with permission from Golovan LA, Kuznetsova LP, Fedotov AB, Konorov SO, SidorovBiryukov DA, Timoshenko VYu, Zheltikov AM, Kashkarov PK (2003) Appl Phys B 76(4): 429–433 @ 2003, Springer)

369

PS 2 25 mA/cm 2 50 mA/cm 2 100 mA/cm c-Si

1.2

0.6

0.0 1.8

a

s-polarized third harmonic

[001] PS

[110]

f

b

p-polarized

E

third harmonic

1.2 s 0.6

p

0.0 0

30

60

90

120

150

180

210

Polarization angle (degree)

them, and SH generation was connected with contributions of the surface dipoles, which, in contrast to micropor-Si, are not compensated in mesopor-Si. The experiment on TH generation in reflection geometry from birefringent mesopor-Si revealed that the TH generation efficiency in this material is up to 50 times higher than that in c-Si, with the TH signal increasing with the rise of porosity (Golovan et al. 2003) (Fig. 2). Note that this TH signal enhancement cannot be reduced to trivial variations of the relevant Fresnel factors caused by the pore formation. Estimations show that cubic susceptibility of mesopor-Si is at least not lower than one of c-Si. It is worth noting that the pore and nanocrystal sizes are less than the fundamental wavelength (1.064 μm); however, the sizes of Si nanocrystals in mesopor-Si formed from p++-Si (110) become comparable with the TH wavelength inside this material, which is about 150 nm (the average refractive index is about 2.3 at 355 nm for a mesoporous-Si layer of 65% porosity). Thus, EMA is not applicable for the TH generation description in mesopor-Si. Equation (6) allows us to calculate cubic susceptibility tensor components and relations between them. Whereas c-Si is characterized by two nonzero independent ð3Þ ð3Þ components χ 1111 and χ 1122 , for mesopor-Si formed at (110) surface we obtain five ð3Þ ð3Þ ð3Þ different components of the effective cubic susceptibility: χ eff , 1111 , χ eff , 3333 , χ eff , 1122 , ð3Þ

ð3Þ

χ eff , 1133 , and χ eff , 3311. Here subscript 3 corresponds to the optical axis of birefringent

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mesopor-Si. This result agrees with the point-group symmetry analysis (Zheltikov ð3Þ 2001). It is instructive to compare the data concerning the χ eff component ratio found in experiment and calculated according the electrostatic EMA. In the experiment the fundamental wavelength ranging from 1.1 to 1.5μm (out of the phase-matching region, see below) TH signal polarized along 110 axis  was  registered in transition geometry with pumping polarized along [001] and 110 (Zabotnov et al. 2005). ð3Þ Experimentally obtained ratio of the TH signals allowed finding the χ eff component   ð3Þ ð3Þ ð3Þ ratio r ¼ χ eff , 1111 þ 3χ eff , 1122 =χ eff , 3333 (the phase mismatch was taken into account). The calculated r value is in qualitative agreement with the experimentally found ones: increase of the porosity results in rise of the r ratio and the nonlinear ð3Þ ð3Þ susceptibility component χ eff , 3333 is smaller than χ eff , 1111 . However, the calculations predict higher r values. This discrepancy could not be corrected by taking into account dynamic depolarization effect and indicate imperfection of the simple EMA approach in the case of nonlinear susceptibility of mesopor-Si. In contrast, analogous analysis carried out for oxidized mesopor-Si demonstrated good agreement of the experiment and EMA theory, which is obviously caused by its lower refractive index, longer wavelength in the Si oxide nanoparticles, and, therefore, better applicability of EMA (Golovan et al. 2006). Finally, let us turn to harmonic generation in such form of macropor-Si silicon (macropor-Si) as Si nanowire (SiNW) ensembles with the nanowires of about 100 nm in diameter (Sivakov et al. 2010). The TH signal demonstrates both fall and one- or two-orders-of-magnitude rise in comparison with c-Si depending on the ensemble structure (Golovan et al. 2012a). The possible reason of the TH generation efficiency increase is effect of the strong scattering resulting in light trapping and increase of the photon lifetime. This fact was earlier known for other macroporous semiconductors (Golovan et al. 2005, 2015) and was recently found in SiNW ensembles (Efimova et al. 2016). The same reasons seem be responsible for the increase of coherent anti-Stokes Raman scattering resonant signal (corresponding to Si phonon frequency of 520 cm1) in SiNWs in comparison with c-Si (Golovan et al. 2012b).

Light Self-Action Nonlinear-optical effects of two-photon absorption (TPA), optical bleaching, and nonlinear refraction (NLR) are often called “light self-action” since intense light propagating through a medium results in variations of its optical parameter, which, in its turn, effect the light propagation. Although these processes are different by their nature and have different typical time, they need no phase matching. Formally, these processes are four-wave interactions and described in terms of cubic nonlinear susceptibility χ(3). In order to describe TPA effect into account the following equation for the light intensity I propagating along z axis is usually written down:

Nonlinear Optical Properties of Porous Silicon

dI ¼ αI  βI 2 , dz

371

(7)

where α is a linear absorption coefficient and β is TPA coefficient. NLR originates from a dependence of the refractive index on the light intensity n ¼ n0 þ n2 I,

(8)

where n2 is NLR index and n0 is a refractive index at low light intensity. The nonlinearity described in Eq. (8) is often referred as Kerr-like nonlinearity. Coefficients n2 and β are connected with real and imaginary parts of the cubic susceptibility (for the degenerative case of the same excitation and probing wavelength λ) (Sutherland 1996): n2 ¼

12π 2 Reχ ð3Þ ðω; ω,  ω, ωÞ, n20 c

(9)

β¼

48π 3 Imχ ð3Þ ðω; ω,  ω, ωÞ: n20 cλ

(10)

The way of n2 and β measurement is the so-called z-scan technique, which is based on variation of the focused laser beam cross-section near the focal point and, therefore, variation of the laser intensity (Sheik-Bahae et al. 1990). Self-focusing in the sample results in the decrease of the signal transmitted through the diaphragm in far zone (close aperture) for the sample before focus and increase of the signal for the sample behind the focus (Lettieri et al. 1999). Open aperture signal indicate two-photon absorption in the sample. The z-scan technique due to its elegance has gained wide researchers’ acceptance. However, this technique makes serious demands to the sample homogeneity since the light transmits through varying area of the sample. That is why for nanostructured materials alternative technique, I-scan, is often employed. In I-scan measurements, the light intensity is controlled in broad limits by means of optical attenuator, whereas the laser-beam cross section has no variation (Simos et al. 2005; Bazaru et al. 2010; Gayvoronsky et al. 2011). TPA is also manifested in two-photon-excited photoluminescence (PL) (Malý et al. 1994; Diener et al. 1998, 1999). Anisotropy of por-Si cubic susceptibility results in anisotropy of the PL signal (Diener et al. 1998). Below, we will consider effect of various factors on the cubic susceptibility of porous silicon. The cubic susceptibility values obtained in different experiments are collected in Table 2.

Influence of Quantum-Size Effect on Cubic Susceptibility of Microporous Silicon For nonlinear-optical properties of the micropor-Si, the quantum-size effects (variation of the electron structure due to restrictions in the carrier movement) in Si nanocrystals play a principal role. That is why the researchers’ interest in 1990s was

Quantumsize effect. Nonlinear refraction

Quantumsize effect. Optical bleaching

Effects

Micropor-Si

Por-Si type c-Si

60–75

60–75

Porosity, %

20 ps

532 (pump) 600–750 (probe) 532 (pump) 633 (probe)

z-scan

30 ps 45 ps

604

DFWM

z-scan

ps

1064 900 860 1064

ns

0.5 ns

200 fs

1540

665

z-scan

130 fs waveguide, SPM DT

Experimental technique z-scan

Pulse duration 200 fs

Wavelength, nm 1220 1800 1540

Table 2 Cubic susceptibilities of por-Si responsible for the light self-action

100μs–10 ms depending on laser pulse energy density

35 ps

1.12 μs

Relaxation time

1.9109

7.5109

1.21012

4.5  0.7109 1.9  0.3108 2.5  0.3108 5.6  1.7108

3.71012

1.8  0.61011

8.21010

~ 108

Im χ(3), e.s.u. 6.1  1.21012 3.2  1.01012 2.01012

Re χ(3), e.s.u. 1.4  0.61011 3.6  0.61011 1.31011

Lettieri et al. (1999) Liu et al. (1999)

Lettieri and Maddalena (2002)

Henary et al. (1995) Klimov et al. (1994)

Dinu et al. (2003) Tsang et al. (2002)

Reference Bristow et al. (2007)

372 L. A. Golovan

Macropor-Si (SiNWs) Micropor-Si

49

Mesopor-Si (110)

73

69 aged 77 refreshed 70

73

49

Mesopor-Si

Mesopor-Si (110)

458

532

cw

4 ns

42 ps

200 fs

1550

1064 above 10 MW/cm2

42 ps

42 ps

1064

1064 below 3 MW/cm2

z-scan of suspension of SiNWs

I-scan

I-scan

I-scan

1 ms

2.01011 91013 11.21011 (o) 6.91011 (e) 3.41011 (o) 0.51011 (e)

1.1108 5.31012 10.8108 (o) 9.7108 (e) 2.6108 (o) 2.7108 (e) 4.2105

~101

5.9109 (o) 5.8109 (e) 1.4109 (o) 1.5109 (e) 1.71011

4.7106 (o) 3.0106 (e) 3.4106 (o) 2.2106 (e) 1.0108

DT is differential transmittance, SPM is self-phase modulation, DFWM is degenerative four-wave mixing, cw is continuous wave o and e are odinary and extraordinary waves, correspondingly

Thermal effect

Surface state effects

Matsumoto et al. (1995)

Apiratikul et al. (2009) Gayvoronsky et al. (2011)

Uklein et al. (2017)

Gayvoronsky et al. (2011)

Nonlinear Optical Properties of Porous Silicon 373

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focused on the energy spectra and dynamics of the transitions. For this purpose, differential transmission (DT) technique registering the difference between transmittances of excited and unexcited spectra was employed. The excitation is carried out by a short pumping laser pulse and a probing monochromatic or continuum light. Transmission is often detected depending on the time delay with respect to pumping pulse. The DT demonstrates optically induced bleaching (Klimov et al. 1994; Henary et al. 1995; Dobryakov et al. 2000; Lettieri and Maddalena 2002) or absorption (Klimov et al. 1995; Malý et al. 1996). The bleaching effect is caused by partial occupation of the upper states, which reduces light absorption. The bleaching spectra exhibit a sharp discrete structure with the peaks corresponding to the saturation of transition to the energetic levels of the nanocrystals. The process is characterized by fast relaxation and high nonlinear susceptibility. The exciting and probing photon energy is of great importance for the DT sign. An accurate resonance (photon energy coincides with the energy of the states occurred due to quantum-size effect) results in effective optical bleaching (Henary et al. 1995). An excitation photon energy of 3.1 eV instead of 2.34 eV resulted in changed bleaching with absorption in a broad spectrum, which could be explained by more efficient excitation of molecular-like complexes in Si nanocrystals by higher-energy photons (Klimov et al. 1995). However, some publications (Lettieri et al. 1999) reported negligible TPA in micropor-Si. As far as Re χ (3) value is concerned, it can reach large values three orders of magnitude exceeding one for c-Si (Henary et al. 1995) and could be connected with resonance absorption by the Si nanocrystal energy states. However, employment of the lasers with off-resonant photon energy (e.g., 1.17 eV, which is below band gap of micropor-Si) also results in huge Re χ (3) absolute values (Lettieri et al. 1999). The authors connected the giant effect to increased exciton transition strength in case of the carrier confinement in silicon nanowires. On the other hand, in micropor-Si, the value of Re χ (3) comparable with one in c-Si (Liu et al. 1999) and, moreover, a two-order-of-maginitude less (Bazaru et al. 2010), which agrees the EMA predictions, were also reported. The discussed results, however, were subjected to some criticism because of not taking into account influence of free carriers generated by the laser pulse (Bindra 2005), which is especially important for excitation by pico- and nanosecond laser pulses. Moreover, although nanocrystals in mesopor-Si do not demonstrate any energy quantization, this material is also characterized by high-effective light self-action.

Effect of the Surface States The effects of the surface states were found both in micro- and mesopor-Si under irradiation by picosecond pulses with photon energy of 1.17 eV. The recombination of carriers rapidly trapped in states localized at the surface of Si crystallites is responsible for slow (microsecond) component of nonlinear absorption (Malý et al. 1996). Mesopor-Si demonstrates huge cubic susceptibility values and TPA and NLR coefficients for relatively low laser intensity (below 2 MW/cm2) (Gayvoronsky et al. 2011) (Fig. 3a). Despite strong in-plain birefringence of the mesopor-Si sample, the TPA coefficient does not depend on the light polarization, whereas NLR coefficient does. The possible reason of huge TPA can be a resonance

Nonlinear Optical Properties of Porous Silicon

Total transmittance (%)

a 100

b

1.9

98 96 94 92

||

90 0

On-axis transmittance

Fig. 3 (a) Total transmission and (b) on-axis transmission in the far field, of mesopor-Si film, normalized to its linear transmission as a function of the peak laser pulse intensity at a wavelength of 1.064 mm for (⊥, dashed curves) ordinary and (||, solid curves) extraordinary waves. (Gayvoronsky et al. 2011) (Reprinted with permission from Gayvoronsky VYa, Golovan LA, Kopylovsky MA, Gromov YV, Zabotnov SV, Piskunov NA, Kashkarov PK, Timoshenko VYu (2011), Quant Electron 41 (3): 257–261 @ 2011, Turpion)

375

5

10

15

1.7 ||

1.5 1.3 1.1 0.9

0

5

10

15

Peak laser intensity (MW/cm2) of the photon energy to a certain transition in the mesoporous silicon, e.g., surface states or defects in silicon oxide covering the surface, characterized with a broad band at 1.18 eV (Memon et al. 2003). The effect is saturable. Taking into account laser energy density necessary for the saturation ~ 2 mJ/cm2 and the thickness of the sample, one can estimate the defect density as ~1018 cm3. Detailed study reveals effect of por-Si surface on nonlinear optical properties: aged mesopor-Si film demonstrates negative values for both Re χ (3) and Im χ (3), whereas refreshing the sample in HF:С2H5OH mixture results in the sign changes. Very impressive results on ambient humidity effect on the por-Si χ(3) value were recently found; in particular, the humidity reduction from 75% to dry air result in fall of the Re χ(3) value and turn of its sign from positive (4.1109 e.s.u.) to negative (4.9109 e.s.u.), i.e., selffocusing turns to self-defocusing (Uklein et al. 2017).

Cubic Nonlinearity in Meso- and Macroporous Silicon The nonlinear-optical properties of c-Si should be inherited to some extent by por-Si structure. Let us compare NLR and TPA coefficients for both these materials. High porosity (~70%) mesopor-Si demonstrates two-times lower value of Re χ(3), which qualitatively agrees with EMA predictions (Eqs. 6 and 9). However, huge Re χ(3)

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values were also reported in mesopor-Si (Gayvoronsky et al. 2011) at rather high laser intensities (above 10 MW/cm2). As far as the TPA coefficient is concerned, reported β values for mesopor-Si differ significantly. In particular, for mesopor-Si waveguide TPA coefficient practically did not differ from one for c-Si (Im χ(3) was twice less than for c-Si) (Apiratikul et al. 2009). In contrast, TPA coefficients two-orders-of-magnitude exceeding ones for c-Si were found in I-scan experiments (Gayvoronsky et al. 2011) in mesopor-Si with rather high laser pulse intensity (Fig. 3a). In the latter experiments, the in-plane birefringent mesopor-Si films exhibited significant TPA anisotropy, which agrees with a negative type of birefringence in the employed film. The porosity increase results in stronger TPA anisotropy and less β values. For macropor-Si TPA coefficient β was estimated to be 20 cm/MW for 1.76 μm and 2 cm/MW for 2 μm (Tan et al. 2004), which is one and two orders of magnitude smaller than that for c-Si (Dinu et al. 2003). The β value fall with wavelength increase is casued by approaching photon energy to the half of the band gap. For such macropor-Si structure as SiNW array, four-order of magnitude rise of nonlinear-optical parameters in comparison with c-Si were reported (Huang et al. 2012), which is possibly caused by enhanced light scattering and rise of the photon lifetime in SiNW layer.

Free-Carrier Effects Free-carrier plasma dispersion (FCD) and free-carrier absorption (FCA) significantly affect the laser pulse propagation since they vary the refractive index and absorption coefficient, respectively. These variations depend on the excess carrier concentration ΔN generated in medium as a result of the laser action: ΔαFCA ¼ σ FCA ΔN,

(11)

ΔnFCD ¼ kFCD ΔN,

(12)

where σ FCA is the free-carrier absorption cross section and kFCD is the free-carrier dispersion coefficient, which can be found in the frames of Drude model (Beresna et al. 2009). In its turn, ΔN is determined by the laser radiation intensity, the freecarrier lifetime τc, and linear absorption coefficient or, in case of infrared radiation, TPA coefficient. The experiments (Apiratikul et al. 2009) on the influence of the 150 fs laser pulse (wavelength of 1.49 μm, pulse energy of 0.12 nJ) on cw radiation (1.56 μm, 1 mW) transmission carried out in silicon-on-insulator (SOI) and mesopor-Si waveguides revealed that the latter material is characterized with less τc value (cf. 0.2 ns for mesopor-Si and 1.1 ns for SOI waveguide). The faster freecarrier lifetime in mesopor-Si could be explained by the much higher surface recombination rate in the material with a developed surface and the much faster Auger recombination lifetime. Employment of visible pulses significantly shortened the carrier lifetime in mesopor-Si up to 3 ps (Dao and Hannaford 2005). The FCA was found in experiments to have stronger effects on the light propagation in mesopor-Si waveguide than in SOI one. FCA cross section in porous

Nonlinear Optical Properties of Porous Silicon

377

silicon waveguide was found to be approximately two-orders-of-magnitude larger than that reported for bulk silicon, which is connected with the higher collision frequency due to the lower mobility of the heavily doped por-Si skeleton. Analogously, kFCD value is more than an order of magnitude higher than one reported for c-Si. Carrier population time and electron-hole pare dephasing time are often measured by means of four-photon spectroscopy. Measurements carried out by means of coherent anti-Stokes Raman scattering in micropor-Si allowed the carrier lifetime to be estimated. Its value ranged from 1.5 to 3.5 ps and increased with the porosity rise; dephasing time for electron-hole pare was estimated from the same experiment as 20 fs (Tomasiunas et al. 1996).

Thermal Effects Typically, the thermal effect provides the huge nonlinearities; however, it is the slowest one. This effect is common for various materials and is caused by laserinduced heating of the sample. The structure of por-Si results in poor heat transfer in this material (Lysenko et al. 1999; Koyama and Fauchet 2000). Thus, it is necessary to take the heating into account for many optical processes in por-Si except for the processes of subpicosecond duration. Mesopor-Si is characterized by the thermooptical coefficient dn/dT ranged 105 – 104 K1, which decreases with the rise of porosity and wavelength (Lysenko et al. 1999; Moretti et al. 2005). Thermal effects are responsible for huge nonlinear absorption and transmittance hysteresis in case of Ar-ion laser radiation (Matsumoto et al. 1995).

Control of the Nonlinear-Optical Process Efficiency The efficiency of such nonlinear-optical processes as SH and TH generation depends on both effective nonlinear susceptibility of the nanocomposite and phase matching. The latter one is of great importance for the effective frequency conversion process since it is only when the nonlinear polarization wave generated in the nonlinearoptical medium propagates with the same velocity as the wave of doubled, tripled, or, generally, sum frequency. Formally, for the nonlinear-optical interactions wave vector mismatch value Δk = kg  kp can be calculated, where kg = ωn(ω)/c is wave vector for the wave at frequency ω propagating in the medium, n(ω) is refractive index at ω, kp is wave vector for nonlinear polarization wave; Δk = 0 corresponds to the phase matching, i.e., the highest efficiency of the interaction (Shen 1984; Boyd 2003). In por-Si structures, the phase matching could be achieved by employing birefringent films or photonic-crystal structures.

Phase Matching in Birefringent Mesoporous Silicon The in-plain birefringence of free-standing PS films formed from (110) p++-Si substrate is high enough to compensate for the normal dispersion of the material and to achieve phase matching for both SH and TH generation.

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Calculations of the phase-matching conditions for SH generation in birefringent mesopor-Si indicate possibility to achieve phase-matched SH generation (Kashkarov et al. 2002). Figure 4 presents wave-vector mismatches Δk versus the angle of incidence θ calculated for both type of phase matching in a negative crystals: oo-e (fundamental radiation is ordinary wave, SH is extraordinary wave), Δk ¼ ke2  2ko1 , and oe-e (fundamental radiation has ordinary and extraordinary components, SH is extraordinary wave), Δk ¼ ke2  ko1  ke1 , where superscripts indicate the wave vectors for ordinary (o) and extraordinary waves (e) and subscripts indicate the fundamental radiation (1) and SH (2). As one can see, birefringence in the sample of 65% porosity is too high and phase matching cannot be achieved at any angle of incidence for Fig. 4a. However, the slight variations in porosity is enough for phase matched SH generation (Fig. 4b). Birefringence reduction due to filling the pores with dielectric liquids also allows the phase matching to be achieved (Fig. 4c and d for ethanol- and glycerol-filled mesopor-Si films of 65% porosity indicate phase matching at the angles of incidence of 40 and 13 , correspondingly). Experimental results on SH generation in the birefringent mesoporous Si (Golovan et al. 2001) agree with the calculations very well. The polarization dependence for the film in air has fourfold symmetry for normal incidence (Fig. 5a) and twofold symmetry for oblique incidence (Fig. 5b) since for normal incidence Δk is less for the oe-e interaction, whereas at high θ the phase mismatch is smaller for oo-e interaction. The dependences for the film filled with glycerol or ethanol exhibit a twofold symmetry at any angle of incidence, implying the oo-e interaction, which is preferential according to the calculations (Fig. 4c and d). In more detail, the dependence of the SH generation efficiency on the angle of incidence is shown in Fig. 5c. The mesopor-Si film was rotated perpendicular to the optical axis, which allows Δk to be controlled; experimental geometry corresponds to oo-e interaction. For the mesoporous Si film with air-filled pores, the maximal SH signal was observed at the angle of incidence of about 57 , whereas, when the pores were filled with ethanol and glycerol, the maximum is reached at lower angles. For the case of ethanol- and glycerol-filled pores, the SH generation efficiency is one and two orders of magnitude higher, respectively, than for the sample in air that is indicative of phase-matched SH generation. Symmetry of the cubic susceptibility tensor responsible for the TH implies phase matching for the ooe-e interaction only. For this type of interaction, a more useful way to check achievement of the phase-matched generation was variation of the fundamental wavelength. Figure 6a displays clearly pronounced maximum as a function of the fundamental wavelength at 1.635 μm. The maximal level of the TH signal exceeds one at 1.2 μm by two orders of magnitude. No sign of phase matching has been observed for other experimental geometries such as ooo–o and eee–e. Additional proof of the phase matching is orientation dependence of the TH signal (Figs. 6b–6d). In the regime of phase matching (Fig. 6b), it can be accurately approximated with a function proportional to sin42ψ, where ψ is an azimuthal angle (see inset in Fig. 6d); this means that ooe-e interaction prevails, whereas off the phase matching (Fig. 6c, d) ooo–o and eee–e interactions have significant contributions as well (Zabotnov et al. 2004).

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a

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Fig. 4 Calculated mismatch wave-vector mismatch Δk versus the angle of incidence of the radiation at the fundamental frequency for (110) PS of two different porosities (a and b), and (110) PS with the pores filled with ethanol (c) and glycerol (d). The optical axis is in the plane of incidence. Solid and dashed lines correspond to oo-e and oe-e interactions, respectively (Kashkarov et al. 2002) (Reprinted with permission from Kashkarov PK, Golovan LA, Fedotov AB, Efimova AI, Kuznetsova LP, Timoshenko VYu, Sidorov-Biryukov DA, Zheltikov AM, Haus JW (2002) J Opt Soc Am B 19(9): 2273–2281 @ 2002, OSA)

Birefringence in oxidized mesoporous Si is also so strong that allows phase matching of the TH generation in this material to be achieved, which was proved by variation of the TH signal and its orientation dependences on the fundamental wavelength (Golovan et al. 2006).

Harmonic Generation in Porous-Silicon Multilayers Since porosity and refractive indices of the por-Si film are determined by the etching current density, by means of periodical variation of this parameter one can form structures with alternating layers of different porosity (and, therefore, different refractive indices), i.e., one-dimensional photonic crystals (Vincent 1994). Field enhancement and specific dispersion laws in these optical structures (Golovan et al. 2000) seem to be very promising for the light frequency conversion, lightby-light control, including switching, and sensing. Por-Si-based waveguides should also be mentioned among promising photonic applications (Rong et al. 2008).

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Angle of incidence (deg) Fig. 5 Polarization (a, b) dependence of the SH signal in a free-standing (110) mesopor-Si film of 65% porosity with the pores filled with dielectric liquids (ethanol, glycerol) for normal (a) and oblique (b) incidences. The sketches demonstrate the geometry of the experiment. Dependence of the SH signal on the angle of incidence (c), the fundamental radiation, and SH were polarized along and perependicular to [001] direction (Golovan et al. 1999) (Reprinted with permission from Kashkarov PK, Golovan LA, Fedotov AB, Efimova AI, Kuznetsova LP, Timoshenko VYu, Sidorov-Biryukov DA, Zheltikov AM, Haus JW (2002) J Opt Soc Am B 19(9): 2273–2281 @ 2002, OSA)

In micropor-Si-based photonic band-gap (PBG) structures, the control of the harmonic generation efficiency by the period of the structure and the angle of incidence was experimentally demonstrated (Golovan et al. 1999): the most efficient SH generation was found when SH was near the PBG (structure A with PBG at 0.44–0.5 μm). In this case, SH signal several-orders-of-magnitude exceeds one from c-Si and significantly exceeds SH signal from multilayers with another PBG positions (B and C) (Fig. 7). SH enhancement for the fundamental wavelength is at the edge of the PBG or coinciding with the microcavity wavelength as reported by

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Fig. 6 (a) The TH signal as a function of the fundamental wavelength for mesoporous Si films. The inset illustrates orientation of the sample and polarization arrangement in experiments on TH generation in the ooe–e geometry; A shows the orientation of the Glan prism, used as a polarization analyzer. (b) Dependences of the TH signal on the angle between the [001] optical axis and the polarization of the pump field in the cases of phase matching (b) and out of phase matching (c and d). The zero of the abscissa axis corresponds to the orientation of the [001] axis along the direction of pump polarization. The TH signal in plots (b–d) is normalized to the maximum of the TH signal for a given dependence. The experimental data for the intensity of phase-matched TH (b) are fitted (solid curve) with a function proportional to sin42ψ. The inset illustrates orientation of the sample and polarization arrangement in experiments; the TH radiation is polarized along the polarization of the pump field (Zabotnov et al. 2004) (Reprinted with permission from Zabotnov SV, Konorov SO, Golovan LA, Fedotov AB, Zheltikov AM, Timoshenko VYu, Kashkarov PK, Zhang H (2004). JETP 99(1): 28–36 @ 2004, Springer)

Dolgova et al. (2001, 2002a, b). The same effect takes place for the TH generation (Dolgova et al. 2002a; Martemyanov et al. 2004). Employment of the near-field optical microscopy allowed the field enhancement in por-Si microcavities with both empty pores and infiltrated with glucose oxidase (Dolgova et al. 2001; Martin et al. 2009) to be found. Por-Si-based PBG structures formed by in-plane birefringent layers introduce new features of the harmonic generation. Since PBG position depends on the light polarization, dependences of the SH and TH signals on polarization angle and angle of incidence strongly varies if the fundamental wavelength is in PBG or coincides with microcavity wavelength (Soboleva et al. 2005; Petrov et al. 2006), whereas pumping with the fundamental wavelength off the PBG results in no variation. In por-Si (70% porosity) waveguide critically coupled with por-Si microring fourwave mixing resulting in generation at frequency ω3 = 2ω1  ω2, where ω1 and ω2 are incident radiation, was reported. Frequencies ω1, ω2, and ω3 correspond to transmittance resonances of the microring. Due to high Q-factors at these frequencies (about 5000), the effect could be found even under cw excitation in the 1550 nm telecom band, with the powers at ω1, ω2 being below 700 and 70 μW, respectively (Simbula et al. 2016).

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Fig. 7 SH generation in reflection geometry in por-Si-based PBG structures formed by 12 pairs layers of relatively low and high porosity formed by etching of (100) p-type c-Si (10 Ωcm): reflection spectra of the multilayers with different position of the PBG (a), dependence of the SH signal on the angle of incidence of the fundamental radiation (b) (Golovan et al. 1999) (Reprinted with permission from Golovan LA, Zheltikov AM, Kashkarov PK, Koroteev NI, Lisachenko MG, Naumov AN, Sidorov-Biryukov DA, Timoshenko VYu, Fedotov AB (1999). JETP Letters 69 (4):300–305 @ 1999, Springer)

Enhancement of the harmonic generation efficiency could be caused by slowing down the light propagation in photonic-crystal structures. The effect of the groupvelocity decrease for the propagating laser pulse in a media is of great interest due to its various applications in photonics. Macropor-Si can be employed for slowing down the light propagation (Wehrspohn et al. 2007; Baba 2008; Gutman et al. 2008; Corcoran et al. 2009) and significant enhancement of the nonlinear-optical interaction efficiency. For example, in the silicon waveguide with a macropor-Si – two-dimensional photonic crystal as a cladding due to dispersion engineering through the use of optimized photonic-crystal waveguides – the group velocity of the laser pulse can achieve c/40, where c is the speed of light. The intensity of the fundamental wave increases as much as the phase velocity is higher than the group velocity. As a result, the TH generation efficiency increases up to 107 at pumping with wavelength near 1.5 μm and peak power 10 W, which allows TH radiation (green light) to be seen with naked eye. The maximal TH generation efficiency is achieved for the fundamental wavelength 1.557 μm, which is characterized by the lowest group velocity in the used spectral range (Corcoran et al. 2009).

Control of PBG The smallest variation of the refractive indices of the por-Si layer can cause detectable shift of the photon band gap (PBG) of the photonic-crystal structure. This fact opens new possibility to all-optical control of the reflectivity and PBG in multilayer structures. Laser-induced variations of the por-Si refractive indices are mainly indebted to instant Kerr effect, free-carrier generation by one- or two-photon absorption, and thermal

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effects. Note that according to Drude model free-carrier generation result in decrease of the refractive index (Boyd 2003) and blue PBG shift, whereas the other two effects in por-Si can cause increase of the refractive index and red PBG shift. The variations of the reflectivity under laser pulse irradiation were found in 1D mesopor-Si-based PBG structures pumped by visible (0.532 μm, 25 ps) (Beresna et al. 2009) or near infrared (NIR) laser pulses (Afonina et al. 2010) and probed at the PBG edge in NIR regions. In the former case, increase of reflectivity was found for about 300 ps; the time of the enhanced reflectivity increases with the laser pulse fluence rise (Beresna et al. 2009). The effect was explained by charge-carrier generation by laser pulses. In macropor-Si (two-dimensional photonic crystal) (Fig. 8a) rise of the refractive index due to Kerr-like nonlinearity results in red shift of the PBG as it was detected in experiments (Kitzerow et al. 2008). The 2D photonic crystal had the PBG edges at approximately 1.3 and 1.6 μm. Employing 2.0 μm pumping and 1.3 μm probing allowed 103 decrease in reflectivity to be detected, which evidenced a redshift of the PBG edge (Fig. 8b). The broadening of the reflectivity trace is explained by slowing down the pump and probe pulse propagation in photonic crystal. High density of carriers can be optically injected and results in the shift of PBG edge. Due to a decrease of the dielectric permittivity of Si with the carrier density rise the PBG edge is shifted to the blue side, with the typical reflectivity rise time 400 fs (Leonard et al. 2002). Detailed calculations based on absorption characteristics of the photonic crystal at the pump wavelength and the variation of the photonic crystal dispersion curves with injected carrier density are in agreement with the maximum shift of about 30 nm observed in experiment (Kitzerow et al. 2008). The free-carrier generation due to TPA also results in significant variation of the macroporous Si reflection (blue shift of its PBG) with the switching time 1 ps (Tan et al. 2004). It is instructive to compare how the Kerr effect and free-carrier generation caused by TPA influence the 2D photonic crystal reflectivity. To obtain significant TPA pumping at wavelength of 1.76 μm and probing at 1.6 μm (i.e., near the red PBG edge) were employed. On the one hand, Kerr-effect-induced PBG red shift results in increase of the signal. On the other hand, free carriers generated by TPA cause blue PBG shift. As one can see from Fig. 8c, the instantaneous Kerr effect brings about rise of reflectivity, but the effect of TPA-pumped free carriers is somewhat stronger and 1 ps later the reflectivity falls. Note that, in contrast to Kerr effect, free-carrier one need some time and both the effect are of one order. The n2 value for macroporSi was estimated to be 5.21015 cm2/W, which is an order of magnitude less than one for c-Si (note, however, difference between nondegenerative and degenerative in frequency Kerr effects as well as difference in wavelengths).

Applications Thus, por-Si possesses unique nonlinear-optical properties, which depend on its porosity and pore and Si nanocrystal sizes. For some por-Si films, nonlinear-optical parameters exceed ones for crystalline silicon. Ability to employ por-Si for formation

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Fig. 8 (a) Optical pump/probe orientation for Si 2D photonic crystal. (b) Temporal response of reflectivity change at the 1.3 μm band edge when the photonic crystal is pumped with a 2.0 μm pulse. (c) Temporal response of the reflectivity change at the 1.6 mm band edge for different pump intensities at 1.76 mm. The inset shows the dependence of the carrier-induced reflectivity change on pump intensity for low pump powers (Kitzerow et al. 2008) (Reprinted with permission from Kitzerow H-S, Matthias H, Schweizer SL, van Driel HM, Wehrspohn RB (2008). Advances in Optical Technologies 2008:780784 @2008, Hindawi)

of birefringent films, photonic crystals, and waveguides opens up new possibilities for enhancement of nonlinear-optical interaction efficiency. All these facts make por-Si a very promising material for frequency conversion and light-by-light control. The discussed nonlinear-optical properties of porous silicon open up various possibilities for their employment in sensors and photonic devices. Employment of por-Si structures for frequency doubling is restricted by low efficiency of the SH generation in centrosymmetrical medium. Embedding the pores with non-centrosymmetrical medium could be a solution; however, this problem is still to be solved. Nevertheless, four-wave mixing, perhaps, could be used for frequency conversion.

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Por-Si could be used for nonlinear-optical sensing. In particular, light self-action seems to be a promising way of sensing due to higher sensitivity of this process to the variations of the ambience (Uklein et al. 2017). Por-Si-based waveguides are of great interest for various applications. Typically, nonlinear-optical properties of the waveguides are characterized by the figure of merit (FOM) value, allowing us to compare nonlinear refraction and nonlinear absorption: FOM ¼

n2 βλ

(13)

Due to high NLR coefficients of birefringent mesopor-Si, the FOM value ranges from 21 to 143 depending on the porosity and light polarization, whereas for c-Si different estimates yield FOM ranging from 0.17 to 2.9 (Gayvoronsky et al. 2011). Strong nonlinear refraction will result in a laser-induced phase shift 2πn2 I(t)/λ, which could be considered as a broadening of the spectrum. Effective control of the por-Si optical parameters by intense light allows us to produce logic gates, i.e., devices, which are made transparent or opaque under the controlling light irradiation. Among first por-Si logic gate, we can mention paper by Matsumoto et al. (1995), which reported logic gates based on laser-induced absorption in micropor-Si film. The Ar-ion laser radiation with intensity of order of tens W/cm2 was employed to control transmitting beam. Thus, logic operations of the inverse and NOR (the latter was with two controlling beams) were realized. Typical time of the signal relaxation was of the order of 1 ms, which indicates important role of thermal processes in the case, although charge carrier accumulation effects should be also taken into account. Optical AND logical function was realized on a basis of photoluminescence in micropor-Si excited by Ar+-ion radiation (0.488 μm) (Fig. 9) (de la Mora et al. 2014). In addition, the por-Si film serves as a controllable reflector for the excitation radiation. The film was irradiated by controlling nanosecond Nd:YAG laser pulse (0.532 μm), which resulted in increase of the film reflection due to laser-induced generation of the carriers. Thus, photoluminescent signal could occur when both laser beams were on. Optical gating effects could be enforced if the por-Si photon-crystal structures are used since the smallest variation of the refractive indices of the por-Si layer causes detectable shift of the photon band-gap edge (Kashkarov and Zheltikov 2000). Employing idler (1.125–1.145 μm) and signal (0.518–0.521 μm) waves of optical parametric oscillator as probe and pump beams, correspondingly, allowed switching between reflected and diffracted laser beams (optical pendulum effect) in 1D photonic-crystal mesopor-Si structure of 360 layer pairs to be demonstrated (Fig. 10) (Novikov et al. 2015). The effect was controlled by laser heating of por-Si caused by absorption of the visible nanosecond pulse. Another approach for all-optical modulator is employment of Mach-Zehnder interferometer with two unbalanced arms: one arm contains por-Si waveguide as a nonlinear element and other one is a fiber delay line (Xiao and Wu 2015). Modeling taking in account

386 Fig. 9 Experimental setup for the photoluminescent logic gate AND (de la Mora et al. 2014) (Reprinted with permission from de la Mora MB, Torres-Torres C, Nava R, Trejo-Valdez M, ReyesEsqueda JA (2014). Optics &Laser Technology 59:104–109 @2014, Elsevier)

L. A. Golovan 488 nm beam

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Fig. 10 Experimental setup for optical switching in photonic crystal. Idler beam of optical parametric oscillator is shown in red. Signal beam of the same laser system is shown in green. Insets illustrate the propagation of light after the por-Si-based-photonic crystal in the absence (left) and presence (right) of a 70 MW/cm2 light beam (Novikov et al. 2015) (Reprinted with permission from Novikov VB, Svyakhovskiy SE, Maydykovskiy AI, Murzina TV, Mantsyzov BI (2015) J Appl Phys 118(19): 193101 @ 2015 AIP Publishing)

two-photon absorption, self-phase modulation, and free-carrier absorption and dispersion evidences that for pulsed pumping pulse and cw probe the output undergoes a significant fall. Due to less carrier recovery time then in the case of crystalline silicon-on-insulator waveguides the proposed device makes possible higher operation speeds. Apart from logic gates, high optical nonlinearity of por-Si is the base for developing optical memory and optical switching devices based on optical

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bistability. These systems have two stable transmission states, with the system exhibiting hysteresis cycle (Gibbs 1985). The bistable system could be realized due to nonlinear absorption or reflection in a Fabri–Pérot resonators (films or microcavities) (Fig. 11a, b). The former approach was realized in the works by Matsumoto et al. (1995). It is worth mentioning that the hysteresis in input power–output power was more pronounced when the excitation irradiation was chopped with low frequency (100 Hz), which indicates charge carrier accumulation and thermal effects. The latter approach was used in micro- (Takahashi et al. 2000) and mesopor-Si-based (Pham et al. 2011) microcavities. Nonlinear reflection results in shift of cavity frequency and variation of the transmission. Depending on history, different transmission values could be found at equal input intensity (Fig. 11c). The effects were detected both for cw radiation of 0.800–0.816 μm modulated at 100 Hz at intensity of 480 W/cm2 (Takahashi et al. 2000), and for nanosecond pulses 0.532 μm with intensities above 100 kW/cm2 (Pham et al. 2011); employed

Fig. 11 (a) Experimental setup used to measure optical bistability. The por-Si samples are placed on a rotation stage (R) and the angle is adjusted to couple the laser to the cavity modes. Neutral density filters are used to attenuate the pulses to the linear operating range of the two high speed detectors (S1 and S2). (b) Transmission and reflection spectrum of a typical porous silicon microcavity and corresponding simulation. (c) Optical hysteresis of porous silicon microcavities at different excited intensities. At low input intensity the transmitted curve shows a straight line which represent a lineartransmission, but at high intensities the input-output curves exhibit hysteresis like patterns in a clockwise direction (Pham et al. 2011) (Reprinted with permission from Pham A, Qiao H, Guan B, Gal M, Gooding JJ, Reece PJ (2011). J Appl Phys 109(9): 093113 @ 2011, AIP Publishing)

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Fig. 12 (a) Output power vs. input coupled power for the porous silicon waveguide. The open squares indicate the measured data, whereas the curves show theoretical fits obtained from numerical simulation. The dashed curve indicates the fit obtained by neglecting free carrier effects, whereas the solid curve includes both two-photon absorption and free-carrier absorption. (b) Experimentally measured output spectra for three different input intensities, for the porous silicon waveguide, showing blue-shift associated with free-carrier dispersion (Apiratikul et al. 2009) (Reprinted with permission from Apiratikul P, Rossi AM, Murphy THE (2009) Opt Exp, 17(5): 3396–3406 @ 2009, OSA)

intensities are below the damage threshold. Note that transmission of the microcavity decreases with time at the scale of pulse duration. Por-Si is a perspective material for fabrication of the waveguides for near IR spectral region (Apiratikul et al. 2009; Shokrollahi and Zare 2013; Suess et al. 2014; Simbula et al. 2016). Broad range of the possible por-Si effective refractive indices opens up new possibilities to tailor the waveguides, whereas well-developed surface increases their sensitivity to the molecules. The consideration of the short laser pulse propagation in the por-Si waveguide needs taking into account both practically instant nonlinear responses such as two-photon absorption and the nonlinear effects caused by laser-induced generation of the nonequilibrium carriers, which takes of order of picoseconds (cf. experimental data and results of simulation without and with FCA in Fig. 12a). Due to free-carrier dispersion the transmitting wave gets additional time-dependent phase shift, which is manifested as the output spectrum blue shift at higher input intensities (Apiratikul et al. 2009), the shift increases with the input intensity (Fig. 12b). Note that SOI waveguides do not demonstrate this effect. However, continuum generation in por-Si-based waveguides, which could be expected analogously to SOI (Kuyken et al. 2011), has not been yet reported. Biocompatibility and biodegradability of por-Si, its low toxicity, and welldeveloped surface make por-Si particles very promising for imaging and therapy. They can be functionalized by sensitizers such a ruthenium complexes of porphirines. Both PL signal and therapeutic action could be excited by TPA of NIR radiation, which allows deep penetration into biological tissues (Chaix et al. 2016; Knezevic et al. 2016).

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Thus, por-Si demonstrates extremely interesting and unusual nonlinear-optical properties. Its nonlinear susceptibility can be both much lower and much higher than one for c-Si strongly depending on pore and Si nanocrystal sizes. Many factors influence the nonlinear susceptibility value, including surface states, quantum-size effects, carrier generation, and enhanced photon lifetime in por-Si film. The efficiency of the nonlinear-optical processes in por-Si can be controlled by its optical anisotropy, filling the pores, formation of periodic structures, and waveguides. All these features make por-Si extremely promising material for frequency conversion, light-by-light control, and devices based on it, e.g., switches, nonlinear-optical sensors, and devices for nonlinear-optical theranostics.

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Color of Porous Silicon Leigh Canham

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mesoporous Silicon Layers, Multilayers, and Photonic Crystals . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mesoporous Silicon Membranes and Powders . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Black Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Color Matching . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The visual color of a material is often not important for many applications but can be crucial for those that involve consumer acceptance and branded products. Solid silicon is gray, but porous silicon can have varied colors depending on its physical form and pore contents. Silicon chip-based layers can exhibit vivid colors, tunable across the visible spectrum through their lowered refractive index and optical interference with the underlying bulk silicon. Highly columnar morphologies, referred to as “black silicon,” include highly porous forms. Even white silicon is possible via photonic crystals. Polydisperse mesoporous silicon microparticle powders are typically dark brown through light tan, depending on bandgap widening, particle size, and the level of oxidation, which is useful for matching skin tone in cosmetic products, but disadvantageous with various foodstuffs, beverages, and oral care products. The color of such powders can be better tuned chemically by the impregnation of specific food nutrients that themselves have vivid colors. Some such natural pigments can themselves benefit

L. Canham (*) School of Physics and Astronomy, University of Birmingham, Birmingham, Worcestershire, UK e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_27

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with improved fading resistance as a result of UV protection via oxidized porous silicon impregnation. Keywords

Black silicon · Color matching · Mesoporous silicon layers · Mesoporous silicon membranes

Introduction The color of the semiconductor in your computer chip or the color of your implanted biomaterial or pharmaceutical tablet is of little consequence. The color of your toothpaste, face cream, beverage, or food is another matter. Consumers are used to the manufactured brands of brilliant white toothpaste with blue, red, or green stripes. Brown is not a popular color for bathroom products (see the handbook review ▶ “Porous Silicon for Oral Hygiene and Cosmetics”). There are brown and even black popular foodstuffs – think of bread, peanuts, cereals, chocolate, coffee, and marmite (see handbook chapter ▶ “Porous Silicon and Functional Foods”). However, these are in the minority, and once again the consumer associates specific foods with specific colors. In this review we detail how the color of mesoporous silicon can be tuned, like many other properties (see handbook chapter ▶ “Tunable Properties of Porous Silicon”). This has been achieved by both control of the physical structure of silicon at the nanoscale and chemical means. The “physical color” of porous silicon films, membrane flakes, and photonic crystals is much more easily tuned than those of milled microparticle powders. The latter display various shades of brown, rather than the gray of solid silicon, and this has to date been an obstacle for applications in certain high-volume consumer products.

Mesoporous Silicon Layers, Multilayers, and Photonic Crystals The very first studies of mesoporous silicon noted the different colors of anodized and so-called stain films (Uhlir 1956; Turner 1958; Archer 1960). Uhlir referred to his surfaces as having a “matte black, brown or red deposit” (Uhlir 1956). Turner commented that “several orders of interference colors can be seen as the film thickens” (Turner 1958). The first use of such colored silicon in the late 1950s was in p-n junction delineation (Iles and Coppen 1958; Whoriskey 1958; Robbins 1962). Porous silicon, with its lower refractive index than solid silicon, induces optical interference effects as etched films on wafers. A colorimetric analysis for layer thicknesses below 500 nm, at quantified porosities, showed that interference color directly related to the optical thickness of anodized single-layer structures (Lazarouk et al. 1997). Figure 1 shows examples for stain-etched p + wafers. The visual color of a given layer can be further changed by plasmonic effects of deposited metal nanoparticles (Lublow et al. 2012) or through controlled oxidation of the silicon skeleton.

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Fig. 1 Varied “physical colors” from ultrathin single layers of stain-etched mesoporous silicon (LHS- A. Loni unpublished 2009) and anodized mesoporous silicon photonic crystals (RHS – Gooding Group Univ. New South Wales, Australia http://www.rsc.org/Publishing/ChemTech/Vol ume/ 2009/02/biosensors.asp)

Many natural organisms have evolved to optimize their visual appearance using layers of modulated refractive index – so-called structural color (Xu and Gao 2013). They can have not only very vivid colors but also change their color in response to external stimuli. Increasingly, we are learning to utilize such “biomimetic” designs in synthetic structures to achieve specific functionalities such as colorimetric sensors (Wang and Zhang 2013). The color palettes achieved via porous silicon photonic crystals are impressive and include white (see Fig. 1 and Mangaiyarkarasi et al. 2008). For detail on the optics, the reader is referred to the reviews of Wehrspoon and Schilling (2003), Sailor (2012), Pacholski (2013), and Agarwal (photonic crystal review in this handbook). Of particular interest from a sensing perspective is that the color of such structures can vary reversibly, or non-reversibly, with substances entering and leaving the mesopores, as first demonstrated by Arwin and co-workers (Bjorkland 1996). Figure 2 shows the reversible effect of a volatile solvent like ethanol entering the pores. In this regard, mesoporous silicon can truly be described as a “silicon chameleon,” not only for its tunable color of electroluminescence under bias (Canham 1993) but also its changing color upon pore filling (Fig. 2), analyte binding, or chemical reaction (Bonanno and DeLoiuse 2010). Multilayers of mesoporous silicon of modulated porosity, when detached from the wafer, have the typical appearance of metal-based “glitter” or even “gold leaf” (Fig. 3) and yet are pure silicon. Unlike conventional glitter, they can be loaded for sustained release of actives like fragrance, thereby having a dual cosmetic role (see handbook chapter ▶ “Porous Silicon for Oral Hygiene and Cosmetics”).

Mesoporous Silicon Membranes and Powders To achieve reasonable quantities (e.g., 100–1 kg) of porous silicon powder, thick (e.g., 100–200 μm) mesoporous membranes can be detached from large-diameter wafers using the electrochemical “lift-off” technique (see handbook chapters

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Fig. 2 Reversible color changes due to ethanol wetting (LHS) and evaporation from mesopores (RHS) Fig. 3 Golden silicon flakes. Mesoporous multilayer structures removed from the substrate after anodization

Fig. 4 Physical colors of dark gray solid silicon powder and brown porous silicon powders. The brown hue of mesoporous silicon is tunable via porosity, microparticle size, and oxide content (Loni A (2008) Unpublished data. Intrinsiq Materials Ltd.)

▶ “Porous Silicon Formation by Anodization” and ▶ “Porous Silicon Membranes”) and converted to powders via mechanical means (see handbook chapter ▶ “Milling of Porous Silicon Microparticles”). Both the membranes and powders are typically brown in color (see Fig. 4). For powders with irregular acicular-shaped microparticles, the color is now dominated by optical absorption and scattering, rather than interference. The brown hue gets lighter; the higher the porosity, the smaller the particle size and the higher the oxide content. Large batches of multicolor porous silicon powders with defined particle size distributions (e.g., d50 in the range of 1–25 μm) have not been achieved via “structural color.” Instead more progress has been made using pigment impregnation. Figure 5 shows examples of the three primary colors achieved using food-grade additives. Of particular interest are hydrophobic and light-sensitive nutrients such as lycopene and beta-carotene that have vivid colors. They can impart a desired hue to

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Fig. 5 Varied “chemical colors” from mesoporous silicon powders, oxidized (800 C 3 h air) and then impregnated with nutrients or food additives (Canham LT (2009) Unpublished data. Intrinsiq Materials Ltd. UK). The light brown color was tuned to red using carmine, yellow using curcumin, and blue using a commercial food dye formulation (E133, E122). Solvent loading was used with 1–2 wt% pigment

mesoporous structures (Canham et al. 2010), in return be UV protected (Canham and Aston 2012; Pavlikov et al. 2012), and potentially have their bioavailability improved as well when ingested (Canham 2007). Control over silicon particle shape, porosity, and polydispersity could provide structural control of color of powders in the future. Of relevance here are the so-called silicon colloids made by “bottom-up” routes. Porous silicon microspheres of 0.5–5.5 μm diameter scattered yellow, orange, and red colors when under white light illumination (Fenollosa et al. 2010).

Black Silicon Porous silicon films of one particular color have recently shown potential in a number of specific application areas. The so-called black silicon has been etched into a morphology that almost completely suppresses optical reflectivity over a very broad spectral range (Koynov et al. 2006). Its visual appearance however does not directly impact most of the uses currently under development. The first optically black silicon structures were probably made by anodization as early as the 1950–1960s. Koltun studied films generated at lower current densities than Uhlir and Turner (Uhlir 1956; Turner 1958; Koltun 1964). He described their persistent black color being due to a “high degree of dispersion” of silicon. Interestingly, he also recorded reflectivity from his “photocells,” but focused on the infrared rather than the visible range (Koltun 1964). Sometimes also referred to as silicon “nanograss” because of its columnar morphology, black silicon also became an unwanted by-product of reactive ion etching (Jansen et al. 1995) but has subsequently been deliberately realized using

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Fig. 6 Color matching: porous silicon and instant coffee for burst aroma release. The jar on the left contains a commercial instant coffee/mesoporous silicon blend after boiling water addition. The jar on the right shows the mesoporous silicon component trapped in the meniscus of boiling water

pulsed laser ablation and metal-assisted etching (Her et al. 1998; Koynov et al. 2006; Chen et al. 2011). It is now being widely investigated for its low reflectivity in solar cell applications (Ma et al. 2006; Branz et al. 2009; Yuan et al. 2009; Koynov et al. 2011; Oh et al. 2012): photodetectors (Su et al. 2013) and photoelectrodes (Ao et al. 2012). Other potential uses are in microsystems (Roumanie et al. 2008), sensing (Gervinskas et al. 2013), its tunable wetting (Dorrer and Ruhe 2007; Zhang et al. 2013), and its antibacterial properties (Ivanova et al. 2013).

Color Matching Manipulation of color in silicon-based devices and nanostructures can be for a myriad of technical reasons (Cao et al. 2010; Doan and Sailor 1992; Seo et al. 2011; Kuznetsov et al. 2012). Highlighted here are very different applications where the intrinsic color of porous silicon is important for consumer acceptance. The example chosen in Fig. 6 is from the beverage industry (see handbook chapter ▶ “Porous Silicon and Functional Foods”). Instant coffee is an example of a beverage, which in both dried powder and liquid form has the same color as mesoporous silicon powder. Six properties of mesoporous silicon are important here for it to be utilized: its ability to entrap coffee aroma under ambient storage and thermally release it into the headspace above boiling water, its ability to stay afloat on boiling water due to its hydrophobicity and low density, its taste and mouthfeel, its low oral toxicity, and, last but not least, its low cost and an appropriate color.

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References Ao X, Tong X, Kim DS, Zhang L, Knez M, Muller F, He S, Schmidt V (2012) Black silicon with controllable macropore array for enhanced photoelectrochemical performance. Appl Phys Lett 101:111–901 Archer RJ (1960) Stain films on silicon. J Phys Chem Solid 14:104–110 Bjorklund RB, Zangooie S, Arwin H (1996) Colour changes in thin porous silicon films caused by vapor exposure. Appl Phys Lett 69(20):3001 Bonanno LM, DeLoiuse LA (2010) Integration of a chemically responsive hydrogel into a porous silicon photonic sensor for visual colorimetric readout. Adv Funct Mater 20(4):573–578 Branz HM, Yost VE, Ward S, Jones KM, To B, Stradins P (2009) Nanostructured black silicon and the optical reflectance of graded-density surfaces. Appl Phys Lett 94:231121 Canham LT (1993) The silicon chameleon. Nature 365:695. doi:10.1038/365695a0 Canham LT (2007) Nanoscale semiconducting silicon as a nutritional food additive. Nanotechnology 18:185704, 6 pages Canham LT, Aston R (2012) Method of protecting skin from UV radiation using a dermatological composition having porous silicon. US Patent 8128912 B2 Canham LT, Loni A, Godfrey A (2010) Colouring techniques. International Patent WO/2010/ 038065 Cao L, Fan P, Barnard ES, Brown AM, Brongersma ML (2010) Tuning the colour of silicon nanostructures. Nano Lett 10(7):2649–2654 Chen T, Si J, Hou X, Kanehira S, Miura K, Hirao K (2011) Luminescence of black silicon fabricated by high-repetition rate femtosecond laser pulses. J Appl Phys 100:073–106 Doan V, Sailor MJ (1992) Luminescent color image generation on porous silicon. Science 256 (5065):1791–1792 Dorrer C, Ruhe J (2007) Wetting of silicon nanograss: from superhydrophilic to superhydrophobic surfaces. Adv Mater 20(1):159–163 Fenollosa R, Ramiro-Manzano F, Tymczenko M, Meseguer F (2010) Porous silicon microspheres: synthesis, characterization and application to photonic microcavities. J Mater Chem 20:5210–5214 Gervinskas G et al (2013) Surface-enhanced Raman scattering sensing on black silicon. Ann Phys 525(12):907–914 Her TH, Finlay RJ, Wu C, Deliwala S, Mazur E (1998) Microstructuring of silicon with femtosecond laser pulses. Appl Phys Lett 73:1673 Iles PA, Coppen PJ (1958) On the delineation of p-n junctions in silicon. J Appl Phys 29:1514 Ivanova EP et al (2013) Bactericidal activity of black silicon. Nat Commun 4:2838, 7 pages Jansen H, De Boer M, Legtenberg R, Elwenspoek M (1995) The black silicon method: a universal method for determining the parameter setting of a fluorine-based reactive ion etcher in deep silicon trench etching with profile control. J Micromech Microeng 5:115–120 Koltun MM (1964) Nature of film on surface of silicon photocell during anodic etching. Russ J Phys Chem 38(3):381–383 Koynov S, Brandt MS, Stutzmann M (2006) Black non-reflecting silicon surface for solar cells. Appl Phys Lett 88:203107 Koynov S, Brandt MS, Stutzmann M (2011) Black thin film silicon. J Appl Phys 110:043–537 Kuznetsov AI, Miroshnichenko AE, Fu YH, Zhang J-B, Luk’yanchuk B (2012) Magnetic light. Sci Rep 2:492 Lazarouk S, Jaguiro P, Katsouba S, Maiello G, La Monica S, Masini G, Proverbio E, Farrari A (1997) Visual determination of thickness and porosity of porous silicon layers. Thin Solid Films 297:97–101 Lublow M, Kubala S, Veyan J-F, Chabal YJ (2012) Colored porous silicon as support for Plasmonic nanoparticles. J Appl Phys 111:084–302 Ma LL, Zhou YC, Jiang N, Lu X, Shao J, Lu W, Ge J, Ding XM, Hou XY (2006) Wide-band “black silicon” based on porous silicon. Appl Phys Lett 88:171–907

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Mangaiyarkarasi D, Breese MBH, Ow YS (2008) Fabrication of three dimensional porous silicon distribution Bragg reflectors. Appl Phys Lett 93:221–905 Oh J, Yuan HC, Branz HM (2012) An 18.2 % efficient black silicon solar cell achieved through control of carrier recombination in nanostructures. Nat Nanotechnol 7:743–748 Pacholski C (2013) Photonic crystal sensors based on porous silicon. Sensors 13:4694–4713 Pavlikov AV, Lartsev AV, Gayduchenko IA, Timoshenko VY (2012) Optical properties of materials based on oxidized porous silicon and their applications for UV protection. Microelectron Eng 90:96–98 Robbins H (1962) Junction delineation in silicon. J Electrochem Soc 109(1):63–64 Roumanie M et al (2008) Enhancing surface activity in silicon microreactors: use of black silicon and alumina as catalyst supports for chemical and biological applications. Chem Eng J 135: S317–S326 Sailor MJ (2012) Chapter 5.3 Optical reflectance measurements. In: Porous silicon in practice. Wiley VCH, Weinheim Seo K, Wober M, Steinvurzel P, Schonbrun E, Dan Y, Ellenbogen T, Crozier KB (2011) Multicolored vertical silicon nanowires. Nano Lett 11:1851–1856 Su Y et al (2013) High responsivity MSM black silicon photodetector. Mater Sci Semicon Proc 16(3):619–624 Turner DR (1958) Electropolishing silicon in hydrofluoric acid solutions. J Electrochem Soc 105(7):402–408 Uhlir A (1956) Electrolytic shaping of germanium and silicon. Bell Sys Tech J 35:333–347 Wang H, Zhang KQ (2013) Photonic crystal structures with tunable structure color as colorimetric sensors. Sensors 13:4192–4213 Wehrspoon RB, Schilling J (2003) A model system for photonic crystals: macroporous silicon. Phys Stat Solidi A 197(3):673–687 Whoriskey PJ (1958) Two chemical stains for making p-n junctions in silicon. J Appl Phys 29:867 Xu J, Gao Z (2013) Biomimetic photonic structures with tunable structural colors. J Colloid Interface Sci 406:1–17 Yuan HC, Yost VE, Page MR, Stradins P, Meier DL, Branz HM (2009) Efficient black silicon solar cell with a density graded nanoporous surface: optical properties, performance limitations and design rules. Appl Phys Lett 95:123501, 3 pages Zhang T, Zhang P, Li S, Li W, Wu Z, Jiang Y (2013) Black silicon with self-cleaning surface prepared by wetting process. Nanoscale Res Lett 8:351, 5 pages

Electrical Transport in Porous Silicon Sanjay K. Ram

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Transport Characteristics and Mechanisms: General Overview . . . . . . . . . . . . . . . . . . . . . Contact Phenomena in Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characteristics of Current Versus Voltage (I–V) Behavior in Porous Silicon Diode . . . . . . . . . . Temperature-Dependent Conduction Behavior in Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Anisotropy in Electrical Transport of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conductivity Versus Porosity in Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effective Medium Theory Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Percolation Theory Approach . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Attempts on Classification of Electrical Properties of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

404 405 405 408 410 412 413 413 415 415 416 417

Abstract

The future development of porous silicon (PS)-based optoelectronic devices depends on a proper understanding of electrical transport properties of the PS material. Electrical transport in PS is influenced not only by each step of processing and fabrication methods but also by the properties of the initial base substrate. This chapter endeavors to chronologically document how the knowledge base on the nature of carrier transport in PS and the factors governing the electrical properties has evolved over the past years. The topics covered include the proposed electrical transport models including those based on effective

S. K. Ram (*) Department of Physics and Astronomy, Aarhus University, Aarhus C, Denmark Interdisciplinary Nanoscience Center – iNANO, Aarhus University, Aarhus C, Denmark e-mail: [email protected]; [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_28

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medium theories, studies on contacts, studies on physical factors influencing electrical transport, anisotropy in electrical transport, and attempts to classify the PS material. Keywords

Annealing · Anodization · Band transport · Coulomb blockade · DC conductivity · Effective medium approximation (EMA) · Anisotrophy · Fermi level · Heterojunction · Hopping · Ideality factor · Meyer-Neldel rule (MNR) · Percolation · Poole–Frenkel (PF) transport · Quantum confinement · Quantum dot (QD) · Quantum wells · Quantum wire (QW) · Resistivity · Space charge limited current (SCLC) · Thermionic emission · Trapping · Tunneling · Workfunction

Introduction An important attraction of porous silicon (PS) has been its customizable morphology which can be tailored to change its optoelectronic properties to suit the required application. In case of luminescence, an important property of PS, morphology can be modified to tune the intensity and the peak position of luminescence over a wide range of wavelengths (Marsh 2002). However, the versatile microstructural nature of porous silicon that imparts to it these exciting possibilities is also the main hindrance in the studies of its electrical properties. The inhomogeneous microstructure of PS impedes the comparability between studies of different laboratories (Foll et al. 2002; Bisi et al. 2000). Even within a sample of the material, the complex and inhomogeneous microstructure and crystallites (Islam et al. 2001, 2005) can result in lack of uniformity in the observed electrical properties (Dutta et al. 2002). In the past decades, electrical transport properties of PS have been found to be dependent not only on its microstructure (Kocka et al. 1996a) but also on several factors like the base c-Si selection (Zimin and Bragin 2004), anodization method (Dutta et al. 2002), metal contact formation (Simons et al. 1995; Martin-Palma et al. 2002), annealing (Zimin and Komarov 1998), electrical measurement method (Boarino et al. 2009), etc. In addition, the electrical properties are strongly influenced by external factors such as ambient atmosphere (Zhang et al. 1995) and residual electrolyte (Parkutik 1996). Thus, the understanding of the transport properties of carriers through such a disordered system is challenging. The early research on the electrical transport properties of porous silicon carried out in the 1980s revealed that the resistivity of the porous silicon layer was a few orders of magnitude higher than the original substrate (Beale et al. 1985). While quantum confinement model has been successfully used to explain the luminescence properties of PS, applying it to explain the transport properties of PS has been difficult and less unequivocal. In this chapter, we present an overview of the electrical properties of porous silicon based on the literature published in the last three decades.

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Electrical Transport Characteristics and Mechanisms: General Overview The earliest reports on the electrical properties of PS were about the high electrical resistivity in this material. Electrical resistivity in PS is five orders of magnitude higher than in intrinsic Si due to the depletion of free carriers. One reason behind the depletion is the widening of energy gap due to quantum confinement and decreased thermal generation of free carriers. The depletion of free carriers can also occur due to their trapping, which occurs during the preparation of PS either because the binding energy of dopant impurities are increased or because of the formation of surface states. Even after etching, the concentration of dopants remains unchanged, although the dopants are in neutral state (Bisi et al. 2000). The electrical conductivity of PS can be dependent on voltage and/or temperature. The commonly observed thermally activated temperature dependence of DC conductivity behavior in PS is expressed by a relation: σ ðT Þ ¼ σ 0 expðE a =k B T Þ,

(1)

where activation energy (Ea) and conductivity prefactor (σ 0) are material property used to explain transport mechanism. In most reports, the value of Ea has been found to be 0.5 eV. This value is half of the energy of the bandgap deduced by luminescence and also comparable to the Ea in intrinsic Si 0.5 eV, suggesting that mechanisms other than quantum confinement are in play as well. All these indicate the disordered nature of the PS skeleton and its local crystalline structure as they are expected to have strong geometrical effects on the conductivity (Bisi et al. 2000). Thus, there has been little consensus on any predominant transport mechanism in PS. This is evident from Table 1 which shows the reported studies on the electrical transport characteristics of PS and the different conduction mechanisms proposed by various workers to explain the observed transport behaviors. In the electrical measurements of PS samples, an important factor is the direction of the current flow, that is, whether the measurement is done in sandwich configuration or in coplanar configuration. Another important consideration is whether the PS material is freestanding or is still supported by the mother substrate.

Contact Phenomena in Porous Silicon The characteristics like linear (ohmic), symmetric (quasi-linear), or nonlinear (rectifying or Schottky types) behavior of I–V characteristics in PS have been reported depending on the type of metal contact (Simons et al. 1995), measuring (Diligenti et al. 1996), and device configuration (coplanar or sandwich) (Kanungo et al. 2010), whether the PS is freestanding or attached with base substrate, microstructure (Dutta et al. 2002), and thickness of PS layer (Ben-Chorin et al. 1994; Balagurov et al. 2000). Here the term microstructure includes the effect of all the processing methods

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Table 1 Summary of electrical transport models described for porous silicon Year of study 1993

1993

References Mares et al. (1993)

PS physical form

Koyama et al. (1993) Ben-Chorin et al. (1994) Ben-Chorin et al. (1995a) Lehmann et al. (1995) Schwarz et al. (1995)

Mesoporous film (n+) Mesoporous membrane

1996

Kocka et al. (1996a, b)

Nanoporous membrane

1996

Lee et al. (1996) Diligenti et al.(1996) Ray et al. (1998) Mathur et al. (1998)

Nanoporous membrane Mesoporous membrane

1994 1995 1995 1995

1996 1998 1998

1998 2000 2001

2002

PS device form AuCa/ PS/cSi/Al Al/PS/Al

Nanoporous Nanoporous

Macroporous

Al/PS/cSi/Al Al/PS/cSi/Al Al/PS/cSi/Al Al/PS coplanar (CP) Au/PS/ Au Al/PS (CP) 4 probe CP Al/PS/cSi/Al Al/PS/ cSi/Al

Hamilton et al. (1998) Balberg (2000) Mikrajuddin et al. (2001)

Axelrod et al. (2002) Dutta et al. (2002)

Nanoporous

Al/PS/cSi/Al Al/PS/cSi/Ag-Al

2004

Forsch et al. (2004)

Nanoporous membrane

2007

Islam et al. (2007)

Nanoporous

Al/PS (CP) and Al/PS/Al Al/PS/cSi/Al

2002

Electrical transport model Tunneling between thermally vibrating surface states (Berthelot-type conduction) Band transport Poole-Frenkel process Hopping at Fermi level Surface trap dominated Transport in nearly extended states and process similar to multiple trapping model Two transport channels: hopping at transport edge and thermionic emission across energy barriers Quantum confinement model Tunneling between the Si nanocrystals (Si-NCs) Thermionic emission across spatially varying bandgaps Tunneling of carriers to localized states near band edges, at high temperatures and variable range hopping near Ef at low temperature Coulomb blockade Two transport channel via nanocrystal network and purely amorphous phase Activated conduction in moderate temperature, at higher temperatures, conductivity obeys Vogel-TammannFulcher law Thermally activated hopping within a fractal network of nanocrystallites Generalized effective medium approximation (EMA) using porosity (Po) and uniformity of PS Poole-Frenkel transport

Carrier generation-recombination in depletion region formed on PS side (continued)

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Table 1 (continued) Year of study 2008

2009

References Bouaicha et al. (2008) Islam et al. (2009)

PS physical form

PS device form Al/PS/cSi/Al

Nanoporous

Al/PS (CP)

Electrical transport model EMA by considering Si nanoparticles with size distribution embedded in SiO2 and vacuum Mott hopping and Efros-Shklovskii hopping at low temperature

and the initial base substrate’s properties. An ohmic contact on PS like in any semiconductor material is crucial for the development of a device. Rectifying behavior can originate due to either unstable contact or work-function value of the metal chosen, or it can be due to both. For example, according to the work function of Al, it should provide ohmic contact to PS, but instead it shows a rectifying behavior. This is due to the existence of large density of surface states on unmodified PS which create barrier against the current flow. Therefore, methods or techniques like annealing, oxidation, derivatization by organic groups and polymer, nitridation, halogenations, and metal (like Cu, Ag, In, etc.)-induced modification of the porous silicon surface are used to stabilize PS by passivation of defect states in order to obtain a stable and reliable electrical contact to PS (Dutta et al. 2002; Zimin and Komarov 1998; Zimin and Bragin 1999). This topic is the focus of the dedicated chapter ▶ “Ohmic and Rectifying Contacts to Porous Silicon” elsewhere in this handbook. Transition from nonlinear (rectifying) to linear (ohmic) behavior of Al contacts on PS depending on postdeposition treatments has been reported by many workers (Dutta et al. 2002; Zimin and Bragin 1999). In some cases, the range of linear part extends only up to few volts, while a broad range of voltages have also been reported for linear part of I–V characteristics. Surface oxidation has been considered a very efficient way of passivating the defects and improving the stability of PS to a large extent. Although it helps in coplanar configuration to obtain ohmic contacts, in sandwich design the contacts may still show improved rectifying behavior because the Fermi level (Ef) gets pinned at the PS/c-Si interface where a large number of volume traps and interface states exist (Islam et al. 2007). Zimin et al. (1995) reported a coplanar Al ohmic contact to n-type PS having a contact resistivity of the order of 103 to 102 Ω cm for low porosity and low resistivity samples, but they observed a rectifying behavior for high resistive PS, both n- and p-type. MartinPalma et al. (1999) reported the same for the Al/PS/c-Si/Al sandwich structure, where rectifying behavior can be seen even after prolonged exposure to the atmosphere. Use of noble metals (Pd, Pt, and Ru) (Kanungo et al. 2009) or their ions are also reported to modify and stabilize the surface of PS in order to obtain reproducible metal contacts thereafter.

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Characteristics of Current Versus Voltage (I–V) Behavior in Porous Silicon Diode There are two types of diode structures in PS that exist in the literature, structure-1: metal1/PS/c-Si/metal2 and structure-2: metal1/PS/metal2, where the former is the most widely investigated device structure. The structure-1 leads to three different junction formations at the interfaces of (1) metal1/PS, (2) PS/c-Si, and (3) c-Si/ metal2. The I–V characteristics usually show asymmetric behavior due to current rectification. Typically the junction, c-Si/metal2 is made to be ohmic with negligible series resistance, so that it plays no role in I–V characteristics of the diode. The effect of metal1 in the junction of metal1/PS on I–V behavior is not very clear as discussed in section “Contact Phenomena in Porous Silicon.” There has been much contention in assigning the possible contribution of metal1/PS and PS/c-Si interfaces to the electrical properties of PS diode structures. In some cases (Pulsford et al. 1994; Giebel and Pavesi 1995), no dependence of the type of metal1 was found, suggesting that the interface of PS/c-Si instead of metal1/PS may be responsible for rectification (Pulsford et al. 1994). This led to a suggestion that the transport of carriers within the PS layer thickness and across the PS/c-Si heterojunction governs the device characteristics (Ben-Chorin et al. 1995b). The I–V characteristics in PS have been contradictorily attributed to both the bulk (PS) properties and junction properties in the reports (Ben-Chorin et al. 1994; Zimin 2000). Some authors have attributed the electrical behavior of thin PS structures to junctions under both reverse and forward bias condition (Zimin 2000; Futagi et al. 1993; Pavesi et al. 1994; Bhattacharya et al. 2000), while others have contributed the electrical behavior under reverse bias conditions to junctions and that under forward bias conditions to bulk conduction mechanisms (Ben-Chorin et al. 1995b). The electrical response of a c-Si/PS heterojunction is determined by the interplay of band edge offsets and density of defect states in the PS layer. Therefore, another possibility is that a large density of states associated with mid-gap defects present in PS pins the Ef and significant band bending and depletion occur only inside silicon rather than at the metal/PS interface (Ben-Chorin et al. 1995b; Pulsford et al. 1993). The thickness of PS layer is also one of the factors that influence the contact behavior. As thin PS layer show rectifying characteristics while thick PS layer show an almost symmetric one, Ben-Chorin (Ben-Chorin et al. 1994) proposed that the rectifying barrier is at the interface between PS and the doped substrate. It was also suggested by them (Ben-Chorin et al. 1994) that the usual diode structure formed by a metal contact, a PS layer, and a doped substrate can be visualized as a series combination of a voltage-dependent resistance and a rectifying barrier. Their study shows that the temperature- and voltage-dependent conductivity relationship follows Poole-Frenkel (PF)-type conduction, where transport mechanism in high fields involves field-enhanced thermal excitation from Coulombic traps: σ ðF, T Þ ¼ σ 0 e kT e Ea

pffiffiffiffiffiffiffiffiffiffiffiffiffi F =F 0 ðT Þ

(2)

Electrical Transport in Porous Silicon

where F is electric field and

409

pffiffiffiffiffiffiffiffiffiffiffiffiffi kT =q F 0 ðT Þ ¼ pqffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi =π ϵ 0 ϵ r

In this type of conduction, charge carriers are thermally excited from traps to some transport band, and barrier energy reduction due to the electric field enhances conduction. Charges will move along special paths with the lowest barrier energies, those which offer the lowest resistance (Ben-Chorin et al. 1994). Further, based on their observations that the conductivity scales correctly with the thickness, they ruled out the possibility of space charge-limited current (SCLC) type of conduction (Ben-Chorin et al. 1995a). In another report, a power law behavior, J / Vn/dm with exponent, n = 2 typical of SCLC was observed (Peng et al. 1996). They argued that the total current of the device is dominated by carrier transport in the high-resistivity PS layer which can be modeled as sandwiched between two conducting materials and the band bending at the metal1/PS interface is so small that the Schottky junction can be neglected. Superlinear behavior of current with rise in voltage (n < 2) (Koyama and Koshida 1993) was also observed by Kocka et al. (1996a, b; Fejfar et al. 1995); however, behavior was linear at lower voltages. In addition, Ea obtained from temperature dependence of transport was also found to be field dependent, where Ea decreases at high voltages as expected from the PF mechanism. However, the initial rise in Ea at low voltages (Kocka et al. 1996a) is explained either by assuming a parallel combination of two transport paths with different Ea (through and over the barrier between nanocrystals) of which one is voltage dependent (e.g., PF-type) or by assuming a series combination of Schottky contact and of SCLC controlled by traps inside the nanocrystals. The contacts dominate at low V and the SCLC at high V. Within this model the Ea represents the thermal generation of carriers from the Fermi level to the transport paths. The influence of temperature on the I–V characteristics of PS/c-Si structures was reported by Theodoropoulou et al. (2004), which also elucidated the mechanisms dominating under different bias conditions. According to this study, for the reverse bias condition, the ohmic bulk resistance dominates at first, and junctional resistance comes into play at a later time. On the other hand, under forward bias conditions, ohmic bulk conduction dominates, but with time gives way to PF-type conduction in the bulk. The time at which the change of mechanisms occurs is a function of temperature, with the change occurring later in lower temperatures. Based on the analyses of reverse I–V characteristics, Islam et al. (2007) proposed that the characteristics of PS/c-Si heterojunctions are found to behave like the Schottky junctions where transport is governed by carrier generation-recombination in the depletion region formed on the PS side. Fermi level of c-Si gets pinned to the defect levels at the interface resulting in ln(I ) / V1/2. In addition, there are several reports which attributed observed I–V behavior of metal1/PS diodes to the formation of a Schottky barrier between PS and the metal1 (Canham 1997). In a reported case (Ray et al. 1998) of low value of ideality factor, n  1 with low series resistance in the low voltage range, it was proposed that PS/cSi junction characteristics are controlled by carrier diffusion in the PS and the

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observed rectification of electron current is due to the barrier between c-Si and PS. However, in most of the cases, high values of ideality factor, n (>5) with either high (Ben-Chorin et al. 1995b; Futagi et al. 1993; Pulsford et al. 1993) or low values of series resistance due to PS layer (Pulsford et al. 1993), suggest most of the applied voltage does not drop on the barrier, but rather on the PS layer, which impedes a determination of reliable junction parameters in PS/c-Si heterojunctions using the conventional analysis of forward I–V characteristics (Koshida and Koyama 1992; Pulsford et al. 1993). According to the report (Koshida and Koyama 1992), tunneling of carriers dominates at high voltages.

Temperature-Dependent Conduction Behavior in Porous Silicon Most of the observations of temperature-dependent electrical conduction in PS indicate that the transport of electrons is thermally activated, with Ea in the range of 0.3–0.7 eV (Koyama and Koshida 1993; Ben-Chorin et al. 1994; Kocka et al. 1996b; Lee et al. 1996; Fejfar et al. 1995; Lubianiker et al. 1996; Lubianiker and Balberg 1997). The measured activation energies can be related to activation of carrier over mobility edges or to typical energy barriers for carrier hopping. In the former, the activation energy indicates the energy difference between the Fermi energy and the mobility edge. In the latter, the activation energy is related to a typical barrier height separating neighboring localized states (Bisi et al. 2000). In addition, disorder-induced localization of free carrier affects the free carrier motion in a way similar to amorphous silicon. However, in many cases, it has also been observed that there is a distinguishable change in the electrical transport behavior for high and low temperature regions usually at some critical temperature (Islam et al. 2009; Zimin 2006; Mikrajuddin and Shi 2000). The Ea is found to have relatively high value in the relatively high temperature region to a low value in the low temperature region (Ben-Chorin et al. 1995a; Islam et al. 2009; Fejfar et al. 1995; Lubianiker and Balberg 1997). However, in some cases, the conductivity is almost temperature independent below 200 K (Koyama and Koshida 1993; Ben-Chorin et al. 1995a). The critical temperature around which the electrical transport behavior changes is found to depend on the size of Si nanocrystals, porosity, their size distribution (Zimin 2006), and their microstructures (Zimin 2006). According to Ben-Chorin et al. (1994), PS is like a disordered assembly of three dimensional quantum wells with large bandgaps of the nanocrystallites (NCs). The transport in PS mainly involves dangling bonds (surface states) near Ef and hopping of carriers takes place in these surface states in the entire temperature range. The thermally generated carriers at the mobility edge do not take any part in the transport, making the bending of bands at the metal1/PS interface irrelevant. On the other hand, Kocka et al. (1996b) assumed that the transport in PS takes two paths – one through the surface grain barrier and the other over the surface grain barrier. This mechanism is field dependent in accordance with PF-type effect.

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In 2000, Balberg (2000) classified the electrical transport in PS by reanalyzing experimental data collected from various published papers about the temperature dependence of DC dark conductivity, according to Meyer-Neldel rule (MNR) lnðσ 0 Þ ¼ BMN þ E a =EMN

(3)

where BMN and EMN are MNR parameters. The relationship between conductivity prefactor σ 0 and activation energy Ea as per Eq. 1 of various PS samples shows two well-separated straight lines suggesting two different mechanisms. One of the lines is similar to amorphous Si, and most of the PS data falling on this line belong to data of high temperature thermally activated region, suggesting extended state transport. The other line, which accumulates most of the data from low temperature regions, represents intercrystallite hopping-tunneling conduction. It must be noted that most measurements in the group similar to a-Si behavior were taken from nanosize porous layers, while the second group obeying the MNR in low temperature regions were low porosity samples. This means two different conduction mechanisms take place in different “parts” of the samples. The amount each of them contributes to the conductivity depends on the sample and the measurement temperature. Nevertheless, most of the currently available models suggest that the temperature dependence of electrical conductivity obeys the Arrhenius relationship with a single activation energy for the entire range of temperatures (Ben-Chorin et al. 1994; Pulsford et al. 1994; Fejfar et al. 1995; Lubianiker and Balberg 1997; Diligenti et al. 1996). However, according to Mikrajuddin et al. (2001), Arrhenius-type activated conduction behavior may be exhibited in moderate temperature region if both continuous networks of blocked and unblocked sites appear in a PS layer. At higher temperatures, continuous networks of unblocked sites and blocked sites occupying discrete positions can be found in PS site, and conductivity obeys Vogel-Tammann-Fulcher (VTF) law, which can be expressed as σ ¼ σ 0 T 1 exp½B=ðT  T 0 Þ,

(4)

where σ 0, B, and T0 are constants. Mikrajuddin et al. (2001) derived the VTF behavior by using mean-field approximation of Ising model and used the model to fit published experimental data to validate it. However, at low temperatures carrier conduction is often found to be assisted by variable range hopping (VRH): lnðσ Þ / ðT x =T Þm

(5)

where m gives the information about type of carrier conduction and depends on dimensionality (mainly arising from crystallite sizes) and the temperature region. Tx is a constant for the material and often “x” is replaced with the name of transport mechanisms. So far Mott hopping transport where a T 0.25 (m = 0.25) dependence is followed at low temperatures was considered for most of the varieties of PS: nanoporous (Islam

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et al. 2009), mesoporous (Zimin 2006), and macroporous (Mathur et al. 1998; Ben-Chorin et al. 1995a). Berthelot-type conduction (m = 1) was also reported and attributed to the fractal structure of PS (Mares et al. 1993; Mehra et al. 1998), and variable range hopping has been observed in porous-amorphous Si of different compositions (Yakimov et al. 1995, 1996). Islam et al. (2009) critically analyzed their temperature-dependent conductivity data over a wide range of temperatures for the applicability of various transport mechanisms. Their findings suggest extended state conduction for T > 300 K, Berthelot-type conduction when 180 < T < 280 K, Mott hopping in the range 140 < T < 180 K, and Efros-Shklovskii hopping for T < 120 K. A clear cross over from Mott to Efros-Shklovskii VRH transport is observed at low temperatures. It is interesting to note that Berthelot-type conduction in Islam et al. (2009) was also observed in the similar range of temperature (190–270 K) as in Mares et al. (1993). Similarly, the temperature range of Mott hopping transport observed in various types of PS is 140–180 K for nanoporous (Islam et al. 2009), 110–200 K for mesoporous (Zimin 2006), and 100–150 K for macroporous (Mathur et al. 1998).

Anisotropy in Electrical Transport of Porous Silicon The cubic lattice structure of c-Si makes it an isotropic optical medium. However, nanostructuring of c-Si by porosification of low-symmetry Si surfaces or the formation of micrometer-sized Si periodic structures converts the resulting c-Si into a strongly birefringent material (see handbook chapter ▶ “Optical Birefringence of Porous Silicon”). Anisotropy in electrical properties of PS was first demonstrated by Forsh et al. in a mesoporous system obtained by (110) c-Si (Forsh et al. 2004). They observed substantial decrease in the conductivity measured in parallel to the surface, i.e., (001) crystallographic direction (σ ||), compared to the conductivity in sandwiched configuration, (110) crystallographic direction (σ ⊥). The conductivity in both directions follows exponential dependence on V1/2 and σ ⊥ > > σ|| at any voltage. PF mechanism was used to explain the dependence of the conductivity on the electric field in the PS. According to Forsh et al. (Forsh et al. 2004, 2005), an increase in thermal emission of carriers across the potential barriers at the boundaries of NCs can be due to the electric fieldinduced enhancement of thermal ionization of impurity atoms and reduction in fluctuations of the potential profile (barriers at boundaries of NCs). Previously, the PF mechanism of conduction was observed in sandwiched configuration PS device prepared from (100)-oriented p-type c-Si wafers (Ben-Chorin et al. 1994). The temperature-dependent conduction shows the widening of the gap between the conductivities in both directions at lower temperatures, which means the Ea in (110) direction (Ea)⊥ is smaller than Ea in (001) direction (Ea)||. Apparently, the material has a certain distribution of potential barriers by height. As the length of the percolation path (constituted by Si-NCs) in the perpendicular (110) direction is shorter than that in the parallel (001) direction owing to the shape anisotropy of NCs, the average height of potential barriers in the perpendicular direction will also be lower than that in the parallel direction. This will lead to higher values of σ ⊥ and lower values of its (Ea)⊥ compared to σ ||.

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Earlier work on both freestanding nanosized PS and mesoporous anodized from (100) c-Si show almost isotropic behavior (Kocka et al. 1996a). It was argued that the transport is controlled by the more or less homogeneous (isotropic) “tissue” part of PS, in which c-Si “islands” are embedded. However, in 2006 Borini et al. demonstrated anisotropic behavior in the conductivity of (100) mesoporous by measuring temperature-dependent conductivity of their sample using two different electrode configurations (Boarino et al. 2009; Borini et al. 2006). The authors observed that the electronic transport parallel to the sample surface (σ ||) is strongly inhibited at room temperature but not along the perpendicular direction (σ ⊥). This behavior was well correlated with the typical microstructure of the mesoporous where, due to the presence of branched columnar morphology, the σ || pathways are poorly interconnected, with several bottlenecks in which potential barriers are built up. Thus, the transport is strongly inhibited in the longitudinal (parallel to the sample surface direction), while in the transverse direction (perpendicular to the sample surface) the bottlenecks can be easily bypassed following the alternative pathways available. It was also shown (Borini et al. 2006) that such electrical anisotropy can be reversibly removed by heating the samples (increasing temperature from 20  C to 100  C) when σ || increases almost six orders of magnitude equaling σ ⊥. The rise of temperature allows the charge carriers to overcome the nanoconstrictions (Coulomb blockade due to charges trapped in the nanoconstrictions), opening the longitudinal percolative pathways. The increase in temperature can remove the Coulomb blockade of a fraction of NCs, until the percolation threshold is reached and exceeded.

Conductivity Versus Porosity in Porous Silicon Effective Medium Theory Approach Among many other microstructural parameters, porosity is one such physical parameter, which is generally used to describe the degree of porous nature of a PS layer. Porosity has been well researched with the fabrication methods and environment. Therefore, if this physical parameter could be correlated with electrical conductivity of the PS using any analytical way, it could serve an important role in tailoring the microstructure to obtain desired device properties. However, not much work has been done to explore this correlation. Effective medium approximation (EMA) as proposed by Bruggeman was used to some extent to determine a correlation between the effective conductivity of the PS layer and porosity of the layer: n X i

vi

σ i  σ eff ¼0 σ i þ 2σ eff

(6)

According to Dutta et al., up to certain low values of porosity, this theory worked well but failed to explain the effective conductivities of mid to highly porous layer

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(Dutta et al. 2002). The major challenge in the pore shape is the possibilities of different pore branching formations during PS growth (Saha et al. 1998). PS layers with “identical porosity” might have different surface-to-volume ratios, leading to different effective conductivities. They followed generalized EMA (GEMA), which accounts for the general form of the shape of the inclusions, where they included the extent of pore branching by “uniformity factor” (Saha et al. 1998): n X vi i

1=t

1=t

σ i  σ eff ¼0 φp 1=t 1=t σi þ 2σ eff 1  φp

(7)

where φp is the percolation volume fraction and t is a nonlinearity correction factor. So for spherical inclusions with φp = 2/3 and t = 1, the GEMA reduces to Bruggeman’s EMA. The theoretically calculated values match well with the experimental values up to the porosity range 70%. But beyond this range of porosity, the calculated value underestimates the effective conductivity. Similar problem of mismatch between calculated and experimental results of effective conductivity was also observed by Bouaicha et al. in the porosity range exceeding 65% (Bouaicha et al. 2006): vox

σ QD=QW  σ eff σ ox  σ eff σ v  σ eff þ vv þ vQD=QW ¼0 σ ox þ 2σ eff σ v þ 2σ eff σ QD=QW þ 2σ eff

(8)

where ΔE

σ QD=QW ¼ σ Si e4kT

(9)

They assumed in their theoretical calculation that the nanoporous silicon is formed by three phases: vacuum, oxide, and c-Si nanocrystallites (quantum dots (QD) for nanoporous or quantum wire (QW) for mesoporous structure) having the same mean-size dimension. The contribution of the latter phase in the total electrical conductivity was developed analytically by using the quantum confinement theory. This assumption worked well when the porosity was within 30–65%, and beyond that theoretical values were too low compared to the experimental ones. However, for large porosities (greater than 65%), where the PS structure exhibit visible luminescence, they could successfully obtain a perfect agreement between the theory and the experiment for all porosities when they considered that the base medium is vacuum in which silicon crystallites are incorporated (Khardani et al. 2006). This means that for the case of high porosities, the role of porosity is substituted by the quantum dot volume fraction in the fitting procedure. They successfully extended this work to obtain a good correlation between effective conductivity of mesoporous Si material and their corresponding porosity. In 2008, however, they further modified their work by considering the Si-NCs as being formed by multiple-sized crystalline dots (John and Singh 1994) embedded in silicon dioxide and vacuum (Bouaicha et al. 2008):

Electrical Transport in Porous Silicon

vox

N X σ ox  σ eff σ v  σ eff σ QDi  σ eff þ vv þ vQDi ¼0 σ ox þ 2σ eff σ v þ 2σ eff σ QDi þ 2σ eff i¼1

415

(10)

As a result, they obtained a good agreement between theory and experiment for all porosities. In this case (Eq. 9), all values of ΔE are considered including those < ΔE0. This avoids the tendency of the medium to be an insulator for higher porosities unlike what happens when the PS medium is considered to have three phases with single mean-sized QD.

Percolation Theory Approach A different approach to correlate porous silicon conductivity with material porosity was described in Aroutiounian and Ghulinyan (2003). In this work, the conductivity was shown to be mainly crystalline for porosities much lower than the percolation threshold at 57%, while a fractal behavior was observed at porosities near percolation threshold. For higher values of porosities, the conductivity was described as a quasi-one-dimensional hopping. The report concluded that in PS with increasing porosity, at lower temperatures, the dimension of the channels of electrical current flow decrease from 3 to 1, as described by the Mott law for amorphous semiconductors. However, the model results described in this work show some deviation from the experimental results. In spite of workers having presented models to fit their experimental data for a range of porosity, none of the works have attempted to fit the data of others with their models. If these models were tested to fit a wider range of published data, one could hope to find a more comprehensive model that could find wider application to make porosity a useful parameter in predicting the electrical behavior of PS.

Attempts on Classification of Electrical Properties of Porous Silicon The study of a heterogeneous material as PS could benefit greatly from a classification system that would allow a more systematic understanding and correlation of its properties. A wide variety of PS has been classified into some broad microstructural groups based on porosity. However, the same agreement has not been reached in correlating electrical properties of these PS groups to parameters related to porosity of the material. This is because electrical current which flows through the Si network depends largely on the size of Si structure and its surroundings and is not directly linked to pore size. In 1997 a comprehensive review of the properties of porous silicon was published by Canham that included a classification proposed by Ben-Chorin (Canham 1997). Ben-Chorin classified PS into two broad and distinct classes, “low porosity” and “nanosized porous Si” material, and explained the plausible electrical transport

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behavior of these two groups. The “low porosity” material is prepared from highly doped c-Si wafers (resistivity < 1 Ωcm), and quantum confinement does not play any role in transport. The nanosized PS is prepared from low-doped c-Si wafers and mostly under illumination. Crystallite size in such material is usually 100) is not trivial, and so a continuous homogeneous filling of such pores has not been achieved yet (Dolgiy et al. 2012). In the case of electrodeposition, an aqueous metal-salt solution is used as electrolyte. In Table 1 a summary

Table 1 Various electrolyte compositions used for the deposition of ferromagnetic metals Metal Ni

Electrolyte “All chloride” NiCl2

Ni

“Watts”

Co

“Sulfate” CoSO4

NiCo Fe

Watts + CoSO4 “Sulfate” FeSO4

Ni-Fe (Aravamudhan et al. 2007)

NiSO4, NiCl2, FeSO4

Co, Fe, Cu (Fortas et al. 2015)

CoSO4, FeSO4, CuSO4

Fe (Bardet et al. 2017)

FeSO4

Composition 170 g/l NiCl2 40 g/l H3BO3 45 g/l NiCl2 300 g/l NiSO4 45 g/l H3BO3 120 g/l CoSO4 30 g/l H3BO3 2:1 16 g/l FeSO4 10 g/l H2SO4 200 g/l NiSO4 8 g/l FeSO4 5 g/l NiCl2 25 g/l H3BO3 3 g/l saccharin 0.2 M CoSO4 0.02 M FeSO4 0.01 M CuSO4 0.4 M H3BO3 0.01 M saccharine 0.081 M C2H3O2Na 0.1 M FeSO4, ALS, saccharine, acetic acid

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of employed electrolytes can be found. For a review dedicated to this impregnation technique, see the chapter in this handbook ▶ “Porous Silicon and Electrochemical Deposition.” During the process two kinds of metal deposition are occurring. One possible reaction is electroless metal deposition: 2M2þ þ Si þ H2 O ! 2M þ SiO2 þ 4Hþ Furthermore electrodeposition takes place under cathodic conditions: M2þ þ 2e ! M Gaseous hydrogen is formed, if the current density exceeds a certain value (Bandarenka et al. 2012) which can influence and hamper the precipitation of the metal structures. 2Hþ þ 2e ! H2 In general the deposition of metals into porous silicon is a cathodic process and reduces the metal-salt ions to metal (e.g., Ni2+ + 2e = Ni). Imperfections and variations of the pore shape such as dendritic branches can also lead to inhomogeneities of the metal deposits, and thus a homogeneous filling of the pores is extremely difficult to obtain. In the case of high-aspect-ratio pores, it is important to ensure that the exchange of electrolyte is sufficient along the entire pore length. If it is insufficient, pores can be blocked, and a continuous growth of a metal wire along the whole length is inhibited. The degree of pore filling depends on the applied current density as well as on the pulse duration of the current (Granitzer et al. 2009). By choosing the deposition parameters in an adequate way, the geometry and spatial distribution of the ferromagnetic deposits are tunable, and thus tailoring of the magnetic properties of the composite is possible. The metal structures can be deposited in a broad size range (spheres according to the pore diameter, ellipsoids with a long axis of 100–500 nm and needles up to a few micrometers in length). In Fig. 2 an overview of various Ni fillings within porous silicon can be seen. A further approach to fabricate a Ni/porous silicon composites is electroless deposition of Ni into luminescent porous silicon powder (Nakamura and Adachi 2012). The porous silicon has been immersed in an aqueous NiCl2 solution at 80  C, whereas the Ni has been deposited on the surface of the porous silicon particles (Nakamura and Adachi 2012). Not only electroless deposition or electrodeposition of magnetic materials into the pores of porous silicon has been used but also the growth of magnetic nanowires by the solgel method has recently been performed (Zheng et al. 2016). The resulting nanowires offer a diameter of about 200 nm which is smaller than the pore diameter (Zheng et al. 2016).

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Fig. 2 Scanning electron micrographs (BSE) showing different arrangements of deposited Ni nanostructures within the pores of the porous silicon (Granitzer et al. 2008b). The pore diameter in all templates is around 60 nm, and the length is around 30 μm. First row: the spatial distribution of the deposited metal varies between two thirds to one third of the depth of the porous layer. Second row: the porous layers are filled between surface and bottom of the pores, but the shape of the precipitated Ni structures differs (from left to right – wires, ellipsoids, particles). Third row is zoomed areas of row two

Magnetic Properties of the Composite Characteristics of ferromagnetism are a spontaneous magnetization and the occurrence of a hysteresis (Bozorth 1993). The mechanisms responsible for the formation of magnetic domains are exchange interaction and anisotropy effects (Bertotti 1998). If the size of the ferromagnetic substance is reduced to below a certain critical radius qffiffiffiffiffiffiffiffi r d ¼ 36 μKM1 2 (K1 . . . anisotropy constant, MS . . . saturation magnetization), the 0

S

domain structure is reduced to one single domain. Such nanosized structures and especially the arrangement of magnetic nanostructures on surfaces or in three dimensions give rise to novel properties which depend not only on the geometry of the structures but also on the interactions between them. The magnetic characterization of Ni deposited on luminescent porous silicon powder offers a superparamagnetic behavior which indicates a particle size in the

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order of nanometers (Nakamura and Adachi 2012). Also the luminescence properties are influenced by the Ni deposition. A decrease of the photoluminescence intensity could be caused by an increase of surface defects due to desorption of hydrogen atoms and the formation of a low-quality oxide layer during the Ni deposition (Nakamura and Adachi 2012). Employing templates with well-separated pores for the incorporation of magnetic nanostructures improves the coercivity and squareness of a hysteresis compared to magnetic thin films, and the magnetic easy axis is turned from the in-plane to the outof-plane direction. The deposition of ferromagnetic thin films on porous silicon results in an in-plane magnetization (Dai et al. 2007). There are three possible parameters of the samples which can be modified to determine the magnetic behavior, namely, the kind of metal deposited within the pores, the morphology of the porous template, and the electrochemical conditions which can be tuned during the deposition procedure resulting in different geometries of the deposits. All these features influence the magnetic properties, whereas the morphology of the porous silicon template mainly influences the dipolar coupling between adjacent pores and thus the magnetic anisotropy between easy- and hard-axis magnetization (Granitzer et al. 2012b). With increasing distance between the pores, the magnetic interaction between metal structures of adjacent pores decreases. The magnetostatic energy Em 2

p of a couple of two parallel dipoles of length l at a distance r is given by:Em ¼ 2πμ 0   1 1 pffiffiffiffiffiffiffiffi , where p is the pole strength ( p =  μ0  π  d2  m/4) (Samwel r 2 2 r þl

et al. 1992). A further reason for enhanced magnetic anisotropy is the elongated shape of the deposits and pore walls with a minimum of side branches which strongly influence the stray fields (Bryan et al. 2012). In the case of nanowire arrays, the magnetic anisotropy is mainly composed of shape anisotropy and magnetocrystalline anisotropy, whereas in the case of Ni wires, the shape anisotropy is the dominating factor (Vega et al. 2011). Preliminary investigations showed that the Ni wires embedded in porous silicon are polycrystalline, and thus the magnetocrystalline anisotropy is negligible. Different loading of the pores can be achieved by modifying the deposition parameters (Rumpf et al. 2010) leading to nanostructures of distinct geometry and in various spatial distributions along the pores (Rumpf et al. 2008). Figure 3 shows three magnetization curves of porous silicon samples with equal morphology but different geometry of the Ni deposits, varying between spherical particles (~60 nm), ellipsoids (~300 nm), and wires (~1 μm). The coercivity varies between 280 Oe (wires) and 500 Oe (spherical particles) and the squareness (MR/MS) between 0.2 (wires) and 0.52 (particles), MR being the magnetic remanence and MS the saturation magnetization. Magnetic remanence, especially the squareness, provides information about coupling mechanisms between metal nanostructures (Stoner and Wohlfarth 1948). Ferromagnetic structures with a random distribution of orientations offer a squareness of 0.5. In comparison structures with the easy axis aligned parallel to the applied

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Fig. 3 Hysteresis loops of Ni particles ( full line), Ni ellipsoides (dotted line), and Ni wires (dashed line) deposited within the pores of porous silicon, whereas the morphology has been the same. The coercivity and the remanence decrease with increasing elongation of the metal nanostructures. The magnetic field has been applied perpendicular to the sample surface

Table 2 Variation of the coercivity and squareness with modification of the pore diameter Pore diameter PS 60 nm (Vega et al. 2011) PS 35 nm (Granitzer et al. 2012b) AAO 80 nm (Vazquez et al. 2004) AAO 30 nm (Vazquez et al. 2004)

HC,II (Oe) 280 660 360 600

HC,⊥ (Oe) 180 190 250 260

(MR/MS)II 0.48 0.89 0.35 0.70

(MR/MS)⊥ 1.5 3.47 1.44 2.3

PS porous silicon, AAO anodic aluminum oxide

magnetic field exhibit a squareness of 1, whereas with the easy axis perpendicular to the magnetic field a value of 0 (Coey 2009). A decrease of the squareness is caused by a diminution of the magnetic remanence. The smaller values determined from specimens with embedded Ni wires within porous silicon compared to the ones with Ni particles arise from demagnetizing effects. If the morphology of the template is modified and the geometry of the deposits is equivalent (wires), the magnetic characteristics also change as summarized in Table 2. In all samples the shape anisotropy is the dominating factor because of the high aspect ratio. The differences in the magnetic behavior are mainly due to magnetic interactions and the thickness of the Ni wires.

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Comparing the magnetization of porous silicon/metal nanocomposites with oxide heterostructures consisting of CoFe2O4 nanopillars in a BiFeO3 matrix (Chen et al. 2013), the magnetic moment is about one order of magnitude higher in the case of the porous silicon composite. Other systems with 2D arrangements of magnetic nanostructures (Enders et al. 2010) need big areas or a magnetic material with high magnetization. One of the characteristics of the porous silicon system is the 3D arrangement of the ferromagnetic nanostructures embedded within high-aspect-ratio pores which yields to a high magnetic moment. There is also a report on Ni deposition within porous silicon which describes the formation of Ni silicide during the electrodeposition process (Dolgiy et al. 2013) which reduces the magnetic moment of the specimen. A hybrid material comprised of porous silicon and ferromagnetic clusters of mainly insulating compounds of cobalt and boron shows a single-domain behavior exhibiting a blocking temperature of about 470 K (Ryzhov et al. 2014). Combining two different ferromagnetic materials, e.g., Ni and Co, within the pores of porous silicon results either in the formation of an alloy or in the formation of individual nanostructures of these two materials which can interact by exchange coupling (Rumpf et al. 2016).

Conclusion Intrinsic ferromagnetism of nanostructures silicon is very weak, but the nanocomposites consisting of deposited magnetic nanostructures offer a strong ferromagnetic behavior. Porous silicon with its tunable morphology is an adequate template material for metal deposition to gain 3D arrays of deposited nanostructures. In the case of ferromagnetic metal precipitation, the magnetic properties of the nanocomposite can be tailored on the one hand by the deposition procedure and on the other hand by the modification of the template morphology. The obtained system exhibits a quasi-“ferromagnetic semiconductor” material which offers its ferromagnetic properties at room temperature. As the silicon substrate is integrable in today’s microtechnology, the system is promising for various applications in sensor technology, magneto-optics, and also the emerging field of spintronics.

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Granitzer P, Rumpf K, Pölt P, Simic S, Krenn H (2008b) Three-dimensional quasi-regular arrays of Ni nanostructures grown within the pores of a porous silicon layer – magnetic characteristics. Phys Status Solidi C 5:3580 Granitzer P, Rumpf K, Poelt P, Albu M, Chernev B (2009) The interior interfaces of a semiconductor/metal nanocomposite and their influence on its physical properties. Phys Status Solidi (c) 6:2222 Granitzer P, Rumpf K, Ohta T, Koshida N, Poelt P, Reissner M (2012a) Porous silicon/Ni composites of high coercivity due to magnetic field-assisted etching. Nanoscale Res Lett 7:384 Granitzer P, Rumpf K, Ohta T, Koshida N, Reissner M, Poelt P (2012b) Enhanced magnetic anisotropy of Ni nanowire arrays fabricated on nano-structured silicon templates. Appl Phys Lett 101:033110 Gusev SA, Korotkova NA, Rozenstein DB, Fraerman AA (1994) Ferromagnetic filaments fabricated in porous Si matrix. J Appl Phys 76:6671 Herino R (1997) Impregnation of porous silicon. In: Canham LT (ed) Properties of porous silicon. INSPEC, London Jansen R (2012) Silicon spintronics. Nat Mater 11:400 Jansen R, Dash SP, Sharma S, Bin BC (2012) Silicon spintronics with ferromagnetic tunnel devices. Semicond Sci Technol 27:83001 Jeske M, Schultze JW, Thönissen M, Münder H (1995) Electrodeposition of metals into porous silicon. Thin Solid Films 255:63 Koda R, Fukami K, Sakka T, Ogata YH (2012) Electrodeposition of platinum and silver into chemically-modified microporous silicon electrodes. NRL 7:330 Kopnov G, Vager Z, Naaman R (2007) New magnetic properties of silicon/silicon oxide interfaces. Adv Mater 19:925 Koshida N, Koyama H (1992) Visible electroluminescence from porous silicon. Appl Phys Lett 60:347 Laiho R, Lähderanta E, Vlasenko L, Vlasenko M, Afanasiev M (1993) Magnetic properties of lightemitting porous silicon. JOL 57:197 Lehmann V, Grüning U (1997) The limits of macropore array fabrication. Thin Solid Films 297:13 Lehmann V, Stengl R, Luigart A (2000) On the morphology and the electrochemical formation mechanism of mesoporous silicon. Mater Sci Eng B 69–70:11 Nakamura T, Adachi S (2012) Properties of magnetic nickel/porous-silicon composite powders. AIP Advances 2:032167 Ogata YH, Kobayashi K, Motoyama M (2006) Electrochemical metal deposition on silicon. Curr Opin Solid State Mater Sci 10:163 Rumpf K, Granitzer P, Krenn H (2008) Porous silicon/metal hybrid system with ferro and paramagnetic behavior. IEEE Trans Magn 44:11 Rumpf K, Granitzer P, Pölt P (2010) Influence of the electrochemical process parameters on the magnetic behavior of a silicon/metal nanocomposite magentic thin films. ECS Trans 25:157 Rumpf K, Granitzer P, Hilscher G, Pölt P (2011) Interacting low dimensional nanostructures within a porous silicon template. J Phys Conf Ser 303:12048 Rumpf K, Granitzer P, Hilscher G, Albu M, Pölt P (2012) Magnetically interacting low dimensional Ni-nanostructures within porous silicon. Microelectron Eng (c) 90:83 Rumpf K, Granitzer P, Michor H (2016) Porous silicon nanocomposites with combined hard and soft magnetic properties. Nano Res Lett 11:398 Ryzhov VA, Pleshakov IV, Nelitailor AA, Glebova NV, Pytashev EN, Malkova AV, Kisilev IA, Matveev VV (2014) Magnetic study of nanostructured composite material based on cobalt compounds and porous silicon. Appl Magn Res 45(4):339–352 Samwel EO, Bissel PR, Lodder JC (1992) Internal field corrections in perpendicular columnar structured alumite films. J Magn Magn Mater 115:327 Stoner EC, Wohlfarth EP (1948) A mechanism of magnetic hysteresis in heterogeneous alloys. Phil Trans R Soc A: Phys Math Eng Sci 240:599 Vager Z, Naaman R (2004) Bosons as the origin for giant magnetic properties of organic monolayers. Phys Rev Lett 92:087205

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Paramagnetic and Superparamagnetic Silicon Nanocomposites Klemens Rumpf and Petra Granitzer

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Superparamagnetic Nanoparticles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Infiltration of Iron Oxide Nanoparticles into Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetic Behavior of the Composite . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biomedical Applications . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Superparamagnetic nanostructures are of interest for applications such as highdensity data storage and biomedical theranostics. In this updated review, the paramagnetic properties of nanostructured silicon are outlined and progress with tuning the magnetic properties of nanocomposites consisting of mesoporous silicon, and infiltrated superparamagnetic iron oxide nanoparticles are discussed. The magnetic behavior of the system depends on the nanoparticle size as well as on the magnetic coupling between them. Both influence the so-called blocking temperature; the transition between superparamagnetic behavior and blocked state. A particle size-related assessment shows that the blocking temperature increases with increasing particle size if the distances between the particles are equal. The blocking temperature can be decreased by weakening the magnetic interaction between the particles. Special attention is paid to iron oxide nanoparticles which are of interest due to their monodispersity and strong magnetic behavior but also because of the biocompatibility of porous silicon-iron oxide nanocomposites.

K. Rumpf (*) · P. Granitzer Institute of Physics, University of Graz, Karl-Franzens-University Graz, Graz, Austria e-mail: [email protected]; [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_31

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Keywords

Porous silicon · Magnetic nanoparticles · Superparamagnetic behavior

Introduction In the case of paramagnetic materials, each atom possesses a permanent dipole moment due to a remaining electron spin or orbital magnetic moments. Without external magnetic field, the magnetic moments offer a random orientation resulting in no net magnetization. If a magnetic field is applied, the free rotating dipoles align preferentially in field direction, but there is no interaction between individual dipoles. Silicon, especially H-terminated silicon surfaces, are in general diamagnetic and free of paramagnetic defects (see handbook chapter ▶ “Diamagnetic Behavior of Porous Silicon”). Oxidation leads to interface defects (Si/SiO2), so-called dangling bonds. They are paramagnetic in the neutral charge state which corresponds to a one-electron spin S = 1/2 configuration (Bardeleben and Cantin 1997). These paramagnetic defects (Pb-center) are superimposed by the diamagnetic contribution. Due to the large surface area in nanostructured silicon, much higher concentrations of interfacial defects can occur than in bulk silicon. The interface between Si/SiO2 has been investigated by many authors according to the dangling bond of the silicon atom, the Pb-center which is the dominant paramagnetic defect in porous silicon (Cantin et al. 1997). Their concentration depends on the position of the Fermi level, and it can also be modified by the H-termination. So oxidation of the surface can increase the Pb-center defect concentration. Two further paramagnetic defects in the oxide layer have been observed, the oxygen vacancy defect E0 (Pointdexter et al. 1981) and the EX defect in SiO2 (Pointdexter et al. 1981). EPR studies give information about these defects from analysis of g-tensors (Cantin et al. 1995) and hyperfine data (von Bardeleben et al. 2005). A special kind of paramagnetism is the so-called superparamagnetism which is dedicated to small magnetic particles which fall below a critical radius of single domain particles. After the incorporation of such particles into the pores of porous silicon, the composite system offers specific magnetic properties which will be discussed in a later section.

Superparamagnetic Nanoparticles If a ferro/ferrimagnetic material is fabricated in a nanoscopic size range and goes below a critical value, the particle becomes superparamagnetic (SPM) which means that the thermal energy dominates over the anisotropy energy and the whole particle behaves like a paramagnetic spin (Bertotti 1998). Below this so-called superparamagnetic blocking radius the particles lose their remanence. The transition takes place at the so-called blocking temperature TB = KV/25kB (K . . . anisotropy constant, V . . . particle volume).

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SPM nanoparticles are of interest for magnetic applications such as high-density data storage in fabricating particles with high anisotropy constant and thus overcome the superparamagnetic limit (Frey et al. 2009). Due to magnetic interactions between them, new properties arise such as soft magnetic alloys (Suzuki 1999) and hard magnetic materials with an improved energy product (Skomsky and Coey 1993). But also in biomedicine SPM nanoparticles are employed for diagnostics and therapeutics, e.g., magnetic resonance imaging (Yallapu et al. 2011), hyperthermia (Lee et al. 2011), and cancer therapy (Lee et al. 2011). Especially iron oxide nanoparticles play an important role due to their low toxicity. Such magnetic nanoparticles are often prepared by chemical synthesis and in general they are coated with a shell of a few nanometers which often is a metal-oxide (Wei et al. 2010), silica (Joo et al. 2009), or an organic surfactant (Shukla et al. 2009) to prohibit agglomeration and to stabilize them. Thus magnetic exchange interaction is excluded, and only dipolar coupling can take place. A further approach to stabilize the particles is the incorporation in a nonmagnetic matrix as, for example, polymers (Munoz-Bonilla et al. 2012), silica (Lee et al. 2008), zeolites (Lukatskaya et al. 2009), or porous silicon (Granitzer et al. 2010a). Furthermore the use of a matrix adds another degree of freedom and allows to tune the properties depending on the particle arrangement within the matrix.

Infiltration of Iron Oxide Nanoparticles into Porous Silicon Because of the adjustable morphology, porous silicon (Lehmann 2002) is employable as matrix. For example, iron oxide nanoparticles have been deposited onto hydroxyl functionalized porous silicon samples resulting in a self-organizing dendrite-like arrangement on the surface which offers a new composite material (Balakrishnan et al. 2006). The used magnetite nanoparticles which are superparamagnetic show dendrite-like formation caused by a diffusion-limited aggregation model (Witten and Sander 1981). If Fe3O4-nanoparticles are infiltrated into the pores of mesoporous silicon, the achieved system leads to a composite material (Fig. 1) showing a ferromagnetic-like behavior at low temperatures (T < TB) and superparamagnetism at higher temperatures (T > TB) (Granitzer and Rumpf 2011). This transition temperature (blocking temperature TB) can be influenced by the particle size but also by the distance between the particles. In principle the distance between the particles within the pores can be tuned either by the thickness of the organic coating or by the filling density. A further degree of freedom is to vary the distance between particles of adjacent pores by the morphology of the matrix.

Magnetic Behavior of the Composite The superparamagnetic behavior of the Fe3O4/porous silicon system above a blocking temperature TB is examined by temperature-dependent magnetization measurements (Granitzer et al. 2010b). For more details on this technique and others,

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Fig. 1 Scanning electron image of magnetite nanoparticles within porous silicon (Granitzer and Rumpf 2011). The size of the particles is 8 nm, and the oleic acid shell is about 2 nm

see handbook chapter ▶ “Magnetic Characterization Methods for Porous Silicon”. In the case of 8 nm particles, zero field cooled (ZFC)/field cooled (FC) investigations performed at an applied field of 500 Oe show a rather high-blocking temperature TB at 170 K which indicates magnetic interactions between the particles. The strength of the coupling can be modified by varying the packing density of the particles within the pores. This effect can be achieved by changing the concentration of the particle solution (Granitzer et al. 2011) (Fig. 2b). By varying the particle size, TB is also strongly influenced which can be seen in Fig. 2a. Furthermore, a shift of the blocking temperature to lower temperatures with higher applied fields is observed. This behavior of superparamagnetic particles is known to be proportional to H2/3 at high fields and proportional to H2 for lower fields (Goya and Morales 2004). The blocking temperatures determined for different applied magnetic fields are summarized in Table 1. Superparamagnetic behavior of porous silicon loaded with a magnetic material could also be achieved by the deposition of Ni within luminescent stain-etched porous silicon (Nakamura and Adachi 2012) or Co particles grown by electrochemically assisted infiltration within the pores. The gained hybrid material offers light emission in the visible when excited by UV light and superparamagnetic behavior and offers also biocompatibility (Munoz Noval et al. 2011). Furthermore porous silicon with absorbed parabenzoquinone molecules results in a nanocomposite which exhibits paramagnetic properties (Antropov et al. 2012) which could be due to the occurrence of additional dangling bonds due to the nanostructuring but less due to the loading of the porous silicon with organic molecules.

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Fig. 2 (a) Shift of the blocking temperature with the particle size in using the same particle concentration. (b) Change of TB with the concentration of the particle solution. In the latter case, the particle size is 8 nm for all concentrations. In all cases a template with equal morphology has been used

Iron oxide nanoparticles are of particular interest due to their magnetic behavior (Gubin 2009) but also because of their low toxicity which renders them applicable in biomedicine (Roca et al. 2009). The infiltration of iron oxide nanoparticles into porous silicon which also is biocompatible (Canham 1995) results in a system of interest for magnetically guided drug delivery (Gu et al. 2010).

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Table 1 Shift of the blocking temperature with increasing applied magnetic field to lower temperatures. The size of the infiltrated particles is 8 nm. The concentration of the magnetite solution was fixed at 8 mg Fe/ml Magnetic field [Oe] 5 500 1000

Blocking temperature [K] 170 75 50

Biomedical Applications Due to the low toxicity of both materials, porous silicon and iron oxide particles, the system is of interest for biomedical applications. The handbook chapter ▶ “Biocompatibility of Porous Silicon” reviews the important in vivo data on the former material. Drug delivery with porous silicon is under discussion in employing particles, films, chip implants, and composite materials, whereas microparticles play a key role because they are compatible to existing drug delivery concepts (Anglin et al. 2008). On the one hand, nanostructured PS tablets are utilized to carry a cocktail of drugs or nutrients which will be delivered in a predetermined time within the body, and on the other hand, percutaneous implants of PS with radioactive content can provide radiation to tumor cells (Anglin et al. 2008). R.E. Serda et al. reports on a multistage delivery system applicable for biological imaging. The loading of PS microparticles with superparamagnetic (SPM) iron oxide nanoparticles (NPs) is presented as well as the examination of their cellular uptake by macrophages. Furthermore the influence of 3-aminopropyltriethoxysilane on the PS surface and retention of the iron oxide NPs are investigated (Serda et al. 2010). For biosensing applications there are various approaches such as the fabrication of arrays of micro-test tubes and microbeakers consisting of macroporous silicon with incorporated iron oxide nanoparticles (Ghoshal et al. 2011). Furthermore optical interferometer biosensors based on porous silicon are used for immunoassaying by combing superparamagnetic particles with an interferometer porous silicon platform (Ko et al. 2012). The handbook chapters ▶ “Porous Silicon Immunoaffinity Microarrays” and ▶ “Porous Silicon Optical Biosensors” provide overviews of these application areas. The combination of porous silicon with Fe3O4-particles and additional loading with a molecular payload is of interest for controlled transport in applying an external magnetic field. The loaded molecules (enzymes) can be transported and subsequently released in an appropriate solution (Thomas et al. 2006). The in vitro cytotoxicity of porous silicon particles loaded with magnetite nanoparticles (see the related handbook chapter ▶ “Cell Culture on Porous Silicon”) has been investigated in using the cell viability of human liver cancer cells and rat hepatocytes (Kinsella et al. 2013). In vivo studies have been carried out on a carcinoma rat model to figure out the biodistribution properties of the composite material (Kinsella et al. 2013). Furthermore after in vitro cytotoxicity tests, in vivo assays have been carried out on New Zealand rabbits showing low inflammatory response and no necrosis

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effect in the tissues of the treated eyes or in the fibrous tissue formed around the agglomerated explanted flakes (Munoz Noval et al. 2013). The composite system has been investigated in relation to the magnetic preconditions for biomedical applications which means no magnetic remanence at room temperature (Rumpf et al. 2013). The magnetic moment is attempted to be high as possible due to closely packed particle loading. So far experiments to transport porous silicon loaded with iron oxide nanoparticles of different sizes (between 5 and 8 nm) in water succeeded. Size-dependent magnetization measurements and subsequent cytotoxicity evaluation of Fe3O4/porous silicon showed that the nanocomposite is an encouraging material for drug delivery (Granitzer et al. 2013). The infiltration process of the iron oxide particles into the porous silicon depends on the porous silicon morphology, on the particle size, and also on the concentration of the particle solution. The infiltration process has been elucidated with respect to these parameters, and the magnetic properties of the nanocomposites have also been characterized with respect to particle size, concentration, and template morphology (Granitzer et al. 2015). Recently porous silicon with superparamagnetic iron oxide particles has been used as vaccine platform, whereas magnetic resonance contrast enhancement has been examined (Lundquist et al. 2014). Furthermore it has been shown that porous silicon nanoparticles coated with natural oxide are biocompatible and can be used as contrast agents for magnetic resonance imaging (Gongalsky et al. 2015).

Conclusions Porous silicon offers in general a diamagnetic behavior, although due to surface modification and oxidation, some paramagnetic defects can occur. The magnetic properties of the material are expanded, for example, by the infiltration of superparamagnetic nanoparticles. The achieved nanocomposite shows a transition between superparamagnetic behavior and blocked state depending on the size of the loaded particles and their particle-particle distance which allows the tuning of the magnetic properties. Concerning the applicability the system is not only of interest for magnetic – but also for biomedical applications due to the biocompatibility of both materials.

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Photoluminescence of Porous Silicon Bernard Gelloz

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence Measurement . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence of Individual Nanocrystals from Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . Photoluminescence of Porous Silicon Layers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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The photoluminescence of mesoporous silicon and silicon nanocrystals has received enormous study over the last 25 years. The spectroscopic nature and efficiency of various emission bands from the near-infrared to the ultraviolet are briefly reviewed, as are mechanistic studies on individual nanocrystals. Improvements in surface passivation and size control of silicon nanocrystals have led to impressive photoluminescence quantum efficiencies in the visible range. Keywords

Electroless techniques · F-band · Nanocrystals · Photo-excitation process · Photoluminescence (PL) · Quantum efficiency · S-band · UV band

B. Gelloz (*) School of Engineering, Nagoya University, Nagoya, Japan e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_32

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Introduction The demonstration in 1990 that porous silicon could emit efficient tunable visible photoluminescence (PL) at room temperature and attributed to quantum-size effects in crystalline silicon (Canham 1990) has induced considerable worldwide research activities in order to (1) identify the various PL bands and their respective properties and emission mechanisms, (2) optimize the PL efficiency, (3) optimize the PL stability, and (4) tailor the PL spectrum (peak wavelength and FWHM). This chapter reviews briefly the specificities of porous silicon PL measurements, the PL of individual silicon nanocrystals from porous silicon, and the PL of porous silicon layers.

Photoluminescence Measurement The PL of porous silicon is quite broad (see Fig. 1a for spectra examples) and inhomogeneous in nature and may involve several phenomena taking place in the silicon nanostructure as well as in the tissue material surrounding it. The chemistry of the various interfaces (e.g. Si/air, Si/SiOx) in porous silicon may also play a very significant role in the luminescence. Thus, extreme care is necessary in the PL measurement procedure and its interpretation (Pelant and Valenta 2012). When using a single broadband grating, care should be taken to eliminate contribution of the short wavelength part to the long wavelength part of the spectrum due to the second order of grating diffraction. Fine structures in the PL have sometimes been observed. At room temperature, these structures (peaks modulating the otherwise generally Gaussian-shaped PL spectrum) were mostly attributed to thin film interference (Kim et al. 2003; Hooft et al. 1992). For the visible range, such interference effect can mostly be observed for layer thicknesses ranging from about 0.5 to 3 μm. At low temperature (20%). It is limited by the blinking effect, characterized by periods where an excited nanocrystal does not emit any light for an extended period of time and which has been seen earlier in various other semiconductor nanodots. A dot whose surface is not perfectly passivated does not emit light. The surrounding matrix could affect the PL lifetime as well as the emission spectral tunability at the high-energy end of the visible range (Kusova et al. 2010).

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Table 1 Summary of studies and results related to the photoluminescence of single silicon nanocrystals System studied Si nanopillars oxidized to produce luminescent silicon cores (Valenta et al. 2002; Sychugov et al. 2005)

Silicon nanodot with oxide shell prepared by gas phase pyrolysis (Martin et al. 2008)

Individual silicon nanocrystals within porous silicon grains (Valenta et al. 2008; Martin et al. 2004; Credo et al. 1999; Mason et al. 1998, 2001; Gargas et al. 2008)

Organically capped nanosilicon (Kusova et al. 2010)

Comments Efficiency: 35%. FWHM ~130 meV (single band). Emission polarized in arbitrary directions. Blinking kinetics appear to be different from that of porous silicon particles (Sychugov et al. 2005) FWHM ~100 meV; satellites at ~150 meV attributed to coupling to LO or TO Si–O–Si phonon modes of SiO2. Strong electron–phonon coupling to Si–O–Si phonons limiting the tunability of light emission versus size at high emission energies Efficiency: 88% (Credo et al. 1999; Mason et al. 1998); 10–20% for 1 exciton per nanodot; decreases as the 0.7th power of excitation at higher excitation (Valenta et al. 2008). FWHM ~120 meV (Valenta et al. 2008; Credo et al. 1999; Mason et al. 1998); splitting of ~160 meV (attributed to the stretching vibration of the Si–O–Si bridge bond) (Credo et al. 1999; Mason et al. 1998). Efficiency limited by blinking; in blinking, OFF state is due to Auger recombinations and exciton–exciton scattering (Valenta et al. 2008). Blinking obeys a power law statistics (Martin et al. 2004). Ellipsoid shapes of silicon dots; degree of polarization strongly anisotropic; depends on anodization conditions (Gargas et al. 2008) Efficiency: 20% (for an ensemble of colloidal nanodots). FWHM >100 meV; satellite peak at ~150 meV. Decay time: 10 ns at 550 nm

Photoluminescence of Porous Silicon Layers Depending mostly on the degree of quantum confinement and on the chemical state of its surface, porous silicon could luminesce from the near-infrared (1.5 μm) to the near-UV as a result of distinct emission bands having different origins (Table 2; Cullis et al. 1997; Bisi et al. 2000). The near-infrared band has not been as extensively studied as the visible bands. It was observed in both partially oxidized porous silicon and oxygen-free samples. It has been related to both quantum-size effect and surface states (Koch et al. 1993). The UV band has been observed only in oxidized porous silicon. It has been related to oxide luminescence, with the silicon nanocrystals playing a potential role in the photoexcitation process (Qin et al. 1996). The F-band, with emission peaks around 415–470 nm, FWHM of 0.38–0.5 eV, and quantum efficiencies of 0.1% at best, has been reported in various thermally or

From near-infrared to yellow 590–1,300

surface including oxygen atoms

From yellow to blue 425–630

From near-infrared to blue 400–1,300

hydrogenterminated surface

Hot PL band

Sband

Label IR band

Spectral range/ typical peak wavelength (nm) Near IR 1,100–1,500

Table 2 Porous silicon luminescence bands

Typical: 1–10% (Billat 1996; Bsiesy et al. 1991; Gelloz and Koshida 2000; Gelloz et al. 1998a,b); record: 23%(Gelloz et al. 2005) 0.01%

μs range

ps range

–

Best efficiency

A few ns to ~150 μs

Typical lifetime at RT 10 ns to 10 μs

–

Yes

Yes

Directly electrically excitable No

(continued)

Generally proposed origin/ comments States at the silicon surface (Koch et al. 1993; Fauchet et al. 1995; Canham 1995) Quantum confinement in Si nanocrystals; indirect bandgap transitions; blueshift upon size reduction; (Cullis et al. 1997; Bisi et al. 2000; Wolkin et al. 1999) Fast relaxation of excitation to surface states related to Si = O species sets a limit to the size effect and a minimum emission wavelength of ~590 nm. Quantum confinement in Si nanocrystals; direct bandgap transitions; observed only under high excitation; redshift upon size reduction (de Boer et al. 2010)

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Long-lived band

UV band

Label F-band

Table 2 (continued)

UV: ~350 (Qin et al. 1996) 270–290 (Gelloz et al. 2014) Blue-green 450–540

Spectral range/ typical peak wavelength (nm) Blue-green 420 ~ 470

1–8 s up to ~ 200 K; ~1 s at RT

ps–ns

Typical lifetime at RT ~10 ns

2% at 300 K; 8.5% at 4 K (Gelloz and Koshida 2012) (including the F-band emission)

Best efficiency 0.1%

–

No

Directly electrically excitable No

Oxide-related species; emission energy strongly dependent on excitation energy (Gelloz and Koshida 2012, 2009; Kovalenko et al. 1999; Wadayama et al. 2002; Kux et al. 1995)

Generally proposed origin/ comments Oxide-related defects (Cullis et al. 1997; Bisi et al. 2000; Koyama and Koshida 1997; Qin et al. 1997)/contamination by organic groups (Loni et al. 1995) The Hot PL band may contribute to the green part (Prokofiev et al. 2009) Oxide-related defects (Qin et al. 1996; Gelloz et al. 2014)

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chemically oxidized PSi samples (Koyama and Koshida 1997; Qin et al. 1997; Cullis et al. 1997; Bisi et al. 2000; Canham 1997). The PL lifetime is in the nanosecond range. Many reports have attributed this band to oxide-related defects or contamination (occurring upon storage and prolonged exposure to air) by organic chromophores (e.g., carbonyl groups) (Loni et al. 1995). Recently, another origin has been suggested: direct bandgap core luminescence (Γ-Γ transitions) (Prokofiev et al. 2009). Using layers of oxide-embedded silicon nanocrystals obtained by sputtering, de Boer et al. (2010) have clearly identified this direct bandgap band and referred to it as the “Hot PL band” because it can be observed only under high excitation. Indeed, getting a reasonable probability of radiative Γ-Γ transitions, a constant generation of hot carrier at the Γ15-point (direct gap valley) by Auger recombination of multiple excitons, is necessary in order to compete with the otherwise very fast (1–10 ps) non-radiative relaxation toward the Δ-valley (indirect gap valley). The Hot PL band characteristics can be found in Table 2. This Hot PL band has only been clearly identified in a partially oxidized system (de Boer et al. 2010). It could be a part of the so-called F-band, in particular the green part, but is unlikely to be the F-band itself, most importantly because (1) it is red-shifted as the crystalline core decreases, reaching the yellow range of the visible spectrum, thus not matching any more the blue emission of the F-band, and (2) a fast blue band (peak wavelength 420 nm, independent of nanocrystal core size; lifetime 10 ns) was still observed as an independent oxide-related band (likely to be part of the F-band) in addition to the Hot PL band (de Boer et al. 2010). This second point is also consistent with the observation of a fast (nanosecond range) efficient blue emission peaked at 410 nm (FWHM 50 nm) in heavily oxidized porous silicon not containing any significant amount of nanocrystals, thus supporting the attribution of the blue part of the F-band to oxide-related species (Gelloz and Koshida 2009, 2012). This latter fast blue band has been observed together with a long-lived band (bluegreen), which has been studied in details recently (Gelloz and Koshida 2009, 2012). It is mostly characterized by (1) a long decay time (several seconds) in the range 4–180 K, (2) a thermally activated quenching from about 180 K (activation energy 0.2 eV), and (3) a spectrum strongly dependent on the excitation energy. The best PL external quantum efficiencies of this blue band was 2% at room temperature and 8.5% at 4 K (Gelloz and Koshida 2012). An emission band showing some similarities with this long-lived band has been observed previously by a few other groups (Kovalenko et al. 1999; Wadayama et al. 2002; Kux et al. 1995). Two of them (Kovalenko et al. 1999; Wadayama et al. 2002) got an emission band peaked roughly around 540 nm for similar excitation wavelengths (337 and 325 nm). Kovalenko et al. (1999) got a decay time of 0.5 s [shorter than that observed by Gelloz et al. (Gelloz and Koshida 2009, 2012)] at 15 K, in “as prepared” (but probably slightly aged) as well as in oxidized porous silicon, and attributed the emission to quantum confinement in very small silicon nanocrystals. Wadayama et al. (2002) reported an emission decaying very slowly (>1 s) at room temperature in porous silicon having been subjected to rapid thermal oxidation (1,000  C) and quenching in liquid nitrogen. The fast cooling in liquid nitrogen was the necessary step for the very slow band to be observed, which was not understood. Kux et al. (1995) also

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observed this band in oxidized porous silicon (using rapid thermal oxidation) and suggested it could be related to SiOH groups. Gelloz et al. (Gelloz and Koshida 2009, 2012) attributed the origin of this band to molecular-like species in or at the surface of the oxide tissue in oxidized porous silicon. Mechanisms based on molecular species are supported by the fact that it could also be observed in fully oxidized porous silicon, with no silicon nanocrystals left in the nanostructure, and also in pure porous glass. Moreover, a similar band has been observed recently in both silica and Al2O3 nanoparticles. The origin of the band was not completely clear but was hypothesized to be luminescence from photogenerated OH radicals (Anjiki and Uchino 2012). The most studied and technologically interesting band is the so-called S-band (S for slow; decay times are rather long compared to those of direct bandgap semiconductors). It is electrically excitable and its properties (e.g., emission spectrum, efficiency, chemical activity) can be in principle engineered. Its main characteristics are summarized in Table 3. It originates mostly from exciton recombinations in Si nanocrystals as indicated by polarization memory of PL, PL saturation under high excitation due to Auger recombinations (Mihalcescu et al. 1995), and resonant excitation and hole-burning experiments (evidencing phonon-mediated recombinations and singlet–triplet exciton state splitting) (Cullis et al. 1997; Bisi et al. 2000). Very high confinement energy (>0.7 eV) results in the breakdown of k-conservation rules and direct recombinations become possible (Kovalev et al. 1998). Figure 1a shows the progressive shift of the PL band of porous silicon having its surface terminated by silicon–hydrogen bonds (oxygen-free) across the visible region obtained by changing the anodization conditions (Wolkin et al. 1999). Generally, its intensity increases and its peak wavelength decreases when the porosity increases. The efficiency usually decreases in the order n-type, p-type, n+type, and p+-type porous silicon due to differences in nanostructures. However, surface states, in particular those introduced by oxygen via Si=O bonds, can greatly influence the peak wavelength, as shown in Fig. 1b. In zones II and III, localized states lie inside the bandgap, setting a limit to the emission blueshift expected from the size reduction and pinning the emission peak wavelength at about 590 nm for nanocrystals below ~2.5 nm in size (Wolkin et al. 1999). White PL can be obtained using partially oxidized porous silicon emitting two-peak PL spectra resulting from the superposition of the blue-green emission of the F-band (or of both the F-band and the long-lived band) and the yellow-red-emitting S-band (Gelloz and Koshida 2005; Gelloz et al. 2009; Dohnalova et al. 2010). While most data relate to porous silicon layers, recently a lot of effort was devoted to the fabrication of porous silicon powders, in particular for bio-imaging purposes. Some powders were obtained by breaking down anodized porous layers followed by various types of oxidation steps used to tune the emission spectra and improve the efficiency and stability (Park et al. 2009; Xia et al. 2012; Tu et al. 2012). Porous silicon layers have also been made using electroless techniques. The technique is used with thin films of microcrystalline silicon produced by PECVD and leads to PL intensities (efficiency 1–10%) comparable with that obtained from anodized crystalline silicon (Solomon et al. 2008).

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Surface passivation is essential for high emission quantum efficiency to reduce the competitive defect-related non-radiative recombinations and good stability. Silicon nanocrystals can be passivated by an oxide shell (Cullis et al. 1997; Takazawa et al. 1994; Petrovakoch et al. 1992; Yamada and Kondo 1992; Gelloz et al. 2005; Gelloz and Koshida 2005, 2007, 2006; Xia et al. 2012; Tu et al. 2012) [one of best technique is high-pressure water vapor annealing (Gelloz et al. 2005; Gelloz and Koshida 2005, 2007, 2006)] or by replacing Si–H bonds of as-anodized Table 3 Some characteristics of the S-band at 300 K Property Peak wavelength

Typical values/characteristics 1,300–400 nm

External quantum efficiency

Layers: 1–10% (Billat 1996; Bsiesy et al. 1991; Gelloz and Koshida 2000; Gelloz et al. 1998a, b); record: 23% (Gelloz et al. 2005) Powders: 10–16% (Park et al. 2009; Xia et al. 2012; Tu et al. 2012) (Values for red-orange PL) 150–180 nm

FWHM

Bare layer

Porous silicon microcavity

Decay

13 nm (emission: 740 nm) (Squire et al. 1998) 10–40 meV (emission: 1.5–2.2 eV) (Araki et al. 1996) Decay time: ns to ~150 μs from blue to red; increases at low temperature due to singlet–triplet exciton state splitting (Cullis et al. 1997; Bisi et al. 2000) Multiexponential decay

Comments Porosity dependent; generally related to nanocrystal sizes; can be strongly influenced by surface states (Wolkin et al. 1999) Key factors: nanocrystal density (related to porosity), surface passivation (Gelloz et al. 2005); exciton localization (Billat 1996; Bsiesy et al. 1991; Gelloz and Koshida 2000; Gelloz et al. 1998a, b) Inhomogeneous broadening (mostly size effect) (Cullis et al. 1997; Bisi et al. 2000) Photonic crystal effect (Pavesi et al. 1996; Araki et al. 1996; Gong et al. 2010; Squire et al. 1998) Depends on quantum confinement strength (effect on wavelength) and surface passivation (limited by non-radiative recombinations) (Cullis et al. 1997; Bisi et al. 2000) Multiexponential shape attributed to exciton migration, carrier escape from Si dots, distribution of dot shape emitting at the same energy (Cullis et al. 1997; Bisi et al. 2000), or more recently to the blinking effect (Dunn et al. 2009) (continued)

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Table 3 (continued) Property Degree of polarization (nonresonant photoexcitation)

Fine structure at low temperature

Resonant excitation

Typical values/characteristics 0.2; decreases with increasing detection wavelength (Cullis et al. 1997; Bisi et al. 2000) Phonon replica at 56 and 19 meV

Nonresonant excitation

Multiple-peak structures; peak separation ~64 meV

Excitation density

Stability/surface chemistry

Output scales linearly with excitation density until Auger effects become predominant (Cullis et al. 1997; Mihalcescu et al. 1995; Bisi et al. 2000) Poor/Si–H bonds (as-anodized) Good/Si–H bonds replaced by more stable Si–C bonds (Buriak 2002)

Improved/Si–H bonds replaced by more stable Si–Ag bonds (Sun et al. 2005) Good/oxide shell

Comments Can be anisotropic (effect of crystallite shape)

Consistent with TO and TA Si phonons (Cullis et al. 1997; Calcott et al. 1993) Consistent with TO Si phonons (Xu and Adachi 2010) Critical excitation level: more than one exciton per silicon nanocrystal. See Table 1 for more details

Si–H bonds are easily oxidized, even in air Replacing hydrogen by long organic chains also enhance stability by steric hindrance effect (Buriak 2002; Boukherroub et al. 2000)

Si–SiO2 interfacial defects should be minimized. Best techniques: high-temperature oxidation (Cullis et al. 1997; Takazawa et al. 1994; Petrovakoch et al. 1992; Yamada and Kondo 1992), high-pressure water vapor annealing (Gelloz et al. 2005; Gelloz and Koshida 2005, 2007, 2006), chemical oxidation (Xia et al. 2012; Tu et al. 2012)

porous silicon by stronger and more stable bonds, such as Si–C bonds (Buriak 2002; Boukherroub et al. 2000). A striking effect of surface termination has recently been confirmed by Dohnalova et al. (2013), who reported fast (nanosecond range) direct bandgap-like blue emission using silicon colloids (prepared via a chemical route, not from porous silicon) with carbon surface termination. The phonon-less transitions were explained by drastic modifications of electron and hole wavefunctions (these transitions are not direct bandgap Γ-Γ transitions).

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Conclusion Recent developments such as fast phonon-less transitions from carbon-terminated nanocrystals (Dohnalova et al. 2013), fast direct bandgap transitions (Prokofiev et al. 2009; de Boer et al. 2010), and very high values of luminescence quantum efficiencies of silicon nanocrystals in layers [in porous silicon, 23% (Gelloz et al. 2005; Gelloz and Koshida 2005), and in other assemblies, 18–100% (Ledoux et al. 2000) and 60% (Jurbergs et al. 2006)] show that the luminescence of nanocrystalline silicon is progressively paving its way toward applications.

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Dunn K, Derr J, Johnston T, Chaker M, Rosei F (2009) Multiexponential photoluminescence decay of blinking nanocrystal ensembles. Phys Rev B 80(3):035330 Elhouichet H, Oueslati M (2002) The role of ambient ageing on porous silicon photoluminescence: evidence of phonon contribution. Appl Surf Sci 191(1–4):11–19 Fauchet PM, Tsybeskov L, Peng C, Duttagupta SP, Von Behren J, Kostoulas Y, Vandyshev JMV, Hirschman KD (1995) Light-emitting porous silicon: materials science, properties, and device applications. Ieee J Select Top Quantum Electr 1(4):1126–1139 Gargas DJ, Sirbuly DJ, Mason MD, Carson PJ, Buratto SK (2008) Investigation of polarization anisotropy in individual porous silicon nanoparticles. Microelectron J 39(9):1144–1148 Gelloz B, Koshida N (2000) Electroluminescence with high and stable quantum efficiency and low threshold voltage from anodically oxidized thin porous silicon diode. J Appl Phys 88(7):4319–4324 Gelloz B, Koshida N (2005) Mechanism of a remarkable enhancement in the light emission from nanocrystalline porous silicon annealed in high-pressure water vapor. J Appl Phys 98(1):123509 Gelloz B, Koshida N (2006) Highly enhanced photoluminescence of as-anodized and electrochemically oxidized nanocrystalline p-type porous silicon treated by high-pressure water vapor annealing. Thin Solid Films 508(1–2):406–409 Gelloz B, Koshida N (2007) Highly efficient and stable photoluminescence of nanocrystalline porous silicon by combination of chemical modification and oxidation under high pressure. Jpn J Appl Phys Part 1 46(4B):2429–2433 Gelloz B, Koshida N (2009) Long-lived blue phosphorescence of oxidized and annealed nanocrystalline silicon. Appl Phys Lett 94:201903 Gelloz B, Koshida N (2012) Blue phosphorescence in oxidized nano-porous silicon. ECS J Sol State Sci Technol 1(6):R158–R162. https://doi.org/10.1149/2.025206jss Gelloz B, Nakagawa T, Koshida N (1999) Enhancing the external quantum efficiency of porous silicon LEDs beyond 1% by a post-anodization electrochemical oxidation. Mater Res Soc Symp Proc 536:15–20 Gelloz B, Nakagawa T, Koshida N (1998b) Enhancement of the quantum efficiency and stability of electroluminescence from porous silicon by anodic passivation. Appl Phys Lett 73(14): 2021–2023 Gelloz B, Kojima A, Koshida N (2005) Highly efficient and stable luminescence of nanocrystalline porous silicon treated by high-pressure water vapor annealing. Appl Phys Lett 87(3):031107 Gelloz B, Mentek R, Koshida N (2009) Specific blue light emission from nanocrystalline porous Si treated by high-pressure water vapor annealing. Jpn J Appl Phys 48(4):04C119 Gelloz B, Mentek R, Koshida N (2014) Ultraviolet and long-lived blue luminescence of oxidized nano-porous silicon and pure nano-porous glass. ECS J Solid State Sci Technol 3(5):R83–R88. https://doi.org/10.1149/2.022405jss Gong YY, Ishikawa S, Cheng SL, Gunji M, Nishi Y, Vuckovic J (2010) Photoluminescence from silicon dioxide photonic crystal cavities with embedded silicon nanocrystals. Phys Rev B 81 (23):Artn 235317. https://doi.org/10.1103/Physrevb.81.235317 Hooft GW, Kessener YARR, Rikken GLJA, Venhuizen AHJ (1992) Temperature-dependence of the radiative lifetime in porous silicon. Appl Phys Lett 61(19):2344–2346 Jurbergs D, Rogojina E, Mangolini L, Kortshagen U (2006) Silicon nanocrystals with ensemble quantum yields exceeding 60%. Appl Phys Lett 88(23):Artn 233116. https://doi.org/10.1063/1.2210788 Kim YY, Lee KW, Ahn EJ, Shim S (2003) Identification of luminous region in porous silicon layers. J Appl Phys 93(1):274–277 Koch F, Petrovakoch V, Muschik T (1993) The luminescence of porous Si – the case for the surfacestate mechanism. J Lumin 57(1–6):271–281 Kovalenko NP, Doycho IK, Gevelyuk SA, Vorobyeva VA, Roizin YO (1999) Geminate and distantpair radiative recombination in porous silicon. J Phys Condens Matt 11(24):4783–4800 Kovalev D, Heckler H, Ben-Chorin M, Polisski G, Schwartzkopff M, Koch F (1998) Breakdown of the k-conservation rule in Si nanocrystals. Phys Rev Lett 81(13):2803–2806 Koyama H, Koshida N (1997) Spectroscopic analysis of the blue-green emission from oxidized porous silicon: possible evidence for Si-nanostructure-based mechanisms. Solid State Commun 103(1):37–41

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Kusova K, Cibulka O, Dohnalova K, Pelant I, Valenta J, Fucikova A, Zidek K, Lang J, Englich J, Matejka P, Stepanek P, Bakardjieva S (2010) Brightly luminescent organically capped silicon nanocrystals fabricated at room temperature and atmospheric pressure. ACS Nano 4(8): 4495–4504 Kux A, Kovalev D, Koch F (1995) Slow luminescence from trapped charges in oxidized porous silicon. Thin Solid Films 255(1–2):143–145 Ledoux G, Guillois O, Porterat D, Reynaud C, Huisken F, Kohn B, Paillard V (2000) Photoluminescence properties of silicon nanocrystals as a function of their size. Phys Rev B 62 (23):15942–15951 Loni A, Simons AJ, Calcott PDJ, Canham LT (1995) Blue photoluminescence from rapid thermally oxidized porous silicon following storage in ambient air. J Appl Phys 77(7):3557–3559 Martin J, Cichos F, Chan IY, Huisken F, Von Borczyskowski C (2004) Photoinduced processes in silicon nanoparticles. Israel J Chem 44(4):341–351 Martin J, Cichos F, Huisken F, von Borczyskowski C (2008) Electron-phonon coupling and localization of excitons in single silicon nanocrystals. Nano Lett 8(2):656–660 Mason MD, Credo GM, Weston KD, Buratto SK (1998) Luminescence of individual porous Si chromophores. Phys Rev Lett 80(24):5405–5408 Mason MD, Sirbuly DJ, Carson PJ, Buratto SK (2001) Investigating individual chromophores within single porous silicon nanoparticles. J Chem Phys 114(18):8119–8123 Mihalcescu I, Vial JC, Bsiesy A, Muller F, Romestain R, Martin E, Delerue C, Lannoo M, Allan G (1995) Saturation and voltage quenching of porous-silicon luminescence and the importance of the auger effect. Phys Rev B 51(24):17605–17613 Park JH, Gu L, von Maltzahn G, Ruoslahti E, Bhatia SN, Sailor MJ (2009) Biodegradable luminescent porous silicon nanoparticles for in vivo applications. Nat Mater 8(4):331–336 Pavesi L, Guardini R, Mazzoleni C (1996) Porous silicon resonant cavity light emitting diodes. Solid State Commun 97(12):1051–1053 Pelant I, Valenta J (2012) Luminescence spectroscopy of semiconductors. Oxford University Press, New York Petrovakoch V, Muschik T, Kux A, Meyer BK, Koch F, Lehmann V (1992) Rapid-thermal-oxidized porous Si – the superior photoluminescent Si. Appl Phys Lett 61(8):943–945 Prokofiev AA, Moskalenko AS, Yassievich IN, de Boer WDAM, Timmerman D, Zhang H, Buma WJ, Gregorkiewicz T (2009) Direct bandgap optical transitions in Si nanocrystals. Jetp Lett 90 (12):758–762. https://doi.org/10.1134/S0021364009240059 Qin GG, Song HZ, Zhang BR, Lin J, Duan JQ, Yao GQ (1996) Experimental evidence for luminescence from silicon oxide layers in oxidized porous silicon. Phys Rev B 54(4):2548–2555 Qin GG, Liu XS, Ma SY, Lin J, Yao GQ, Lin XY, Lin KX (1997) Photoluminescence mechanism for blue-light-emitting porous silicon. Phys Rev B 55(19):12876–12879 Solomon I, Rerbal K, Chazalviel JN, Ozanam F, Cortes R (2008) Intense photoluminescence of thin films of porous hydrogenated microcrystalline silicon. J Appl Phys 103(8):Artn 083108. https:// doi.org/10.1063/1.2909914 Squire EK, Snow PA, Russell PS, Canham LT, Simons AJ, Reeves CL (1998) Light emission from porous silicon single and multiple cavities. J Lumin 80(1–4):125–128 Sun J, Lu YW, Du XW, Kulinich SA (2005) Improved visible photoluminescence from porous silicon with surface Si–Ag bonds. Appl Phys Lett 86(17):Artn 171905. https://doi.org/10.1063/1.19240426 Sychugov I, Juhasz R, Linnros J, Valenta J (2005) Luminescence blinking of a Si quantum dot in a SiO2 shell. Phys Rev B 71(11):115331 Takazawa A, Tamura T, Yamada M (1994) Photoluminescence mechanisms of porous Si oxidized by dry oxygen. J Appl Phys 75(5):2489–2495 Tu CC, Zhang QF, Lin LY, Cao GZ (2012) Brightly photoluminescent phosphor materials based on silicon quantum dots with oxide shell passivation. Opt Express 20(1):A69–A74 Valenta J, Juhasz R, Linnros J (2002) Photoluminescence spectroscopy of single silicon quantum dots. Appl Phys Lett 80(6):1070–1072 Valenta J, Fucikova A, Vacha F, Adamec F, Humpolickova J, Hof M, Pelant I, Kusova K, Dohnalova K, Linnros J (2008) Light-emission performance of silicon nanocrystals deduced

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Chemiluminescence of Porous Silicon Jianmin Wu

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Detection Methods . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porous Si Chemiluminescent System . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Chemiluminescence from Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

463 464 466 467 471 471

Abstract

Visible chemiluminescence (CL) arising from the oxidation of porous silicon is reviewed from the perspectives of its discovery, excitation and detection, potential mechanisms, and future applications. CL can be more sensitive to surface states than PL, and efficiencies are generally much higher for one-electron oxidizing agents like the permanganate ion than two-electron oxidizers like the persulfate ion. Applications in biosensing and forensics are highlighted. Keywords

Chemiluminescence · Electrochemiluminescence · Porous silicon · Biosensing · Forensics

Introduction Since the first discovery of porous silicon (pSi) by Uhlir (1956) and Turner (1958), significant attention has been directed toward visible luminescence properties of this material (Canham 1990). The first visible photoluminescence (PL) in porous silicon J. Wu (*) Department of Chemistry, Zhejiang University, Hangzhou, Zhejiang, China e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_130

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was reported by Pickering et al. at low temperatures (Pickering et al. 1984), but it was not until 1990 that Canham reported this type of PL at room temperature (Canham 1990). Since silicon is an indirect gap semiconductor in which interband transitions need phonons, the transition probabilities in bulk silicon are 100 times smaller than that in direct bandgap materials. This results in quantum efficiency as low as 0.0001% with radiative recombination in bulk silicon. Thus, strong visible light emission from porous silicon with high quantum efficiency (1%–10%) has attracted great interest in studying the mechanism behind this emission process. Furthermore, interest in this field has been expanded to various formats of luminescence from pSi. For example, electroluminescence (EL) of pSi can be observed by electrical injection of electron-hole pair (Koshida and Koyama 1992), while electrochemiluminescence (ECL) was observed when the pSi was immersed in electrolyte and applied with positive and negative potential (Bressers et al. 1992; Canham et al. 1992; Gee 1960). Chemiluminescence (CL) is the emission of light resulting from chemical reactions (Kooij et al. 1998; Campbell 1988). CL differs from PL and EL in that the electronic excited state is the product of chemical reaction rather than of the absorption of a photon or electrical energy. However, for some time, there has been some confusion on the differences among CL, EL, and ECL from pSi. Strictly, CL is operated in an open-circuit mode without need of external electrical power, while ECL needs a driving force by applying positive or negative potential on pSi electrode and involves electrochemical reactions on pSi/electrolyte interface. In contrast, EL emission is only driven by electrical force without involving any electrochemical reaction. The main topic of this chapter will be focused on CL emission, although, unlike other types of semiconductor quantum dots or carbon quantum dots, the applications of pSi-based CL have not been extensively exploited.

Detection Methods The instrumental setup for CL detection is similar to that of ECL (Fig. 1a) (Tan et al. 2014), except that the CL intensity or its spectrum should be measured in an opencircuit condition. Usually, a high-sensitivity cooled CCD camera is needed to acquire a CL image emitted from the pSi/liquid junction. For the measurement of CL intensity, a photomultiplier is usually employed due to the low intensity of CL generated from pSi. Spectra of CL can be acquired by a high-sensitivity spectrometer equipped with a high quantum efficiency CCD array. Either pSi particle or pSi film can generate CL, but the application of these two forms is different. Similar with other commonly used quantum dots, pSi particles can be potentially used as labels to report the immunoreaction or other kinds of biochemical reaction, as well as bio-imaging agents. In contrast, when pSi film was used as the CL emitter, the porous layer is still attached on the bulk silicon. The advantage of this method is that multiplex assay can be achieved on a CL sensor array by patterning the pSi film. Our recent work on ECL demonstrated that the pSi film can

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Fig. 1 a An imaging system for CL and ECL detection. b Mechanism for negative ECL image and (c) positive image. d Example of fingerprint image. e Fingerprint with residue TNT molecules on pSi chip. f Cross-sectional relative gray values over 15 parallel ridges within the area indicated by the black rectangles in (d) and (e)

visualize fingerprint images or chemicals transferred onto the film (Fig. 1) (Tan et al. 2014). Similarly, CL emission from pSi chip can also be potentially applied in the visualization of chemical patterns.

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Porous Si Chemiluminescent System

Fig. 2 Spectra of porous Si during chemiluminescence (a) and photoluminescence (b). Chemiluminescence was from vapor phase above concentrated HNO3 (McCord et al. 1992)

Intensity (arbitrary units)

In 1992, McCord et al. found that p-type pSi, when immersed in an aqueous nitric acid (HNO3) or peroxodisulfate (S2O82
), gave a weak luminescence in the visible (McCord et al. 1992). The spectrum of CL generated by pSi in contact with HNO3 vapor was similar to that of PL, but the maximum wavelength of CL (λmax = 780 nm) was longer than that of PL (λmax = 700 nm) (Fig. 2). Bressers et al., in the study of ECL of pSi, also noted luminescence emission in the presence of S2O82
at the opencircuit potential (Bressers et al. 1992). McCord et al. found a strong flash when dry pSi was exposed to concentrated HNO3 (McCord et al. 1992). However, this phenomenon may result from strong exothermic reactions between pSi and concentrated HNO3, causing the deflagration and explosion of the material (Koch and Clement 2007). A weak luminescence can be observed during the chemical etching of silicon in concentrated HNO3/HF solutions (McCord et al. 1992). Meulenkamp et al. investigated the luminescence emission from p-type and n-type pSi in the presence of various oxidizing agents (Meulenkamp et al. 1993). They indicated that the strongest emission from both types of pSi were obtained with Ce4+ and MnO4
ion solution. Light emission induced by Ce4+ was found to persist for long periods (several hours). S2O82
gave a weak emission, while no CL was observed with H2O2 and diluted HNO3 solutions. Meulenkamp et al. showed that CL can be also observed when pSi was immersed in acetonitrile solution containing phenanthrene radical cation (Meulenkamp et al. 1994). Both McCord et al. and Meulenkamp et al. noted high similarity between luminescence of pSi and that of siloxene (Si6O3H6). In general, the structures of siloxene (Fig. 3) are believed to consist of linear chains or sixfold rings of Si interconnected by oxygen or Si layers with alternating OH or H terminations (Deak et al. 1992; Van de Walle and Northrup 1993). The presence of oxygen in a planar array of silicon atoms results in molecular orbitals confined to Si rings or chains which are responsible for the luminescence properties. The important property of the material is that the luminescence energies can be tuned over a larger spectral range by the substitution of hydrogen with various ligands, such as OH

b a

500

600 700 800 Wavelength (nm)

900

Chemiluminescence of Porous Silicon

467

Fig. 3 The chemical structure of siloxene

groups, alcohols, or halogens (Brandt et al. 1992). This phenomenon is probably caused by the introduction of different surface states after the substitution reactions. The mechanism of chemiluminescence generated by the reaction between siloxene and oxidant is still unclear, but later works proved that the chemiluminescence of pSi involves radiative electron-hole recombination during the process of pSi oxidation. Similar radiative recombination was also found in the ECL of silicon nanocrystal (Ding et al. 2002).

Mechanism of Chemiluminescence from Porous Silicon Mechanism of pSi-derived CL involves at least three steps, which include charge transfer between chemical reagents and pSi, recombination of electron-hole pair delocalized in the quantum-confined Si nanocrystal, and surface chemistry reactions. When porous silicon is immersed in a solution containing strong oxidizing agents such as Ce4+, MnO4
, and IrCl62
, visible CL is observed. Holes can be efficiently injected from oxidizing agents in the valence band of quantum-confined orbital of the Si nanocrystal. For example, Ce4+ ion, a typical one-electron oxidizing agent, is reduced at HF-pretreated silicon by injecting holes into valence band (Meulenkamp et al. 1993, 1994). IrCl62
also follows the hole injection mechanism as that of Ce4+ ion: Ce4þ ¼ Ce3þ þ hvb þ

(1)

Upon the hole injection, a subsequent anodic oxidation on pSi surface will take place: Si þ 2H2 O þ 4hvb þ ¼ SiO2 þ 4Hþ

(2)

If the hole is injected into the valence band of n-doped nanocrystals, radiative recombination of electron in conduction band with the hole will dominate over the processes, giving a strong CL emission. On contrary, if the rate of reaction (2) dominates over the rate of electron-hole recombination, the CL intensity will be very

468

1.0 Intensity (a.u.)

Fig. 4 Emission spectrum of a p-type porous silicon electrode in a 0.1 M Ce4+, 1 M H2SO4 solution (Kooij et al. 1998; Campbell 1988)

J. Wu

0.5

0.0

Intensity (a.u.)

Fig. 5 Time evolution of the emission intensity at 650 and 790 nm, respectively (Kooij et al. 1998; Campbell 1988)

Intensity (a.u.)

600

700 800 wavelength (nm)

900

1.0

0.5 650 nm 0.0 1.0

0.5 790 nm 0.0

0

100

200

300 time (s)

400

500

600

low or even cannot be detected. Therefore, the quantum efficiency of CL greatly depends on the competition between electron-hole recombination and reaction (2). As shown in Fig. 4, a broad emission peak was found with λmax at around 720 nm under open-circuit conditions in a 0.1 M Ce4+, 1 M H2SO4 solution (Kooij et al. 1998; Campbell 1988). By monitoring the CL spectra during chemical reaction, the emission maximum shifted to shorter wavelength (740–720 nm) was observed. An interesting phenomenon observed by Kooij et al. indicated the time evolution of the emission intensity at 650 and 790 nm was different (Kooij et al. 1998; Campbell 1988). The transient for the longer wavelength passed through a maximum at a shorter time than that for the shorter wavelength (Fig. 5). After around 10–15 min, the CL intensity decreases to an undetectable value. The decrease in CL intensity was attributed to the gradual oxidization of pSi surface, resulting in the hindered charge transfer. The blue shift was ascribed to the selective excitation of inhomogeneous Si nanocrystal with different size distributions. Those larger Si nanocrystal regions will be oxidized first giving longer wavelength emission, since the larger Si

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469

nanocrystals have low barrier for hole transfer. However, due to the non-direct bandgap nature of larger Si nanocrystal, the intensity of light emission in longer wavelength is lower. As the oxidation proceeds, the barriers for charge transfer increased, resulting in the injection of holes into smaller Si nanocrystal, which have a wider and direct bandgap due to quantum confinement effect. Thus, the wavelength of CL shifts blue and its intensity increases. Finally, when all the Si nanocrystals are oxidized, CL emission will gradually fade. Similar luminescent process was also found in ECL, which undergoes a regular process including activation, strong emission, and fading (Tan et al. 2014). The CL spectra of n-type and p-type pSi are similar, since the essential steps in CL emission consist of oxidation of Si-containing groups (e.g., Si-H, Si-Si) and energy transfer to a luminescent Si center (Campbell 1988). Therefore, the range of wavelength shift in CL spectra is not so significant, but the intensity of CL is greatly affected by surface chemistry of pSi. In contrast to one-electron oxidizer, two-electron oxidizing agents such as H2O2 and S2O82
do not inject holes in the first step. Instead, they need to capture electron from conduction band of n-doped Si, forming a radial intermediate (Kooij et al. 1996): ▪
S2 O8 2
þ e ¼ SO2
4 þ SO4

(3)

Compared with the redox pair of S2O82
/SO4 ▪
, the SO4 ▪
/SO42
pair has a more positive potential. Therefore, the formed sulfate radicals, SO4 ▪
, can either capture a second conduction band electron or inject a hole (h+) into the valence band of semiconductor electrode: 2
þ SO▪
4 ¼ SO4 þ h

(4)

The injected holes can recombine with electrons from the conduction band with the emission of light. Nevertheless, the hole injection process must compete with second electron capturing process. It has been reported that hole injection only occurs under exceptional conditions for the two-electron oxidizing agents (Kooij et al. 1996). Similar with S2O82
, H2O2 also involves two steps. The capture of conduction band electron results in the formation of radical intermediate (OH▪), followed by a second electron capturing process, which dominates over the hole injection process at pSi /electrolyte interface (Kooij et al. 1996). The hindered hole injection was attributed to a strong interaction of surface-generated OH▪ and SO4▪
with the silicon surface. Reasonably, either S2O82
or H2O2 gives no CL or weak CL. It should be emphasized that strong CL emission can be found in other types of semiconductor quantum dots (e.g., CdSe) in the presence of S2O82
or H2O2 (Poznyak et al. 2004). The different CL behaviors probably due to the different competition results between the recombination of electron-hole pair and reaction of intermediate radical species on these semiconductor surfaces. The reduction of MnO4
and NO3
involves complex reactions. Strong CL emission can be observed when MnO4
is acted as an oxidizing agent. Gerischer

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et al. found that MnO4
can inject holes into n-type silicon (Gerischer and Lubke 1988). Kooij et al. found that reduction of HNO3 by hole injection and weak CL emission can only be observed when the concentration of HNO3 is high. From the results of different pSi-based CL systems, the quantum efficiency of CL is highly correlated with the ability of hole injection caused by the reduction of oxidizing reagents. Those (Ce4+, MnO4
) with high capability of hole injection can generate strong CL emission, while oxidizing agents (S2O82
, H2O2, and HNO3) with low ability of hole injection at interface of pSi/electrolyte only produce weak CL or even give no CL. It is reasonable to expect the generation of a band-edge CL if one electron and one hole are simultaneously injected into a Si nanocrystal from the same solution. Under this circumstance, both p-type and n-type pSi can act as the CL emitter. Kooij el al. proposed that there is an analogy between CL and ECL emission during anodic polarization of pSi (Hory et al. 1995; Kooij et al. 1997). In this way, the holes injected in reaction (1) are trapped in Si-Si surface bonds, as described in reaction (5): H

H hVB

Si

ð5Þ

Si Si

Si

bond can react with water to form Si-OH bond and surface radical intermediate (reaction 6): H

H

H 2O

SiH

H

Si

ð6Þ

Si OH

Si

The radical intermediate likely has energy level in the bandgap of pSi. Electron in such levels can be thermally excited to the conduction band (reaction 7): H

H

Si Si OH

Si

Si O

+

e–CB + H

ð7Þ

Accompanied with surface oxidation, radiative recombination of the injected electron in conduction band with a hole in valence band of Si nanocrystal results in CL emission. However, there is a competitive reaction (8), in which the radical intermediate subsequently captures a second hole from the valence band. This reaction will not lead to light emission:

Chemiluminescence of Porous Silicon

471 H

H +

Si Si OH

hVB

Si

Si O

+ H

ð8Þ

Conclusions In conclusion, the quantum efficiency of CL from pSi depends on many factors. Among them, hole injection capability of oxidizing agents exerts a great influence on the CL intensity. The reduction of different oxidizing agents follows different pathways, ultimately leading to different quantum yields of CL emission. Before the radiative recombination of electron-hole pair, several competitive reactions occurred on surface of pSi will lead to diminishing CL emission. Chemiluminescence work on CdTe quantum dots (QDs) indicated that several factors, such as the size of particles, aggregate state, surface capping, and microenvironment of QDs surrounded by micelles, can significantly affect the CL quantum efficiency of CdTe QDs (Wang et al. 2005). This work gives a new pathway to further improve the efficiency of CL from pSi. Until now, the applications of chemiluminescence from pSi have been rare. Unlike other types of semiconductor quantum dots, the size of pSi particle is not small enough to act as a labeling agent in many bioanalytical applications. Our recent work reveals that the kinetics of ECL from pSi is highly correlated with the nature of surface-adsorbed chemicals (Tan et al. 2014). Based on this finding, an image contrast technology was successfully established to visualize chemical patterns left on a pSi chip (Fig. 1b). Owing to the similarity between ECL and CL, future application of CL emitted from pSi can be expected.

References Brandt MS, Fuchs HD, Stutzmann M, Weber J, Cardona M (1992) The origin of visible luminescence from “porous silicon”: a new interpretation. Solid State Commun 81:302 Bressers PMMC, Knapen JWJ, Meulenkamp EA, Kelly JJ (1992) Visible light emission from a porous silicon/solution diode. Appl Phys Lett 61:108 Campbell AK (1988) Chemiluminescence: principles & Applications in Biology & Medicine. VCH, Weinheim, 608pp Canham LT (1990) Silicon quantum wire array fabrication by electrochemical and chemical dissolution of wafers. Appl Phys Lett 57:1046 Canham LT, Leong WY, Beale MIJ, Cox TI, Taylor L (1992) Efficient visible electroluminescence from highly porous silicon under cathodic bias. Appl Phys Lett 61:2563 Deak P, Rosenbauer M, Stutzmann M, Weber J, Brandt MS (1992) Siloxene: chemical quantum confinement due to oxygen in a silicon matrix. Phys Rev Lett 69:2531 Ding Z, Quinn BM, Haram SK, Pell LE, Korgel BA, Bard AJ (2002) Electrochemistry and electrogenerated chemiluminescence from silicon nanocrystal quantum dots. Science 296:1293 Gee A (1960) Electrochemiluminescence at a silicon anode in contact with an electrolyte. J Electrochem Soc 107:787

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Gerischer H, Lubke M (1988) On the etching of silicon by oxidants in ammonium fluoride solutions: a mechanistic study. J Electrochem Soc 135:2782 Hory MA, Hérino R, Ligeon M, Muller F, Gaspard F, Mihalcescu I, Vial JC (1995) Fourier transform IR monitoring of porous silicon passivation during post-treatments such as anodic oxidation and contact with organic solvents. Thin Solid Films 255:200 Koch EC, Clement D (2007) Special materials in pyrotechnics: VI Silicon – an old fuel with new perspectives. Prop Explos Pyrotech 32:205 Kooij ES, Noordhoek SM, Kelly JJ (1996) Reduction of peroxodisulfate at porous and crystalline silicon electrodes: an anomaly. J Phys Chem 100:10754 Kooij ES, Rama AR, Kelly JJ (1997) Infrared induced visible emission from porous silicon: the mechanism of anodic oxidation. Surf Sci 370:125 Kooij ES, Butter K, Kelly JJ (1998) Hole injection at the silicon/aqueous electrolyte interface: a possible mechanism for chemiluminescence from porous silicon. J Electrochem Soc 145:1232 Koshida N, Koyama H (1992) Visible electroluminescence from porous silicon. Appl Phys Lett 60:347 McCord P, Yau SL, Bard AJ (1992) Chemiluminescence of anodized and etched silicon: evidence for a luminescent siloxene-like layer on porous Silicon. Science 257:68 Meulenkamp EA, Bressers PMMC, Kelly JJ (1993) Visible chemiluminescence and electroluminescence of porous silicon. Appl Surf Sci 64:283 Meulenkamp EA, Cleij TJ, Kelly JJ (1994) Electroluminescence and chemiluminescence of porous silicon in nonaqueous solution. Electrchem Soc 141:1157 Pickering C, Beale MIJ, Robbins DJ, Pearson PJ, Greef R (1984) Optical studies of the structure of porous silicon films formed in p-type degenerate and non-degenerate silicon. J Phys C 17:6535 Poznyak SK, Talapin DV, Shevchenko EV, Weller H (2004) Quantum dot chemiluminescence. Nano Lett 4:693 Tan J, Xu L, Li T, Su B, Wu J (2014) Image-contrast technology based on the electrochemiluminescence of porous silicon and its application in fingerprint visualization. Angew Chem Int Ed 53:9822 Turner DR (1958) Electropolishing silicon in hydrofluoric acid solutions. J Electrochem Soc 105:402 Uhlir A (1956) Electrolytic shaping of germanium and silicon. Bell Syst Tech J 35:333 Van de Walle CG, Northrup JE (1993) First-principles investigation of visible light emission from silicon-based materials. Phys Rev Lett 70:1116 Wang Z, Li J, Liu B, Hu J, Yao X, Li J (2005) Chemiluminescence of CdTe nanocrystals induced by direct chemical oxidation and its size-dependent and surfactant-sensitized effect. J Phys Chem B 109:23304

Thermoluminescence of Porous Silicon Valeriy Skryshevsky

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermoluminescence Measurement for Electronic Trap Detection . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermoluminescence Glow Curves of Nanoporous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Thermoluminescence-Based Dosimetry of Ionizing Radiations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scintillator-Porous Silicon Composites . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

474 475 475 479 483 484 484

Abstract

In this updated and expanded review, the thermoluminescence (TL) of porous silicon (PS) is discussed as both a method for characterization of its electronic states and as a dosimeter of ionizing radiations. The observed shape of the TL peaks in PS at low temperatures (4–250 K) is explained by a quasi-continuous spectrum of electron traps with activation energy in range of 0.03–0.4 eV. The high-energy peaks observed at 100–300
C are associated with radiation-induced defects E` ( Si•) and nonbridging oxygen hole centers (Si-O•) that are generated in the insulating SiOx layer which covers the PS surface. TL of PS is not currently used for radiation dosimetry due to the low activation energies of the traps and strong fading. Nevertheless, related PS materials (like oxidized PS, silicon nanoparticles in solid matrix, various nanocomposites of scintillation materials in PS) are considered as promising for dosimetry due to high luminescence quantum yield, emission in the visible region, and their biocompatibility that allows to create in vivo dosimetry systems of high spatial resolution. Commercial scintillator-PS composites are also under development.

V. Skryshevsky (*) Institute of High Technologies, Taras Shevchenko National University of Kyiv, Kyiv, Ukraine e-mail: [email protected]; [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_35

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Keywords

Thermoluminescence · Electronic trap · Porous silicon · Photoluminescence · Dosimeter

Introduction Thermally stimulated luminescence or thermoluminescence (TL) is the emission that arises during the heating of the pre-irradiated substance (McKeever 1988; Furetta 2003). During the excitation by light, X- and γ-rays, or other radiation at low temperatures, the electrons can be captured onto electron traps. If the end of excitation is followed by heating, trapped electrons can be released and recombine with free holes (or holes trapped at a recombination centers) causing the light emission. The probability W per unit time that electron will escape from a trap obeys the Boltzmann equation: W ¼ S expðEa =kTÞ

(1)

where S is the frequency factor, depending on the frequency of the number of hits of an electron in the trap which can be considered as a potential well, Ea is the thermal activation energy required to liberate a trapped charge carrier, k is Boltzmann’s constant, and T is the absolute temperature (Rivera 2011). TL is a powerful technique extensively used for dosimetry of ionizing radiations since the irradiation results in generation of electron-hole pairs in material followed by their capture in traps and contains information about the absorbed dose. The efficiency of material as dosimeter is characterized by luminescence yield Y, i.e., the number of photons emitted after exposure to irradiation (in photon/MeV). TL dosimeters are used in a variety of medical applications including diagnostic radiology and radiotherapy. Among TL dosimetry, the most widespread is radiation monitoring of personal working around sources of ionizing radiations enabling detection of small doses (Moscovitch and Horowitz 2007; Kortov 2007). Another TL application is the analysis of the electronic states in the material, which are defined by both the material and technology of its production. For example, for porous silicon (PS), different contact methods to define the electronic states in metal/PS structures have been already applied: thermally stimulated depolarization currents (TSDC) and thermally stimulated current (TSC) (Ciurea et al. 1998; Anastasiadis and Triantis 2000; Brodovoy et al. 2002), optical charging spectroscopy (OCS) (Ciurea et al. 2000), and deep-level transient spectroscopy (DLTS) (Pinčik et al. 1999; Skryshevsky et al. 2006). The above methods determine the parameters of traps related with both PS material and metal/PS interface and often differ from results obtained from TL experiments. The primary advantages of the TL method are that it is contactless and defines energetic spectra of bulk and/or surface states. The primary limitation of TL is obvious; it can be applied only for luminescent materials.

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Thermoluminescence Measurement for Electronic Trap Detection In the usual TL experiments, the samples are irradiated at low temperature (when kT730 nm, (5) λ 5 h Minutes

10–10 5–1.5  10

p+n(L)

p+n+(L)

Au

ITO

ECO

3

3–1

Au

2 min exposure in 10% HNO3

5

p and p+n (L) p+n(L)

1 min L

Au

EL threshold V-mA/cm2 6 h 80 h Hours Seconds

Table 3 EL characteristics of most devices based on porosified pn junction

650

690

670–780

650

Emission peak (nm) 640 700 600 630

1.1–0.08

0.8–0.07

0.2

0.01

Highest efficiency EQE-EPE (%) 0.18 10 2 0.180.16–0.016

References Chen et al. (1993) Steiner et al. (1993) Loni et al. (1995) Linnros and Lalic (1995) Peng and Fauchet (1995) Lalic and Linnros (1996a, b) Nishimura et al. (1998) Gelloz et al. (1998a)

492 B. Gelloz

HWA ECO + HWA 4

4

4

2.2–7  10 2–10 2–10

4

3.5–4  10 3–10 4

1.5–2

EL threshold V-mA/cm2

Hours > 1 week Days, EQE is stable Stable Stable

1 month

Stability >7 h

700 820

680

640 640

620–770

Emission peak (nm) 460–550

10 –

3

1.07–0.37

0.51–0.05 0.21–0.02

0.1

Highest efficiency EQE-EPE (%)

References Fauchet et al. (1997), Tsybeskov et al. (1995, 1996) Fauchet et al. (1997), Tsybeskov et al. (1995, 1996) Gelloz et al. (1998a) Gelloz et al. (1998b) Pavesi et al. (1999) Gelloz and Koshida (2000, 2004a) Gelloz and Koshida (2006) Gelloz et al. (2006)

D and L mean that anodization was conducted in the dark and under illumination, respectively. EQE and EPE are external quantum efficiency and external power efficiency, respectively

ITO ITO

Anneal in N2 or in 10% O2 in N2 ECO ECO ECO ECO

p+p(D)

Alpoly Si ITO ITO Al/n+ ITO

n+(L) n+(L) p(L) n+(D) n+(L) n +p n+

Posttreatment H2O2 oxidation

Structure n(UV)

Contact Au

Table 4 EL characteristics of most devices based on partially oxidized porous silicon

Electroluminescence of Porous Silicon 493

n+(D)

n(L)

n(L:UV)

n(L:UV) n(L:UV)

n(L:UV)

n(L:UV)

Au

Au

Au

Au Au

Au

Au

Al electroplating

Sb electroplating

Ga electroplating Sn electroplating

Posttreatment Polypyrrole electrodeposition PANI chemical deposition Polyaniline chemical deposition In electroplating

0.1

0.1

500

3–400

EL threshold V-mA/cm2 2– 500°C

O C

C

Si C

C

C

Si C

C

ð10Þ Modification strategies that generate functional nanostructures from the high carbon-content porous Si have been developed (Jalkanen et al. 2009; Salonen et al. 2006; Bjorkqvist et al. 2004b, 2005; Sciacca et al. 2010).

Porous Silicon Nitrides Silicon nitride as a surface modification for porous Si has been less well studied. Routes into this chemistry somewhat parallel the industrial processes used to prepare silicon nitride: direct reaction of Si with N2 at high temperature, and chemical vapor deposition from N2, NH3, and silane precursors (for thin films). Porous Si can be nitrided by heating it in NH3 or N2 ambients (Bjorkqvist et al. 2003; Morazzani et al. 1996; Lai et al. 2012) or by plasma-assisted CVD. A low-temperature (600  C) process that preserves porosity and pore size has been developed (James et al. 2010). These methods tend to incorporate substantial quantities of silicon oxide (Lai et al. 2011). The chemical stability of nitrided porous Si is similar to the thermally grown oxide (Bjorkqvist et al. 2003; James et al. 2009, 2010; Lai et al. 2011).

Attachment of Biomolecules to Functionalized Porous Si Most of the above surface modification chemistries are amenable to attachment of functional molecules such as antibodies (Andrew et al. 2010; Tinsley-Bown et al. 2000; Gu et al. 2012; Serda et al. 2010; Lowe et al. 2010; Wu et al. 2009; Rossi et al. 2007; Meskini et al. 2007; Bonanno and DeLouise 2007; Park et al. 2006; Starodub et al. 1996) and other therapeutics (Wu et al. 2008, 2011a, b; Chhablani et al. 2013;

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Sailor and Park 2012; Sweetman et al. 2015; Nieto et al. 2015; Salonen 2014; Guan et al. 2014; Canham 2014; Zilony et al. 2013; Tzur-Balter et al. 2013a, b; Tang et al. 2013; Secret et al. 2013). In addition, sugars and polyethers are readily attached to porous Si and are useful as antifouling coatings in biomedical applications (Schwartz et al. 2005; Kilian et al. 2007a, b; Godin et al. 2010; Velleman et al. 2010). For more detailed information on attachment choices and methodology, see the dedicated chapter ▶ “Biomolecule Attachment to Porous Silicon.”

Perspective Historically, it often seemed like the chemistry of porous Si was too unstable and unpredictable to be worth the trouble. Whereas the chemical instability challenged researchers trying to make electroluminescent devices or Li ion batteries, it has been effectively harnessed in areas like biomedical implants and drug delivery devices where chemical instability (in particular the harmless dissolution of the material) has been a key advantage. The advent of very stable carbonization (Salonen et al. 2002), metallization (Tsuboi et al. 1998; Fuertes et al. 2009; Harraz et al. 2001, 2002b, 2003a, b; Kawamura et al. 2005; Ogata et al. 2000; Sasano et al. 2003a, 2005; Hamm et al. 2004; Fukami et al. 2009; Panarin et al. 2007; Fukami et al. 2008; Sasano et al. 2003b; Steiner et al. 1995a, b; Jeske et al. 1995; Coulthard and Sham 1997; Hamadache et al. 2002; Presting et al. 2004; Ogata et al. 2006; Liu et al. 2011), and oxidation (Gelloz et al. 2006, 2012; Gelloz and Koshida 2012) reactions has inspired a revisitation of some of the older applications where low stability was a problem. For the applications where controlled degradability is desired, the chemistry of porous Si has enabled a range of functionalities and tunabilities.

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Biocompatibility of Porous Silicon Suet P. Low and Nicolas H. Voelcker

Contents Biocompatibility . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Biodegradability . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Cytotoxicity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Fate of Porous Silicon Particles in the Body . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . In Vivo Behavior of Porous Silicon Implants . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Toward In Vitro and In Vivo Biosensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Porous Silicon for Tissue Engineering . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Localized Drug Delivery . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Vaccine Development . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The biocompatibility of porous silicon is critical to its potential biomedical uses, both in vivo within the human body for therapy and diagnostics, and in vitro for biosensing and biofiltration. Published data from cell culture and in vivo studies are reviewed, and a number of emerging applications for bioactive or biodegradable silicon are discussed.

S. P. Low (*) Mawson Institute, University of South Australia, Adelaide, SA, Australia e-mail: peng.low@flinders.edu.au N. H. Voelcker ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Future Industries Institute, University of South Australia, Adelaide, SA, Australia e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_38

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Keywords

Biocompatibility · Biosensors · Cytotoxicity · Drug delivery · Lactate dehydrogenase (LDH) · Tissue reengineering · Vaccination

Biocompatibility The term “biocompatibility” is defined as “the ability of a material to perform with an appropriate host response in a specific situation” (Williams 2008). A biocompatible material can be inert, where it would not induce a host immune response and have little or no toxic properties. A biocompatible material can also be bioactive, initiating a controlled physiological response. For porous silicon, bioactive properties were initially suggested based on the observation that hydroxyapatite (HA) crystals grow on microporous silicon films. HA has implications for bone tissue implants and bone tissue engineering (Canham 1995). An extension of this work showed that an applied cathodic current was able to further promote calcification on the surface (Canham et al. 1996). More recently, Moxon et al. showed another example of bioactive porous silicon where the material promoted neuron viability when inserted into rat brains as a potential neuronal biosensor, whereas planar silicon showed significantly fewer viable neurons surrounding the implant site (Moxon et al. 2007).

Biodegradability A comprehensive review on pSi biodegradability is covered in chapter ▶ “Biodegradability of Porous Silicon”. We discuss this here as the degradation rate, and products can influence its biocompatibility in biomedical applications. Porous silicon is instable in aqueous solutions and degrades into orthosilicic acid (Si(OH)4) (Allongue et al. 1993) as a result of oxidative hydrolysis (Scheme 1). Silicic acid is a nontoxic small molecule and the common form of bioavailable silicon in the human body (Carlisle 1972, 1982). Silicic acid does not accumulate within the human body and has been shown to be absorbed readily by the gastrointestinal tract of humans and is rapidly excreted via the urinary pathway (Reffit et al. 1999). Although silicic acid at concentrations of 2 mM has been reported to be cytotoxic to fibroblasts and macrophages (Tanaka et al. 1994), high concentrations of silicic acid up to 100 mM have been tested in vitro on cells with no apparent affect on their viability (Mayne et al. 2000). The rate of dissolution can be controlled by the porosity of porous silicon (Anderson et al. 2003) and by its surface chemistry (Canham et al. 1999, 2000). Silicon with medium porosity (62% porosity) shows slow degradation, whereas higher porosity silicon (>80% porosity) showed exponential release of silicic acid over time (Anderson et al. 2003). Surface modification has been applied to the porous silicon surface to impart protection against hydrolytic attack and has the dual role of being able to change the surface chemistry (Low et al. 2006). By

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Scheme 1 Proposed mechanism for porous silicon degradation in aqueous solutions, adapted from Allongue et al. (1993). (a) A Si–H-terminated surface immersed in H2O. (b) The Si–H bond undergoes hydrolytic attack and is converted to Si–OH and produces a hydrogen molecule. (c) The Si–OH at the surface polarizes and weakens the Si–Si backbonds, which are then attacked by H2O, producing HSi(OH)3. (d) In solution, the HSi(OH)3 molecule is quickly converted to Si(OH)4 releasing a second hydrogen molecule

applying different surface modifications, porous silicon degradation rates can be tuned anywhere from minutes to months (Godin et al. 2010). This makes porous silicon as an ideal transient material for localized drug delivery or cell delivery purposes. The degradation rate of porous silicon increases with increasing pH (Anderson et al. 2003), and the local tissue pH therefore has to be taken into consideration when designing porous silicon for a certain biomaterial application. Different methods for surface modification and subsequent effect on cells are covered in chapters ▶ “Functional Coatings of Porous Silicon” and ▶ “Silicon–Carbon Bond Formation on Porous Silicon”. In brief, the functional groups presented on the surface of porous silicon allow for the attachment of biological factors and proteins in culture medium, which in turn influence cell attachment. Several in vitro culture studies have shown that surface modification of the porous silicon surface can modulate cell attachment and growth (Low et al. 2006; Yang et al. 2010). Neuroblastoma (Low et al. 2006; Yang et al. 2010; Gentile et al. 2012; Khung et al. 2006), human embryonic kidney cells (Sweetman et al. 2011), B50 cells (Mayne et al. 2000; Bayliss et al. 1997, 1999), and primary mesenchymal cells (Clements et al. 2011; Noval et al. 2012) are a few cell types that have been successfully cultured on porous silicon surfaces.

Cytotoxicity As discussed above, the degradation products of porous silicon have been shown to be relatively harmless and have opened the use of this material in biological environments. The interaction of porous silicon and cells is covered in chapter ▶ ”Cell Culture on Porous Silicon”, but silicic acid is not the only degradation product that may induce cytotoxicity. It has been recently demonstrated that porous silicon is capable of producing reactive oxygen species (ROS) (Belyakov et al. 2007; Kovalev et al. 2004). ROS have important physiological roles such as signalling

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molecules to regulate cell proliferation, apoptosis, and differentiation (Finkel and Holbrook 2000). ROS generation by porous silicon is directly related to the surface chemistry (Kovalev et al. 2002, 2004), and therefore, porous silicon particles are more susceptible to generating ROS. Recent investigations have demonstrated that untreated particles generate ROS at concentrations that lead to cell death, whereas simple surface stabilization with oxidation was able to mitigate this effect (Low et al. 2010; Santos et al. 2010). The size of porous silicon particles is also an important factor. Particles smaller than 3 μm have been demonstrated to be cytotoxic to monocytes (Ainslie et al. 2008); loss in Caco-2 cell metabolic activity was seen with particles between 1.2 and 25 μm in size (Santos et al. 2010). In contrast, particles below 500 nm were demonstrated to be nontoxic to lymphoma cells (De Angelis et al. 2010), and particles smaller than 1 μm did not cause any cytotoxic effects with macrophage and endothelial cells (Godin et al. 2012).

Fate of Porous Silicon Particles in the Body The retention of porous silicon in the body has recently been shown to be transient. Intravenous injection of porous silicon nanoparticles into mice leads to its accumulation within the liver and the spleen, demonstrating rapid removal from the circulatory system (Bimbo et al. 2010; Park et al. 2009). Another study intravenously injected oxidized and aminosilane-functionalized porous silicon microparticles into mice. The study revealed that surface chemistry and charge affected microparticle distribution within the body (Tanaka et al. 2010a). In this study, porous silicon microparticles after intravenous administration were also found to accumulate within the liver and spleen. The enzyme lactate dehydrogenase (LDH) is often used as an indicator of tissue damage, and this study found LDH levels were only increased after multiple administrations of the particles and cytokine levels remained stable. There was no difference in LDH levels between particles with different surface chemistries. Other studies have shown that intravenously administered particles that accumulated within the liver and spleen degraded over a period of 4 weeks, and cells within these organs retained their normal morphology (Park et al. 2009). Tanaka et al. showed in a mouse model that injected porous silicon particles loaded with siRNA accumulated primarily within the liver and spleen (Tanaka et al. 2010b). Clearance or degradation of the particles within these organs occurred within 3 weeks for the spleen but significantly longer in the liver, indicating that degradation kinetics of the porous silicon particles was organ dependent. No tissue injury or inflammatory cytokines were detected for the organs investigated, and the morphology of the cells within these organs remained unchanged. This study again demonstrated that porous silicon did not cause any adverse tissue effects when injected into the body. For drug delivery applications, ingestion of porous silicon is of particular interest. Porous silicon is reasonably stable at low pH (Anglin et al. 2008), showing degradation kinetics that are suitable for drug delivery to the intestine. An investigation into

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the distribution of ingested porous silicon microparticles showed that they passed through the gastrointestinal tract without signs of uptake of particles within the lining of the gastrointestinal tract (Bimbo et al. 2010). Another study utilized 18F-radiolabeled thermally hydrocarbonized porous silicon particles to investigate the accumulation and retention of ingested particles in a rat model. Once again, the particles were stable in stomach acid, remained in the GI tract, and did not cross the intestinal cell layer (Sarparanta et al. 2011). Another article by the same authors attached the protein hydrophobin class II to the thermally carbonized particles. This conveyed mucoadhesive properties to the particles, allowing them to attach to gastric cells. The particles were stable in gastric fluid, and in vivo experiments showed that the particles were retained within the stomach cavity for a period of time. However, upon entering the intestinal tract, the particles lost their adhesive properties and were quickly expelled (Sarparanta et al. 2012). Particle size can also be used to control the distribution throughout the body and can be a form of targeted drug delivery. Porous silicon particles larger than 519 nm in diameter are unable to cross the placenta into a fetus and can help prevent fetal exposure to administered drugs (Refuerzo et al. 2011).

In Vivo Behavior of Porous Silicon Implants A range of in vitro cell culture studies have been carried on porous silicon to demonstrate the suitability of this material as a support for mammalian cells. Implants, on the other hand, can experience a variety of tissue and host immune responses, such as generalized cytotoxic effects, microvascularization, and hypersensitivity to the implant. To date, there have been limited numbers of studies on the effects of porous silicon in vivo. In 2000, Rosengren et al. implanted unmodified porous silicon into the abdominal wall of rats with flat silicon and titanium as controls. An inflammatory response was observed with a resultant fibrous capsule forming around the implant with minimal cell death at the cell-porous silicon interface, and it was noted that the tissue response was similar for porous silicon, flat silicon, and titanium (Rosengren et al. 2000). Fibrous encapsulation of an implant is a common tissue response to a foreign body (Ratner and Bryant 2004). The factor determining the outcome of the implantation is the thickness of the capsule and the degree of inflammation around the capsule, often measured by the frequency of inflammatory cells such as macrophages and foreign body giant cells. Excessive capsule thickness or inflammation can cause pain or discomfort around an implant and can ultimately result in implant rejection. For active pSi implants, such as devices for drug delivery, this encapsulation may also influence the rate of drug release. With the aim of developing a drug delivery vehicle, hydrosilylated and thermally oxidized porous silicon microparticles were injected into the vitreous of rabbit eyes (Cheng et al. 2008). It was noted that the particles settled into the inferior vitreous cavity over a few days. Degradation of the hydrosilylated particles took considerable time (>4 months) in comparison to untreated particles which degraded within a period of 3–4 weeks. No adverse effects were observed in ocular tissues including the retina and the lens. Furthermore, normal ocular pressure was maintained.

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Fig. 1 Thermally oxidized and amino-silanized porous silicon membranes containing cultured limbal cells were implanted under the conjunctiva of rats. (a) Images of the implant site after 0, 3, 6, and 9 weeks showing gradual dissolution of the membrane. (b) Histological analysis of the implant site with the porous silicon (PS). Small amount of inflammatory cells (IC) are found and the formation of a fibrous capsule around the porous silicon membrane (F) (Low et al. 2009)

Implants of porous silicon membranes under the rat conjunctiva demonstrated similar results (Fig. 1). A small inflammatory response was initially observed, but histological examination of the implant site showed that a thin fibrous capsule formed around the porous silicon membrane with only a small fraction of inflammatory cells surrounding the implant site (Low et al. 2009). The capsule around was significantly thinner than the fibrous capsule surrounding the surgical sutures used to hold the membranes in place. Tissue erosion and vascularization were absent, indicating that porous silicon was highly biocompatible within tissues of the eye. Outside of the eye, the bioactive properties of pSi implants were investigated in contact to nerve tissue. Porous silicon films on bulk silicon supports were implanted into the sciatic nerve of a rat. Nerve tissue could hence grow on the porous region or the flat region. The authors observed that the formed fibrous capsule formation was significantly thinner on the porous silicon region in comparison to the flat silicon region. They postulated that the porous nature allowed for the implant to anchor strongly to the tissue and thus prevent sheer forces that may influence the formation of fibrous capsules. They also determined that a greater percentage of axons formed on the porous silicon, further highlighting the bioactivity of porous silicon in terms of promoting neural cell formation (Johansson et al. 2009).

Toward In Vitro and In Vivo Biosensors Porous silicon is rousing interest in the biosensor community because of several unique intrinsic material properties. First and foremost are the optical properties which include photoluminescence, thin-film reflectance, and photonic effects (Jane et al. 2009). Second, the material has a high surface area (allowing higher binding density as compared to flat

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surfaces), the ability to introduce size exclusion layers (filtering out undesired molecules), and a well-developed surface chemistry with a range of options for bioreceptor immobilization. The photoluminescence properties of porous silicon have been exploited since the initial discovery of this effect (Canham 1990). Quenching of the photoluminescence signal has been utilized to detect proteins and enzyme activity (Letant et al. 2004), selectively capture streptavidin biomolecules (Letant et al. 2003), selectively detect myoglobin from a serum solution (Starodub et al. 1999), and capture DNA (Chan et al. 2000). However, the simplest form of a porous silicon biosensor utilizes thin-film interference effects, resulting in a characteristic Fabry-Perot fringe pattern. Changes in the position of the fringe pattern indicate the binding or loss of molecules within the porous layer (Brecht and Gauglitz 1995). This technique has been utilized to detect down to femtomolar concentrations of proteins and DNA binding to the porous silicon surface (Lin et al. 1997; Steinem et al. 2004; Szili et al. 2011). Porous silicon structures with alternating layers of low and high porosity show 1D photonic effects with sharp stop bands. Depending on the interface between the layers, these structures are termed Bragg mirrors or rugate filters (Pavesi and Dubos 1997). The binding to or release of molecules from the porous layer leads to shifts in the spectral peak (Guillermain et al. 2007). The photonic properties of porous silicon have been used by the Sailor group for the in situ monitoring of cell viability. This concept has been coined the “smart Petri dish.” A light source is aimed at an incident angle which is reflected away from the detector. Cells attached to the porous silicon surface scatter some of the light back to the detector, leading to a small spectral peak. Change in cell morphology as a result of cell death leads to an increase in light scattering and therefore an increased detector signal. This allows the label free and in situ monitoring of cell viability without the need for adding dyes into the cell culture medium (Schwartz et al. 2006). The described optical effects could also be used for implanted biosensors which combine the aspects of biocompatibility and biodegradability with the optical effects which are retained upon implantation. Monitoring of a sensor implanted underneath the skin can be accomplished by merely irradiating the sensor with a light source and collecting the reflected spectra. A drawback is the fouling of the sensor when placed into a complex biological environment which will interfere with the sensor readout. The Gooding group utilized hydrosilylation and subsequent conjugation of oligoethylene oxide (OEG) moieties to produce a non-fouling layer, which effectively prevented the adhesion of proteins while still maintaining reflectivity, even when placed into human blood plasma (Kilian et al. 2007). This feat bodes well for the possibility of in situ monitoring in biological fluids and in vivo.

Porous Silicon for Tissue Engineering The biocompatible, bioactive, and biodegradable properties of porous silicon render this material a suitable scaffold for tissue reengineering. For example, with relevance to neural engineering, it has been demonstrated that porous silicon is able to support the growth of neuronal cells that still maintain their action potential capabilities (Ben-Tabou de Leon et al. 2004). Another study has found

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that porous silicon with an average pore diameter of 300 nm is able to guide axonal growth (Johansson et al. 2005, 2008) suggesting a potential application for porous silicon in nerve regeneration. The early studies identifying bioactive porous silicon in the ability to form HA crystals have generated interest in its use as a bone matrix alternative. Porous siliconsubstituted HA structures implanted into the femur displayed better bone integration around the implant compared to a nonporous HA implant (Porter et al. 2006). This was attributed to the porous silicon degradation, revealing voids for bone ingrowth and allowing dynamic remodeling. The biodegradable nature of porous silicon makes it an ideal carrier for cell therapy applications. Here, stem cells on a suitable carrier are implanted into a host, for example, in order to regenerate tissue function. After delivery of the cells into the host, it is desirable that the carrier be degradable in vivo. As a potential treatment for ocular surface disease, human limbal stem cells isolated from the cornea have been expanded on porous silicon membranes as carriers and used to demonstrate cell outgrowth from membranes in an animal model. The stem cell migrated from the porous silicon membrane into the surrounding tissue, and histological analysis of the porous silicon membranes after 8 weeks showed low inflammatory response and absence of vascularization of the implant and significant implant degradation (Low et al. 2009).

Localized Drug Delivery Porous silicon has also been shown to be an effective platform in sustained drug delivery exploiting the large drug loading capacity that stems from the large internal surface. A study looked at five different orally received drugs and their compatibility with a porous silicon delivery vehicle. The drugs were loaded into thermally carbonized and thermally oxidized porous silicon particles. Water-soluble drugs usually display fast drug release profiles, while poorly soluble drugs show slow release kinetics (Salonen et al. 2005). Porous silicon particles were able to moderate the drug release kinetics, on the one hand reducing the release rate of water-soluble drugs while on the other hand enhancing release kinetics for poorly soluble drugs (Wang et al. 2009). Coupled with its stability at low pH (Anglin et al. 2008), porous silicon makes an ideal carrier for oral drug delivery into the small intestine. A tenfold increase in permeation of insulin across intestinal cell layers was achieved when delivered with porous silicon particles over traditional soluble permeation solutions (Foraker et al. 2003), suggesting that apart from moderating drug release, porous silicon can enhance drug absorption by the body. Another significant advantage of using porous silicon particles is to be loaded with a high payload of drug so that a single dose of the drug delivery system suffices for continuous therapeutic effects, avoiding repeated drug administrations. Here, amino-functionalized porous silicon particles have been used to deliver siRNA to successfully silence a gene for an oncoprotein in vivo where the silencing effect was maintained for several weeks, whereas traditional applications required multiple administrations (Tanaka et al. 2010b).

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The porous silicon particles have also been used for sustained peptide delivery in rats. In this case, surface modification was used to modulate peptide release from the particles. Thermally carbonized, thermally oxidized, and undecylenic acidconjugated thermally carbonized particles were loaded with a peptide. They found that as peptide release was partially related to the degradation kinetics of pSi, the thermally oxidized particles, which degraded the fastest, had the greatest release over a 2-week period both in vitro and in vivo, whereas the more stable thermally carbonized particles released less peptide (Kovalainen et al. 2012). This was also demonstrated with a ghrelin antagonist; sustained release was achieved with this peptide over 17 h when loaded into pSi particles, and without the particles, the peptide lost its activity within 4 h (Kilpeläinen et al. 2009). These studies demonstrate the advantages of using pSi as a carrier vehicle for protein delivery. Bioactivity of proteins can be preserved, and the lifetime of a loaded protein can be extended, leading to a sustained drug delivery profile.

Vaccine Development A further application for biocompatible porous silicon relates to vaccination using antigen-loaded particles. Porous silicon particles were conjugated to antigens that specifically target the toll-like receptors on dendritic cells (DC). This stimulated phagocytosis by dendritic cells, maturing the cells to become antigen-presenting cells (Fig. 2) (Meraz et al. 2012). The activated DCs increased proinflammatory cytokines IL-1β, TNF-α, and IL-6, and when the activated DCs were injected into

Fig. 2 Pseudocolored SEM images of dendritic cells at low (top) and high (below) magnification. (a) Cells only; (b) dendritic cells phagocytosing porous silicon particles and (c) porous silicon particles loaded with liposaccharide antigen being taken up by the dendritic cells (Meraz et al. 2012)

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mice, they migrated into the lymphatic system where they activated T cells by upregulation of cell surface receptors and presenting the antigen along with major histocompatibility (MHC), all of which play a role in mediating an active immune response. This study demonstrated effective stimulation of the immune system with antigen-loaded porous silicon which is highly relevant to the development of vaccines for various diseases.

Summary Since the discovery that porous silicon can stimulate the formation of HA crystals in simulated body fluid, the use of porous silicon in biomaterial applications has soared. Apart from bioactivity, properties such as in vitro and in vivo biocompatibility, biodegradability, high surface area, tunability of pore size and porosity, and finally ease of surface modification have contributed to this increasing interest. These properties open exciting avenues for neural, ocular, and bone tissue engineering and also for drug and vaccine delivery. Combining the biocompatibility with the material’s optical properties of porous silicon enables diagnostic applications such as smart tissue cultureware and implantable biosensors.

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Biodegradability of Porous Silicon Qurrat Shabir

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanism of Biodegradation and Degradation Products . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Kinetics of Degradation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

There is increasing interest in biodegradable silicon nanostructures for drug delivery and theranostic formulations, as well as “transient” microelectronic implants. The medical biodegradability of mesoporous silicon is now established both in vitro and in vivo. This updated review highlights the techniques used to date to characterize this phenomenon, the degradation kinetics, and the various factors that can influence the kinetics of dissolution into orthosilicic acid. Recent in vivo studies have started to address the in vitro to in vivo correlation for specific tissue microenvironments. Keywords

Porous silicon · Biodegradability · Orthosilicic acid · Drug delivery · Transient electronics · Degradation kinetics · In-vitro · In-vivo

Q. Shabir (*) Department of Molecular and Clinical Pharmacology, University of Liverpool, Liverpool, UK e-mail: [email protected] # Her Majesty the Queen in Right of United Kingdom 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_39

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Introduction There is growing interest and acceptance in replacing permanent prostheses by temporary ones in the human body. These would in effect help the body to heal itself and require biomaterials to have “biodegradability” within physiological environments (Ratner et al. 2004). Currently four different terms are found in the literature to signify that a material or device will eventually disappear after having been introduced into a living organism: biodegradation, bioerosion, bioabsorption, and bioresorption (Ratner et al. 2004). Unfortunately no agreed distinctions exist; we will use the biodegradability term here. Figure 1 shows examples of biodegradable materials and their uses. The in vitro discovery in 1995 that high-porosity mesoporous silicon (pSi) can be rapidly biodegradable, unlike solid crystalline silicon (Canham 1995), and subsequent in vivo demonstrations of biodegradability and biocompatibility (Bowditch et al. 1999; Park et al. 2009; Sarparanta et al. 2012; Tolli et al. 2014) have been very significant in this regard. The huge surface area (e.g., 100–500 m2/g) of pSi coupled with its nanostructured skeleton promotes solubility in water and biological media. An increasing level of research is being conducted on the use of both porous silicon and silica materials in drug and nutrient delivery (see handbook chapters ▶ “Drug Delivery with Porous Silicon” and “▶ Porous Silicon and Functional Foods”). A biodegradable porous matrix offers the dual advantages of sustained release at target sites in the body and gradual biological elimination after administration (Ahuja and Pathak 2009). The performance of porous silicon in this regard should be compared with that of biodegradable polymers that can “microencapsulate” drugs (Park et al. 2005) and mesoporous biodegradable silicas that can also entrap them (Finnie et al. 2009). Biodegradable silicon-based microelectronic devices are now also under development (so-called transient electronics) for “smart” medical implants (Yin et al. 2015; Kang et al. 2015, 2016). This review discusses the hydrolysis mechanism underlying mesoporous silicon biodegradability; the factors affecting typical kinetics of that biodegradability, together with techniques used to date to tune those kinetics and time scales achieved.

Mechanism of Biodegradation and Degradation Products In vitro degradation studies of porous silicon have shown release of orthosilicic acid from both anodized films and microparticles using molybdate blue assays or ICP analysis (Anderson et al. 2000; Anglin et al. 2008; Chiappini et al. 2010). POLYMERS

PLGA/PLA Grafts, sutures, implants, depots

CERAMICS

Hydroxyapatite/Tri calcium phosphate Orthopaedic devices &tissue engineering scaffolds

SEMICONDUCTORS

METALS

Mg / Fe alloys

Mesoporous Silicon

Coronary Stents Paediatric implants

Brachytherapy Tuneable drug delivery

Fig. 1 Biodegradable materials of different classes and their medical uses

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Mesoporous silicon membranes/microparticles in simulated body fluids change color, becoming transparent as biodegradation proceeds to completion (Fig. 2). These results are more evident as the released silicic acid forms a blue-colored complex with molybdenum blue, and the color intensity increases with time (Fig. 3). Both in vitro (Canham 1995) and in vivo studies (Bowditch et al. 1999; see Fig. 4) have used electron microscopy to reveal mesoporous film corrosion and disappearance. The first in vivo study (Bowditch et al. 1999) of implanted discs used a combination of electron microscopy and monitoring of disc weights (Canham 2014). Biodegradation of silicon nanoparticles is higher in alkaline solutions particularly ones containing bicarbonates (Yin et al. 2015). Multistage nanovectors made from porous silicon have shown quicker biodegradation at higher pH in physiological buffers (Martinez et al. 2013). Additional characterization methods to monitor corrosion in nonmedical environments are discussed in the forthcoming handbook chapter “Corrosion Behavior of Porous Silicon”. Porous silicon in aqueous conditions undergoes hydrolysis to form orthosilicic acid, and the reaction is catalyzed by OH-; hence, the rate of dissolution increases with increasing pH. Dissolution of unoxidized silicon can be described with a simplified two-step process: Si þ 2H2 O ! SiO2 þ 2H2 SiO2 þ 2H2 O ! SiðOHÞ4 The oxidative first step involves electronic carrier (hole) injection and is dependent on both electronic bandgap and doping of the semiconductor. Complete hydrolysis of the oxide phase then generates orthosilicic acid, which is the natural bioavailable form of silicon, freely diffusible in human tissues, and readily excreted via the kidneys (Jugdaohsingh et al. 2002; Refitt et al. 1999). Bioresorbable nanoporous silicon membranes have been seen to degrade in 30 h at a thickness rate of

Fig. 2 In vitro biodegradation of an 83% porosity mesoporous silicon membrane

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Fig. 3 Molybdenum blue assay for orthosilicic acid released from mesoporous silicon membranes at different time points

Fig. 4 In vivo biodegradation of a 30% porosity mesoporous silicon layer in the subcutaneous site of the guinea pig. Plan view HREM images of porosified silicon disc surfaces (a) prior to implantation, (b) after 4 weeks in vivo, and (c) after 12 weeks in vivo (Canham 2014)

9 μm/day in cerebrospinal fluids (Kang et al. 2016). Porous silicon nanoparticles after intravitreal administration have been reported to be degraded over a period of 125 days, and degradation profiles were measured by ICP-OES ( Gu et al. 2012). The biocompatibility of porous silicon is reviewed in detail elsewhere in this handbook (chapter ▶ “Biocompatibility of Porous Silicon”) so is not discussed here.

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Kinetics of Degradation The kinetics of biodegradation is affected by physical parameters like degree of crystallinity, porosity, surface area, and pore size distribution. A striking example is the difference in solubility between amorphous and polycrystalline silicon (Shabir et al. 2011). Microporous porous silicon layers degrade faster than mesoporous followed by macroporous layers which suggest that degradation is dependent on surface area (Peckham and Andrews 2015). The kinetics is also tunable by pore wall surface chemistry which affects wettability by body fluid and resistance to initial hydrolysis. Mesoporous silicon is often manufactured by electrochemical etching techniques (see handbook chapter ▶ “Routes of Formation for Porous Silicon”) resulting in hydrogenterminated surfaces (Si-Hx). For drug delivery applications, a less reactive surface is crucial, and the hydrogen termination of the freshly etched pSi is normally replaced (Li et al. 2009). By converting the reactive groups into a more stable oxidized, hydrosilylated, or (hydro) carbonized form, the pSi surface can be modified in terms of hydrophilicity and resistance to hydrolysis (Canham et al. 1999). Such changes in pore wall chemistry have been shown to significantly change biodegradation kinetics, as summarized in Table 1. The hydrolysis of silica-based surfaces is also strongly pH dependent (Iler 1979). Comparison of hydrolysis rates at pH 2 and 9 shows an increase in excess of three orders of magnitude in the alkali fluid. Significant differences are therefore expected and observed between physiological environments of varying pH. Examples of relevance to medical uses are the low-pH condition inside lysozymes (Gu et al. 2012) and the low-pH microenvironment in polymer-pSi composites due to polymer biodegradation products (Henstock et al. 2014; McInnes et al. 2009, 2012). Another is the widely varying stabilities observed in foodstuffs and beverages for oral consumption (Canham 2007). The implications of the latter case are discussed in the handbook chapter ▶ “Porous Silicon and Functional Foods.” In vitro Raman microspectroscopy is now also being developed for diagnostics of the processes of uptake and biodegradation of porous silicon nanoparticles (Tolstik et al. 2016).

Conclusions There has been growing interest in development of nanostructured porous siliconbased medical therapy over the past few years. Porous silicon dissolves in body fluids into orthosilicic acid, a benign bone nutrient bioavailable from the diet. To make pSi more compatible with loaded drugs and nutrients, various strategies are used to make the nanostructured surfaces less reactive, resulting in slower biodegradation in body fluids. As expected, nanoparticles completely biodegrade much faster than microparticles and films of similar morphology. Composites of biodegradable polymers and porous silicon are likely to exhibit much slower biodegradation rates of the semiconductor component. Enhanced in vivo degradation in tumors

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Table 1 Biodegradation kinetics with differing pSi structures, surface chemistries, and biological environments pSi structure and physical parameters

Biological fluid/body site

Degradation kinetics

Native oxide (autoclaved)

Low porosity layer on n + Si discs (30% porosity and 30 μm thickness)

Blood plasma (subcutaneous site)

>3 months half-life

Native oxide

Multilayer microparticles (~67% porosity) Multilayer microparticles (~67% porosity) Multilayer microparticles (~67% porosity) Microparticles (40 nm APD)

Vitreous humor (eye)

1 week halflife

Vitreous humor (eye)

5 weeks half-life

Vitreous humor (eye)

16 weeks half-life

Phosphate buffered saline

100% after 2 days

PEGylation

Microparticles (40 nm APD)

Phosphate buffered saline

100% after 3–4 days

Silicon native oxide

Nanoparticles (126 nm diameter, 7.5 nm APD) Nanoparticles (80–120 nm diameter, ~5 nm APD) Microparticles (20–50 μm, 446 m2/ g, 1.5 ml/g, 10.7 nm APD) Microparticles (20–50 μm, 367 m2/ g, 0.84 ml/g, 7.6 nm APD) Films and microparticles

Phosphate buffered saline

100% after 4h

Phosphate buffered saline

3 h half-life

Hon et al. (2012)

Phosphate buffered saline

80% after 96 h

Phosphate buffered saline

~5% after 300 h

Phosphatebuffered saline

Difficult to quantify but slow kinetics Difficult to quantify but slow kinetics 80% after 7 days ~80% after 12 days ~60% after 30 days

TzurBalter et al. (2013) TzurBalter et al. (2013) McInnes et al. (2009, 2012) Henstock et al. (2014) Tzur Balter et al. (2015)

Surface chemistry

Thermal oxidation

Hydrosilylation

Silicon native oxide

Rapid thermal oxidation (800C)

Silicon native oxide

Hydrosilylation (dodecyl groups)

Composites with PLLA

Composites with polycaprolactone

Microparticles

Simulated body fluids

Hydrosilylation (dodecyl groups)

Microparticles (2–18 um)

Healthy tissue Tumors

Silicon native oxide

Nanoparticles (200–400 nm of 185 m2/g)

Phosphatebuffered saline

Reference Bowditch et al. (1999) Canham (2014) Cheng et al. (2008) Cheng et al. (2008) Cheng et al. (2008) Godin et al. (2010) Godin et al. (2010) Park et al. (2009)

Maher et al. (2016)

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compared to healthy tissue has been recently reported. There is much potential to tailor the silicon surfaces in terms of chemistry, pore size, pore structure, and porosity making it a versatile carrier system for controlled release applications.

References Ahuja G, Pathak K (2009) Porous carriers for controlled/modulated drug delivery. Indian J Pharm Sci 71(6):599–607 Anderson SHC, Elliott H, Wallis DJ, Canham LT, Powell JJ (2000) Dissolution of different forms of partially porous silicon wafers under simulated physiological conditions. Phys Status Solidi A 197:331–335 Anglin EJ, Cheng L, Freeman WR, Sailor MJ (2008) Porous silicon in drug delivery devices and materials. Adv Drug Deliv Rev 60(11):1266–1277 Bowditch AP, Waters K, Gale H, Rice P, Scott EAM, Canham LT, Reeves CL, Loni A, Cox TI (1999) In-vivo assessment of tissue compatibility and calcification of bulk and porous silicon. Mater Res Soc Symp Proc 536:149–154 Canham LT (1995) Bioactive silicon structure fabrication through nanoetching techniques. Adv Mater 7:1033–1037 Canham LT (2007) Nanoscale semiconducting silicon as a nutritional food additive. Nanotechnology 18:185704 Canham LT (2014) Porous silicon for medical use: from conception to clinical use, chap 1. In: Santos HA (ed) Biomedical uses of porous silicon. Woodhead Publishing, Cambridge, UK, pp 3–20 Canham LT, Reeves CL, Newey JP, Houlton MR, Cox TI, Buriak JM, Stewart MP (1999) Derivatized mesoporous silicon with dramatically improved stability in simulated human blood plasma. Adv Mater 11(18):1505–1507 Cheng L, Anglin E, Cunin F, Kim D, Sailor MJ, Falkenstein I, Tammewar A, Freeman WR (2008) Intravitreal properties of porous silicon photonic crystals: a potential self-reporting intraocular drug-delivery vehicle. Br J Ophthalmol 92(5):705–711 Chiappini C, Liu X, Fakhoury JR, Ferrari M (2010) Biodegradable porous silicon barcode nanowires with defined geometry. Adv Funct Mater 20(14):2231–2239 Finnie KS, Waller DJ, Perret FL, Krause-Heuer AM, Lin HQ, Hanna JV, Barbe CJ (2009) Biodegradability of sol-gel silica microparticles for drug delivery. J Sol-Gel Sci Technol 49:12–18 Godin B, Gu J, Serda RE, Bhavane R, Tasciotti E, Chiapinni C, Lu X, Tanaka T, Decuzzi P, Ferrari M (2010) Tailoring the degradation kinetics of mesoporous silicon through PEGylation. J Biomed Mater Res 94(4):1236–1243 Gu L, Ruff LE, Qin Z, Corr M, Hedrick SM, Sailor MJ (2012) Multivalent porous silicon nanoparticles enhance the immune activation potency of agonistic CD40 antibody. Adv Mater 24(29):3981–3987 Henstock JR, Ruktanonchai UR, Canham LT, Anderson SI (2014) Porous silicon confers bioactivity to polycaprolactone composites in vitro. J Mater Sci 25(4):1087–1097 Hon NK, Shaposhnik Z, Diebold ED, Tamanoi F, Jalali B (2012) Tailoring the biodegradability of porous silicon nanoparticles. J Biomed Mater Res 100(12):3416–3421 Iler RK (1979) Chemistry of silica: solubility, polymerization, colloid and surface properties and biochemistry. Wiley, New York Jugdaohsingh R, Anderson SH, Tucker KL, Elliott H, Kiel DP, Thompson RP, Powell JJ (2002) Dietary silicon intake and absorption. Am J Clin Nutr 75(5):887–893 Kang et al (2015) Dissolution chemistry and biocompatibility of silicon and germanium-based semiconductors for transient electronics. Appl Mater Interf 7:9297–9305 Kang SK et al (2016) Bioresorbable silicon electronic sensors for the brain. Nature 530:71–76

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Li HL, Zhu Y, Xu D, Wan Y, Xia L, Zhao X (2009) Vapour-phase silanization of oxidised porous silicon for stabilizing composition and photoluminescence. J Appl Phys 105:114–307 Maher S, Kumeria T, Wang Y, Kaur G, Fathalla D, Fetih G, Santos A, Habib F, Evdokiou A, Losic D (2016) From the mine to cancer therapy: natural and biodegradable theranostic silicon nanocarriers from diatoms for sustained delivery of chemotherapeutics. Adv Healthc Mater. https://doi.org/10.1002/adhm.201600688 Martinez JO, Chiappini C et al (2013) Engineering multi-stage nanovectors for controlled degradation and tunable release kinetics. Biomaterials 34(33) 8469–8477 Martinez JO, Evangelopoulos E, Chiappino C, Liu X, Ferrari M, Tasciotti E (2014) Degradation and biocompatibility of multistage nanovectors in physiological systems. J Biomed Mater Res 102:3540–3549 McInnes SJP, Thissen H, Choudbury NR, Voelcker NH (2009) New biodegradable materials produced by ring opening polymerisation of poly(l-lactide) on porous silicon substrates. J Colloid Interface Sci 332:336–344 McInnes SJ, Irani Y, Williams KA, Voelcker NH (2012) Controlled drug delivery from composites of nanostructured porous silicon and poly(l-lactide). Nanomedicine 7(7):995–1016 Park JH, Ye M, Park K (2005) Biodegradable polymers for microencapsulation of drugs. Molecules 10:146–161 Park J-H, Gu L, von Maltzahn G, Ruoslahti E, Bhatia SN, Sailor MJ (2009) Biodegradable luminescent porous silicon nanoparticles for in vivo applications. Nat Mater 8(4):331–336 Peckham J, Andrews GT (2015) Comparative study of the biodegradability of porous silicon films in simulated body fluid. Biomed Mater Eng 25(1):111–116 Ratner BD, Hoffman AS, Schoen FJ, Lemons JE (eds) (2004) Biomaterials science: an introduction to materials in medicine, 2nd edn. Elsevier, San Diego, p 851 Refitt DM, Jugdaosingh R, Thompson RPH, Powell JJ (1999) Silicic acid: its gastrointestinal uptake and urinary excretion in man and effects on aluminium excretion. J Inorg Biochem 76:141–147 Sarparanta M, Bimbo LM, Rytkonen J, Makila E, Laaksonen TJ, Laaksonen P, Nyman M, Salonen J, Linder MB, Hirvonen J, Santos HA, Airaksinen AJ (2012) Intravenous delivery of hydrophobin-functionalized porous silicon nanoparticles: stability, plasma protein adsorption and biodistribution. Mol Pharm 9:654–663 Shabir Q, Pokale A, Loni A, Johnson DR, Canham LT, Fenollosa R, Tymczenko M, Rodríguez I, Meseguer F, Cros A (2011) Medically biodegradable hydrogenated amorphous silicon microspheres. SILICON 2011:173–176 Tolli MA, Ferreira MPA, Kinnunen SM et al (2014) In-vivo biocompatibility of porous silicon biomaterials for drug delivery to the heart. Biomaterials 36(29):8394–8405 Tolstik E, Osminkina LA, Matthaus C, Burkhardt M, Tsurikov KE, Natashina UA, Timoshenko VY, Heintzmann R, Popp J, Sivakov V (2016) Studies of silicon nanoparticles uptake and biodegradation in cancer cells by Raman Spectroscopy. Nanomedicine 12(7):1931–1940 Tzur Balter A, Shatsberg Z, Beckerman M, Segal E, Artzi N (2015) Mechanism of erosion of nanostructured porous silicon drug carriers in neoplastic tissues. Nat Commun 6:6208 Tzur-Balter A, Rubinskia A, Segal E (2013) Designing porous silicon-based microparticles as carriers for controlled delivery of mitoxantrone dihydrochloride. J Mater Res 28(2):231–239 Yin L, Farimani AB, Min K, Vishal N, Lam J, Lee YK, Aluru NR, Rogers JA (2015) Mechanisms for hydrolysis of silicon nanomembranes as used in bioresorbable electronics. Adv Mater 27:1857–1864

Part III Characterization

Characterization Techniques and Challenges with Porous Silicon Leigh Canham

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Glossary of Characterization and Modelling Techniques . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Skeleton Dimensionality and Nanostructure Packing Density . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Pore Size Distribution and Morphology: Euclidean or Fractal? . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Mechanical Strength at High Porosity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Metastable Silicon Hydride Surface . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . The Effects of Very Low Thermal Conductivity . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding Comments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Mesoporous silicon is a complex nanostructure whose optoelectronic properties and morphology have received intense study over the last 25 years. Its properties often depend on both its skeleton size distribution and the chemical nature of its high internal surface area. This expanded review provides a glossary of about 100 characterization techniques applied to date to porous silicon; highlighting those techniques receiving dedicated reviews in this section of the handbook; and linking all of them to other parts of the handbook dealing with specific structures, properties and applications. It then also collates some of the lessons learned with regard characterization, highlighting potential issues that need to be considered and artifacts that can arise. These have in the past both complicated data interpretation and even caused problems in reproducing published data.

L. Canham (*) School of Physics and Astronomy, University of Birmingham, Birmingham, Worcestershire, UK e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_40

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Keywords

Mesoporous Silicon · Characterization techniques With Porous Silicon · Silicon Nanocrystallite Size Distribution · Metastable Silicon Hydride Surface

Introduction High porosity mesoporous silicon is a complex nanostructure whose optoelectronic properties and morphology have received intense continuous study over the last 25 years, following the publication of its dramatic luminescence properties in 1990. There are a series of reviews that historically chart progress in understanding and exploitation of its luminescent, optical, and electrical properties (Fauchet et al. 1995; Hamilton 1995; Cullis et al. 1997; Bisi et al. 2000; Boarino et al. 2009; Torres-Costa and Martin-Palma 2010; Chao 2011; Golovan and Timoshenko 2013; Pacholski 2013). This review collates a large number of techniques (both experimental and theoretical) applied so far to porous silicon and then highlights the five problem areas shown in Fig. 1 with regard to characterization that have often hindered progress in optoelectronic applications. They also have relevance to the many other application areas under more recent development (see handbook chapter ▶ “Porous Silicon Application Survey”). The latter objective is to alert the reader to some general issues, prior to other handbook reviews in this section that analyze in detail the insight gained from specific techniques, like Deep Level Transient Spectroscopy,

Densely packed interconnected nanostructures

Difficult microscopy

Inaccurate size metrology

Hierarchical porosity

Morphological complexity

Complex theoretical modelling

Low mechanical strength

Degraditon upon liquid removal

Properties change during processing

Unstable surface chemistry

“Aging” during storage

Properties change with time

Low thermal transport

High temperature rises

Properties change during study

Fig. 1 Problem areas for characterization of mesoporous and microporous silicon

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X-Ray Diffraction, Raman spectroscopy, Electron Paramagnetic Resonance, Acoustic Transmission Spectroscopy, and so on.

Glossary of Characterization and Modelling Techniques A large number of characterization techniques have now been applied to porous silicon. Tables 1 and 2 provide an alphabetically arranged list of the majority of techniques, experimental and theoretical respectively, linking each technique to reviews elsewhere in the handbook.

Skeleton Dimensionality and Nanostructure Packing Density The length scale at which dramatic quantum size effects occur in silicon lies in the range 1–3 nm (see Fig. 1 in handbook review ▶ “Electronic Band Structure in Porous Silicon”). Silicon quantum dots or wires of this size push the spatial resolution of many microscopy techniques, and when very small size variations cause large changes in properties, accurate metrology becomes increasingly important and difficult. This is not helped if those nanostructures are very close together, interconnected to varying degrees, and arranged in complex three-dimensional networks. The first challenge therefore relates to those properties that are tunable by size metrology of the silicon skeleton: how do we accurately extract the silicon nanocrystallite size distribution throughout its volume? The handbook review ▶ “Microscopy of Porous Silicon” discusses such issues and progress to date.

Pore Size Distribution and Morphology: Euclidean or Fractal? The size of mesopores (2–50 nm) and particularly micropores (0–2 nm) also makes their accurate metrology via microscopy difficult. The handbook review ▶ “Gas Adsorption Analysis of Porous Silicon” discusses how mesopore size can be evaluated over the entire sample volume accessible, but it is not an accurate technique for micropores. However porous silicon is normally not exclusively riddled with only one class of pore. There is now significant evidence that many electrochemical etching regimes, for example, produce more than one class of porosity in a given structure; macropore walls can themselves be mesoporous; structures can contain both mesopores and micropores. In extreme cases, so-called “hierarchical” porosity is present with pore diameters covering a huge range of length scales (Xiu et al. 2007, Xu and Li 2008, Cozzi et al. 2016, Liu et al. 2016). This type of wide-ranging length scale of porosity, together with a “fractal” geometrical arrangement of pores (see Fig. 2), is a topic only briefly mentioned elsewhere in this handbook (see, e.g., the chapter ▶ “Mesoporous Silicon”), and so it will be highlighted here.

ATS AES AFM BS CC

Acoustic Transmisssion spectroscopy

Auger electron spectroscopy Atomic force microscopy Autoradiography Brillouin spectroscopy

Cell culture

CT CM CAM

CV DLTS DTA DRIFTS

Computed Tomography Confocal microscopy

Contact angle measurements

Cyclic voltammetry

Deep level transient spectroscopy Differential thermal analysis Diffuse reflectance infrared Fourier transform spectroscopy

Chemography Colour/hue mapping

Acronym(s) AM

Experimental Characterization technique applied to porous silicon Acoustic microscopy

Related Review(s) in Handbook Acoustic characterization of porous silicon Microscopy of porous silicon Acoustic characterization of porous silicon Porous silicon phononic crystals Chemical characterization of porous silicon Microscopy of porous silicon Porous silicon in brachytherapy Acoustic characterization of porous silicon Phonon frequencies in porous silicon Cell culture on porous silicon Porous silicon and tissue engineering scaffolds Methods to evaluate spatial uniformity in porous silicon Colour of porous silicon Methods to evaluate spatial uniformity in porous silicon Chemotherapy with porous silicon Microscopy of porous silicon Porous silicon microneedles and nanoneedles Cell culture on porous silicon Porous silicon immunoaffinity microarrays Functional coatings on porous silicon Porous silicon and li-ion batteries Porous silicon ballistic hot electron emitter Electrical characterization techniques for porous silicon Characterization of porous silicon by calorimetry Characterization of porous silicon by infrared spectroscopy

Table 1 Experimental characterization techniques. Those techniques that receive dedicated reviews in this section of the handbook are highlighted in bold text, as are all the related characterization reviews. In most cases the related reviews linked to a specific characterization technique mention that technique; otherwise published papers on that topic have utilized the technique

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DC

DLS, pcs, QELS ERDA EBIC EELS ET ER EDX/EDAX ENDOR EPR EXAFS GAA GA HSP HM IS IH ICP-AES IR/FTIR IC IA LDA

Diode characteristics (I-V,I-T, C-V) Dosimetry

Dynamic light Scattering

Elastic recoil detection analysis Electron beam induced conductivity Electron energy loss spectroscopy Electron Tomography Electroreflectance Energy dispersive X-ray microscopy/analysis Electron nuclear double Resonance Electron paramagnetic Resonance Extended X-ray absorption fine structure Gas adsorption analysis Gravimetric analysis

High speed photography Hyperspectral monitoring Impedance spectroscopy Implant histology Inductively coupled plasma -atomic emission spectroscopy Infrared spectroscopy

Ion chromatography Isothermal adsorption Laser diffraction analysis

(continued)

Electrical characterization techniques for porous silicon Ohmic and rectifying contacts to porous silicon Porous silicon in brachytherapy Thermoluminescence of porous silicon Porous silicon suspensions and colloids Porous silicon nanoparticles Chemical characterization of porous silicon Electrical characterization techniques for porous silicon Microscopy of porous silicon Microscopy of porous silicon Electronic band structure in porous silicon Chemical characterization of porous silicon Characterization of porous silicon by EPR and ENDOR Characterization of porous silicon by EPR and ENDOR Photoluminescence of porous silicon Gas adsorption analysis of porous silicon Pore volume(porosity) in porous silicon Melt intrusion in porous silicon Energetics with porous silicon Methods to evaluate spatial uniformity in porous silicon Porous silicon formation by anodization Biocompatibility of porous silicon Biodegradability of porous silicon Characterization of porous silicon by infrared spectroscopy Silicon carbon bond formation on porous silicon Porous silicon for oral hygiene and cosmetics Biomolecule adsorption and release from porous silicon Milling of porous silicon Microparticles

Characterization Techniques and Challenges with Porous Silicon 561

Acronym(s) LS LM Pl,el,cl,Tl

MFM MRI MALDI MP MC MBA MSR Ns, FNS NMR NRA OPPS OR OS OT PA PC

Experimental Characterization technique applied to porous silicon Liquid sorption

Lorentz microscopy Luminescence spectroscopy

Magnetic force microscopy Magnetic Resonance imaging Matrix assisted laser desorption/ionization Mass Spectrometry

Mercury Porosimetry Microbial culture Molybdate blue assay Muon spin spectroscopy Nanoindentation Noise spectroscopy Nuclear Magnetic Resonance

Nuclear reaction analysis Optical pump-probe spectroscopy

Optical reflectometry

Optical Scattering Optical transmittance Photoacoustics

Photoconductivity

Table 1 (continued) Related Review(s) in Handbook Solvent loading of porous silicon Color of porous silicon Magnetic characterization methods for porous silicon Photoluminescence of porous silicon Thermoluminescence of porous silicon Chemiluminescence of porous silicon Magnetic characterization methods for porous silicon Theranostic imaging with porous silicon Porous silicon based Mass Spectrometry Porous silicon Immunoaffinity microarrays Pore volume in porous silicon Drug delivery with porous silicon Biodegradability of porous silicon Effects of irradiation on porous silicon Mechanical properties of porous silicon Porous silicon gas sensing Mesopore diffusion within porous silicon NMR cryoporometry characterization of mesoporous silicon Chemical characterization of porous silicon Optical gain in porous silicon Nonlinear optical properties of porous silicon Porous silicon and solar cells Optical characterization of porous silicon multilayers Milling of porous silicon Microparticles Optical characterization of porous silicon multilayers Thermal properties of porous silicon Thermal isolation with porous silicon Electrical characterization techniques for porous silicon

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PS PAS PET PIGE QENS RS RBI RBS SEM STM SHG SIMS SPECT SANS SAXS SE SLIM SFG SPS SPR DSC,fc,IMC TDS TSCS

Polarization spectroscopy Positron annihilation spectroscopy

Positron emission Tomography Proton induced gamma ray emission Quasi-elastic neutron Scattering Raman spectroscopy

Reflectance-based interferometry Rutherford backscattering spectroscopy Scanning electron microscopy

Scanning tunnelling microscopy Second harmonic generation Secondary ion Mass Spectrometry

Single photon emission computed Tomography

Small angle neutron Scattering Small angle X-ray Scattering Spectroscopic Ellipsometry Spectroscopic liquid infiltration method Sum frequency generation spectroscopy Surface Photovoltage spectroscopy Surface Plasmon Resonance Thermal calorimetry Thermal desorption Spectrometry Thermally stimulated (discharge) current spectroscopy

(continued)

Optical birefringence of porous silicon Microporous silicon Effects of irradiation on porous silicon Theranostic imaging with porous silicon Chemical characterization of porous silicon Mesopore diffusion within porous silicon Raman spectroscopy of porous silicon Phonon frequencies in porous silicon. Porous silicon optical biosensors Effects of irradiation on porous silicon Microscopy of porous silicon Macroporous silicon Microscopy of porous silicon Nonlinear optical properties of porous silicon Chemical characterization of porous silicon Porous silicon gettering Theranostic imaging with porous silicon Chemotherapy with porous silicon Pore volume (porosity) in porous silicon Microporous siicon Refractive index of porous silicon Optical characterization of porous silicon multilayers Characterization of porous silicon by infrared spectroscopy Electrical characterization techniques for porous silicon Functional coatings on porous silicon Characterization of porous silicon by calorimetry Effects of irradiation on porous silicon Electrical characterization techniques for porous silicon

Characterization Techniques and Challenges with Porous Silicon 563

Acronym(s) TGA THG TMP TEM TT UI UV-Vis VPL XRD XPS XRR XRT ZPM

Experimental Characterization technique applied to porous silicon Thermogravimetric analysis

Third harmonic generation Transient microwave photoconductivity Transmission electron microscopy

Triangle testing Ultrasound imaging UV-visible absorption Variable stripe length technique X-ray diffraction X ray photoelectron spectroscopy X-ray reflectivity X-ray topography Zeta potential measurements

Table 1 (continued) Related Review(s) in Handbook Drug delivery with porous silicon Solvent loading of porous silicon Nonlinear optical properties of porous silicon Electrical characterization techniques for porous silicon Microscopy of porous silicon Sintering of porous silicon Porous silicon and functional foods Theranostic imaging with porous silicon Porous silicon suspensions and colloids Optical gain in porous silicon X-ray diffraction in porous silicon Chemical characterization of porous silicon Ultrathin porous silicon films Oxidation of macroporous silicon. Porous silicon suspensions and colloids

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Table 2 Theoretical modelling techniques. The related characterization reviews in this section of the handbook are highlighted in bold text Theoretical modelling technique applied to porous silicon Ab initio molecular orbital calculations Bond polarizability model Density functional theory

Acronym AIMOC BPM DFT

Digital image processing Effective Mass theory Effective medium approximation

DIP EMT EMA

Fast Fourier transform

FFT

Finite difference time domain models Finite element analysis

FDTD

Molecular dynamics

MD

Monte Carlo simulations

MCS

Percolation theory Phonon confinement model Rigorous coupled wave analysis Tight binding approach Transfer matrix method

PT PCM RCWA TB TMM

Wave Cascade matrix method

WCMM

FEA

Related review(s) in handbook Characterization of porous silicon by infrared spectroscopy Raman spectroscopy of porous silicon Electronic band structure in porous silicon Gas and liquid doping of porous silicon Porous silicon gettering Microscopy of porous silicon Electronic band structure in porous silicon Refractive index of porous silicon Electrical transport in porous silicon Nonlinear optical properties of porous silicon Optical characterization of porous silicon multilayers Porous silicon diffraction gratings Porous silicon electrochemical biosensors: Performance and commercial prospects Mechanical properties of porous silicon Electronic band structure in porous silicon Thermal properties of porous silicon Homoepitaxy on porous silicon Thermal isolation with porous silicon Electrical transport in porous silicon Raman spectroscopy of porous silicon Porous silicon diffraction gratings Electronic band structure in porous silicon Optical characterization of porous silicon multilayers Optical characterization of porous silicon multilayers

Extremely regular arrays of large macropores with smooth sidewalls and Euclidean geometry are a regular feature of the porous silicon literature (see Fig. 3a), as are highly directional mesopore arrays with minimal pore branching under specific anodization conditions (Canham 1990; Ouyang et al. 2005). Very different fractallike etching patterns were first revealed from electrochemically etched macropores, also under certain conditions (Harsanyi and Habermeier 1987). Seminal investigations of the anodization formation mechanisms by Smith and co-workers (Chuang et al. 1989; Smith and Collins 1992; Tondare et al. 2008) then provided strong microscopic evidence for the broader occurrence of fractal pore morphology. At least with their specific etching conditions (n-substrates with both mesopores and macropores), macropore walls were clearly also mesoporous at decreasing length scales (see Fig. 3b).

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1-st order pore

2-nd order pore

Fig. 2 Fractal-like pore arrangement in a porous silicon membrane (Lysenko et al. 2004)

Fig. 3 LHS Euclidean macropore array in p-type silicon (Kim et al. 2009); RHS Fractal-like oxide replica of the pore volume in n-type silicon (Tondare et al. 2008)

An increasing number of properties of primarily mesoporous silicon are now being modeled using a fractal geometry for the porous silicon surface. Examples include the exterior surface roughness of layers (Happo et al. 1998) and their hydrophobicity (Cao et al. 2008; Gentile et al. 2011), optical absorption (Derlet et al. 1995), electrical transport (Ben-Chorin et al. 1995; Axelrod et al. 2002), gas transport (Lysenko et al. 2004), hydride content (Nychyporuk et al. 2005), vapor adsorption within pores (Moretti et al. 2007), and low temperature thermal conductivity (Valalaki and Nassiopoulou 2014). In contrast, much of the theoretical modeling of the band structure of mesoporous silicon has been based on idealized nanoscale silicon building blocks (quantum wires and dots) or Euclidean geometry-based subtractive models which introduce periodic porosity via supercells (see handbook chapter ▶ “Electronic Band Structure in Porous Silicon”).

Mechanical Strength at High Porosity Crystalline silicon is a strong but brittle material. The introduction of porosity often lowers hardness, stiffness, and fracture strength (see handbook chapter ▶ “Mechanical Properties of Porous Silicon”), and if the structure becomes too weak, it cannot

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often survive common material processing techniques without alteration. Examples include air drying (see handbook chapter ▶ “Drying Techniques Applied to Porous Silicon”), reduction of particle size via communition (see handbook chapter ▶ “Milling of Porous Silicon Microparticles”), and oxidation of layers (see handbook chapter ▶ “Oxidation of Mesoporous Silicon”). The properties of electrochemically etched layers can depend not only on etch parameters but how the material was dried. The properties of microparticles can be sensitive to how they were milled. In addition, for materials with very low strength, characterization techniques normally deemed quite gentle can become destructive. One example is the widespread use of gas adsorption to measure pore size which produces irreversible changes in ultrahigh porosity silica “aerogels” (Scherer et al. 1995) and is likely to run into similar issues with characterization of silicon “aerocrystals” (Canham et al. 1994).

The Metastable Silicon Hydride Surface Processing silicon in hydrofluoric acid solutions has been an invaluable route to generating all classes of porous silicon, but the resulting silicon hydride bonds are metastable in ambient air and gradually replaced by native oxide growth. The effects on the chemical composition of highly porous stain-etched silicon were recorded by infrared spectroscopy in the 1960s (Beckmann 1965), and effects of air storage on many properties (photoluminescence, refractive index, electrical resistivity) emphasized much later for microporous silicon (Canham et al. 1991). If structures are not chemically “passivated” immediately after etching then the properties will evolve with storage time until ambient oxidation is complete. This can take many months (Canham 1997). In addition, many characterization techniques utilize energetic beams that can accelerate this process if carried out in ambient air or even vacuum (see handbook chapter ▶ “Effects of Irradiation on Porous Silicon”). In some cases, such as ion beam analysis in vacuo of chemical composition, “capping” of layers is required to provide reliable data (Giaddui et al. 1998).

The Effects of Very Low Thermal Conductivity The incredibly low thermal conductivity of high porosity silicon is discussed in three chapters of this handbook: ▶ “Thermal Properties of Porous Silicon”, ▶ “Porous Silicon and Thermoelectrics” and ▶ “Thermal Isolation with Porous Silicon”. It can be utilized in specific applications but also introduce challenges with many characterization techniques and processes. Particular care must be taken with characterization techniques that utilize high energy excitation in localized volumes. Examples include micro Raman spectroscopy, cathodoluminescence spectroscopy, and photoluminescence microscopy. The effects of exothermic reactions are also often

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amplified by poor energy dissipation, such as in thermal passivation of mesoporous silicon powders (Loni and Canham 2013). Massive temperature rises in the silicon skeleton can occur, so much so that some degree of sintering or even localized melting occurs during analysis. This becomes more pronounced for porous silicon membranes and powders which are removed from the bulk silicon heat sink and more pronounced when data is collected in vacuum, removing gaseous heat transport. Figure 4 provides an example where Raman analysis at gas pressures was used to demonstrate silicon nanoparticle temperatures up to 800  C under focussed photoexcitation. A particularly striking example of this is the visible light emission from some porous silicon and silicon nanoparticle structures originally ascribed to “photoluminescence“but later revealed to be blackbody thermal radiation by careful experimentation (Costa et al. 1998; Roura and Costa 2002). Some very spectrally broad “cathodoluminescence“spectra published are also likely to be primarily thermal radiation, as discussed in the handbook chapter “Cathodoluminescence of Porous Silicon”. Much of the nonlinear optical properties reported for porous silicon over the period 1992–2002 may also need reinterpretation accounting for thermal effects, as discussed by Roura and co-workers (Roura and Costa 2002). It is an important issue, mentioned at the end of a review on the topic (Golovan and Timoshenko 2013) and discussed in the handbook review ▶ “Nonlinear Optical Properties of Porous Silicon”.

Concluding Comments More than 80 experimental characterization techniques have been applied to varied forms of porous silicon to date. High porosity mesoporous silicon is a fascinating nanostructure with low dimensionality that has a number of novel properties. It also Fig. 4 Temperature of silicon nanoparticles under micro Raman analysis, as a function of gas pressure (Costa et al. 1998)

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can possess chemical instability, mechanical weakness, and low thermal transport. These latter properties can necessitate very careful characterization in order to avoid data misinterpretation and unwanted changes to the nanostructured material.

References Axelrod E, Givant A, Shappir J, Feldman Y, Saar A (2002) Dielectric relaxation and transport in porous silicon. Phys Rev B 65:165–429 Beckmann KH (1965) Investigation of the chemical properties of stain films on silicon by means of infrared spectroscopy. Surf Sci 3(4):314–332 Ben-Chorin M, Moller F, Koch F, Schirmacher W, Eberhard M (1995) Hopping transport on a fractal: ac conductivity of porous silicon. Phys Rev B 51(4):2199–2213 Bisi O, Ossicini S, Pavesi L (2000) Porous silicon: a quantum sponge structure for silicon based optoelectronics. Surf Sci Rep 38(1–3):1–126 Boarino L, Borini S, Amato G (2009) Electrical properties of mesoporous silicon: from a surface effect to coulomb blockade and more. J Electrochem Soc 156(12):K223–K226 Canham LT (1990) Silicon quantum wire array fabrication by electrochemical and chemical dissolution of waters. Appl Phys Lett 57(10):1046–1048 Canham LT (1997) Properties of porous silicon, EMIS datareview series no 18. IEE Press, London Canham LT, Houlton MR, Leong WY, Keen JM (1991) Atmospheric impregnation of porous silicon. J Appl Phys 70(1):422–431 Canham LT, Cullis AG, Pickering C, Dosser OD, Cox TI, Lynch TP (1994) Luminescent anodized silicon aerocrystal networks prepared by supercritical drying. Nature 368:133–135 Cao L, Price TP, Weiss M, Gao D (2008) Super water- and oil-repellent surfaces on intrinsically hydrophilic and oleophillic porous silicon films. Langmuir 24(5):1640–1643 Chao Y (2011) Optical properties of nanostructured silicon. Compr NanoSci Technol 1:543–570 Chuang SF, Collins SD, Smith RL (1989) Porous silicon microstructure as studied by transmission electron microscopy. Appl Phys Lett 55:1540–1543 Costa J, Roura P, Morante JR, Bertran E (1998) Blackbody emission under laser excitation of silicon nanopowder produced by plasma-enhanced chemical-vapour deposition. J Appl Phys 83 (12):7879–7885 Cozzi C, Polito G, Strambini LM, Barillaro G (2016) Electrochemical preparation of in-silicon hierarchical networks of regular out-of-plane macropores interconnected by secondary in-plane pores through controlled inhibition of breakdown effects. Electrochim Acta 187:552–559 Cullis AG, Canham LT, Calcott PDJ (1997) The structural and luminescence properties of porous silicon. J Appl Phys 82(3):909–965 Derlet PM, Choy TC, Stoneham AM (1995) An investigation of the porous silicon optical absorption power law near the band edge. J Phys Condens Matter 7:2507–2523 Fauchet PM, Tsybeskov L, Peng C, Duttagupta SP, von Behren J, Kostoulas Y, Vandyshev JMV, Hirschman KD (1995) Light-emitting porous silicon: materials science, properties and device applications. IEEE J Sel Topics Quant Electron 1(4):1126–1139 Gentile F, Battista E et al (2011) Fractal structure can explain the increased hydrophobicity of nanoporous silicon films. Microelectron Eng 88:2537–2540 Giaddui T, Earwaker LG, Forcey KS, Loni A, Canham LT (1998) Improved capping layers for suppression of ambient ageing in porous silicon. J Phys D Appl Phys 31:1131–1136 Golovan LA, Timoshenko VY (2013) Nonlinear optical properties of porous silicon nanostructures. J Nanoelectron Optoelectron 8:223–239 Hamilton B (1995) Porous silicon. Semicond Sci Technol 10:1187–1207 Happo N, Fujiwara M, Iwamatsu M, Horii K (1998) Atomic force microscopy study of self-affine fractal roughness of porous silicon surfaces. Jpn J Appl Phys 37:3951–3953

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Harsanyi J, Habermeier HU (1987) Fractal micropatterns generated by anodic etching. Microelectron Eng 6(1–4):575–580 Kim JH, Kim KP, Lyu HK, Woo SH, Seo HS, Lee JH (2009) Three dimensional macropore arrays in p-type silicon fabricated by electrochemical etching. J Korean Phys Soc 55(1):5–9 Liu X, Miao R, Yang J, Bie Y, Wang J, Nuli Y (2016) Scalable and cost-effective preparation of hierarchical porous silicon with a high conversion yield for superior lithium ion storage. Energy Techn 4(5):593–599 Loni A, Canham LT (2013) Exothermic phenomena and hazardous gas release during thermal oxidation of mesoporous silicon powders. J Appl Phys 113:173505 Lysenko V, Vitiello J, Remaki B, Barbier D (2004) Gas permeability of porous silicon nanostructures. Phys Rev E 70:017301 Moretti L, De Stefano L, Rendina I (2007) Quantitative analysis of capillary condensation in fractallike porous silicon nanostructures. J Appl Phys 101:024309 Nychyporuk T, Lysenko V, Barbier D (2005) Fractal nature of porous silicon nanocrystallites. APS J Phys Rev B 71:115–402 Ouyang H, Christopherson M, Fauchet PM (2005) Enhanced control of porous silicon morphology from macropore to mesopore formation. Phys Status Solidi A 202(8):1396–1401 Pacholski C (2013) Photonic crystal sensors based on porous silicon. Sensors 13:4694–4713 Roura P, Costa J (2002) Radiative thermal emission from silicon nanoparticles: a reversed story from quantum to classical theory. Eur J Phys 23:191–203 Scherer WG, Smith DM, Stein D (1995) Deformation of silica aerogels during characterisation. J Non-Cryst Solids 186:309–315 Smith RL, Collins SD (1992) Porous silicon formation mechanisms. J Appl Phys 71:R1 Tondare VN, Gierhart BC, Howitt DG, Smith RL, Chen SJ, Collins SD (2008) An electron microscopy investigation of the structure of porous silicon by oxide replication. Nanotechnology 19:225–301 Torres-Costa V, Martin-Palma RJ (2010) Application of nanostructured porous silicon in the field of optics. A review. J Mater Sci 45:2823–2838 Valalaki K, Nassiopoulou AG (2014) Thermal conductivity of highly porous silicon in the temperature range 4.2 to 20K. Nano Res Lett 9, 318 Xiu Y, Zhu L, Hess DW, Wong CP (2007) Hierarchical silicon etched structures for controlled hydrophobicity/superhydrophobicity. Nano Lett 7(11):3388–3393 Xu HJ, Li XJ (2008) Silicon nanoporous pillar array: a hierarchical silicon structure with high light absorption and triple band photoluminescence. Opt Express 16(5):2933–2941

Microscopy of Porous Silicon Raúl J. Martín-Palma and Vicente Torres Costa

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Transmission Electron Microscopy Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Scanning Electron Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Atomic Force Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Confocal Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Other Microscopies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Concluding Remarks . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Traditionally, the structure of porous silicon has been thoroughly characterized by a wide variety of microscopy techniques aiming at carrying out fundamental studies. However, as a consequence of the explosive growth of specific uses of porous silicon in such diverse fields as photonics, chemistry, energy, and biomedicine, the number of microscopies used for its characterization has continued to grow over time. In this updated review, some recent specific applications in which these characterization tools have demonstrated their enormous capabilities are highlighted. Such application areas, underpinned by 15 microscopy techniques, include tissue engineering, cell culture, biosensing, drug delivery, and lithium-ion batteries.

R. J. Martín-Palma (*) · V. Torres Costa Departamento de Física Aplicada, Universidad Autónoma de Madrid, Madrid, Spain e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_41

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Keywords

Acoustic microscopy · Atomic force microscopy · Confocal microscopy · Confocal scanning beam microscopy · Fluorescence microscopy · In-situ microscopy · Micro-Raman spectroscopy · Multiphoton microscopy · Near-field scanning optical microscopy · Photoluminescence · Scanning electron microscopy · Scanning tunneling microscopy · Transmission electron microscopy

Introduction The rather complex structure of porous silicon (PS) provides this material with a large number of appealing physicochemical properties, among which visible luminescence can be highlighted. Additionally, the possibility of controlling its morphology at the micro- and nanoscales makes PS a very versatile material for its use in many different applications in a broad diversity of fields (Canham 1997; Lehmann 2002; Sailor 2011; Hernández-Montelongo et al. 2015), ranging from optics (TorresCosta and Martín-Palma 2010) to biomedicine (Martín-Palma et al. 2010, 2014). Anyhow, the particular structure of porous silicon is itself a matter of study, given its inherent complex surface, bulk, and interface morphology. Besides, there are many different forms of porous silicon, thus making the analysis of its structure an even more complex task. In all, detailed morphological and chemical characterization is a key factor to improve our understanding of the physical mechanisms responsible for the very different behavior of PS with respect to bulk Si, and also to improve the overall behavior of PS-based structures and devices. Within this context, researchers have performed over the years studies on the morphology of porous silicon with notable implications in applied research and the further development of practical devices. Aiming at precisely determining its intricate structure, a host of different microscopy characterization techniques have been used. From these, (high-resolution) transmission electron microscopy (HR)TEM, (high-resolution) scanning electron microscopy (HR)SEM, and atomic force microscopy (AFM) have been extensively used. Other experimental techniques covered in this updated chapter include scanning transmission electron microscopy (STEM) tomography, confocal microscopy, cathodoluminescence (CL), scanning tunneling microscopy (STM), photoluminescence (PL), micro-Raman spectroscopy, near-field scanning optical microscopy (NSOM), multiphoton microscopy, fluorescence microscopy, Fourier transform infrared (FTIR) microscopy, secondary ion mass spectroscopy (SIMS) microscopy, and acoustic microscopy. The main advantage of most microscopy techniques is that they allow the direct characterization of the morphology at the micro- and nanoscales and thus are not based in indirect results to determine, for instance, the nanocrystal or pore size. Also, many of these techniques are nondestructive and can be combined with others for the thorough study of a given sample. Moreover, additional studies (chemical composition, photoluminescence, etc.) can be performed simultaneously. In the following sections, a non-exhaustive review of the most

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remarkable uses of different microscopies applied to the study of porous silicon is presented.

Transmission Electron Microscopy Studies Transmission electron microscopy (TEM) and its high-resolution version, namely, HRTEM (Goodhew et al. 2001), have been extensively used for the characterization of porous silicon at the nanoscale. Furthermore, the combination of TEM/HRTEM with image processing constitutes a powerful technique to perform detailed morphological analysis at the nanoscale. Moreover, the joint use of TEM with energydispersive X-ray spectroscopy (EDX or EDS) or energy-filtering TEM (EFTEM) allows the determination of the chemical composition of the structure of PS with a very high degree of precision. HRTEM studies have allowed determining the lattice parameter, crystallite size, as well as orientation of the silicon nanocrystals present in PS (Cole and Harvey 1992; Cullis and Canham 1991; Münder et al. 1992; Lehmann et al. 1993; St. Frohnhoff et al. 1995; Martín-Palma et al. 2002, 2004; Pascual et al. 2005; Wijesinghe et al. 2009). Pore size, together with nanocrystal arrangement and distribution, has been found to enormously depend on the particular fabrication technique and growth parameters. Under the appropriate fabrication conditions, the electrochemical etch of Si results in PS layers composed of Si nanocrystals with no preferential orientation embedded in an amorphous matrix (Lehmann et al. 1993), as shown in Fig. 1. Furthermore, the lattice parameter of the Si nanocrystals which compose PS has been found to increase with respect to that of bulk Si. This effect is also shown in Fig. 1. Additionally, TEM techniques allow the analysis of cross sections of porous silicon. In particular, TEM has been used to analyze the in-depth porosity profile of PS-based multilayer stacks. An example is shown in Fig. 2. More recently, in situ TEM was used for the characterization of PS-based nanostructures for their use in lithium-ion batteries. As an example, the volume expansion of nonfilling carbon-coated porous silicon microparticles (nC-pSiMPs) during lithiation was characterized by in situ TEM (Lu et al. 2015), as portrayed in Fig. 3. In the nC-pSiMPs, porous silicon microparticles (pSiMPs) consist of many interconnected primary silicon nanoparticles, and only the outer surface of the pSiMPs was coated with carbon, leaving the interior pore structures unfilled.

Scanning Electron Microscopy Scanning electron microscopy (SEM) and high-resolution SEM (HRSEM) (Goodhew et al. 2001) are widely used to study the morphology of porous silicon. It is virtually impossible to give a complete list of works in this field. In this regard, many SEM images of high quality can be found in books, reviews (see for example Refs. (Canham 1997; Lehmann 2002; Sailor 2011)), and even the web! As in the

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Fig. 1 (Left) Morphology of nanostructured porous silicon, consisting in an amorphous matrix with Si crystallites and no preferential orientation embedded in it. This material shows a polycrystalline diffraction pattern. (Right) Image processing was used to study the structure of the individual Si nanocrystals

Fig. 2 Cross-sectional TEM image of a PS-based optical interference filter

case of TEM/HRTEM, in addition to the analysis of the surface morphology, SEM allows studying cross sections of porous silicon. Figure 4 shows typical field emission SEM images (top and cross-sectional views) of nanostructured PS (nanoPS) and columnar macroporous silicon (macroPS).

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Fig. 3 (a) Schematic representation of the in situ TEM device used to study the volume expansion of nonfilling carbon-coated porous silicon microparticles (nC-pSiMPs) during lithiation. (b) Timelapse images of the lithiation of a 500 nm particle. Li transports along and across the carbon layer to react with the Si inside, causing volume expansion. It is observed that the carbon shell remains intact after full lithiation given that the pore structure provides enough space to accommodate the expansion. (c) Lithiation of a 1 μm nC-pSiMP (Reprinted from Lu et al. 2015)

Image processing can also be used in the analysis of images acquired by SEM/HRSEM with the objective of determining several parameters of interest. These include porosity, specific surface area, pore size, and pore size distribution (Ludurczak et al. 2009).

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Fig. 4 Typical field emission scanning electron microscopy images of nanoPS and columnar macroPS. (a) Cross section (thickness ~1.40 μm), (b) top view (pore diameter ~25 nm), (c) cross section (thickness ~6.5 μm), and (d) top view (pore diameter ~0.6–1.2 μm) (Martín-Palma et al. 2015)

Atomic Force Microscopy Detailed analysis of the structure of the surface of PS is an issue of major importance for many different practical uses, including photonic and biomedical applications. In this regard, atomic force microscopy (AFM) (Eaton and West 2010) is a technique which is commonly used for the analysis of the surface structure of PS at the nanoscale. To illustrate the use of this particular technique, Fig. 5 shows typical AFM images of low-porosity and high-porosity PS surfaces. These surfaces may be considered optically flat for practical purposes in the whole visible range (diffuse reflection at the blue end of the visible spectrum is lower than 0.3%). A related technique, namely, scanning tunneling microscopy (STM), has also been used to acquire topographic as well as photon emission maps of the surface of porous silicon (Dumas et al. 1993). In this microscopy, a STM tip is used as a local source of electrons to excite cathodoluminescence (CL), resulting in an image resolution of around 1 nm. In this line, CL mapping of PS can be achieved by modifying a commercial SEM system, basically by adding a collecting mirror and a

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Fig. 5 Surface of two very different porous silicon surfaces determined by AFM

photomultiplier to the SEM (Bruska et al. 1996), aiming at understanding the relationship between the structural and electronic properties in this material. A variation of STM, named scanning tunneling microscopy-light emission (STM-LE), allows the measurement of the visible spectra from individual protrusions on the surface of PS (Ito et al. 1995). Additionally, photoassisted STM has been used to study the surface of porous silicon (Pavlov and Pavlova 1997). In this specific case, electron-hole pairs are excited by light, and STM is used to measure the corresponding tunneling current of excited carriers.

Confocal Microscopy The tremendous growth of bio-applications of PS has made confocal microscopy a very convenient tool for the characterization of this material. Anyway, in spite of the current widespread use of this technique for such uses as cell culture, drug delivery, biosensing, biodegradability assessment, and tissue engineering, an early application of confocal microscopy included reconstructing 3D profiles of porous silicon, as depicted in Fig. 6.

Other Microscopies Although TEM/HRTEM, SEM/HRSEM, and AFM are extensively and routinely used to analyze the morphology and overall physicochemical behavior of porous silicon, this material has been studied using a number of other microscopy techniques. Among them, photoluminescence can be explicitly mentioned given that the most striking property of PS was its light-emitting capabilities in the visible wavelength regime at room temperature. As an example of the myriad of studies in this area, luminescence from individual Si nanocrystals in PS has been spatially isolated

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Fig. 6 (a) Confocal reflected light image (200  200 μm scan size) at 488 nm of a small area of a PS sample. This area was found to be made up of a large number of small “islands” 10–20 μm in diameter. This is a confocal slice of the top surfaces of the islands imaged on a conventional confocal scanning laser microscope. (b) Same as (a) except that the focal plane is now at the base of the islands (Δz = 10.2 μm). The islands are therefore 10.2 μm in height (Reprinted from Ribes et al. 1995a)

and detected (Mason et al. 1998). For this study, a combination of single-particle spectroscopy and shear force microscopy was used. In a subsequent study, the distribution of individual chromophores in porous silicon was analyzed by combining the previous techniques with fluorescence microscopy (Mason et al. 2001). The experimental results link the number and size of quantum dots in PS with its photoluminescence emission. In this line, photoluminescence and reflected light images of porous silicon can be acquired by means of a confocal scanning beam macroscope/microscope (Ribes et al. 1995b). Raman microscopy has also been widely used to determine the structure and other properties of porous silicon at the nanoscale. In particular, parameters like crystallite size, temperature, and stress in PS have been studied by a combination of microRaman and microphotoluminescence spectroscopies (Manotas et al. 1999), as shown in Fig. 7. Besides PS layers and multilayers, more elaborate structures such as microcapsules have also been studied by micro-Raman spectroscopy, aiming at evaluating stress and crystallinity (Naumenko et al. 2012). Fluorescence microscopy is also widely used for very different applications, including biomedical uses of porous silicon. As an example, Fig. 8 shows the experimental results regarding study of the adhesion and migration of human mesenchymal stem cells (hMSCs) on 2D square PS/Si micropatterns (Torres-Costa et al. 2012). The fluorescence microscopy image was taken after 72 h of culture. It is demonstrated that hMSCs respond to the particular structure at the micro- and nanoscales of the 2D surface patterns.

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Moreover, near-field scanning optical microscopy (NSOM) has been successfully applied to the imaging of topography and locally-induced photoluminescence of porous silicon (Rogers et al. 1995). The experimental results are, as in previous cases, consistent with the quantum-confinement model.

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Fig. 8 (a) Fluorescence microscopy images of human mesenchymal stem cells (hMSCs) on nanostructured porous silicon square micropatterns. Actin is stained green and nuclei are stained blue. (b) Detailed image at an intersection and (c) histogram of hMSC population from image (a) with absolute % and area normalized population (left and right columns, respectively) (Reprinted from Torres-Costa et al. 2012)

Multiphoton microscopy is another technique applied to the study of the nonlinear optical response in porous silicon (Palestino et al. 2009). As an example of its use, two-photon-excited fluorescence (TPEF) emission and second-harmonic generation (SHG) from glucose oxidase (GOX) adsorbed on porous silicon were detected simultaneously. And, finally, acoustic microscopy, comprising high-frequency micro-echography, acoustic signature V(z), and acoustic imaging, can be used to investigate the elastic properties of PS in a nondestructive manner (Da Fonseca et al. 1995). Among these properties, thickness, longitudinal wave velocity, density, acoustic impedance, velocities of surface acoustic modes, the presence of elastic gradients, and surface and subsurface roughness and defects can be measured, estimated, or, at least, qualitatively determined.

Concluding Remarks The complex structure of porous silicon makes it a very versatile material which can be used in a wide variety of fields. Several microscopy techniques have allowed to precisely determine the morphology of PS, with typical feature sizes spanning the micro- to nanometric length scales. However, the increased number of applications of porous silicon over the years has made necessary the use of additional microscopy techniques. This is illustrated by the concluding Table 1, where varied microscopy

AFM HRSEM Fluorescent confocal microscopy FTIR imaging microscopy HRTEM

Cell culture Biosensing Intracellular delivery from nanoneedles Surface derivatization patterning Targeted and sustained drug delivery Imprinted pSi via MACE

Multiphoton confocal microscopy

Microscopy technique STEM tomography SIMS microscopy

Field of use Lithium battery anodes Photocathodes

Azeredo et al. (2016)

Coombs et al. (2016) Joo et al. (2016)

Spatial homogeneity of surface chemistry Size and shape of encapsulated nanoparticles Metrology of 3D patterning

Reference Ge et al. (2013) Chandrasekaran et al. (2014) Marinaro et al. (2014) Schmidt et al. (2015) Chiappini et al. (2015)

Purpose 3D pore structure of PS particles from stain etching Depth distribution of nanoparticles impregnated into pSi layer Cell adhesion dependence on surface topology Individual mesopore size metrology in membranes In vivo biodistribution of nucleic acids

Table 1 The widespread use of varied microscopies in varied porous silicon (PS) structures and varied application fields

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techniques have recently underpinned characterization of patterned layers, impregnated layers, nanoparticles, membranes, and nanoneedles of porous silicon in diverse application areas.

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Manotas S, Agulló-Rueda F, Moreno JD, Martín-Palma RJ, Guerrero-Lemus R, Martínez-Duart JM (1999) Depth-resolved microspectroscopy of porous silicon multilayers. Appl Phys Lett 75(7):977 Marinaro G, La Rocca R, Toma A, Barberio M, Cancedda L, Di Fabrizio E, Decuzzi P, Gentile F (2014) Networks of neuroblastoma cells on porous silicon substrates reveal a small world topology. Integr Biol. doi:10.1039/c4ib00216d Martín-Palma RJ, Pascual L, Herrero P, Martínez-Duart JM (2002) Direct determination of grain sizes, lattice parameters, and mismatch of porous silicon. Appl Phys Lett 81:25–27 Martín-Palma RJ, Pascual L, Landa A, Herrero P, Martínez-Duart JM (2004) High resolution transmission electron microscopic analysis of porous silicon/silicon interface. Appl Phys Lett 85(13):2517–2519 Martín-Palma RJ, Manso-Silván M, Torres-Costa V (2010) Review of biomedical applications of nanostructured porous silicon. J Nanophoton 4:042502-1-20 Martín-Palma RJ, Hernández-Montelongo J, Torres-Costa V, Manso-Silván M, Muñoz-Noval A (2014) Nanostructured porous silicon-mediated drug delivery. Expert Opin Drug Deliv 11 (8):1273 Martín-Palma RJ, Hernández-Montelongo J, Muñoz-Noval Á, Manso-Silván M, Torres-Costa V (2015) Silicon-based nanoparticles for biosensing and biomedical applications. Encyclopedia of inorganic and bioinorganic chemistry, Wiley Online (ISBN: 9781119951438), pp 1–11 Mason MD, Credo GM, Weston KD, Buratto SK (1998) Luminescence of individual porous Si chromophores. Phys Rev Lett 80(24):5405 Mason MD, Sirbuly DJ, Carson PJ, Buratto SK (2001) Investigating individual chromopores within single porous silicon nanoparticles. J Chem Phys 114(18):8119 Münder H, Andrzejak C, Berger MG, Klemradt U, Lüth H, Herino R, Ligeon M (1992) A detailed Raman study of porous silicon. Thin Solid Films 221(1–2):27 Naumenko D, Snitka V, Duch M, Torras N, Esteve J (2012) Stress mapping on the porous silicon microcapsules by Raman microscopy. Microelectron Eng 98:488 Palestino G, Martin M, Agarwal V, Legros R, Cloitre T, Zimányi L, Gergely C (2009) Detection and light enhancement of glucose oxidase adsorbed on porous silicon microcavities. Phys Status Solidi C 6(7):1624 Pascual L, Martín-Palma RJ, Landa-Cánovas AR, Herrero P, Martínez-Duart JM (2005) Lattice distortion in nanostructured porous silicon. Appl Phys Lett 87(25):251921-1-3 Pavlov A, Pavlova Y (1997) Investigation of the surface topography of light emitting nanostructures of porous Si and the related photovoltaic effect by photoassisted scanning tunnelling microscopy. Thin Solid Films 297:132 Ribes AC, Damaskinos S, Dixon AE, Carver GE, Peng C, Fauchet PM, Sham TK, Coulthard I (1995a) Photoluminescence imaging of porous silicon using a confocal scanning laser macroscope/microscope. Appl Phys Lett 66:2321 Rogers JK, Seiferth F, Vaez-Iravani M (1995) Near field probe microscopy of porous silicon: observation of spectral shifts in photoluminescence of small particles. Appl Phys Lett 66(24):3260 Sailor MJ (2011) Porous silicon in practice. Wiley, Weinheim Schmidt T, Zhang M, Sychugov I, Roxhed N, Linnros J (2015) Nanopore arrays in a silicon membrane for parallel single molecule detection: fabrication. Nanotechnology 26:314001 St. Frohnhoff M, Marso MG, Berger M, Thönissen HL, Münder H (1995) An extended quantum model for porous silicon formation. J Electrochem Soc 142(2):615 Torres-Costa V, Martín-Palma RJ (2010) Application of nanostructured porous silicon in the field of optics. A review. J Mater Sci 45(11):2823–2838 Torres-Costa V, Martínez-Muñoz G, Sánchez-Vaquero V, Muñoz-Noval Á, González-Méndez L, Punzón-Quijorna E, Gallach-Pérez D, Manso-Silván M, Climent-Font A, García-Ruiz JP, Martín-Palma RJ (2012) Engineering of silicon surfaces at the micro- and nanoscales for cell adhesion and migration control. Int J Nanomedicine 7:623 Wijesinghe TLSL, Li SQ, Breese MBH, Blackwood DJ (2009) High resolution TEM and triple-axis XRD investigation into porous silicon formed on highly conducting substrates. Electrochim Acta 54:3671

X-Ray Diffraction in Porous Silicon Jeffery Coffer

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of Strain . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of pSi Multilayers . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of Loaded/Infiltrated pSi . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

X-ray diffraction (XRD) is a useful, complementary tool in the structural characterization of porous silicon (pSi), providing information not readily available from direct visualization techniques such as electron microscopies. This review outlines key considerations in the use of diffraction techniques for analyses of this material in both thin film form and freestanding porous Si nano or microparticles. Examples of the range of content in the analysis of pSi are provided, including formation mechanisms, layer thickness, extent of pSi oxidation, and degree of crystallinity. Such properties influence practical properties of pSi such as its biodegradability. We also focus on selected key properties where XRD has been particularly informative: (a) strain, (b) the structural analysis of pSi multilayers, and (c) an analysis of pSi loaded with small molecules of fundamental or therapeutic interest. Keywords

X-ray diffraction diffraction · XRDnanostructure

J. Coffer (*) Department of Chemistry, Texas Christian University, Ft. Worth, TX, USA e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_42

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Introduction X-ray diffraction (XRD) (Pecharsky and Zavalij 2009) is a complementary tool in the structural characterization of porous silicon (pSi), providing useful information not readily available from direct visualization techniques such as electron microscopies. This review outlines key considerations in the use of diffraction techniques for analyses of this material in thin film form attached to its underlying Si substrate, along with recent results applied to freestanding porous Si nano or microparticles. In terms of instrumentation, a typical x-ray powder diffractometer used in the analysis of pSi is illustrated in Fig. 1. Spectra of pSi in powder form with good signal-to-noise ratios can be obtained using a Cu Kα source operating at 25–30 kV on sample sizes of 10–25 mg. XRD has been utilized for a diverse range of scientific content in the analysis of pSi, ranging from formation mechanisms (Chamard et al. 2001) to layer thickness (Guilinger et al. 1995). Other examples include the use of XRD as an informative probe of the extent of pSi oxidation (Ogata et al. 2001; Buttard et al. 1996a; Pap et al. 2005) as well as the degree of crystallinity (Lehmann et al. 1993); experimental modification of these two parameters strongly influences other unique properties of pSi such as its biodegradability (Shabir 2014; Shabir et al. 2011). Representative examples of the range of information obtained from XRD on pSi are outlined in Table 1. Fig. 1 Typical x-ray powder diffractometer (Phillips XL 300). Key components are identified, including the transformer/power supply, x-ray source, goniometer, and detector.

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Table 1 Areas of pSi research served by XRD. Fundamental topic/ application Formation mechanism Layer thickness Oxidation Crystallinity/amorphization Thermal expansion Wetting by liquids Structural form (film, membrane, particle) Interfacial roughness Homoepitaxy Strain

Porosity gradients/ multilayers Loading/infiltration

Reference Chamard et al. (2001) Guilinger et al. (1995) Ogata et al. (2001), Buttard et al. (1996a), Pap et al. (2005) Lehmann et al. (1993), Deb et al. (2001) Faivre et al. (2000) Bellet and Dolino (1994) Milita et al. (2001), Buttard et al. (2002), Russo et al. (2011) Lomov et al. (2000) Liu et al. (2003) Barla et al. (1984), Young et al. (1985), Bensaid et al. (1991), Bellet et al. (1992), Lehmann et al. (1993), Bellet and Dolino (1996), Lopez-Villegas et al. (1996), Buttard et al. (1999), Abramof et al. (2006), Wijesinghe et al. (2009) Buttard et al. (1996b, 1998) Henschel et al. (2008, 2009), Berwanger et al. (2009), Henschel et al. (2010), Wang et al. (2010), Ge et al. (2013)

We subsequently focus below on three key properties where XRD has been particularly informative: (a) strain, (b) the structural analysis of pSi multilayers, and (c) an analysis of pSi loaded with small molecules of fundamental or therapeutic interest.

Analysis of Strain The most detailed scrutiny has emerged from studies of pSi samples obtained from anodization of p and p+ wafers; traditional high-resolution diffraction (Ogata et al. 2001; Pap et al. 2005), along with double (Buttard et al. 1996a; Young et al. 1985; Lopez-Villegas et al. 1996) and triple diffraction measurements (Wijesinghe et al. 2009), has been evaluated. In the standard diffraction experiments of this type of pSi, two features are observed in the 26–31 region: a sharp peak associated with the reflection (~28 ) and a broad diffuse peak (Ogata et al. 2001; Young et al. 1985). Experiments to date suggest that the relative contributions of each are a function of HF electrolyte concentration (Ogata et al. 2001) and wafer resistivity (Ogata et al. 2001; Buttard et al. 1996a). Importantly, with thermal annealing up to 450  C, the intensity of the sharp feature disappears (Fig. 2; Ogata et al. 2001). Subsequent detailed concurrent XRD, transmission electron microscopy (TEM), and electron diffraction have shown that the broad diffuse peak is not associated with amorphous material, but likely rather a consequence of a random distribution of nanopores (Bensaid et al. 1991). This is not without some controversy, however, as

588

d Intensity

Fig. 2 XRD spectra demonstrating a shift of (111) peak of a p+ pSi film after annealing at (a) as prepared, (b) 350  C, (c) 400  C, and (d ) 450  C (Adapted from Ogata et al. 2001).

J. Coffer

c b a 25 26 27 28 29 30 31 32 2q / degree

some groups propose the possibilities of strained microcrystallites (Lehmann et al. 1993) or pSi oxidation contributing to this phenomenon. Pragmatically, it should also be noted that for a freestanding, oriented pSi membrane, the Bragg condition is only satisfied for the peak near 70 (Ogata et al. 2001). Quantitative analysis of strain (typically by standard Hall-Williamson plots (Williamson and Hall 1953)) for a number of different sample types has led to an establishment of the following useful trends for pSi: • Strain is present in the form of lattice expansion perpendicular to the sample surface (Barla et al. 1984; Young et al. 1985; Bellet and Dolino 1996; LopezVillegas et al. 1996), with typical Δa/a values in the range of 1.49  10 3 to 2.2  10 3 (Lopez-Villegas et al. 1996). • Significantly, strain increases with increasing porosity (Barla et al. 1984; Bellet and Dolino 1996). • Strain increases with surface oxidation brought about by thermal annealing (and correlates with dangling bond concentration) (Ogata et al. 2001; Buttard et al. 1996a; Pap et al. 2005). These trends provide complementary insights into changes in the fundamental structure of pSi that accompany its common manipulation in the laboratory.

Analysis of pSi Multilayers The extensive number of studies of pSi-based biosensors fabricated in multilayer form (of alternating porosities) has provided strong motivation for analyses by XRD (Thust et al. 1996; Lin et al. 1997; Dancil et al. 1999; Chan et al. 2000). Related handbook chapters are ▶ “Porous Silicon Multilayers and Superlattices,” and ▶ “Porous Silicon Optical Biosensors.” Results for this type of pSi sample have

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been reported from double diffraction experiments, with data typically presented in the form of so-called rocking curves. One of the most detailed investigations has been reported by Bellet and coworkers for a structure of alternating porosities of 36% and 60% for 10 periods (Buttard et al. 1996b, 1998). The most unique result of such investigations is the appearance of prominent satellite peaks in the ω/2Θ plots and their analysis by detailed simulations. Excellent agreement between experiment and simulation is found when a linear gradient transition layer (in terms of both porosity and lattice parameter) is employed between layers, with a width of 14 nm for this layer providing optimal results (Buttard et al. 1996b, 1998). For pSi superlattice multilayers of high quality, it is also possible to correlate the observed fringes in the low-order satellite peaks with the total number of periods in the pSi film (Buttard et al. 1998). This is viewed as strong evidence of lateral homogeneity of the entire superlattice film thickness overall.

Analysis of Loaded/Infiltrated pSi One of the interesting fundamental questions associated with a nanoporous material concerns the effect of the nanopore on the structure and associated properties of a loaded or infiltrated substance. For the case of pSi films still attached to its underlying Si substrate, this question has been addressed for a range of simple organic molecules, ranging from alkanes (such as hexane (Henschel et al. 2009)) to alcohols of various sizes (from as small as ethanol (Henschel et al. 2010) to a 19 carbon linear chain alcohol (Henschel et al. 2008; Berwanger et al. 2009)). In such studies, the size of the pore clearly has a strong influence on the structure of the infiltrated species. For example, mesoporous silicon with a 15 nm pore diameter results in the formation of lamellar bilayer structure, as evidenced by the appearance of a discrete Bragg reflection associated with this type of structure. In contrast, mesopores with a 10 nm diameter lack these layering reflections (Henschel et al. 2008). In the long term, it is believed that investigations of this sort may provide useful information concerning the use of mesoporous silicon matrices for inducing nucleation for the crystallization of protein solutions. Recent developments have also shown the ability of freestanding porous Si particles to act as stand-alone carriers for drug delivery (Anglin et al. 2008; Salonen et al. 2008), in vivo imaging (Park et al. 2009), and sensing (Sailor and Link 2005). In addition to ideally providing information regarding relative particle size (from a Scherrer analysis of linewidth) (Patterson 1939), for drug-loaded pSi materials, x-ray diffraction can also reveal details regarding the influence of pore structure on the crystallinity of the drug upon infiltration. For example, recent reports have shown that the crystalline antibacterial drug triclosan can be readily infiltrated into mesoporous SI via a straightforward meltloading procedure (Wang et al. 2010). These studies have shown that a significant broadening and/or loss of triclosan-associated peak intensity in the 20–30 range of 2θ takes place, depending on pSi porosity and loading method. Such changes are associated with nanostructuring or amorphatization of the loaded drug within the

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a 5000

b 1400 Triciosan 1200 X-Ray Intensity

X-Ray Intensity

4000 3000 2000 1000

Triciosan-loaded mesoporous (81%)Si

1000 800 600 400

0

200 20

22

24 26 2q (degrees)

28

30

20

22

24 26 2q (degrees)

28

30

Fig. 3 X-ray diffraction spectra for (a) crystalline triclosan and (b) triclosan-loaded mesoporous Si (81%) (Adapted from Wang et al. 2010).

mesopores. To illustrate this effect, typical XRD patterns for crystalline triclosan and a pSi sample of 81% porosity exposed to molten triclosan at 90  C for 35 min are illustrated in Fig. 3. Significant loss of intensity in the crystalline triclosan reflections near 24 and 25 are clearly observed.

Conclusions The above x-ray diffraction studies of pSi clearly demonstrate the level of sensitive structural information that this technique can provide. Given the increasing importance of this matrix in biosensing and drug delivery, along with emerging areas in energy relevant to battery technology (e.g., Li storage and cycling (Ge et al. 2013)), ample motivation for expanded use of XRD is in place.

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Gas Adsorption Analysis of Porous Silicon Armando Loni

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Main Principles . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application to Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Origin of Hysteresis in “Non-interconnected” Mesopores . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Determination of Effects Associated with Processing . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Limitations . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

594 594 595 596 596 598 598

Abstract

Pore volume and surface area of porous silicon are key parameters to consider when developing applications that rely on the capacity to carry a payload, such as drug delivery, or that are dependent on the degree of “reactivity,” such as sensing or energetics. The ability to define and tune surface areas and pore size distributions is a necessity for clinical use of the material. Herein, the historical assessment of these physical parameters by gas adsorption is reviewed, the methodology behind the measurements is described, the limitations are highlighted, and data related to its use in determining the effects associated with different anodization parameters and post-anodization processing is presented. Keywords

Gas adsorption analysis · Hysteresis · Pore volume · Surface area

A. Loni (*) Ledbury, Herefordshire, UK e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_43

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A. Loni

Introduction Increasingly, porous silicon is being evaluated for the delivery of therapeutic agents, such as hydrophobic drugs, proteins, and peptides (see chapter ▶ “Drug Delivery with Porous Silicon”). Surface area is particularly important for the optimization of large-molecule monolayer adsorption, while pore diameter is particularly important when loading proteins. Pore volume is related to porosity (see chapter ▶ “Pore Volume (Porosity) in Porous Silicon”) and is generally described in terms of open volume (ml) per unit weight (g) of material, while surface area is defined by the exposed internal surface (m2) per unit weight of material; these parameters can be measured using gas adsorption-desorption analysis (Gregg and Sing 1982). As well as surface area and pore volume, information on pore size and shape can also be surmised.

Main Principles The gas adsorption-desorption technique relates to the adsorption of nitrogen (or, less commonly, carbon dioxide, argon, xenon, and krypton), at cryogenic temperatures, via adsorption and capillary condensation from the gas phase, with subsequent desorption occurring after complete pore filling. An adsorption-desorption isotherm is constructed based upon the relationship between the pressure of the adsorbate gas and the volume of gas adsorbed/desorbed. Computational analysis of the isotherms based on the BET (Brunauer-Emmett-Teller) (Brunauer et al. 1938) and/or BJH (Barrett-Joyner-Halenda) (Barrett et al. 1951) methods, underpinned by the classical Kelvin equation, facilitates the calculation of surface area, pore volume, average pore size, and pore size distribution. A variety of instruments designed specifically for gas adsorption-desorption analysis are commercially available. Common to all is the sample preparation and measurement methodology (International Organization for Standardization 2006a): a portion of the porous material to be analyzed (typically >150 mg) is placed in a glass sample tube, dried/degassed (taking care to avoid thermal modification of the structure), and weighed; after attaching to the instrument, the sample tube is evacuated and the free space volume measured by dosing with helium; after evacuating the helium, the tube is immersed in cryogenic fluid (77 K for nitrogen adsorbate) and the adsorbate gas dosed to the tube in incremental volumes, with the pressure (P) being measured in situ relative to the saturation vapor pressure (PSV) of the gas; multilayer adsorption onto the pore walls occurs initially, followed by capillary condensation as the relative pressure is increased; dosing continues until the isotherm reaches a plateau, signifying that the pores are completely filled (P/PSV = 1); thereafter, the pressure is incrementally reduced such that the liquid starts to desorb, the porous structure eventually becoming empty once again (P/PSV = 0). Surface area is obtained relatively quickly from the adsorption portion of the isotherm (in the region of low relative vapor pressure) and follows the complete

Gas Adsorption Analysis of Porous Silicon

a

H1

H2

595

b H2

H2 Pore Blocking

Vad

Vad

Cavitation

Amount odsorbed

Delayed Condensation

0.2 H3

0.4

0.6

H4

0.8

1.0

Delayed Condensation

0.2 Meniscus

0.4

0.6

0.8

1.0

W>Wc

W90%) may be subject to pore collapse during air-drying (see chapter ▶ “Drying Techniques Applied to Porous Silicon”), and this would yield a comparatively lower pore volume (as well as lower surface area) and therefore porosity. With regard to thermal oxidation, while the classical model implies there is a continual shrinkage of pore size due to the associated volumetric expansion of the crystal lattice (Sailor 2012), in practice, the average pore size actually increases; this is accompanied by a shift in the pore size distribution and significant reductions in both surface area and pore volume, probably due to sintering (see chapter ▶ “Sintering of Porous Silicon”) (Loni and Canham 2013).

Gas Adsorption Analysis of Porous Silicon

597

Pore Size Distribution : Anodised & Oxidised pSi

Cumulative Pore Volume (ml/g)

2.5

2

1.5

1

0.5

0 0

100

200

300

S1: 66% porosity

400 500 600 700 Average Pore Diameter (Å) S2: 86% porosity

800

900

1000

S2: Oxidised (800°C, 85 min)

Fig. 2 Pore size distributions for low- and high-porosity porous silicon (also showing the effect of thermal oxidation)

Table 1 Parameters obtained from gas adsorption-desorption analysis after low- and high-currentdensity anodization and after static thermal oxidation in air (porosity calculated gravimetrically and from total pore volume; author’s data) Description S1: anodized (66% gravimetric) S2: anodized (86% gravimetric) S2 then oxidized (800  C, 85 min)

BET surface area (m2/g) 287

Pore volume (ml/g) 0.789 (64% porosity)

Average pore size (nm) 10.9

495

2.267 (84% porosity)

18.3

238

1.199 (75% porosity)

20.1

Nitrogen annealing has been shown to increase pore size through coalescence (Bjorkqvist et al. 2006), similar to oxidation, and also results in an overall reduction in pore volume (and therefore payload capacity) (Limnell et al. 2007), while chemical derivatization of pore walls has been shown to have very little effect on the surface area and average pore size (Buriak et al. 1999). The technique has also proved useful in the characterization of optical grating-type waveguides (Radzi et al. 2012) as well as in the study of low-temperature solid-state interactions in nanoporous silicon (Khokhlov 2008) and in the characterization of other forms of porous silicon including etched silicon powders (Loni et al. 2011), silicon micro-assemblies (Bao et al. 2007), and silicon aerogels (Chen et al. 2012).

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A. Loni

Limitations Gas adsorption-desorption analysis becomes problematic for materials with low surface area (Yanazawa et al. 2000; Suzuki and Oosawa 1997) and with pore sizes >100 nm (Klobes et al. 2006). For mesoporous silicon with porosity less than 50% (equivalent pore size 50 nm – macroporous silicon (macroPS) (Cullis et al. 1997). In spite of a large number of EPR investigations of such different prepared samples, all results in general for as-prepared PS concerning structure and properties of defects were similar. Let us discuss the results obtained by X-band EPR technique at room temperature for PS samples with different porosities (35 –85%) which were electrochemically etched from p- and n-type substrates in HF-based solutions. EPR spectrum of as-prepared PS samples is anisotropic; it is characterized with spin S = 1/2, trigonal symmetry of the g-tensor (eq. (4)) with principal g values in the range gk = 2.0017 2.0024, g⊥ = 2.0078 2.0091 (Δg = 0.0003), where the parallel direction is [111]. Usually spectrum simulations (fitting procedure) are necessary in order to decompose the EPR spectrum in its individual lines, and Hamiltonian parameters can be extracted from the EPR line positions. Typical EPR spectra in p-type PS for three orientations B: B k [001] (a), B k [111] (b), and B k [110] (c) are shown in Fig. 1. Additionally at a higher amplification (*50), an anisotropic central hyperfine interaction with a nuclear spin I = 1/2 for each of the central lines is observed (Fig. 1). From fitting procedure the following principal values of the A tensor (eq. (4), (4a), (6)), which has C3v symmetry, are obtained: Ak = 139*10
4 cm
1, A⊥ = 73*10
4 cm
1 (ΔA = 3 cm
1), and the intensity ratio of the hyperfine lines and the central ones (I = 0) is 0.041 (von Bardeleben et al. 1993a). This result points out that the central atom (on which the electron is bound) is the silicon atom according to the natural isotopic abundance of 29Si (4.7%). The rotation pattern of the EPR signal indicates that the axial directions of the dangling bonds are distributed in the four [111] crystal axes of the original silicon lattice. Typical angular dependence of the EPR line positions of PS is shown in Fig. 2.

632

E. A. Konstantinova [100]

[111]

[011]

2.0090 2.0080 2.0070

X X X

X X

X X X X

X X

geff

2.0060

X X X X X X

X

X

2.0050

X

X X

2.0040

X

X X

X

2.0030

X XX

X X X X X X

2.0020 0

10

20

30

40

50

60

X X X X

70

80

90

Angle Fig. 2 Experimental (crosses and open circles) and theoretical (solid line) rotation patterns of the EPR signal for PS. The angle in degree is that between [100] and the direction of the magnetic field which rotates in the (011) plane. The open circles denote the data obtained from a decomposition treatment of the spectra by a computer simulation program (Mao et al. 1993) Table 1 PS of different types: g-values and hyperfine constants Type of substrate p
(100) p+ (100) p
(100) p (111) p (100) p (100)

g║ 2.0023 2.0017 2.0024 2.0020 2.0018 2.0015

g⊥ 2.0086 2.0091 2.0080 2.0088 2.0085 2.0085

A║, cm
1 – 13910
4 – – 14610
4 –

A⊥, cm
1 – 7310
4 – – 7810
4 –

Reference Mao et al. 1993 von Bardeleben et al. 1993a Uchida et al. 1993 Uchida et al. 1993 Rong et al. 1993 Brandt and Stutzmann 1992

Therefore the EPR signal in PS can be identified with Si dangling bonds at the (111) Si/SiO2 interface (so-called Pb center) by its [111] axially symmetric g- and Atensor and characteristic g and A values. According to the obtained results, PS is characterized by an existence of the crystalline Si phase with retention of the original crystal orientation in PS layers. The Pb center formation is found to be closely related to the surface oxidation. The most reliable and important results are summarized in Table 1. In the PS samples studied in (von Bardeleben et al. 1993a), the peak-to-peak linewidth is always equal to 1.2 G even after thermal annealing. In the PS samples studied in (Uchida et al. 1993), the linewidth increases as the magnetic field is tilted from [111] axis: ΔBpp = 0.26, 0.31 and 0.37 mT for angle equal 19.5 , 52.7 and

Characterization of Porous Silicon by EPR and ENDOR

633

90 , respectively. The angular dependence of the linewidth was explained as a broadening related to a distribution in g⊥ due to a strain of the center. A variation of linewidth values in different papers can be due to different conditions of PS formation and storage. It is expected that the additional linewidth originates from hyperfine interactions with neighboring 1H nuclei (because of the presence of hydrogen at the surface of PS (Grosman et al. 1993a)). Pb center concentrations were estimated, for example, in (von Bardeleben et al. 1993a) and in (Konstantinova et al. 1995): 5109 cm
2 for p+ layers, 51010 cm
2 for n+ layers, and 1010 cm
2 for p-type PS, respectively. It is a surprising low density of spins! For example, the Pb center concentration in bulk Si (at the Si/SiO2 interface) is typically 1012 cm
2. Notice that these values have been calculated by comparison with spin standard sample and taking into account the real surface of PS, which has been determined as 200 m2/cm
3 for p + layers and 600 m2/cm
3 for p-type PS (Bomchil et al. 1989). The low concentration of Pb center can be explained by a small fraction of the Si surface (10–20%) is covered by SiO2 and/or O-Si-H species (Grosman et al. 1993a). So far we have discussed the (111) Si/SiO2 interface, which is characterized by only one type of EPR active defect – Pb center. At the (100) Si/SiO2 interface, two different intrinsic defects Pb0 and Pb1 are present. The Pb0 center is a [111]-oriented Si dangling bond with properties very close to the Pb center at the (111) interface. The most appropriable model of Pb1 defect is a dangling bond located on a silicon dimer configuration (strained bond) as proposed by Edwards (Edwards 1988). Properties of Pb1-like centers in PS are discussed in (Xiao et al. 1994). In this work the results of EPR spectra fitting give evidence about the Pb1 centers’ existence. The ratio of the number of Pb1-like centers to that of Pb0-like centers is related to the PS porosity. Remote hydrogen plasma processing of the annealed PS does not change the ratio significantly, although the total numbers of Pb0-like and Pb1-like centers are reduced. The detailed study of microscopic structure of Pb1 center is represented in (Cantin et al. 1995) The Pb1 defect is characterized by spin equal ½, a monoclinic I point symmetry, and principal values of the g tensor g1 = 2.0029, g2 = 2.0058, g3 = 2.0069. Unlike the Pb and Pb0-like centers, its linewidth of 4.5 G is isotropic and frequency independent, indicating an atomic configuration not influenced by stress distributions. The central 29Si hyperfine interaction of the Pb1 defect is at least a factor of 2 smaller than that of the Pb and Pb0-like centers, implying a delocalization of its electron wave function over more than one Si nucleus.

Observation of Free Electrons in Porous Silicon Pb-like centers are present in all PS samples discussed here, but now we turn our attention to new EPR signal with a g-factor below 2.00. Under photoexcitation (near-infrared or visible) at temperatures below 50 K, a high-intensity EPR signal is observed in both n- and p-type PS (Fig. 3) (von Bardeleben et al. 1993a).

634 Fig. 3 EPR spectrum at T = 4 K of a free-standing p+type 100 μm porous layer under thermal equilibrium (A) and under 1 μm photoexcitation (B). There are two EPR signals vs Pb centers and conduction electron spin resonance (von Bardeleben et al. 1993a)

E. A. Konstantinova

A

B

Pb 3310

3330 MAGNETIC FIELD B(G)

CESR 3350

The effective g-factor of the EPR spectrum is close to the one of conduction electron spin resonance in n-type Si g = 1.9988. The peak-to-peak linewidth is of the order of 2 G. The anisotropy of the EPR spectrum demonstrates directly that the spectrum comes from the monocrystalline Si layer and cannot be associated with the amorphous oxide layer present at the surface. From the high concentration, g-value, the absence of hyperfine splitting, and observation in both n- and p-type PS, the authors (von Bardeleben et al. 1993a) ascribe this spectrum to the spin resonance of optically excited free electrons in quantum-confined Si structures, in which the degeneracy of the six X-valleys has been lifted by the uniaxial stress present in this material. This stress perpendicular to the (100) plane of the substrate will lead to the splitting of the isotropic resonance observed in bulk Si and may give rise to valley repopulation effects. The attribution of this EPR signal to triplet (S = 1) excitons existing in Si nanostructures can be excluded by the very different lifetimes: in range of from μs up to ms
for excitons (Demin et al. 2010) and the order of 100 s
for photocarriers (von Bardeleben et al. 1993a). An isotropic EPR center with approximately similar g-factor (g = 1.9995) was observed also at T = 4.2 K in both p-type and n-type porous silicon but without photoexcitation (Fig. 4) (Young et al. 1997). Angular dependence study showed that this center is isotropic and shows slight asymmetric line shape due to the Dyson effect (Young et al. 1997). By comparing its g value with those of shallow donors in bulk silicon and taking into account the Dysonian line shape, the center was identified due to the conduction-band electrons (not quantum confined!) in silicon microcrystals (Young et al. 1997). The conduction-band electron signal, present in freshly prepared p-type and n-type samples, can be dramatically and surprisingly enhanced by the presence of a polar solvent (as example authors have used acetone (Young et al. 1997)) on the n-type porous silicon surface. The authors believe that most of the donor electrons in an n-type sample can be pulled into the porous layer from the

Characterization of Porous Silicon by EPR and ENDOR

12.0 g = 2.0045(5)

EPR SIGNAL (A. U.)

Fig. 4 EPR spectra of (a) a low-doped n-type PS sample, (b) Si-rich oxide powder, (c) the n-type PS sample with an acetone treatment, and (d) a p-type PS sample, measured at T = 4.2 K and microwave frequency of 9.4440 GHz (Young et al. 1997)

635

8.0

gCB = 1.9995(1) (a) n-type PSi

4.0

(b) Si-rich oxide powder

0.0

(c) n-type PSi with acetone

−4.0

(d) p-type PSi

−8.0 3340

3360 3380 3400 MAGNETIC FIELD (G)

3420

substrate by solvent exposure of the porous layer, but a reasonable model was not established. Notice that in previous work [Von Bardeleben et al. 1993a], the similar center was ascribed to free electrons in quantum-confined Si structures and was detected only under photoexcitation. This difference in results of different authors can be due to different sizes of Si nanocrystals in the PS layers under investigation. Electrons in PS layers with Si nanocrystal size in the order of 10 nm and more are not quantum confined (meso- and macroporous Si), but quantum confinement of electrons takes place in PS with Si nanocrystal size in the order of 1 nm (microporous Si) (Buda et al. 1992; Sanders and Chuang 1992). The authors (Chiesa et al. 2003) report evidence for the generation of conductionband electrons in p+-type PS by the adsorption of NH3 molecules. The g value of the observed signal (g = 1.9984) is, within the experimental error, the same as that of conduction electrons in crystalline, polycrystalline, and porous silicon. The number of free carriers increases with increasing NH3 adsorption, and onset of both a skindepth effect and transition from a Langevin to Pauli paramagnetism (Slichter 1989) is observed. The starting material (p+ crystalline silicon) is a p-type semiconductor, which becomes practically an insulator upon etching (p+ mesoporous silicon), and, in turn, an n-type semiconductor upon adsorption of ammonia. This conversion is triggered only by the adsorption of a gas! The remarkable changes observed are ascribed to effects associated with the chemical interaction between the PS and the adsorbed molecules (Chiesa et al. 2003). Konstantinova et al. (2009a) have investigated the effect of adsorption of dry and wet ammonia on the concentration of equilibrium charge carriers in p- and n-type of PS layers (meso-PS). The new EPR signal with g = 1.9987 attributed to the conduction band electrons has been also detected in p-type and n-type of PS samples (Fig. 5). Notice that only in the atmosphere of wet ammonia the generation of conduction band electrons in p- and n-type silicon nanocrystals takes place. The concentrations of conduction-band electrons were ~1017 cm
3 for the p-type sample and 1.5  1018 cm
3 for n-PS. It is found respectively by means of IR spectroscopy

636

2 g=1.9987 EPR Signal (a.u.)

Fig. 5 EPR spectra at T = 77 K of the n-type PS in vacuum of 10
5 Torr (1) and in an atmosphere of NH3 and H2O mixture at the pressure of PNH3 = 15 Torr, PH2O = 15 Torr (2); subsequent evacuation to 10
5 Torr (3) (Konstantinova et al. 2009a)

E. A. Konstantinova

3

geff = 2.0055 1 3320

3340 3360 3380 Magnetic field (G)

3400

that only in the presence of water molecules the ammonia adsorption results in an increase in the concentration of free-charge carriers in n-type samples up to a level exceeding 1018 cm
3 in agreement with EPR data (Fig. 6). In p-type samples, a non-monotonic dependence of the charge-carrier concentration vs ammonia pressure is observed that evidences about the appearance of free electrons (Konstantinova et al. 2009a). The authors assume the following mechanism of NH3 molecule interaction with PS surface: NH3 + PSi ! NH3+ + (PSi)
. It is surprising that the P (phosphorus) dopant easily detected in n-type bulk Si is not registered in n-type PS (von Bardeleben et al. 1993a). Authors of (von Bardeleben et al. 1993a) conclude that the Fermi level in n+-PS is no longer pinned by the donor impurity but must have moved to the mid-gap position. This conclusion is also supported by EPR observation of the neutral Pb centers under thermal equilibrium conditions, which was not expected for n+ or p+ samples. B (Boron) acceptor is difficult to detect both in the bulk Si and in the PS due to strain broadening effects.

Defects after PS Thermal Annealing in Ultrahigh Vacuum Different papers are devoted to the effect of vacuum annealing (Von Bardeleben et al. 1993a, b; Laiho et al. 1994; Laiho and Vlasenko 1995). Authors of (Von Bardeleben et al. 1993a) have performed a series of isochronal (t = 30 min) thermal annealings in vacuum in the 100–600  C temperature range on a series of 40 μm thick 80% porosity n+ and p+ samples and determined the variations in the Pb center concentration (Fig. 7). The sharp increase of Pb center concentration at 450  C was attributed mainly to a de-passivation (due to hydrogen exo-diffusion) of existing Pb centers. Additionally the high-porosity samples investigated in this work show after 450  C annealing a modified EPR spectrum (Fig. 8); this new EPR spectrum can be decomposed into the

Characterization of Porous Silicon by EPR and ENDOR

1018 N (cm-3)

1017

Fig. 7 Intensity variation of the Pb center concentration as a function of an isochronal (t = 30 min) thermal annealing in vacuum (see the text for details) (Von Bardeleben et al. 1993a)

10-5

100

PNH3 (Тоrr)

101

40 Pb CONCENTRATION (arb. u.)

Fig. 6 Dependence of freecharge carrier concentration vs NH3 pressure in n-type samples (according to the IR spectroscopy data) (Konstantinova 2007)

637

p+

20 n+

0

0

700 350 ANNEAL TEMPERATURE (°C)

Pb center spectrum and an additional isotropic spectrum characterized by a g-factor of 2.0055 (Eq. 2) and a linewidth of 8 G. According to these parameters, the new signal can be identified with the dangling bond defect in amorphous or disordered Si. The formation of amorphous inclusions, as confirmed by the observation of the dangling bond defect, has also been reported for annealed p-type micro-PS (Konstantinova et al. 1995). In the latter case, the dangling bond center could be observed after a 250–300  C annealing. Annealing in the temperature range of 400–600  C and vacuum up to 510
5 Torr performed in (Laiho et al. 1994; Laiho and Vlasenko 1995) increases the EPR line intensity of Pb centers in a way depending on the annealing time. This can be explained by hydrogen de-passivation of the Pb centers. An isotropic line with g = 2.0055, usually attributed to disordered Si dangling bonds, appears after vacuum annealing of some minutes, but at a longer heat treatment, its intensity decreases. At room temperature this decrease is exponential with a time constant of a few minutes,

638 Fig. 8 Total EPR spectrum for B║[110] of a 80% porosity, 450  C annealed, p-type PS (b); subtraction of the Pb center spectrum (a) shows the presence of an isotropic line (c) with g = 2.0055 and linewidth of 8 G (Von Bardeleben et al. 1993a)

E. A. Konstantinova

a

b

c

3425

3442.5 MAGNETIC FIELD B(G)

3460

depending on the level of vacuum. Heat treatment of PS in vacuum leads to amplification of the Pb EPR spectrum after exposure of samples to air at room temperature.

Oxidized Porous Silicon Layers Thermal and anodic oxidation of PS is used to reduce the size of silicon nanocrystals in PS layers (to less than 3–4 nm) in order to achieve quantum size condition of electrons and, respectively, effective visible light emission. The surface of PS exposed to air according to the data of EPR- and IR-spectroscopy analysis by nuclear reaction is covered by SiO2 layer (von Bardeleben et al. 1993a; Bisi et al. 2000). The concentration of defects (Pb centers) at the interface Si/SiO2 is a very important parameter for practical applications of PS in the field of micro- and optoelectronic sensor devices (Bomchil et al. 1989); Bisi et al. 2000). Such defects play a role of non-radiative recombination centers, serve as traps of charge carriers limiting their transport, and can interact with adsorbed molecules changing their properties (Konstantinova et al. 1996; Ben-Chorin et al. 1994). The effect of PS oxidation has been studied by different authors (Morazzani et al. 1993; Pavlikov et al. 2011; Meyer et al. 1993; Salonen et al. 1997; Grosman et al. 1993b; Geobaldo et al. 2001; Sharov et al. 2005). For example, in the papers (Von Bardeleben et al. 1993a; Morazzani et al. 1993), a series of p+ samples of 65% initial porosity were oxidized at 1000  C for 1–10 min in dry oxygen at a pressure 12 mbar. After 1 min oxidation, the samples contain a strongly increased concentration of Pb centers
21012 cm
2. This value is referred to the total internal surface of PS

Characterization of Porous Silicon by EPR and ENDOR Fig. 9 EPR spectrum at 300 K of a 1000  C oxidized p+ layer for B ║ [001] (von Bardeleben et al. 1993a)

639

Pb

EX

3425

3445

3465

(200 m2/cm3) and is two orders of magnitude higher than in the non-purposely oxidized, aged samples. The reason of this effect is exo-diffusion of hydrogen, which had passivated most of the Pb centers present in the as-prepared samples. Combined oxidation/passivation procedures probably to be necessary in order to improve the results of surface quality. For longer annealing times, the Pb center concentration decreases approximately linearly with the oxidation time. This effect may be due to improved interface quality because of Pb center passivation by oxygen. Simultaneously the PL intensity of the samples increases because of the suppression of non-radiative recombination channels taking place through defect states. The authors of (von Bardeleben et al. 1993a; Morazzani et al. 1993) have also found that 1000  C oxidized samples present, in addition to the Pb centers, a different EPR spectrum, which is characterized by an isotropic line with g-factor of 2.0025 and linewidth of 0.5 G (Fig. 9). This spectrum seems to be identical to the EX center observed in (Stesmans and Scheerlinck 1995). The exact microscopic structure of this defect was not identified. Its concentration has varied only a little with the oxidation time. Notice that dangling bond defect characteristics of disordered and amorphous Si are absent in such hightemperature thermally oxidized PS. It was established previously that the formation of such centers in PS is only effective in high-porosity material (more than 75%) (Von Bardeleben et al. 1993c; Konstantinova et al. 1995). In the paper (von Bardeleben et al. 1993a), the results of an EPR study on 10 μm thick p+ PS of 65% porosity anodically oxidized at a constant current in 0.1 mol KNO-H2O solution up to various (5–25 V) potential differences between the sample and a Pt cathode are reported. Figure 10 shows the results for 5 V potential difference and different orientations of the magnetic field. For magnetic field B k [001], the total spectrum is composed of a 5G broad line at g = 2.0056 and a small intensity, isotropic line with a powder line shape and g-factors of gk = 2.0018 and g⊥ = 2.0004, which identify this second spectrum as

640 Fig. 10 EPR spectra for three orientations of the magnetic field B ║ [001] (A), B ║ [111] (B), B ║ [100] (C) of an anodically oxidized p+ PS of 65% porosity (von Bardeleben et al. 1993a)

E. A. Konstantinova

E´ A

B

C

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3445 MAGNETIC FIELD B(G)

3465

the E/ center (Silsbee 1961). The E/ center is one of the main irradiation-induced intrinsic defects in SiO2 that has been attributed to the oxygen vacancy defect. In the anodically oxidized p+ layers, the E/ center is a native defect (von Bardeleben et al. 1993a). This is a bulk defect, whose concentration will scale with the amount of oxide formed. Concerning the main spectrum at g = 2.0056 for magnetic field B k [001], an angular variation of the magnetic field shows it to be anisotropic and to be composed of different lines. The authors of (von Bardeleben et al. 1993a) suppose that the spectrum results from the superposition of Pb centers from different interface planes (111), (100), etc. Pb center concentration does not depend on oxidation potential, which implies that the oxidation of all internal surfaces is already completed. The E/ center concentration increases with further oxidation, as expected for a volume defect in SiO2. The decrease in its intensity for potentials higher than 10 V must be ascribed to a modification of the stoichiometry of the oxide formed under these conditions. An oxidation of PS surface during adsorption of NO2 molecules is discussed in (Sharov et al. 2005). PS was electrochemically fabricated using (100) p+ Si substrate. The interaction between NO2 and PS is a complex and entangled physicochemical process including van der Waals, hydrogen-bond type, ionic, and covalent binding. The PS surface becomes fundamentally modified on NO2 adsorption which is physisorption and weak and strong chemisorption. The adsorption consists of the following: (1) physisorbing nitrogen dioxide dimer on the surface, (2) synthesizing different nitrogen-containing surface molecular groups, (3) strong oxidation and hydration at the surface, (4) forming Pb centers, and (5) causing the appearance of ionic complexes of nitrite anions with Pb+-centers accompanied by increasing the free hole concentration in PS. EPR spectra of PS samples subjected to different times of NO2 adsorption are presented in Fig. 11. According to the g-values emerging from the spectra (gk = 2.0018, g⊥ = 2.009 and g1 = 2.0028, g2 = 2.0056, g3 = 2.0067), we detect the Pb centers (Pb0 and Pb1,

Characterization of Porous Silicon by EPR and ENDOR x 0.25

10000 EPR signal, arb. units

Fig. 11 EPR spectra of as-prepared PS sample (1), samples subjected to NO2 adsorption with the time of adsorption 10 s (2), 1 min (3), and 10 min (4). Adsorption effected in the air (Sharov et al. 2005)

641

1 2 3 4

5000 0 –5000 –10000 3320

3400

1019 1 2 Concentration, cm–3

Fig. 12 The dependences of hole (1) and Pb center (2) concentrations in PS upon NO2 pressure. Adsorption effected in a vacuum (Sharov et al. 2005)

3340 3360 3380 Magnetic field intensity, G

1018

1017

1 before adsorption

2

1016

10–2

10–1

100

101

NO2 pressure, Torr

respectively). The effect of NO2 pressure on Pb center concentration and hole one (emerging from IR-spectra) is represented in Fig. 12. We have noticed that the Pb center concentration is enhanced with an increase in NO2 pressure monotonically as contrasted to non-monotonic dependence of hole concentration (Fig. 12) and suggested that it should be accounted for by capturing holes to Pb centers, passivating Pb centers by NO2, and consequent releasing holes by such passivation. In the presence of atmospheric oxygen, this is not the case, and another oxidation mechanism is present. With an increase in adsorption time up to 1 min, trapping electrons and synthesizing new Pb centers by NO2 take place which result in an increase both in hole and in Pb center concentrations. Then the interaction of atmospheric oxygen with Pb centers that gives rise to silicon oxide commences to dominate over the second process concerned, and the amount of Pb centers reduces (Pb center passivation process). Oxygen is less active than nitrogen dioxide, and the oxidation of surface defects does not start at once but only after a stage of induction. Notice that the hole concentration in PS on NO2 adsorption does not exceed that in

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the initial crystal silicon. Thus, there is a good reason to believe that in as-prepared PS, most holes are captured by Pb centers. Only when NO2 interacts with Pb centers do these centers finish trapping holes and instead release them. Also stain-etched PS samples were successfully studied by EPR technique (Schoisswohl et al. 1995; Cantin et al. 1996). In as-prepared and air-aged (during approximately 2 months) samples, two paramagnetic defects were detected: the Pb center at (111) Si/SiO2 interfaces and the silicon dangling bond center similar to that in amorphous silicon (Schoisswohl et al. 1995). The Pb center concentration was 131018 cm
3, which is an order of magnitude higher than in comparable anodically etched PS. The angular variation of the EPR spectrum of Pb centers proves the predominant monocrystalline character of the material. The silicon dangling bond concentration was 131017 cm
3 that indicates in agreement with the Raman results only for a low fraction (approximately 1 vol %) of disordered silicon (Schoisswohl et al. 1995). In the stain-etched PS samples, which previously were annealed at the 650  C in vacuum and then also air aged at the room temperature, Pb1-centers were detected with principal values of the g tensor g1 = 2.0029, g2 = 2.0058, g3 = 2.0069 (Cantin et al. 1996). Therefore (100) interfaces (additionally to the (111) interfaces) are observed in the 650  C vacuum-annealed PS. The reason of this observation is hydrogen effusion and disappearance of the Si-H, Si-H2 stretching bands. Notice that stain-etched PS samples are under evaluation in some biomedical applications associated with drug chemical reactivity and delivery (Riikonen et al. 2012; Salonen and Lehto 2008; Timoshenko 2014).

EPR Diagnostics of Singlet Oxygen Generation in Porous Silicon Layers EPR spectroscopy can be successfully employed to the diagnosis of the process of singlet oxygen generation in nanocrystals of PS (Konstantinova et al. 2008; Konstantinova et al. 2009b; Demin et al. 2010). Figure 13a shows the EPR spectra of micro-PS samples in oxygen in the dark and under illumination, which were measured at the low power Pmw = 0.64 mW of incident microwave radiation. As can be seen in Fig. 13a, a noticeable change in the EPR signal amplitude of micro-PS in oxygen did not occur under illumination (curve 2a), as compared with that in the dark (curve 1a). At the same time, a considerable variation in the EPR signal amplitude of micro-PS under analogous conditions was detected at Pmw = 200 mW (Fig. 13b; curves 1 and 2). It is well known that, at a sufficiently high intensity of microwave radiation, its absorption by Pb centers was saturated; this manifested itself in a decrease in the EPR signal amplitude (Konstantinova et al. 1995). This effect was observed in our measurements performed with micro-PS in a vacuum (Fig. 13b, curve 3). Note that the relaxation of Pb centers from an excited state to the ground state occurred by energy transfer to crystal lattice phonons in the case of micro-PS samples in a vacuum. At the same time for micro-PS in an oxygen atmosphere, the magnetic dipoles of 3O2 molecules adsorbed on the surface of silicon nanocrystals interacted with the magnetic moments of excited Pb centers to

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Fig. 13 (a) EPR spectra of PS measured at Pmw = 0.64 mW in an oxygen atmosphere (1a) in the dark and (2a) under illumination. (b) EPR spectra of PS in an oxygen atmosphere (1) in the dark and (2) under illumination or (3) in a vacuum at Pmw = 200 mW (Konstantinova 2007)

induce their rapid relaxation to the ground state. Consequently, the relaxation time of Pb centers for micro-PS in an oxygen atmosphere decreased; the saturation effect of microwave absorption weakened; and, as a result, the EPR signal amplitude considerably increased (Fig. 13b, curve 1). The photosensitization of oxygen molecules occurred under the illumination of micro-PS layers placed in an oxygen atmosphere (Konstantinova et al. 2009b); consequently, the concentration of 3O2 molecules decreased. In turn, this caused an increase in the relaxation time of Pb centers; as a result of this, the EPR signal amplitude decreased (Fig. 13b, curve 2). It should be noted that the change in the EPR signal amplitude upon photoexcitation of PS is almost completely reversible in switching on-switching off cycles if the defect formation under illumination is reduced to a minimum. The samples under investigation satisfy this condition, because they were preliminarily oxidized upon illumination in air for several minutes, which resulted in a considerable decrease in the defect generation rate. In order to confirm the decisive role of oxygen in the decrease of the EPR signal amplitude upon photoexcitation of PS, we measured the EPR spectra of micro-PS in an atmosphere of nitrogen because N2 molecules are diamagnetic (Fig. 14). As can be seen in Fig. 14, a noticeable change in the EPR signal amplitude of micro-PS in nitrogen ambient did not occur under illumination (curve 2), as compared with that in the dark (curve 1). The amplitude of both spectra is small because of saturation effect. Therefore the relaxation of spins takes place only through the spin-lattice

644

1,0

EPR signal (arb. units)

Fig. 14 EPR spectra of micro-PS in nitrogen atmosphere: (1) in the dark and (2) under illumination. Experimental conditions: Pmw = 200 mW, Iexc = 650 mW/cm2, pO2 = 1 bar (Konstantinova et al. 2008)

E. A. Konstantinova

1 2

0,5 0,0 -0,5 -1,0 3320

3340

3360

3380

Magnetic field (G)

relaxation channel; the spin-spin relaxation is suppressed because of the diamagnetic nature of N2 molecules. Therefore triplet oxygen molecules are responsible for the effective spin relaxation process and an elimination of saturation effect. Control experiments for meso-PS show that there is no change in EPR spectra amplitude of meso-PS under illumination of the samples in oxygen ambient. This fact points out an absence of singlet oxygen generation effect and confirms the decisive role of excitons in energy transfer process from photoexcited excitons in nanocrystals in PS layers to oxygen molecules adsorbed on their surface. Indeed, the average size of nanocrystals in meso-PS layers is approximately 10 nm. In such systems the exciton binding energy is small in comparison with thermal energy at room temperature (26 meV). The above data were obtained at high microwave powers Pmw. In order to evaluate the limits of applicability of the EPR technique in the study of the generation of singlet oxygen upon photoexcitation of silicon nanocrystals, let us analyze the influence of the microwave power on the EPR signal amplitude. The dependences of the pEPR ffiffiffiffiffiffiffiffisignal amplitude on the square root of the microwave radiation power I Pmw are plotted in Fig. 15. It can be seen that the dependences I EPR EPR pffiffiffiffiffiffiffiffi Pmw obtained in vacuum and oxygen under illumination and in the dark coincide at low microwave powers (Pmw  0.5 mW) and differ substantially at high microwave powers due to the saturation effect (Poole and Horacio 1987). Consequently, at low microwave powers Pmw, the saturation effect is absent, and, correspondingly, the EPR signal amplitudes are identical for micro-PS in vacuum and the oxygen atmosphere irrespective of the illumination (Fig. 15). The last circumstance along with the reversibility of the amplitude of the EPR spectrum after illumination is switched off is an additional argument indicating that the decrease in the EPR signal amplitude at high microwave powers Pmw (Fig. 13) is not associated with the decrease in the number of spin centers in the sample. However, as was noted above, the decrease in the concentration of triplet oxygen either upon evacuation or in the course of photosensitization of oxygen molecules leads to an increase in the characteristic relaxation times of Pb centers and, as a consequence, to a decrease in the absorption of the microwave power. Indeed, the

Characterization of Porous Silicon by EPR and ENDOR

645

Fig. 15 Saturation curves for micro-PS in oxygen (pO2 = 1 bar): (1) in the dark, (2) under illumination, and (3) in vacuum (pO2 = 10
6 bar). The approximation dependences were obtained in the framework of the Bloch theory with due regard for the specific features of the detection system. The errors coincide with the sizes of experimental points (Konstantinova et al. 2008)

saturation curve for micro-PS in oxygen under illumination (Fig. 15, curve 2) lies lower than that obtained under dark conditions (Fig. 15, curve 1) (effect of generation of 1O2 molecules), and the saturation curve for the samples in vacuum (Fig. 15, curve 3) is characterized by a lower amplitude as compared to curve 2, passes through a maximum, and decreases with an increase in the microwave power Pmw (Fig. 15). pffiffiffiffiffiffiffiffi The dependences I EPR Pmw for the samples of PS in oxygen under illumination were approximated by the sum of the saturation curves for PS in the oxygen light dark atmosphere in the dark and in vacuum (Fig. 15): I EPR ¼ α  I vac EPR þ β  I EPR . In this expression, the quantity αdetermines the fraction of nanocrystals involved in the photosensitization of oxygen, and the quantity β determines the fraction of nanocrystals that do not participate in this process (α + β = 1). Actually, PS with nanocrystal sizes that do not exceed 2–4 nm are electron donors for triplet oxygen molecules, because these nanocrystals contain excitons due to the quantum size effect. Therefore, their surface under illumination is predominantly covered by 1O2 molecules that do not contribute to the paramagnetic relaxation. The other part of nanocrystals (with larger sizes), as in the case of meso-PS (see above), does not make a contribution to the photosensitization of oxygen. This is equivalent to the relaxation of Pb centers in the atmosphere of 3O2 molecules under dark conditions. It follows from the aforesaid that the quantity α also determines the percentage of oxygen molecules transforming from the triplet state to the singlet state. The best approximation is achieved at α = 0.41 and β = 0.59. This means that, in our case, approximately 41% of the total number of oxygen molecules transform into the singlet state. The quantity α is conveniently expressed through the experimental I dark
I light

EPR data. Indeed, since α + β = 1, we have α ¼ IEPR . Evidently, the efficiency of dark
I vac EPR

EPR

generation of 1O2 molecules depends on the amount of 3O2 molecules that surround a silicon nanocrystal. Figure 16a shows the dependence of EPR signal amplitudes on the pressure of oxygen ( p) for PS layers in the dark and under illumination. We can express the value of α in terms of experimental data shown in Fig. 16a. The

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Fig. 16 (a) The amplitude of the EPR spectra of micro-PS samples vspO2 (1) in the dark and (2) under illumination. (b) The concentration of 1O2 molecules photosensitized in the PS layers vs pO2 . The parameters are Iexc = 650 mW/cm2 and Pmw = 200 mW

amplitude of an EPR signal at p = 10
5 mbar was chosen as I dark EPR. The dependence of the fraction of photosensitized 1O2 molecules on p thus obtained can be directly converted into the concentration of 1O2 molecules (NSO), taking into account the initial triplet oxygen concentration in silicon pores, which is equal to 2.71019 cm
3 at pO2 = 1 bar (the Avogadro number divided by the molar volume). Figure 16b shows this result. An increase in the intensity of illumination of PS nanocrystals caused an increase in the amount of 1O2 molecules photosensitized on the surface of Si nanocrystals. As a result of this, the EPR signal amplitude decreased (Fig. 17). In this case, a sharp decrease in the value of IEPR was observed up to Iexc = 600 mW/cm2; as Iexc was further increased, IEPR reached an approximately constant value. The latter was likely due to the fact that, at the specified value, the predominant fraction of oxygen molecules that covered a nanocrystal occurred in a singlet state. Figure 17 also shows calculated dependence of NSO on Iexc. It should be noted that the investigation of triplet oxygen by the EPR technique was performed in the millimeter (Q) band of microwave radiation in view of a large width of the EPR spectrum of 3O2 molecules (due to the short lifetime in the excited state (Ruzzi et al. 2013)). The EPR spectra of 3O2 molecules in pores of micro-PS are shown in Fig. 18. The presence of several lines in the EPR spectrum (Fig. 18) is determined by the strong interaction of the rotational angular momentum K and the spin angular momentum S of the oxygen molecule (S = 1). The total angular momentum J takes on values K and K  1. As a rule, the recorded EPR spectrum contains six

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647

Fig. 17 Amplitude of the EPR spectra of micro-PS and the concentration of photosensitized 1O2 molecules vs Iexc measured at Pmw = 200 mW and pO2 = 1 bar (Konstantinova 2007)

Fig. 18 EPR spectra of molecular oxygen in pores of micro-PS: (1) in the dark, (2) under illumination, and (3) within 5 min after the stop of illumination. Experimental conditions: Pmw = 200 mW, Iexc = 650 mW/cm2, and pO2 = 0.5 bar (Konstantinova et al. 2008)

lines designated as C, E, F, G, K, and J, which have the highest intensities. They correspond to the following magnetic dipole transitions J, MJ ! J/, MJ/: {1,
1 ! 1, 0} (C); {2, 1 ! 2, 2} (E); {2, 0 ! 2, 1} (F); {6,
2 ! 4,
1} (G); {2,
1 ! 2, 0} (K), and {2, 1 ! 2, 2} (J) (Tinkham and Strandberg 1955; Vahtras et al. 2002). It can be seen from Fig. 18 that the illumination of the samples results in a decrease in the amplitude of the EPR spectrum. This suggests a decrease in the concentration of triplet oxygen molecules. The data obtained can be explained by the transition of oxygen molecules to the singlet state and can be considered a direct proof of the generation of 1O2 molecules in micro-PS layers. It should be noted that the EPR signal amplitude after the illumination is switched off only partially regains its initial value (before illumination) (Fig. 18). This can be explained by the fact that a considerable number of 1O2 molecules pass from the pores of the sample into the closed volume of the measuring cell, i.e., into the gaseous medium, in which the lifetime of the singlet state increases to approximately 50 min, as compared to approximately 500 μs in pores of PS (Konstantinova et al. 2008). Since the area under the EPR line is proportional to the number of spin centers, it is easy to estimate

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the fraction of the newly formed singlet oxygen molecules from the relationship S , where Slight and Sdark are the areas under the EPR lines of 3O2 α ¼ 1
Slight dark molecules under illumination and in the dark, respectively. Therefore, approximately 30% of oxygen molecules upon photoexcitation of nanocrystals in micro-PS layers transform into the singlet state. It should be noted that similar experiments with meso-PS revealed no changes in the amplitude of the EPR spectrum of triplet oxygen molecules in pores of meso-PS. This indicates that 1O2 molecules are not formed in this material.

Investigation of Porous Silicon by ENDOR Technique The detailed study of PS by the ENDOR technique was performed in the papers (Bratus et al. 1994; Bratus et al. 1995). According to the (Bratus et al. 1994), the ENDOR spectrum of the Pb center is composed of two lines corresponding to the Larmor frequency of the 1H and 19F nuclei; a typical low-temperature spectrum is shown in Fig. 19. The linewidth of the 1H line is equal to 500 kHz at 77 K and 250 kHz at 4.2 K; it does not vary further at lower temperatures. The lower-intensity 19F line is recorded with confidence only at helium temperatures, where its linewidth is equal to 125 kHz. Dependence of intensities of the ENDOR spectra on the magnetic-field essentially repeats the line shape of the EPR absorption spectrum for all crystal orientations. The microwave power level for the observation of the maximum ENDOR signal depends on the spin-lattice relaxation time T1 of a particular sample; as T1 increases, it moves to the lower microwave power range. At 77 K, the ENDOR spectrum for 1H was detected only for 64% porosity as-grown aged samples, pffiffiffiffiffiffiffiffiffiffi which have the longest spin-lattice relaxation time; its relaxation parameter T 1 T 2 determined at 300 K from the saturation curves was equal to 3.2 μs (T2 is the spin-spin relaxation time). For the annealed 80% porosity p+ samples with a threefold smaller pffiffiffiffiffiffiffiffiffiffi relaxation parameter T 1 T 2, the ENDOR signal was not detected at 77 K. The distinguishing features of the ENDOR spectra of the Pb center observed in a variety of PS samples are a constant linewidth and a constant ratio of the amplitudes of the 1H and 19F ENDOR lines for samples of different origin and porosity. In order to produce an effect on the ENDOR spectrum, the samples were treated in different ways in order to modify the paramagnetic Pb center concentration and the oxide layer. It was performed through vacuum annealing leading to H exo-diffusion, re-passivation of Pb centers by low-temperature H2 annealing, and HF etching reducing the SiO2 layer thickness. The thermal annealing has been performed for a set of samples in a vacuum of 10 Torr for 30 min at 350, 400, 550, 740, and 865  C. Some samples were hydrogenated in pure H2 at 253  C for 37 min; the others were dipped in fluoric acid for 1 s. The authors of (Bratus et al. 1994) observed both an increase in the Pb center concentration by a factor of 2
20 and a decrease by about two times, respectively. The ENDOR spectrum varied in intensity according to the EPR intensity and the microwave power level. It is very important that the form of

Characterization of Porous Silicon by EPR and ENDOR 250 Endor Signal (arb. units)

Fig. 19 ENDOR spectrum at T = 4 K and B ║ [001] showing the low-intensity 19F and the 1H distant ENDOR lines (Bratus et al. 1994)

649

200 150 100 50 0 13

13.5

15 14 14.5 Frequency (MHz)

15.5

16

the ENDOR spectrum and the relative intensities of the 1H and 19F signals do not changed during any changes of the EPR intensity. The ENDOR intensity depends on many factors, particularly on the relationship among different relaxation paths in the electron-nucleus system. As this relationship is highly sensitive to structural changes, the authors have concluded that the nearest environment of the dangling bond at the (111) Si-SiO2 interface of PS including the location of the hydrogen and fluorine atoms is extremely stable. In spite of the absence of resolved structure in the 1H ENDOR line, some useful information can be obtained from the sample independent linewidth at helium temperatures. Since the spin of the Pb center S = 1/2 and the nuclear spin I = 1/2, for either 1H or 19F, the ENDOR spectrum is observed at the frequencies νENDOR ¼  A2
νL j, where A is a hyperfine coupling constant and νL is the nuclear Larmor frequency. The ENDOR lines observed at the Larmor frequency of 1H and 19F can thus be described by two unresolved hyperfine components. It is reasonable to suppose that A is only determined by the dipole-dipole interaction, which allows to estimate the distance between the Pb center and the H, F nuclei from the ENDOR linewidth. The simulation of the 1H and 19F ENDOR lines as a superposition of two Lorentzians with equal intensity shows that the maximum separation between them is about 13 of the linewidth. The dipole-dipole interaction constant is determined as 1 bi ¼ 4π μ0 gμB gn μnB ψ 2 r
3 , where the symbols have their usual meanings (see section “Introduction” and Spaeth et al. 1992). Substituting the value of gn μnB for the known nucleus and the spin density ψ 2 = 0.72 according to (Von Bardeleben et al. 1993c), we obtain from the helium temperature linewidth the minimum distances of the 1H and 19F nuclei from the Pb center as 1.3 and 1.4 nm, respectively (Bratus et al. 1994). This estimate gives about the same distance as previously obtained for the nearest pair of Pb centers (Van Gorp and Stesmans 1992). Hence there are no passivated or unpassivated dangling bonds around the individual dangling bond inside the circle about 1.3 nm . The stability of the linewidth and the ratio of 1H to 19F ENDOR intensity shows that the Pb center is not a simple dangling bond with an oxide cap

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structure but is more extended in the interface plane including some passivated (or, perhaps, unpassivated) dangling bonds at a distance of about 1.3 nm . The question of whether the fluorine in PS is just a passivating agent of interface defects or an integral part of the interface structure in the nearby surrounding of the Pb center is still open. It is also possible that F occurs at the interface as a HF molecule (Bratus et al. 1994). The attempts to reveal the ligand hyperfine interaction with Si nuclei for the Pb center have not been successful, probably due to the small intensity of the Si superhyperfine EPR lines (see above – section “Paramagnetic Centers in As-Prepared Samples” and (Von Bardeleben et al. 1993c)). Under certain experimental conditions, new set of lines with smaller intensities and linewidth to be about 40 kHz has been observed (Bratus et al. 1995). This set of lines labeled the “New-spectrum” is symmetrical about νL of 1H and depends on the orientation of the magnetic field. As for free-standing samples, the best resolution of this spectrum was achieved at low radio-frequency power (Prf) and with microwave power Pmw of 0.4 μW. On further increasing of the Pmw and the Prf, the New-spectrum is hard to observe. The reason is SHF interaction with hydrogen nuclei of different shells (Bratus et al. 1995). It is evident that the largest SHF interaction originates from the nearest-neighbor shells, and the broad line is due to the higher shells. The angular dependence of the effective hyperfine constants for different 1H shells is observed (Bratus et al. 1995). This is the evidence that hydrogen nuclei can occupy certain distinct position in the Pb center environment. The bonded hydrogen atoms are present in thermally oxidized PS in the form of H-Si-H, Si-H, Si-O-H, bonds. The atoms bonded to Si were positioned along [111] direction. The bond lengths of Si-H, Si-O, O-H were chosen 0.148, 0.161, and 0.105 nm, respectively; Si-O-H angle was assumed the same as Si-O-Si bond angle to be 148 (Bratus et al. 1995 and references therein). In the case of Si-H2 species, the second hydrogen atom was positioned along one of the three other directions. The data were analyzed using the following spin Hamiltonian (Eq. 4): H ¼ μB geff BS þ

X i

 Si Ai Ii
gn μnB BIi ,

where S = 1/2, I = 1/2, Ai is the SHF interaction tensor and i marked the hydrogen  12 site around the defect; geff ¼ gk cos 2 θ þ gsin 2 θ , where θ is the angle between an axis and a direction of a static magnetic field B; for the other symbols, see “Introduction.” To  the first order  of perturbation theory, the ENDOR transitions are  A  given by hν ¼ gn μnB B  2eff . It is convenient to determine the SHF interactions with different shells in terms of the isotropic a and anisotropic b and b/ constants, which are referred to the principle values of the usual SHF tensor A by AXX = ab + b/, AYY = a-b-b/, AZZ = a + 2b (eq. (4a), (5), (6), (8), and Slichter 1989; Spaeth et al. 1992). In fitting the SHF constants, the authors (Bratus et al. 1995) have ignored the small anisotropy of g value and used an axial approximation (b/=0).

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There is no case in which the New-spectrum can be described except in the case of Si-H species located in the environment of the Pb center beginning at the secondneighbor shell. The other species presumably make a contribution to the broad 1H ENDOR line. The results of calculation of the SHF parameters of the Pb centers with three 1H neighbor shells are as follows: shell – 2, site – 6, distance – 0.681, a = 380 kHz, b = 248 kHz; shell – 3, site – 6, distance – 0.782, a = 201 kHz, b = 142 kHz; and shell – 4, site – 12, distance – 0.1027, a = 110 kHz, b = 84 kHz (Bratus et al. 1995). Annealing at 400  C caused an increase of the ENDOR signal. The New-spectrum is still observed. The ENDOR signal is decreased significantly after annealing at 550  C because hydrogen desorbs from the SiH species completely, but the New-spectrum is beyond delectability. The line shape of the broad line is unchanged for the samples annealed up to 865  C. An unambiguous conclusion about the thermal stability of the nearest hydrogen environment of Pb center is hard to make from these data. Probably the hydrogen remainder belongs to O-H bonds which are more stable than Si-H bonds (Bratus et al. 1995).

Conclusions Structure and properties of defects in porous silicon have been studied with EPR and ENDOR spectroscopy taking advantage of the high specific area of this material. In spite of different types of substrate, different preparations, and storage conditions, the dominant type of defects (paramagnetic centers) in PS is Si dangling bond at Si/SiO2 interface or Pb center. There are two types of Pb centers
Pb0 (at (111), (100) Si/SiO2 interface) and Pb1 (at (100) Si/SiO2 interface) – which are characterized by different parameters of EPR spectra. Pb center concentration is very sensitive to vacuum heating and oxidation. In the first case, the Pb center density is nonlinear vs annealing temperature; in the second one, its behavior is also nonlinear vs annealing time because of the competition of process generation/passivation of Pb centers. During vacuum heating (at temperatures higher than 300–400  C), Si dangling bond similar to that in amorphous silicon is detected in PS. Free-electron EPR signal is observed in PS. EX center defects are detected in high-temperature oxidized PS. In anodically oxidized PS E/ center, a defect studied previously in SiO2 and attributed to positive charged oxygen vacancy is observed. The generation of singlet oxygen upon photoexcitation of silicon nanocrystals in PS layers was investigated, and its concentration depending on pressure of oxygen and intensity of illumination was determined using EPR spectroscopy. The observation of ENDOR with 1H and 19F atoms located at the average distance of approximately 1.3 nm of Pb centers emphasizes the dominant importance of the dangling-bond defect for the interface structure and suggests that H and F ions are probably incorporated at dangling bond sites. This defect structure is not modified by high-temperature thermal annealing or hydrogen passivation treatments.

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Laiho R, Vlasenko LS, Afanasiev MM et al (1994) Electron paramagnetic resonance in heat-treated porous silicon. J Appl Phys 76:4290–4292 Laiho R, Vlasenko LS (1995) Electron paramagnetic resonance of dangling bond centers in vacuum-annealed porous silicon. J Appl Phys 78:2857–2859 Mao JS, Jia YQ, Fu JS et al (1993) Electron paramagnetic resonance observation of trigonally symmetric Si dangling bonds in porous silicon layers: evidence for crystalline Si phase. Appl Phys Lett 62:1408–1410 Meyer BK, Petrova-Koch V, Mushik T et al (1993) Electron spin resonance investigation of oxidized porous silicon. Appl Phys Lett 63:1930–1932 Mogoda AS, Ahmad YH, Badawy WA (2011) Characterization of stain etched p-type silicon in aqueous HF solutions containing HNO3 or KMnO4. Mat Chem Phys 126:676–684 Morazzani V, Chamarro M, Grosman A et al (1993) Partial oxidation of porous silicon by thermal process: study and structure of electronic defects. J Lumin 57:45–49 Pavlikov AV, Konstantinova EA, Timoshenko V (2011) Correlation between spin density and photoluminescence intensity in thermally oxidized porous silicon. Phys Status Solidi C 8:1928–1930 Poole CP, Horacio F (1987) Theory of magnetic resonance, 2nd edn. Wiley, New York Riikonen J, Salomaki M, Wonderen van J et al (2012) Surface chemistry, reactivity and pore structure of porous silicon oxidized by various methods. Langmuir 28:10573–10583 Rong FC, Harvey JF, Poindexter EH et al (1993) Nature of Pb-like dangling-orbital centers in luminescent porous silicon. Appl Phys Lett 63:920–922 Ruzzi M, Sartori E, Moscatelli A et al (2013) Time-resolved EPR study of singlet oxygen in the gas phase. J Phys Chem 117:5232–5240 Sanders GD, Chuang YC (1992) Theory of optical properties of quantum wires in porous silicon. Phys Rev B 45:9202–9213 Salonen J, Lehto VP, Laine E (1997) Thermal oxidation of free-standing porous silicon films. Appl Phys Lett 70:637–641 Salonen J, Lehto VP (2008) Fabrication and chemical surface modification of mesoporous silicon for biomedical applications. Chem Engin J 137:162–172 Schoisswohl M, Cantin JL, Von Bardeleben HJ et al (1995) Electron paramagnetic resonance study of luminescent stain etched porous silicon. Appl Phys Lett 66:3660–3662 Sharov CS, Konstantinova EA, Osminkina LA (2005) Chemical modification of a porous silicon surface induced by nitrogen dioxide adsorption. J Phys Chem B 109:4684–4693 Shih S, Jung KH, Hsieh TY et al (1992) Photoluminescence and formation mechanism of chemically etched silicon. Appl Phys Lett 60:1863–1865 Silsbee RH (1961) Electron spin resonance in neutron irradiated quartz. J Appl Phys 32:1459–1462 Slichter CP (1989) Principles of magnetic resonance, Springer series in solid state sciences, vol 1, 3rd edn. Springer, Berlin Spaeth J-M, Niklas JR, Bartram BH (1992) Structural analysis of point defects in solids, Springer series in solid state sciences, vol 43. Springer, Berlin Stesmans A, Scheerlinck F (1995) Electron-spin-resonance analysis of the natural intrinsic EX center in thermal SiO2 on Si. Phys Rev B 51:4987–4997 Timoshenko VY (2014) Porous silicon in photodynamic and photothermal therapy. In: Canham L (ed) Handbook of porous silicon. Springer, Heidelberg/New York/Dordrecht/London, pp 929–939 Tinkham M, Strandberg MWP (1955) Interaction of molecular oxygen with a magnetic field. Phys Rev 97:951–966 Uchida Y, Koshida N, Koyama H et al (1993) Paramagnetic center in porous silicon: a dangling bond with C3v symmetry. Appl Phys Lett 63:961–963 Vahtras O, Loboda O, Minaev B et al (2002) Ab initio calculations of zero-field splitting parameters. Chem Phys 279:133–142 Van Gorp G, Stesmans A (1992) Dipolar interaction between [111] Pb defects at the (111) Si/SiO2 interface revealed by electron-spin resonance. Phys Rev B 45:4344–4347 Von Bardeleben HJ, Ortega C, Grosman A et al (1993a) Defect and structure analysis of n+-, p+- and p-type porous silicon by the electron paramagnetic resonance technique. J Lumin 57:301–313

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Electrical Characterization Techniques for Porous Silicon Magdalena Lidia Ciurea and Ana-Maria Lepadatu

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Characterization in the Dark . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Application to Sensors . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Characterization Under Illumination . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Electrical Characterization of Carrier Traps . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Contactless Electrical Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary and Outlook . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Porous silicon (PS) is a complex material with a large variety of properties given by the morphology from low to high porosity, by Si skeleton with sizes in a large interval and by its very reactive internal surface with a large area. This chapter covers more than 10 measurement techniques used for the electrical characterization of PS, separately reviewed are electrical investigations in the dark; under illumination; trapping phenomena studies; and contactless techniques. The electrical characterization is completed by taking into account the corresponding models and the PS morphology. These techniques also serve for the characterization of different devices and applications obtained by using PS with targeted properties.

M. L. Ciurea (*) Academy of Romanian Scientists, Bucuresti, Romania National Institute of Materials Physics, Magurele, Romania e-mail: ciurea@infim.ro A.-M. Lepadatu National Institute of Materials Physics, Magurele, Romania e-mail: lepadatu@infim.ro # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_111

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Keywords

Transient microwave photoconductivity · Current-temperature · Current–voltage (I–V) · Deep level transient spectroscopy (DLTS) · Electrical characterization · Optical charging spectroscopy · Space charge limited currents · Surface photovoltage spectroscopy · Traps characterization · X-UV synchrotron radiation

Introduction Porous silicon (PS) is a complex material with properties strongly dependent on its morphology and environment that make it interesting for applications in optoelectronics, micro-optics, catalysis, energy conversion, and sensors and for biomedical applications. The morphology of PS can be tuned by changing the parameters used in the process of electrochemical Si etching and by using Si wafers with different parameters (crystalline orientation, resistivity, and doping). During the last two decades, a vast number of papers focused on PS electrical properties, based on different characterization techniques, were published (Fedotov et al. 2016; Boarino et al. 2009; Islam et al. 2009; Urbach et al. 2007; Aroutiounian and Ghulinyan 2003; Ciurea et al. 1998). Different methods and techniques, generally used for characterization of semiconductor materials and devices (Schroder 2006; Deen and Pascal 2006), could be employed in electrical characterization of PS. In this chapter are presented techniques for electrical investigations of PS in dark and under illumination together with techniques used for traps characterization. The contactless techniques are reviewed, too. In the following, we give several examples for showing the importance of these techniques for PS electrical characterization, the PS electrical behavior being the subject of chapter “Electrical Transport in Porous Silicon,” and the investigation of contacts related to PS stability is presented in chapter “Ohmic and Rectifying Contacts to Porous Silicon.” The electrical characteristics and parameters obtained by using these measurement techniques combined with modeling and information resulting from microstructure investigations lead to a complex characterization of PS, as shown in Table 1. These techniques also serve for the characterization of different devices and applications obtained by using PS with targeted properties.

Electrical Characterization in the Dark The benchmark techniques for steady-state electrical characterization of PS are current–voltage (I – V ) and current–temperature (I – T ) measurements in the dark. In order to obtain an electrical characterization as accurate and complete as possible, I – V and I – T characteristics are modeled taking into account the PS morphology. Thus, the charge transport mechanisms at different temperatures are established, and the parameters of PS, i.e., energy bandgap, density of states, conductivity/resistivity, electrical anisotropy, quantum confinement levels, etc., as well as energy barriers

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Table 1 Techniques for electrical characterization of PS Characterization Techniques In dark Current–voltage I – V Current–temperature I – T

Electron beam-induced conductivity 1/f noise

Under illumination

Photocurrent–voltage Iph – V Photocurrent–temperature Iph – T Photocurrent spectra Iph – λ

Charge traps

Optical charging spectroscopy

Deep-level transient spectroscopy

Contactless

Space charge limited currents Photoelectron emission excited by X-UV synchrotron radiation Transient microwave photoconductivity

Surface photovoltage spectroscopy

Extracted information PS parameters (energy bandgap, density of states, conductivity, quantum confinement levels in nanoPS) Energy barriers (PS/Si substrate, nanoPS/Si oxide) Charge transport mechanisms at different temperatures Distribution of internal electrical field in PS Transport mechanisms Sensing molecules adsorbed on PS surface Optical bandgap and blue shift of edge for nanoPS, density of states, defects Transport mechanisms Photosensitivity for different spectral ranges Trap parameters (activation energy, trap concentration, cross section, lifetime) Trap parameters (carrier type, activation energy, trap concentration, cross section, lifetime) Density of states Conduction processes

References (Kulathuraan et al. 2016; Ménard et al. 2015; Gallach et al. 2012; Kanungo et al. 2010; Islam et al. 2009; Boarino et al. 2009; Martínez et al. 2008; Borini et al. 2006; Ciurea 2005; Martı́nPalma et al. 1999; Iancu and Ciurea 1998) (Jasutis et al. 1995) (Mkhitaryan et al. 2007; Bloom and Balberg 1999) (Wu and Li 2015; Torres et al. 2008; Khalili et al. 2007; Torchynska et al. 2005; Kang et al. 2003; Ciurea et al. 1996)

(Ciurea et al. 2007; Iancu et al. 2003; Draghici et al. 2000) (Simoen et al. 2012; Skryshevsky et al. 2006)

(Matsumoto et al. 1998) (Jacobs et al. 2000)

Transient changes of (Kytin et al. 2003) conductivity changes and dielectric function for discriminating between meso- and nanoPS Semiconductor type, (Burstein et al. 1997) optical bandgap, bandtail width

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Fig. 1 I – V characteristics measured at 30  C on PS films obtained by etching p
-, p-, and p+-type Si wafers (Reproduced with permission from Journal of Applied Physics, volume 118, S. Ménard, A. Fèvre, J. Billoué, G. Gautier, “P type porous silicon resistivity and carrier transport”, pages 105703(1–6). Copyright 2015, AIP Publishing LLC)

(interfaces of PS/Si substrate, PS nanostructures/surrounding Si oxide) are found. I – V measurements were recently reported (Ménard et al. 2015) on PS with different morphologies. The PS films were obtained by etching p+-, p-, and p
-type Si wafers resulting in PS with mesopores for p+ and p Si and macropores filled with mesoporous PS for p
Si. The I – V characteristics taken at 30  C present different dependences in function of PS morphology that in turn depends on Si wafer resistivity (Fig. 1). The authors show that for p
and p Si wafers, the current is limited at low voltages due to the surface layer of PS (related to the pore nucleation), while the more linear I – V dependence at higher voltages is given by the contribution of PS film, including PS/Si wafer interface. In the case of p+ samples, the I – V curve is practically symmetric and approximately linear up to 10 V. The I – V curves measured at five temperatures between 30  C and 200  C allow the use of the high voltage region of “forward” regime with linear I – V dependence for extracting the PS resistivity ρPS and for analyzing ρPS dependence on PS porosity. Therefore, it is shown that ρPS increases exponentially with the porosity increase and it is thermally activated (ln(ρPS) = ln(ρ0)
(EA/kBT )). By modeling the ρPS(T ) dependence with Meyer–Neldel formalism, the authors could conclude that in PS

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films obtained on p+ Si, the charge transport takes place through bandtails and deep centers located in a tissue surrounding the crystallites that are produced by the presence of hydrogen and oxygen, respectively. A similar mechanism was also found in PS samples on p Si with porosity up to a threshold, while for higher porosities, the hopping between crystallites is the dominant mechanism. In PS on p
Si, hopping mechanism was evidenced, too. The mesoporous PS obtained by (100) Si anodization has electrical anisotropy (Borini et al. 2006, 2009) demonstrated by measuring I – V characteristics in planar configuration (parallel with PS film) and in sandwich configuration (perpendicular with PS film). It is shown that the anisotropy can be removed by heating the PS samples and this process is irreversible. There are many other papers that make use of I – V characteristics to study PS layer formation (Kulathuraan et al. 2016), proper contacts/electrodes (Gallach et al. 2012; Martı́n-Palma et al. 1999), and influence of PS surface passivation on I – V characteristics (Kanungo et al. 2010). Concerning PS layer formation, I – V curves were measured on PS layers obtained by etching Si wafers (2–5 Ωcm resistivity) by varying the current density (40–150 mA/cm2). It was observed that PS porosity increases with the current density increase up to 100 mA/cm2 and then decreases. The porosity decrease of PS is explained by breaking of pore walls and therefore by exposing another layer to etching, this layer being positioned deeper in crystalline Si wafer. Consequently, the I – V curve measured on PS layer anodized with current density higher than 100 mA/cm2 is typical for a Schottky barrier. In other papers, I – V technique correlated with impedance spectroscopy was used for characterization of Schottky diodes fabricated on PS layers (Gallach et al. 2012). For this, an Au/NiCr bilayer contact was deposited on columnar PS ensuring a good contact adhesion. I – V measurements were employed also to study the effect of passivation produced by dipping in aqueous acidic solution of PdCl2 and the stability of contacts (Kanungo et al. 2010). For both geometries of contacts, i.e., planar and sandwich, I – V characteristics measured on passivated PS samples (40–60% porosity) are different than those measured on non-passivated ones. So, in planar geometry, I – V curves become linear after passivation, while in sandwich geometry, strong I – V rectifying curves are evidenced (due to PS/Si barrier). The charge transport in passivated PS films takes place mainly through the Si skeleton, but alternative path through Pd particles can exist. Measurements of electron beam-induced conductivity (EBIC) were performed on PS-based diode structures on n-Si and p-Si (Jasutis et al. 1995) for investigation of distribution of internal electrical field in PS. The curves of EBIC current in function of beam position present two peaks, one correlated to the metal/Si contact region and the other one to PS/Si interface that is in fact the built-in electric field between Si and electrolyte from pores (and disappears when electrolyte is removed). Free-standing luminescent PS was investigated by recording 1/f noise characteristics, and the transport mechanisms and paths were determined (Bloom and Balberg 1999). The authors found that the transport takes place on 2 paths, one being the continuous network of Si nanowires, while the other one consists of the network of Si nanocrystals that is responsible for the noise data. On the second

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Fig. 2 (a) I – T characteristics measured on PS layers with nanowires of 2.5–3.0 nm size; (b) best fits of I ~ exp(const  T
m) for different temperature intervals (Reprinted from Physica E, volume 41 (2009), M.N. Islam, S.K. Ram, S. Kumar, Mott and Efros-Shklovskii hopping conductions in porous silicon nanostructures, pages 1025–1028. Copyright (2008), with permission from Elsevier)

path, the conduction is limited by Coulomb blockade tunneling. The noise measurements were also used to investigate (Mkhitaryan et al. 2007) the effect of different gas media (air, polar molecules CO, and ethanol vapors) adsorbed on the surface of PS. The absorbed molecules produce different changes of the noise level. The I – T measurements represent a powerful tool for identifying the charge transport mechanisms. By modeling I – T characteristics measured over a large temperature interval, different mechanisms specific to different temperature ranges can be evidenced (Islam et al. 2009). So, the temperature dependence of dark current in PS layers was studied over a wide temperature range from 15 to 450 K (Fig. 2a) using a bias (1.5 V) corresponding to ohmic behavior. The PS layers were obtained by electrochemical anodization of p-type Si (100) with 6–10 Ωcm resistivity and have a columnar Si skeleton separated by pores, the PS nanowires having 2.5–3.0 nm size. The current depends on temperature by a law of I ~ exp(const  T
m). Depending on the value of exponent m, different conduction mechanisms were found for different temperature intervals (Fig. 2b). At temperatures lower than 120 K, the best fit was found for m  0.55 demonstrating that the carrier conduction is well described by Efros–Shklovskii model, in which the carrier transport takes place by variable range hopping on defect states located near Fermi level within the Coulomb gap. At higher temperatures (140–180 K), Mott variable range hopping is found as m  0.26. By analyzing I – T curves, the transition temperature between the two mechanisms of Efros–Shklovskii and Mott was evidenced at 150 K. For hightemperature interval (180–290 K), m 
0.86 corresponding to a Berthelot-type conduction mechanism, and above room temperature, an Arrhenius behavior (m = 1) was evidenced.

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I – T measurement technique was also used for identifying the quantum confinement levels in PS nanowires and for evaluating the nanowire diameter (Iancu and Ciurea 1998; Ciurea 2005). In Si nanowires with diameter of few nanometers, the quantum confinement effect is strong. Therefore, the electron Hamiltonian for a nanowire can be written in a good approximation as a sum of two parts, a longitudinal part along the nanowire and a transversal part. The transversal part can be modeled by a 1D infinite or finite quantum well that introduces quantum confinement energy levels in the bandgap, while the longitudinal part is described by 2D Bloch functions. Therefore, the electron energy in a nanowire is given in the effective mass approximation by the expressions e ¼ en ðkz Þ þ "

2π 2 ℏ2 2 x mt d2 p, l

#  2π 2 ℏ2 2 2π 2 ℏ2  ¼ en ðkz Þ þ  2 x1, 0 þ  2 x2p, l
x21, 0  mt d mt d

ns e ðk z Þ

þ ep
1, l ;

in which esn(kz) represents the Bloch electron energy (kz being the longitudinal wave vector component) that is shifted (in respect to initial value of en(kz)) in order to make the fundamental quantum confinement level to coincide with the valence band maximum (e0,0 = 0 eV). This means that the confinement energy is measured from the valence band maximum. The energies ep,l of the quantum confinement levels depend on the transversal effective mass mt and nanowires diameter d for a cylindrical symmetry (in the case of square symmetry, d is the square diagonal). xp,l also depends on the symmetry, for a cylindrical nanowire, xp,l = zp,l/π in which zp,l is the p-th zero of the cylindrical Bessel function Jl(z) indexed by orbital quantum number l, while for nanowires with square cross section, xp+2,l = [( p + 2)2 + l]1/2/2. From the I – T curves, the quantum confinement energy levels can be found by evidencing activation energies that are modeled as energy differences between quantum confinement levels. I – T characteristics measured in PS samples with Si nanowires, from 150 to 330 K, have Arrhenius behavior with two activation energies of Ea,1 = 0.55 eV and Ea,2 = 1.5 eV. In the frame of the model, the electrons are excited from the valence band (infinite particle reservoir) to the next quantum confinement level until this level becomes fully occupied (and the process can be continued with the temperature increase). When the electrons are excited to the next quantum confinement level, the slope of I –T curve changes. In order to find which levels are involved, the theoretical ratios of energies are compared with the experimental    ratio Ea,2/Ea,1 = 2.80 of activation energies. One finds that

x23, 0
x21, 0 = x22, 0
x21, 0 ¼ 2:80 for selection rule Δl = 0 as the electrons are thermally excited, meaning that the activation energies Ea,1 and Ea,2 are identified with e1,0 and e2,0, respectively. As the quantum confinement energy levels are known, the average nanowire diameter is easily evaluated. By using this model, the average diameter of PS nanowires is found to be 3.2 nm. This value was validated by high-resolution TEM images.

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Application to Sensors By using the techniques of electrical measurements in dark, the sensor properties of PS are investigated. I – V curves together with the evolution of PS resistance in time after exposure of PS to CO show that PS is a suitable material for CO sensor (Martínez et al. 2008). After PS exposure to CO for 40 min, the current increases with about 1 order of magnitude (I – V curve is nonlinear), and the resistance of PS samples decreases by reducing the surface states acting as traps. Mesoporous PS passivated by anodic oxidation is sensitive to different organic molecules (ethanol, methanol, acetone, chloroform, etc.) that was proved by capacitance–time and conductance–time measurements (Harraz et al. 2014, 2015). By exposing to polar solvents, the capacitance of PS samples increases, and then by solvent evaporation, the capacitance decreases. This process is reversible for polar solvent exposure, and it is reproducible for many exposure–evaporation cycles demonstrating that the capacitive chemical sensor based on mesoporous PS has a good reliability. For nonpolar molecules, this process is irreversible. Electrical measurements of capacitance–time and susceptance–frequency were also used to test the humidity sensing elements fabricated on paper substrates coated with mesoporous PS particles with 100–2000 nm size (Jalkanen et al. 2015). By varying the humidity from 0% to 90%, a reproducible resistance decreasing up to 3 orders of magnitude was observed. The fabrication process for the humidity sensing elements on paper is demonstrated to be robust and relatively simple, and consequently it has direct application in the development of low-cost humidity sensing devices.

Electrical Characterization Under Illumination Another important technique for electrical characterization of PS is related to the current measurements under illumination, i.e., spectral distribution of the photocurrent Iph – λ, photocurrent–voltage characteristics Iph – V and temperature dependence of photocurrent Iph – T. These measurements can be performed under dc regime (being similar to those used in dark investigations), and in this case the total current under illumination (sum between photocurrent and dark current) is measured. Also, these measurements can be done under modulated light evidencing the photocurrent, only. This kind of characterization correlated with PS morphology and photoluminescence measurements gives information about the optical bandgap, density of states, presence of defects, etc. (Ciurea et al. 1996). The PS characterization by photocurrent measurements demonstrates that this material with specific morphology is proper to be used for applications in optical sensors and solar cells. In a recent paper (Wu and Li 2015), the measurements of spectral responsivity and reflectance correlated with current measurements in dark and under illumination are used for characterization of three-layered PS (TLPS) structures. These structures are formed by PS layers with different porosities, i.e., the upmost layer with 20% porosity that is formed by 3–10 nm Si nanocrystals, the middle layer

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Fig. 3 Spectral response: (a) individual PS layers; (b) three-layered PS structure (TLPS); in blue, the sum of responsivities measured on individual layers in figure (a); in orange, the sum of responsivities corresponding to NPS and LP-PS and ten times responsivity of HP-PS; (c) the current–voltage characteristics taken in dark and under illumination (Reproduced with permission from Materials, volume 8, K.-H. Wu, C.-W. Li, “Light absorption enhancement of silicon-based photovoltaic devices with multiple bandgap structures of porous silicon,” pages 5922–5932. Copyright 2015 by the authors; licensee MDPI, Basel, Switzerland)

with high porosity of 70%, and the bottom PS layer with low porosity of 40%. The middle and bottom layers present similar morphology of pipe-shaped micro-rods and micro-holes. The three-layered PS structure was prepared on n+ Si wafers by varying the parameters of electrochemical etching from one layer to the next one leading to three PS layers with different optical bandgap energies (2.5, 1.8, and 1.6 eV), likewise a tandem solar cell. The spectral responsivity of this structure was measured between 300 and 1000 nm wavelengths and compared with the spectral responsivity of each PS layer that was separately obtained by etching under similar conditions (Fig. 3a, b). The samples with one PS layer of low porosity (LP-PS in Fig. 3a) present a spectral responsivity that is narrower, smaller, and blue-shifted (750 nm) in respect to that of crystalline Si. The spectral responsivity of samples with PS layer of high porosity (HP-PS) is much decreased (mainly due to the presence of surface states with high density) and blue-shifted (670 nm) in respect to the responsivity of low

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Fig. 4 Photoconductivity spectra measured on PS samples etched for different times (Reprinted from Microelectronics Journal, volume 39 (2008), J. Torres, H.M. Martinez, J.E. Alfonso, L.D. López C, Optoelectronic study in porous silicon thin films, pages 482–484. Copyright (2007), with permission from Elsevier)

porosity PS, while for the nanoporous PS layer (NPS), the responsivity is broader and the maximum is again blue-shifted (500 nm). The three-layered PS (TLPS) structures have a broadband photoresponse within the solar spectrum, being the envelope of photoresponse spectra of individual PS layers (Fig. 3b). Moreover, current–voltage measurements in dark and under illumination were performed on all samples with one PS layer and on the three-layered PS structure. The Iph – V curves show that the biggest photocurrent was measured (between 0 and 10 V) on the structure with stacked PS layers – TLPS (Fig. 3c). Therefore, this measurement technique represents a powerful tool for showing that the three-layered PS structure is suitable for developing PS-based solar cells. Measurements of spectral distributions of photocurrent and/or photoconductivity were also employed by other research groups for studying the influence of anodization conditions (current density, time, and HF concentration in the HF/ethanol/ distilled water electrolyte) on the PS photoconductivity (Khalili et al. 2007). In typical PS samples (30% porosity) formed by nanowires with 6 nm radius, the spectral distribution curve of photocurrent has a maximum positioned at 600 nm (2.13 eV) corresponding to the optical bandgap energy of PS layers. With the increase of current density, etching time, and HF concentration, the photocurrent maximum shifts toward lower wavelengths being attributed to the increase of porosity and thinning of Si skeleton. The measurement techniques related to photocurrent and photoluminescence were also used to distinguish between the contribution of Si skeleton and surface defects in PS films (Torres et al. 2008). The measured photoluminescence is intense,

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and its peak exhibits a blue shift (from 711 to 582 nm) by increasing the anodization time that is attributed to the decrease of nanowire size. On the contrary, with the anodization time increase, the peak position of photoconductivity spectra remains the same (450 nm), meaning that the main contribution to the photocurrent is given by the surface states with high density. Besides this, the photocurrent intensity decreases with the increase of anodization time (Fig. 4), explained by enlarging of pores that in turn produces the decrease of Si skeleton amount. Through photo- and dark current–voltage measurements, the optimal etching regime (current density) was determined with the aim to prepare PS films with small density of surface defects for preparing PS Schottky diodes with good characteristic (Torchynska et al. 2005). For this goal, the best PS samples that present a good rectifying behavior were obtained by using a very low anodization current density (5 mA/cm2). These Schottky diodes present in the reverse current branch of I – V characteristic a photocurrent higher with 2 orders of magnitude than the dark current. Generally, the photocurrent spectra of PS samples are broadened in respect to that of crystalline Si, and the edge from high energies is shifted to higher energies when the anodization current density is increased due to the decrease of Si nanoparticles. In order to diminish the contribution of Si substrate to the total photoresponse, lateral p+-PS-n+ diode structures were fabricated, in which PS layer was obtained by selective photoelectrochemical etching on Si diaphragm (Kang et al. 2003). Iph – V characteristics under 365 nm illumination with different photon fluxes show rectifying behavior, and the forward current increases with the increase of photon flux. The forward Iph – V curves show two linear dependences corresponding to ~0.7–2.6 V and ~2.7–5 V, respectively. The sensitivity was determined as 0.144 mA/μm for 5 V bias. The UV photoconductivity in PS was explained by a model based on deep electron traps and double injection. In order to enhance the photoconductivity of nanostructured PS, an optical microcavity structure (centered at λ = 700 nm) was fabricated that leads to the enhancement of photoconductance by photon confinement (Urteaga et al. 2009). The optical microcavity structure is a PS multilayer structure consisting in a PS layer of 52% porosity and λ/2 thickness that is bordered by 4 periodic Bragg mirrors with λ/4 thickness each, i.e., PS bilayers of 88% and 52% porosity. The spectral distribution of photocurrent measured in the 550–900 nm interval presents a sharp peak with full width at half maximum FWHM of 17 nm, positioned at ~700 nm due to the enhancement of electric field intensity by a factor up to 14 at resonance energies. The reflectance spectrum presents a similar peak of 17 nm FWHM (around 700 nm).

Electrical Characterization of Carrier Traps PS has a specific morphology with a high surface area and a high density of surface states. An important effort was spent for passivation of PS surface, but the trapping phenomena are little studied. The investigation of defects is important for PS-based

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Fig. 5 Discharge current curve Id – T measured on PS film (Reproduced with permission from Journal of Applied Physics, volume 94, V. Iancu, M.L. Ciurea, M. Draghici, “Modeling of optical charging spectroscopy investigation of trapping phenomena in nanocrystalline porous silicon,” pages 216–223. Copyright 2003, AIP Publishing LLC)

applications as the traps alter the electrical behavior of devices and diminish their performance and reliability. In the literature, the techniques mostly used for traps investigation in different materials (bulk and films) are the classical method of thermally stimulated currents (TSC) and conventional deep level transient spectroscopy (DLTS). Other related methods were developed, such as optical charging spectroscopy (Botila and Croitoru 1973; Petre et al. 1994), zero bias TSC (Chen and Das 1985; Lau et al. 1995), TSC spectroscopy (Muzikov et al. 2012), interface trapped charge DLTS, optical and scanning DLTS (Schroder 2006). Some of these techniques are used for investigation of trapping phenomena in PS. By using the method of optical charging spectroscopy, during the sample heating, the carriers released from traps move under the internal electric field (without any other external bias). The internal electric field at a particular temperature is produced by the still trapped carriers. A typical discharge current curve Id – T measured on PS films obtained by etching (100) p-type Si is shown in Fig. 5 (Iancu et al. 2003; Draghici et al. 2000) and was recorded using the setup from Fig. 6. The trap charging was made by illuminating PS samples with monochromatic light of λ = 0.5 μm wavelength at liquid nitrogen temperature. The maxima and shoulders in Id – T curve correspond to different trapping levels, and the sign of discharge current is controlled by the direction of internal electric field. In order to find the parameters of different traps, the technique of optical charging spectroscopy was combined with modelling (Ciurea et al. 2007). The model takes into account the trapping, detrapping, and retrapping processes with the following hypotheses: PS film thickness smaller than the absorption length meaning that traps are fully occupied and trapping levels are considered as being discrete (with zero energy width). The trapped carrier concentrations nt, pt at given temperature during

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Fig. 6 Setup for measuring discharge currents (optical cryostat, electrometer, light source, monochromator, temperature controller)

sample heating are calculated, and the internal electric field Ez (z,T ) is integrated on the depth z where the traps are located: ðz e Ez ðz, T Þ ¼ ½pt ðz0 , T Þ
nt ðz0 , T Þdz0 : e0 er 0

The total discharge current integrated on the PS film thickness d is ðd A I ðT Þ ¼ jðz, T Þdz  A~j ðT Þ; d 0

in which j(z,T ) is the current density, ~j ðT Þ is its mean value, and A is the illuminated electrode area. In PS samples, the total current has two components, one due to equilibrium carriers je ¼ σ 0 ðT ÞEz ðz, T Þ, with conductivity σ 0 ðT Þ ¼ e½μn ðT Þn0 ðT Þþ μp ðT Þp0 ðT Þ (n0, p0 equilibrium carriers concentrations, μn, μp mobilities) and the other one due to detrapped carriers jne ¼ Δσ ðz, T ÞEz ðz, T Þ , respectively. In PS, diffusion and displacement currents are neglected. By modeling, the discharge current curve was resolved into five trapping levels, and the parameters for each trap, i.e., activation energy, trap concentration, cross section, and lifetime, were found.

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DLTS is more advantageous as from DLTS curves, more information (e.g., type of trap for electrons or holes) can be obtained, but the preparation of structures with Schottky contacts is difficult for high resistivity materials as PS. DLTS technique was reported in literature for studying the effect of molecule adsorption/desorption on localized states in nanoporous and mesoporous Si (Skryshevsky et al. 2006). DLTS curves were measured in vacuum, in air, and in different atmospheres of Ar, N2, CO2, and O2, and then they were analyzed considering the peak intensity and activation energy. The DLTS spectra show peaks for hole traps (positive) and for electron traps (negative) that were fitted by Gaussians functions. The aspect of DLTS curves is strongly dependent on the PS morphology and the nature of adsorbed molecules. For example, the activation energies of peaks in the DLTS curves taken in N2 atmosphere on nanoporous PS increase when the reverse voltage increases, meaning that deep trapping levels are located in a broadband that is associated with a spatially inhomogeneous trap distribution. By performing measurements in O2 atmosphere on mesoporous Si, the shape of DLTS curve changes with the increase of O2 partial pressure, namely, some peaks disappear (e.g., a hole trap), and the other new peaks appear (e.g., two traps or even one trap for electrons) by a strong passivation of deep traps. Consequently, this technique is useful for study of PS films with selective chemical sensitivity that are proper for applications in bio- and chemical sensors. DLTS method was also used for investigation of defects in p-type Czochralski Si wafers containing macropores formed by high-temperature annealing (Simoen et al. 2012). The authors showed that the pore formation process produces deep-level defects in the Si substrate at the depth below macropores region. By using DLTS technique, a broadband of defects located close to the interface with top Schottky Al contact was evidenced, for which the hole capture is anomalously slow being explained to be due to internal interface states of macropores. The deep levels that are located in the Si depth beyond the pore region were attributed to the point defects possibly associated with metal contamination. For the study of point defects in semiconductors, generally, the Laplace-transform deep-level spectroscopy technique was proposed in Dobaczewski et al. (2004) as this technique provides a higher energy resolution with 1 order of magnitude than classical DLTS. The condition to achieve this resolution is to have a very good signal-to-noise ratio associated with the measurements. Laplace DLTS offers the benefit of eliminating the instrumental effect of levels broadening. Due to its high resolution, Laplace DLTS was successfully used in relation with uniaxial stress in lifting the orientational degeneracy of deep states and to evidence the defects symmetry. The density of states in Au/nanocrystalline PS devices was studied by measuring the space charge limited currents as a function of applied voltage and sample thickness (Matsumoto et al. 1998). The authors show that space charge limited currents are entirely controlled by localized states situated at quasi-Fermi level and the distribution of density of states near the Fermi level was determined.

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Contactless Electrical Characterization A special group of techniques for PS electrical characterization is the one represented by contactless techniques. These techniques are advantageous as the problem to contact the surface of PS is avoided. It is known that in order to elucidate electrical transport mechanisms in PS, stable and high-quality contacts at PS surface are mandatory. This is the reason why the use of contactless techniques is so attractive for PS electrical characterization, but also for the study of PS growth mechanisms related to the distribution of electric field (Parkhutik et al. 2003). The conduction processes in PS were studied by using photoelectrons produced by excitation with X-UV synchrotron radiation (Jacobs et al. 2000). The photoelectron emission spectra as a function of sample temperature were recorded on PS samples with ~80% and ~55% porosity. The authors measured the variation of the total electron yield signal in function of X-ray photon energy (98–110 eV including Si L2,3-edge) at different temperatures between 176 and 229 K. They observed Si L2,3-edge emission and its chemical shifts (due to PS oxidation) and the disappearance of these features at lowest temperature. This is explained by a transition from the insulating state to the conducting state of PS. With the decrease of temperature, the total electron yield decreases, and signal inversion at Si L2,3-edge transition is evidenced due to surface charging related to insulator character of PS. This is explained by percolation theory and Coulomb blockade. Another contactless technique is the one of complex transient microwave photoconductivity that was employed in characterization of free-standing meso- and nanoPS (Kytin et al. 2003). NanoPS has 85% porosity and contains crystallites of 2–4 nm size, while mesoPS has 45–50% porosity with 8–10 nm pores. By using this technique (35.1 GHz microwave source and laser pulse excitation of either 337 or 903 nm), transients of conductivity changes, Δσ, and of dielectric function changes, Δε, are obtained. So, by using UV excitation, mesoPS presents a negative Δε, nanoPS a positive Δε, while bulk Si presents also negative Δε. The transients of conductivity changes Δσ has a very high positive value in bulk Si, while in PS Δσ amplitude is smaller and decreases with the decrease of pore size. By using IR excitation only for mesoPS, Δε is negative, and the decay of Δσ and Δε is longer than for UV case. In mesoPS at microwave frequencies, the photoexcited carriers behave as free carriers contributing to dielectric constant decrease. In nanoPS (with smaller nanoparticle size), Δσ is very small but very pronounced, and Δε change is positive due to strong localization of charge carriers that are polarized under microwave field. A powerful technique, better known, is the surface photovoltage spectroscopy giving information about semiconductor type, optical bandgap, and width of bandtails (Burstein et al. 1997). The measurements of surface photovoltage together with conductivity in dark and photoconductivity were used to find the origin of strong PS red photoluminescence. The main results obtained in free-standing PS are the optical bandgap of 2 eV and the width of bandtails of 0.3 eV.

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Summary and Outlook Most techniques employed for electrical characterization of PS use PS samples with electrical contacts. They refer to PS electrical properties in dark and under illumination and to defects. In order to evidence and understand the electrical properties and processes that take place in PS films, the obtained electrical characteristics and results have to be modeled taking into account the PS morphology and microstructure parameters. The measurement techniques in dark supply information about the charge transport mechanisms at different temperatures, conductivity, electrical anisotropy, energy bandgap, density of states, quantum confinement levels, and energy barriers (PS/Si substrate, PS nanostructures/surrounding Si oxide). Additionally, from electrical measurements under illumination, information about photoconductivity, spectral responsivity, defects, and surface states is obtained. The techniques used for investigation of trapping phenomena make complete the picture of electrical characterization. Contactless techniques are also used for PS electrical characterization offering results about electrical transport mechanisms in PS. They have the advantage to avoid the problem related to ohmic and rectifying character of contacts. The electrical characterization techniques used for studying and evidencing the PS properties are also a powerful tool for different applications and devices based on PS. Acknowledgments The contribution to this work was supported by the Romanian National Authority for Scientific Research through the CNCS–UEFISCDI Contract No. PN II-PT-PCCA9/2012 and by the Romanian Ministry of National Education through the NIMP Core Program PN16-480102.

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Magnetic Characterization Methods for Porous Silicon Klemens Rumpf and Petra Granitzer

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetometer . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . MFM . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magneto-optical Microscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Magnetic Characterization . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Analysis of Magnetization Measurements . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

Characterization methods for magnetic materials, especially nanostructured ones such as mesoporous silicon, are reviewed. Besides magnetometers, which are one of the most important instruments to investigate magnetic properties, magnetic force microscopy and magneto-optical microscopy are briefly outlined. Magnetometers measure in an integrative way over the entire sample, whereas magnetic force microscopy and magneto-optical methods probe the magnetic properties of a local region or an individual nanostructure. With magnetometers in general, field- and temperature-dependent measurements can be performed; magnetooptical microscopy can be used to get knowledge about the domain structure, and with magnetic force microscopy, the magnetization reversal of, e.g., a nanowire, can be studied. Keywords

Lorentz microscopy · Magnetic force microscope (MFM) · Magnetization · Magneto-optical Kerr effect · SQUID-magnetometer K. Rumpf (*) · P. Granitzer Institute of Physics, University of Graz, Karl-Franzens-University Graz, Graz, Austria e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_46

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Introduction Due to nanostructuring of materials and the ability to fabricate nanosized objects, the requirements for their characterization are increasing, and the resolution of the various techniques has to be improved continuously. Concerning the investigation of the magnetic properties of nanomaterials on the one hand, the employed technique has to be sufficiently sensitive in the case of a small amount of material, and on the other hand it has to be appropriate to provide information about nanosized objects, e.g., about the arrangement on surfaces or in 3-D templates. Magnetization measurements can give evidence about the coupling mechanism, especially in combination with structural investigations. For the characterization of magnetic materials, many methods are suitable to gain information about the magnetic status, wherein magnetometers play a key role, especially concerning the investigation of nanomaterials. Such instruments enable measurements of magnetization dependence on applied external magnetic field and temperature. In general the magnetic behavior strongly depends on these parameters, as, for example, superparamagnetic particles offer a magnetic phase transition, whereas this transition temperature is correlated with the particle size and the arrangement of the particles (Gubin 2009). The two most employed types of magnetometer for nanomaterial investigations are superconducting quantum interference devices (SQUID) and vibrating sample magnetometers (VSM), but also alternating gradient field magnetometers (AGFM) are used. Further techniques to get knowledge about the magnetic properties of a sample are, for example, scanning techniques such as scanning probe microscopy (Hubert and Schäfer 1998), magneto-optical methods (Thiaville et al. 2005), ferromagnetic resonance (Baberschke 2011), scattering methods such as electron scattering (Zhu 2005) or X-ray scattering (Zhu 2005) or neutron scattering (Zhu 2005). Porous silicon with embedded magnetic nanostructures, which have been discussed in the other handbook chapters, has been investigated magnetically mainly by SQUID and VSM. Further applied methods to get information about the magnetic status of such specimens are magneto-optical or magnetic force microscopy. Thus, in this review, such techniques as SQUID, VSM, magnetic force microscopy, and magneto-optical microscopy will be introduced. More details about these techniques can be found elsewhere (Czichos et al. 2006).

Magnetometer A magnetometer commonly measure samples in an integrative manner, integrating the magnetic moment over the entire volume in contrast to, e.g., magneto-optical methods which give more local information about the specimen. The method is in general noninvasive. The most sensitive instruments are SQUID magnetometers (Fig. 1) which in general allow to measure signals down to a range of 10 8 emu. They are equipped with superconducting pickup coils and a superconducting quantum interference device as flux detector.

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Fig. 1 Image of a superconducting quantum interference device (SQUID) MPMS XL 7, Quantum design. Magnetic field range: 7 T, temperature range: 1.7–400 K (Institute of Physics, Karl-FranzensUniversity Graz)

Vibrating sample magnetometers measure the magnetic moment in the presence of a static or slowly changing external magnetic field. Most setups are employed with fixed pickup coils, and the sample is vibrating between them. The measured signal depends not only on the magnetic moment but also on the amplitude and the frequency of the vibrations. The sensitivity of such systems is in the range of 10 6 emu. An alternating gradient field magnetometer is a modification of the Faraday balance, and this instrument measures the force on a magnetic dipole caused by a magnetic field gradient. The sensitivity which can be reached is comparable to the one of a SQUID. For all magnetometers, the sample size is normally limited to a few square millimeters (2–5 mm across), and thus the magnetization signal can be weak, depending on the type and amount of contained magnetic material, which necessitates a sufficient high sensitivity of the employed instrument.

MFM Further possibilities to investigate magnetic samples are scanning techniques as, for example, an atomic force microscope equipped with a magnetic tip (Saenz et al. 1987), a so-called magnetic force microscope which can be used to investigate especially the domain structure and even the domain walls of magnetic samples (Hsieh et al. 2005). This method utilizes the magnetic interaction between magnetic tip and sample which is recorded. In general a spatial resolution of about 20 nm can be achieved. The resolution is determined by the geometry of the tip, the distance between sample and tip and also the sensitivity of the instrument. By optimizing all factors, also structures in a lower nanosize regime can be monitored, e.g., magnetic

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b 0.5

1 mm

Magnetic Phase (° )

a

∼ 3 μm

0.4 0.3 0.2 0.1 0.0 –0.1 0

2

4

6 8 10 Distance (μm)

12

14

Fig. 2 (a) MFM measurements performed in magnetic mode showing the abrupt change between silicon (right) and Ni-filled porous silicon (left). (b) The sharp contrast of the magnetic phase between nonmagnetic and magnetic material can be seen (Granitzer et al. 2010)

nanoparticles in the range of 10 nm by using special prepared tips (Koblischka et al. 2003). To record individual nanoparticles, the distance between the particles has to be sufficiently large to guarantee an image with excluding a convolution of the tip with the particle and also the effects of magnetic coupling (e.g., local ordering) on the image have to be kept in mind. A precondition for porous samples to be investigated is low surface roughness of the specimens. MFM has been successfully employed in porous alumina templates with deposited magnetic nanowires to figure out the direction of magnetization of the individual wires (Kalska-Szostko et al. 2009) and also to investigate their magnetization reversal (Sorop et al. 2003). In the latter case, an external magnetic field has to be applied during the investigations. In the case of porous silicon with embedded magnetic nanostructures, the investigation of the magnetization of the precipitates with a MFM is quite difficult. In scanning the surface, the roughness has to be taken into account, and the pores have to be completely filled up to the surface. The image depends on the magnetic polarization direction of the tip, out-of-plane or in-plane, in relation to the scanned surface. Standard tips are polarized out-of-plane. This is important in scanning cross-sections of porous samples with embedded magnetic nanostructures because the stray fields of the deposits depend on their geometry (e.g., spherical particles or elongated structures). An MFM image of a porous silicon sample filled with quite densely packed spherical Ni particles can be seen in Fig. 2. All these limitations are often not met, and thus other characterizing methods are more suitable for porous media.

Magneto-optical Microscopy A further local method to investigate magnetization processes as well as domain nucleation is the exploitation of the Faraday and Kerr effect. Magneto-optical techniques are very sensitive and allow the investigation of only monolayer thick

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magnetic films and nanostructure arrays (Gan’shina et al. 2005; Gonzalez-Diaz et al. 2007). In layered systems consisting of noble metal/ferromagnetic metal multilayers, an increase of the magneto-optical effects is observed due to surface plasmon resonance of noble metals. In general magneto-optic imaging is based on the rotation of the plane of polarization of linearly polarized light after interacting with a magnetic sample, whereas the reflected beam is detected (Kerr microscopy) (Giergiel and Kirschner 1996). The magneto-optical Kerr effect is physically based on the magnetic circular dichroism. This effect is used for magnetic imaging using an optical microscope setup. If the diameter of the light beam is larger than the domain size, magnetization curves can be observed (Beaurepaire et al. 2006). The spatial resolution of this method is only limited by the optical resolution. A further optical method to investigate magnetic domains and magnetization reversal processes is based on transmission electron microscopy whereat the Lorentz force affecting the electrons is exploited and used for magnetic imaging. Thus, this technique is called Lorentz microscopy (Petford-Long and Chapman 2005). The direction of the electrons after transmission through a sample is deflected in different directions depending on the direction of the local magnetization. Both techniques, which give local information about the magnetic behavior of a sample, are adequate to investigate porous materials (Pathak and Sharma 2012; Korolev et al. 2007), whereat in the case of transmission electron microscopy the sample preparation of porous materials, especially with embedded deposits, is tricky and can influence and modify the sample. For example, in using focused ion beam preparation, amorphization of porous silicon at the pore walls or sputtering of the metal deposits can occur.

Magnetic Characterization As discussed in a separate handbook chapter ▶ “Ferromagnetism and Ferromagnetic Silicon Nanocomposites,” the ferromagnetic signal of bare porous silicon samples is very weak and two to three orders of magnitude smaller compared to porous silicon with magnetic precipitates. The magnetic status of bare porous silicon depends on the surface treatment and the occurrence of dangling bonds. Due to the weak and difficult observable magnetic signal, the measurements have to be carried out with a sufficiently sensitive magnetometer. The ferromagnetic magnetization signal of a bare porous silicon sample is in the range of 10 6 emu, and thus a VSM is already operating in the upper limit wherefore a SQUID is more suitable to figure out the magnetization curve which in general is superimposed by a strong diamagnetic contribution. In the case of porous materials such as porous silicon with embedded ferromagnetic nanostructures, the choice of the magnetometer is not so important due to the sufficient high magnetization, and thus all types of magnetometer allow the magnetization of the sample to be ascertained. MFM enables the magnetization status of small regions to be derived, in general in the range of a few 10 nm of the sample and

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thus even individual nanostructures. The porous structure and the involved rough surface, especially caused by the metal deposition process (Granitzer et al. 2009a), constrain the potential of this method. Representative magnetization measurements were performed using Ni precipitated in needlelike structures of an aspect ratio up to 100, Co deposited in almost spherical particles with a maximum length of 100 nm and NiCo incorporated in granular elongated structures with a length up to 500 nm (Rumpf et al. 2010). MFM investigations of the cross-section of samples, using an out-of-plane polarized tip, do not give magnetic information in the case of elongated structures because the magnetic easy axis is along the pores. Spherical particles and not perfect cylindrical wires offer stray fields which lead to a usable signal.

Analysis of Magnetization Measurements From magnetization measurements, not only the magnetic status of a sample can be acquired, but it can be also concluded if particles magnetically interact or about the geometry and size of nanostructures. Measuring the magnetization of porous silicon with embedded nanowires in both magnetization directions, perpendicular (easy axis) and parallel (hard axis), to the sample surface results in a magnetic anisotropy which not only gives information about the shape of the ferromagnetic structures but also correlates with the interaction of the ferromagnetic metal nanostructures (Granitzer et al. 2012). The coercivities can also be used to obtain information about the magnetic interactions of the metal structures considering such ones of same size and shape. The coercivity of nanowires depends mainly on contributions due to the shape, magneto-crystalline and magnetoelastic anisotropy (Skomski and Coey 1999). Shape anisotropy affects the strength of the demagnetizing factor, and magnetocrystalline anisotropy depending on the crystal symmetry of the metal structures can be neglected if the crystalline orientation of the deposits is randomly distributed (Granitzer et al. 2009b; Bertotti 1998). Magnetoelastic anisotropy which is usually induced by an external stress is caused on the one hand due to magnetostrictive effects but also in relation with the lattice mismatch between deposited metal and silicon skeleton. Magnetic interaction takes place due to dipolar interactions between the nanostructures, but exchange interactions cannot be excluded in case of a dense metal structure distribution.

Conclusion For magnetic investigations of porous silicon and porous silicon nanocomposites, various magnetic characterization techniques are available. The choice of technique depends on the necessary sensitivity and also on the desired magnetic information. Magnetometers measure in an integrative way over the entire sample, whereas magnetic force microscopy and magneto-optical methods investigate the magnetic

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properties of a local region or even individual nanostructure of the sample. Magnetometers generally allow measurements of dependence on magnetic field and temperature and from the data, not only the magnetization of the sample but also indications of, e.g., the structure and magnetic coupling between nanoparticles can be ascertained. Magneto-optical methods with their local character allow determination of the domain structure of a sample. MFM can be used to figure out magnetization reversal processes if an external magnetic field is applied and also to see the direction of the magnetization, e.g., in the case of a nanowire. Using MFM, also the sample preparation plays an important role because the nature of the surface can influence the gained results. The abovementioned methods are both noninvasive and nondestructive to the examined specimen.

References Baberschke K (2011) Ferromagnetic resonance in nanostructures, rediscovering its roots in paramagnetic resonance. J Phys Conf Ser 324:012011 Beaurepaire E, Bulou H, Scheurer F, Kappler JP (eds) (2006) Magnetism: a synchrotron radiation approach. Springer, Berlin Bertotti G (1998) Hysteresis in magnetism. Academic, San Diego Czichos H, Saito T, Smith L (2006) Springer handbook of materials, measurement methods. Springer, Berlin Gan’shina EA, Kochneva MY, Podgornyi DA, Shcherbak PN, Demidovich GB, Kozlov SN (2005) Structure and magneto-optical properties of “porous silicon–cobalt” granular nanocomposites. Phys Solid State 47:1383 Giergiel J, Kirschner J (1996) In situ Kerr microscopy for ultrahigh vacuum applications. Rev Sci Instrum 67:2937 Gonzalez-Diaz JB, Garcia-Martin A, Armelles G, Navas D, Vazquez M, Nielsch K, Wehrspohn RB, Gösele U (2007) Enhanced magneto-optics and size effects in ferromagnetic nanowire arrays. Adv Mater 19:2643 Granitzer P, Rumpf K, Poelt P, Albu M, Chernev B (2009a) The interior interfaces of a semiconductor/metal nanocomposite and their influence on its physical properties. Phys Stat Sol C 6 (2222) Granitzer P, Rumpf K, Poelt P, Krenn H (2009b) Porous silicon/metal nanocomposite with tailored magnetic properties. Phys Stat Sol A 206(1264) Granitzer P, Rumpf K, Pölt P, Plank H, Albu M (2010) Ferromagnetic nanostructure arrays selfassembled in mesoporous silicon. J Phys Conf Ser 200:72037 Granitzer P, Rumpf K, Ohta T, Koshida N, Reissner M, Poelt P (2012) Enhanced magnetic anisotropy of Ni nanowire arrays fabricated on nano-structured silicon templates. Appl Phys Lett 101:033110 Gubin SP (ed) (2009) Magnetic nanoparticles. Wiley-VCH, Weinheim Hsieh CT, Liu JQ, Lue JT (2005) Magnetic force microscopy studies of domain walls in nickel and cobalt films. Appl Surf Sci 252:1899 Hubert A, Schäfer R (1998) Magnetic domains: the analysis of magnetic microstructures. Springer, Berlin Kalska-Szostko B, Brancewicz E, Mazalski P, Sveklo J, Olszewski W, Szymanski K, Sidor A (2009) Electrochemical deposition of nanowires in porous alumina. Acta Phys Pol A 115:542 Koblischka MR, Hartmann U, Sulzbach T (2003) Resolving magnetic nanostructures in the 10 nm range using MFM ambient conditions. Mater Sci Eng C 23:747 Korolev FA, Gan’shina EA, Demidovich GB, Kozlov SN (2007) Impedance and magneto-optical properties of porous silicon-cobalt nanocomposites. Phys Solid State 49:528

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Pathak S, Sharma M (2012) Magneto-optical Kerr effect measurements on highly ordered nanomagnet arrays. J Appl Phys 111:07E331 Petford-Long AK, Chapman JN (2005) Lorentz microscopy In: Hopster H, Oepen HP (eds) Magnetic microscopy of nanostructures. Springer, Berlin Rumpf K, Granitzer P, Pölt P (2010) Synthesis and magnetic characterization of metal-filled doublesided porous silicon samples. Nanoscale Res Lett 5:379 Saenz JJ, Garcia N, Gruetter P, Meyer E, Heinzelmann H, Wiesendanger R, Rosenthaler L, Hidber HR, Guentherodt HJ (1987) Observation of magnetic forces by the atomic force microscope. J Appl Phys 62:4293 Skomski R, Coey JMD (1999) Permanent magnetism. Institute of Physics Publishing, Bristol Sorop TG, Untiedt C, Luis F, de Jongh LJ, Kröll M, Rasa M (2003) Magnetization reversal of individual Fe nanowires in alumites studied by magnetic force microscopy. J Appl Phys 93:7044 Thiaville A, Miltat J, Garcia JM (2005) Magnetic force microscopy: images of nanostructures and contrast modeling In: Hopster H, Oepen HP (eds) Magnetic microscopy of nanostructures. Springer, Berlin Zhu Y (ed) (2005) Modern techniques for characterizing magnetic materials. Springer/Kluwer, Boston

Chemical Characterization of Porous Silicon Mihaela Kusko and Iuliana Mihalache

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 682 Chemical Composition of Electrochemically Etched Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . 682 References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 688

Abstract

We provide a literature survey of a number of classical techniques used to quantify the chemical composition of porous silicon, highlighting their general merits and potential limitations with the material. Much of the early literature was focused on photoluminescent material, but increasingly there are studies on nanocomposites where chemical composition analysis is required to assess the degree and uniformity of impregnation or surface attachment. Keywords

EDX · XPS · AES · SIMS · RBS

M. Kusko (*) Laboratory of Nanobiotechnology, National Institute for Research and Development in Microtechnologies (IMT – Bucharest), Bucharest, Romania e-mail: [email protected] I. Mihalache Laboratory of Nanotechnology, IMT-Bucharest, National Institute for Research and Development in Microtechnologies, Bucharest, Romania e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_47

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Introduction The chemical composition of porous silicon (PS) plays an important role in determining its properties (see chapters 19–37) and applications (chapters 72–100). This review surveys five classical techniques used in this regard, highlighting their general merits and potential issues with porous silicon, their application to different chemical forms of porous silicon, and their use in development of structures for diverse applications. Both electron and ion spectroscopies are “classic” methods of in-depth and surface chemical analysis at the nanoscale. Therefore, energydispersive X-ray spectroscopy (EDX), X-ray photoelectron spectroscopy (XPS), Auger electron spectroscopy (AES) on one hand, and secondary ion mass spectroscopy (SIMS) and Rutherford backscattering spectroscopy (RBS) on the other hand have been generally used to analyze the porous layers, mainly to determine their compositional profile. Each of them has advantages and disadvantages which represent critical factors in deciding the most appropriate one for achieving the information of interest and are briefly presented in Table 1.

Chemical Composition of Electrochemically Etched Porous Silicon The as-prepared PS layers were firstly investigated with these complementary methods in order to achieve a complete image of the large internal area chemical map where about 20% of the silicon atoms are located, looking forward to accomplish a general model for porosification process and also a mechanism for the most renowned property of this material, photoluminescence (see chapter ▶ “Photoluminescence of Porous Silicon”). Table 2 provides example of the compositional studies of freshly prepared and passivated/oxidized PS. While the associated analyses have been dedicated mainly to understand the role of the surface states on PL, including the time instability, and to find physicalchemical methods for stabilization and/or enhancement, the extension of the application areas towards nanocomposites and the attachment of complex molecules has given more importance to these methods (see, e.g., chapters ▶ “Ferromagnetism and Ferromagnetic Silicon Nanocomposites”, ▶ “Porous Silicon Optical Biosensors”, ▶ “Porous Silicon Immunoaffinity Microarrays”). Table 3 contains examples of applications where the corresponding assay analysis methods represented a key process to certify formation of more complex structures on the surface through both introduction of functional groups and implantation/deposition of metallic ions and the consequent development of hybrid devices based on the PS matrix. It is notable that the compositional analyses performed to study the surface oxidation process and its influence on the luminescence properties demonstrated also the higher chemical reactivity of the PS layers possessing higher porosity, mainly because higher porosity means a larger internal surface.

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Table 1 Evaluation of chemical characterization methods by analytical capabilities for PS samples Method EDX

XPS

AES

SIMS

RBS

Advantages Quantitative without reference samples Used in conjunction with electron microscopy (T)SEM Chemical binding state ID and elemental composition (surface concentrations and the chemical shifts)

Can reveal the chemical composition of the silicon surface during the etching process Some organic molecules can be identified Smallest volume of any analytical technique, depth profiling analysis 10–50 nm lateral spatial resolution Elemental composition, phase transition, and some chemical state ID

Depth resolution 0.5 nm The atomic ratio H/Si in PS Ultimate detection limit – ppb for B in Si Molecular specificity – polymers Fast analyzing method Nondestructive Elemental composition and depth profiling of individual elements (2–20 nm resolution) Quantitative without reference samples Investigate changes in porosity

Disadvantages No chemical state ID

Reference Garratt Reed and Bell (2003)

No chemical depth profile Samples for XPS must be compatible with the ultrahigh vacuum environment It probes only the very outer surface, not the huge internal surface Cannot detect H or He and consequently the ratio of elements in the sample that is not entirely accurate Only a lateral resolution of a few to 100 μm can be reached Surface may be damaged by the incident electron beam Surface electrostatic charging Quantitative analysis depends on the element (light elements >0.1%, heavier elements >1%); typical accuracy 10% Destructive method Quantification needs very similar standards Calibration is critical

Heide (2011), Watts and Wolstenholme (2003)

Watts and Wolstenholme (2003), Thompson et al. (1985)

Vickerman et al. (1989)

No chemical state ID No chemical structure No organic analyses Only a lateral resolution of ~1 mm The chemical composition of PS can change during ion beam (RBS) analysis While it is very sensitive for heavy elements (ppm), it has a low sensitivity for light elements

Kimura (2006)

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Table 2 Compositional analysis contributing to PL mechanism elucidation Type of PS As-prepared

Investigation techniques XPS

AES

SIMS

RBS

Passivated surface PS

EDX

XPS

AES RBS

Oxidized PS

XPS

AES

SIMS

RBS

Motivation for using technique The presence of a multiphase system, amorphous and crystalline, directly evidenced by XPS, is responsible for visible PL The electron structure of the chemical bonds and defects that might influence PL, like the oxygen role Composition of the silicon surface during porosification in fluorohydrogenate ionic liquids Thickness of thin films and elemental composition (quantitative AES) The presence of H and H2 bonds on the Si surface atoms; the character of the confined energy states in PS vs. crystalline silicon Reveals that some oxidation occurs as part of the etching process or as a result of exposure to atmospheric oxygen PS porosity – beam effect strongly depends on the porosity of the sample The porosity profile of the PS multilayer structure – five low/high porosity bilayers Detailed information on the nature of the surfaces, the extended defects, the amount of hydrogen passivation, and their oxidation state The effect of plasma fluorination used for surface passivation on PL – tuning of optical and dielectric properties Quantitative depth profiling of the Si, O, N, and S elements – concentration distribution The existence of Si, O, and C has been evidentiated in different PS samples, which is consistent with the XPS results Study the various oxidation states present before and after aging of PS and the effect upon the optical properties Fraction of oxidized layer (SiO2 present) over the cross section of PS samples induced by electrochemical oxidation in H2SO4 Comparison of SiO2/Si (bulk) and SiO2/PS interfaces shows characteristic oxygen depletion in the last case after thermal oxidation Refractive index change and the PL blue shift observed are explained as features of aging-induced oxidation

Reference Perez et al. (1992) Domashevskaya et al. (1998) Raz et al. (2010)

Galiy et al. (1998) Dorigoni et al. (1996) Collins et al. (1992) Kótai et al. (2000) Torres-Costa et al. (2004) Yu et al. (2005)

Pan et al. (2004)

Xiong et al. (2001) Feng et al. (2006) Thogersen et al. (2012) Salem et al. (2006)

Cwil et al. (2006)

Beresna et al. (2007)

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Table 3 Utilization of the compositional analysis for novel applications development Applications Optoelectronics

Results and comments Control of different organic molecules (alkenes and nonconjugated dienes) attachment process to the surface through Si-C bonds Selenization treatment Passivation and origin of the PL intensity enhancement achieved by Se bonding on Si surface Metal (Ag, Cu, Au) Clarify the reaction deposition mechanism and quantify the metal deposition; PL enhancement by Au film deposition is attributed to the oxidation inhibiting effect of the Au film Ultrathin metallic Chemical films (Au/Ti/Ni composition and thermal evaporation, electronic structure Ti electron beam (electron evaporation) concentration in the conduction band close to the surface of PS and at Si-Ti Si-Au interfaces) Electroplated PS with The particle size Fe and Co; Fe, Co, was calculated and Ni galvanic using static deposition from dielectric constant; sulfate salts Chemical composition of PS surface and layer depth profile; modifications of line shapes evidence metal reaction

Analysis PS modification XPS Passivated luminescent PS based on organic monolayers covalently attached

Reference Boukherroub et al. (2001), Lin et al. (2011), Hong et al. (2010), Harraz et al. (2002)

AES

Vdovenkova et al. (1997, 1999), Hamadache et al. (2003), Domashevskaya et al. (2012)

(continued)

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Table 3 (continued) Applications

Sensors

Results and comments Surface passivation by coating with CHx layer or Lewis acid-mediated hydrosilylation Thin metallic layer Penetration of the analyses metallic layer into the PS pores and effect on EL Lanthanide-doped Optical activation mesoPS of lanthanides within the whole area Er, Fe ions Analysis of incorporated in PS in-depth Er and Fe concentrations Infrared PL of The Er depth Er-doped PS distribution in the PS is not influenced Impregnation with by annealing up to different elements 1,000  C, although (Ni, Cu, Au, Pt, In, it strongly Fe) influences the oxygen content of the Si skeleton and the Er IR PL intensity Compact low The deposition resistivity silicide homogeneity contacts on Si through the whole thickness of the layer and the effect on the PS optical properties PS surface XPS confirms the functionalization with preliminary organic layers for functionalization biosensing steps and presence of thick protein film covering the PSi surface Mechanism of grafting PEG monolayers; optimization of silane deposition and further surface modification with aldehyde and PEG

Analysis PS modification SIMS Long-time stabilization of CHxmodified PS PL properties

Reference Mahmoudi et al. (2007), Stewart and Buriak (2001), Kleps et al. (2000), Gaponenko (2001), Bondarenko et al. (2003)

RBS

Henley et al. (2000), Herino (2000), Ramos et al. (2000)

EDX XPS

Jia et al. (2010), Guo et al. (2009), ArroyoHernandeza et al. (2003), Fang et al. (2006), Buriak et al. (1999), Lawrie et al. (2009), Baratto et al. (2000)

(continued)

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Table 3 (continued) Applications

SIMS

RBS

Energy

Results and comments Presence of reactive functional groups grafted on the PS surface (amine from APTS molecules or carboxylic functions from acrylic acid) Surface Monitorization of functionalization by Lewis acidalkynes and alkenes mediated hydrosilylation, consistent surface functionalization down the length of the pore DNA hybridization Using mesoPS for sensing in situ synthesis significantly increases the quantity of DNA probes attached NO gas sensor based Study of on PS DC-sputtered Au distribution in the pores Electrocatalytic EDX confirms the activity of metalpresence and the porous Si uniformity nanoassemblies distribution of Pt particles on/into PS layers PS surface passivation Finding of an for crystalline Si solar efficient surface cell passivation of a PS surface PS nanostructures as Clarifies that the hydrogen reservoirs amount of hydrogen produced by Si-H bond reaction with OH is limited by the Si-O and Si-F bonds formed during electrochemical etching

Analysis PS modification

EDX

XPS

Reference

Miu et al. (2010), Bilyalov et al. (2000), Zhan et al. (2011), Bahruji et al. (2009), Li and Buriak (2006)

(continued)

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Table 3 (continued) Applications

Biomedicine

Analysis PS modification Photoelectrochemical system for water splitting

SIMS

Functionalization with catalysts

EDX

PS particles for drug delivery

SIMS

Pt incorporation within the calcium phosphate layers on PS

Results and comments Predicts the treatments necessary to accelerate the photooxidation of nano-silicon by water for hydrogen production PS matrix impregnated with transition metalmediated dehydrogenate silanes (zirconocene and titanocene) Calibration of impregnation process of PS particles with Fe2O3 Diffusion studies of Pt complex-based antitumor compounds

Reference

Kleps et al. (2010), Li et al. (2000)

Acknowledgements The authors thank M. Simion, A. Bragaru, and T. Ignat for assistance in conducting literature searches and L. Canham for valuable suggestions.

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Lawrie JL, Xu Z, Laibinis PE, Molinari M, Weiss SM (2009) DNA oligonucleotide synthesis in mesoporous silicon for biosensing applications. In: Fauchet PM (ed) Frontiers in pathogen detection: from nanosensors to systems. Proceedings of SPIE, vol 7167. p 71670R Li YH, Buriak JM (2006) Dehydrogenative silane coupling on silicon surfaces via early transition metal catalysis. Inorg Chem 45:1096–1102 Li X, St. John J, Coffer JL, Chen Y, Pinizzotto RF, Newey J, Reeves C, Canham LT (2000) Porosified silicon wafer structures impregnated with platinum anti-tumor compounds: fabrication, characterization, and diffusion studies. Biomed Microdevices 2:265–272 Lin L, Sun X, Tao R, Feng J, Zhang Z (2011) The synthesis and photoluminescence properties of selenium-treated porous silicon nanowire arrays. Nanotechnology 22:075203 Mahmoudi B, Gabouze N, Guerbous L, Haddadi M, Beldjilali K (2007) Long-time stabilization of porous silicon photoluminescence by surface modification. J Lumin 127:534–540 Miu M, Kleps I, Danila M, Ignat T, Simion M, Bragaru A, Dinescu A (2010) Electrocatalytic activity of platinum nanoparticles supported on nanosilicon. Fuel Cells 10(2):259–269 Pan LK, Ee YK, Sun CQ, Yu GQ, Zhang QY, Tay BK (2004) Band-gap expansion, core-level shift, and dielectric suppression of porous silicon passivated by plasma fluorination. J Vac Sci Technol B 22:583 Perez JM, Villalobos J, McNeill P, Prasad J, Cheek R, Kelber J, Estrera JP, Stevens PD, Glosser R (1992) Direct evidence for the amorphous silicon phase in visible photoluminescent porous silicon. Appl Phys Lett 61:563 Ramos AR, Conde O, Paszti F, Battistig G, Vazsonyi E, da Silva MR, da Silva MF, Soares JC (2000) Ion beam synthesis of chromium silicide on porous silicon. Nucl Instrum Methods Phys Res B 161–163:926–930 Raz O, Shmueli Z, Hagiwara R, Ein-Eli Y (2010) Porous silicon formation in fluorohydrogenate ionic liquids. J Electrochem Soc 157:H281–H286 Salem MS, Sailor MJ, Harraz FA, Sakka T, Ogata YH (2006) Electrochemical stabilization of porous silicon multilayers for sensing various chemical compounds. J Appl Phys 100:083520 Stewart MP, Buriak JM (2001) Exciton-mediated hydrosilylation on photoluminescent nanocrystalline silicon. J Am Chem Soc 123:7821–7830 Thogersen A, Selj JH, Marstein ES (2012) Oxidation effects on graded porous silicon anti-reflection coatings. J Electrochem Soc 159:D276–D281 Thompson M, Baker MD, Christie A, Tyson JF (1985) Auger electron spectroscopy. Chemical analysis. Wiley, New York Torres-Costa V, Pászti F, Climent-Font A, Martín-Palma RJ, Martínez-Duart JM (2004) RBS characterization of porous silicon multilayer interference filters. Electrochem Solid-State Lett 7:G244–G246 van der Heide P (2011) X-ray photoelectron spectroscopy: an introduction to principles and practices. Wiley, Chichester Vdovenkova T, Strikha V, Vikulov V (1997) Auger electron spectroscopy study of the electronic structure of porous silicon-metal interfaces. J Electron Spectrosc Relat Phenom 83:159 Vdovenkova T, Strikha V, Tsyganova A (1999) Silicon nanoparticles characterization by Auger electron spectroscopy. Appl Surf Sci 144:69–72 Vickerman JC, Brown A, Reed NM (1989) Secondary ion mass spectrometry: principles and applications. Clarendon, Oxford Watts JF, Wolstenholme J (2003) An introduction to surface analysis by XPS and AES. Wiley, New York Xiong ZH, Liao LS, Yuan S, Yang ZR, Ding XM, Hou XY (2001) Effects of O, H and N passivation on photoluminescence from porous silicon. Thin Solid Films 388(1–2):271–276 Yu J-I, Kim DL, Lee DY, Yun J-G, Bae I-H, Lee JH (2005) Optical properties in porous Si investigated by an anodization current variation of photoluminescence spectra. Phys E 28:93–95 Zhan C, Chu PK, Ren D, Xin Y, Huo K, Zou Y, Huang NK (2011) Release of hydrogen during transformation from porous silicon to silicon oxide at normal temperature. Int J Hydrog Energy 36:4513–4517

Acoustic Characterization of Porous Silicon G. Todd Andrews

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Brillouin Light Scattering Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Acoustic Transmission Spectroscopy . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Characterization of Elastic Properties . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Effects of Chemical Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Attenuation of Acoustic Waves . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Phonon-to-Fracton Crossover . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Multilayered Films and Related Structures . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

692 692 692 696 697 698 699 700 701 701

Abstract

Experimental techniques used to characterize the elastic properties and to probe acoustic wave behavior in porous silicon are identified and the primary results obtained by application of these approaches are summarized. Particular emphasis is placed on Brillouin light scattering and acoustic transmission spectroscopy that, collectively, have been used for elastic moduli determination and for studying the effects of chemical modification on elastic wave velocities, phonon-tofracton crossover, acoustic phonon attenuation, and elastic wave propagation in multilayered porous silicon films. Keywords

Acoustic characterization · Porous silicon · Elastic properties · Elastic waves · Acoustic waves · Brillouin scattering · Acoustic transmission spectroscopy · G. T. Andrews (*) Department of Physics and Physical Oceanography, Memorial University of Newfoundland, St. John’s, NL, Canada e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_105

691

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Microacoustic techniques · Elastic constants · Acoustic wave velocities · Acoustic phonons · Phonon · Fracton · Phonon-to-fracton crossover · Rayleigh waves · Acoustic attenuation · Phononic crystal · Superlattice · Multilayer · Chemical modification

Introduction Initial efforts to characterize the acoustic properties of porous silicon predate the discovery of visible room temperature luminescence from this material in 1990. Since that time, several studies have been carried out using a variety of approaches to investigate elastic and acoustic wave behavior in this system (see Table 1 for chronology). These studies have been primarily aimed at determination of elastic properties and the dependence of these properties on porosity and pore network morphology, but interesting work has also been done on the effects of chemical modification on acoustic wave behavior, phonon-to-fracton crossover, acoustic wave attenuation, and elastic wave propagation in multilayered porous silicon films. Two techniques that have been widely employed are Brillouin light scattering and acoustic transmission spectroscopy. This review provides a brief introduction to these two techniques followed by a summary of the salient results obtained from acoustic characterization studies of porous silicon using these and other complementary approaches.

Brillouin Light Scattering Spectroscopy Brillouin scattering is the inelastic scattering of light by thermal acoustic phonons. Figure 1 shows a photograph of the typical apparatus used in a Brillouin scattering experiment together with a corresponding schematic diagram. In this technique, light from a single-mode laser impinges upon a target material and the light it scatters in some predefined direction is subject to frequency analysis, the standard spectroscopic instrument being a multipass Fabry-Perot interferometer. Depending upon the scattering geometry employed and the nature of the sample, the scattered light spectrum contains features (typically peaks) due to surface, interface, film-guided, and/or bulk acoustic phonons in the target. These features appear at frequencies different from that of the incident light frequency due to the inelastic nature of the scattering, with the shifts in frequency typically lying in the gigahertz range. From these shifts, one can determine phonon velocities and, often, a complete set of material elastic constants from which quantities such as compressibility, bulk modulus, and Young’s modulus may be deduced. In some cases, it is also possible to obtain acoustic attenuation from the feature width.

Acoustic Transmission Spectroscopy Figure 2 shows a schematic of a typical acoustic transmission spectroscopy setup. In this approach, ultrasonic or hypersonic acoustic waves are generated and detected by tiny transducers in indirect contact with the sample via a coupling medium (often

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Table 1 Chronology of experimental studies on characterization of acoustic/elastic properties of porous silicon Technique X-ray diffraction

Reference Barla et al. 1984 Da Fonseca et al. 1995b Cho et al. 1996

Substrate type p (
0.01 Ω cm)

Porosity 34, 54, 72

p (0.01–0.08 Ω cm)

20–55

Type not specified (0.1–0.2 Ω cm) (10–20 Ω cm)

41, 56 50, 58, 60

p (0.01 Ω cm)

30

Brillouin spectroscopy

Andrews et al. 1996 Bellet et al. 1996 Beghi et al. 1997

p (5 Ω cm) p (0.01 Ω cm) n (0.5 Ω cm) p (20 Ω cm)

70 36–90 30, 40 60, 70

Microacoustic methods Brillouin spectroscopy

Boumaiza et al. 1999 Lockwood et al. 1999

Not specified

20–47

p (10–35 Ω cm)

80

Brillouin spectroscopy

Andrews and p (0.01 Ω cm) Clouter 2001

Brillouin spectroscopy

Fan et al. 2001

p (1.48–1.84 Ω cm)

70

Brillouin spectroscopy Brillouin spectroscopy

Fan et al. 2002a Fan et al. 2002b

p (0.005–1.84 Ω cm)

57–83

p (1.48–1.84 Ω cm) p (1.0–1.05 Ω cm)

70 75

Brillouin spectroscopy

Fan et al. 2003

p (1.48–1.84 Ω cm)

70

Brillouin spectroscopy Brillouin spectroscopy

Andrews et al. 2004 Andrews et al. 2007

p (0.01 Ω cm)

25–40

Acoustic transmission spectroscopy

Aliev et al. 2009

p (5.1–6.9 Ω cm) p (0.017 Ω cm) n (0.016 Ω cm) p (0.001–0.015 Ω cm)

59 62 56 25–85

Acoustic microscopy Phase velocity scanning of interference fringes Brillouin spectroscopy Nanoindentation

25–29

Primary results Elastic Properties Elastic Properties Elastic Properties, Sound Attenuation Elastic Properties Elastic Properties Phonon-Fracton Crossover, Elastic Properties Elastic Properties Elastic Properties, Phonon-Fracton Crossover Chem Modification Effects Chem Modification Effects Elastic Properties Chem Modification Effects Chem Modification Effects Elastic Properties Elastic Properties Elastic Properties (continued)

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Table 1 (continued) Technique Nanoindentation

Reference Fang et al. 2009

Substrate type p (0.01–0.02 Ω cm)

Porosity 60, 73, 79

Nanoindentation

Oisten and Bergstrom 2009 Parsons and Andrews 2009 Polomska and Andrews 2009 Aliev et al. 2010

p (0.001–0.005 Ω cm) n (0.5–1 Ω cm)

Not specified

p (0.005–0.02 Ω cm)

46–59

p (0.005–0.02 Ω cm)

30–62

p (0.01–0.015 Ω cm)

47, 61

Brillouin spectroscopy Brillouin spectroscopy Acoustic transmission spectroscopy Acoustic transmission spectroscopy Acoustic transmission spectroscopy Brillouin spectroscopy Brillouin spectroscopy

Acoustic transmission spectroscopy Brillouin spectroscopy Acoustic transmission spectroscopy Acoustic transmission spectroscopy

Thomas et al. p (0.001–0.005 Ω cm) 2010

58–73

Aliev et al. 2011

p (various resistivities)

25–85

Parsons and Andrews 2012 PolomskaHarlick and Andrews 2012 Aliev et al. 2012

p (0.005–0.02 Ω cm)

50, 58

p (0.005–0.02 Ω cm)

33–72

p (0.025 Ω cm)

30–75

Parsons and Andrews 2014 Aliev and Goller 2014

p (0.005–0.02 Ω cm)

45, 54

p (0.001–0.005 Ω cm)

49, 50, 67, 69

p (0.001–0.005 Ω cm)

51, 65

Aliev and Goller 2015

Primary results Elastic Properties, Chem Modification Effects Elastic Properties Sound – Multilayered Structures Elastic Properties – Superlattices Sound – Multilayered Structures Sound – Multilayered Structures Elastic Properties Sound – Multilayered Structures Elastic Properties – Superlattices Sound – Multilayered Structures Sound – Multilayered Structures Sound – Multilayered Structures Sound – Multilayered Structures

gallium-indium eutectic due to its compatibility with the silicon surface and relatively low acoustic attenuation). The transducers are connected to a vector network analyser (VNA in Fig. 2) which is used to measure the magnitude and phase of the incident and transmitted voltage signals as a function of frequency. From this data, one can obtain the

Acoustic Characterization of Porous Silicon

M

BS

695

HWP Nd:YVO4 LASER VNDF

VNDF

M CONTROL UNIT

A VNDF S

L

L

P f/2.8

BF

532 ± 5 nm

TANDEM PMT FABRY-PEROT INTERFEROMETER

Fig. 1 Top: Photograph of a typical Brillouin light scattering setup. The light source (a single-mode laser), spectrometer (a tandem multipass Fabry-Perot interferometer housed in a black enclosure), and sample holder can be seen in the middle top, middle right, and bottom left of the photograph, respectively. Bottom: Schematic of the Brillouin light scattering setup shown in the top panel. The meanings of the symbols are as follows: HWP half-wave plate, BS beam splitter, M mirror, VNDF variable neutral density filter, L lens, S sample, P prism, A aperture, BF bandpass filter, PMT photomultiplier tube (Reprinted from Physica Status Solidi (a), 204, G.T. Andrews, A.M. Polomska, E. Vazsonyi, J. Volk, Brillouin light scattering study from porous silicon films and multilayers, 1372–1377, 2007 # 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim. https://doi.org/10.1002/pssa.200674344)

acoustic transmission spectrum and, by fast Fourier transforming the transmitted signal, the traversal times of the acoustic waves through the sample. From these times and knowledge of the relevant sample thicknesses, acoustic wave velocities can be determined. Acoustic attenuation can also be obtained from time domain data.

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Characterization of Elastic Properties Early work on the elastic properties of porous silicon by x-ray diffraction (Barla et al. 1984), phase velocity scanning of laser interference fringes (Cho et al. 1996), Brillouin spectroscopy (Andrews et al. 1996), and microacoustic techniques (da Fonseca et al. 1994, 1995a, b) revealed elastic moduli and acoustic wave velocities that are lower than those for crystalline silicon and that decrease with increasing porosity. To quantify the porosity dependence, semiempirical relations of the form V x ¼ V cSi ð1  ξÞηx were x fitted to experimental velocity versus porosity data for porous silicon made from p-type substrates with various dopant concentrations. Here, the Vx are acoustic wave velocities (x = Rayleigh (R), transverse (T), or longitudinal (L)) for porous silicon with porosity ξ, and the V cSi are the corresponding velocities for bulk crystalline silicon. Best-fit ηL x values of
1.1 and
0.6 were obtained for longitudinal acoustic waves in porous silicon formed from p+- (da Fonseca et al. 1995a, b) and p++-type (Aliev et al. 2009) substrates, respectively, while ηR  1.3 for Rayleigh waves in porous silicon made from p+ substrates (da Fonseca et al. 1995b). Analogous expressions for elastic moduli Cx have also been fitted to experimental data with the corresponding exponent
2.0 < γ x

PSi Coupling liquid 1 Transducer 1

VNA port 1

Si Coupling liquid 2 Transducer 2

VNA port 2

Fig. 2 Left: Photograph of a typical acoustic transmission spectroscopy setup (courtesy of G. Aliev). The vector network analyzer (VNA), and the transducers used to generate and detect acoustic waves in the sample, can be seen at the top centre and bottom centre (at the position indicated by the small white rectangle) of the photograph, respectively. Upper Right: Close-up photograph of the transducers with a sample in place between them (courtesy of G. Aliev). Bottom Right: Schematic of the experimental setup used for the acoustic transmission measurements. (Reprinted from [G.N. Aliev, B. Goller, Journal of Applied Physics, 116, 094,903, 2014] with the permission of AIP Publishing. https://doi.org/10.1063/1.4894620)

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<
3.2 (Note: Cx
ρV 2x ¼ CcSi ð1  ξÞ2ηx þ1 ¼ CcSi ð1  ξÞγx ). More complicated x x formulae have also been used (Boumaiza et al. 1999). The sensitivity of ηx or, equivalently, γ x, to substrate dopant concentration, coupled with the fact that porous silicon film morphology depends on the resistivity of the parent substrate, suggested that the elastic properties of porous silicon depended not only on porosity but also on pore network morphology. For longitudinal acoustic waves in porous silicon films fabricated from p-type substrates using a 1:1 solution of aqueous HF and ethanol, this dependence was found to be of the form ηL = 0.23 log R + 1.15, where R is the substrate resistivity in Ω∙cm (Aliev et al. 2011) (see Fig. 3). Over the range of R investigated,
0.6 < ηL <
1.3, leading to the conclusion, based on the results of Phani et al. (1986), that this type of porous silicon possesses a relatively ordered pore morphology. It was also asserted in the same study that acoustic wave velocities in different crystallographic directions have the same porosity dependence, thereby requiring that all three elastic constants have the same porosity dependence. The influence of film morphology on elastic properties was also revealed in Brillouin scattering and nanoindentation experiments that showed that elastic moduli for films of similar porosity formed from substrates with different dopant types and concentrations have very different elastic properties (Bellet et al. 1996, Andrews et al. 2004, Andrews et al. 2007). In particular, acoustic wave velocities and elastic moduli for films made from heavily doped p-type substrates are generally higher than those of the same porosity made from lightly doped p-type substrates (Bellet et al. 1996, Andrews et al. 2004, Andrews et al. 2007).

Effects of Chemical Modification The effects of chemical modification on the elastic properties of porous silicon films have been studied in some detail. Brillouin scattering work showed that both natural and electrochemical oxidation of
70% porous silicon result in a reduction of acoustic mode frequencies relative to as-prepared samples, with the frequencies decreasing with increasing oxidation time as the silicon skeleton is transformed into SiO2 (Fan et al. 2003). In another Brillouin study, an
5% reduction in surface acoustic phonon velocities for air-aged porous silicon films dipped in dilute hydrofluoric acid relative to undipped films was attributed to increased porosity resulting from native oxide removal by the acid (Andrews and Clouter 2001). Nanoindentation experiments show that thermally oxidized porous silicon films have increased Young’s modulus relative to as-prepared samples, with the modulus increasing with oxidation temperature (Fang et al. 2009). Two Brillouin scattering studies have also been done on porous silicon films passivated with various organic molecules (Fan et al. 2001, Fan et al. 2002b). In the first of these, estimates of film coverage by passivating molecules were obtained from a simple formula that exploits the sensitivity of acoustic mode velocities to passivation. In the second, differences in phonon frequencies for films passivated with 1-decene,

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Fig. 3 (a)–(c) Experimental values of longitudinal acoustic velocity v0L in porous silicon for the layers produced from Si wafers of indicated crystallographic orientation and resistivity are depicted by symbols fitted by v0L ¼ v0L ð1  φÞκ with different fitting parameter κ. (d) Dependence of K on wafer resistivity. Symbols correspond to samples with orientation: • (100), Δ (110), o (111), and from literature « (100) (Polomska 2010) and ◊ (100) (Fan et al. 2002a). Inset: κ vs VHF/VEth, volume ratio of etchant HF to ethanol. (Reprinted from (G.N. Aliev, B. Goller, P.A. Snow, Journal of Applied Physics, 110, 043, 534, 2011) with the permission of AIP Publishing. https://doi.org/ 10.1063/1.3626790)

decyl aldehyde, undecylenic acid, and ethyl undecylenate were correlated with changes in film density and elastic constants resulting from differences in the bonding and physical properties (mass, chain length, dipole moment) of the passivating molecules.

Attenuation of Acoustic Waves There has been relatively little work done on attenuation of acoustic waves in porous silicon (see Table 2 for a summary of attenuation coefficients). Phase velocity scanning of laser interference fringes was used to measure surface acoustic wave

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Table 2 Attenuation coefficients for surface (S), bulk transverse (T), and bulk longitudinal (L) acoustic waves in porous silicon Reference Aliev and Goller 2015

Thomas 2011 Aliev et al. 2012 Cho et al. 1996

Attenuation coefficient (dB/cm) αL= 29.6 αT = 9.3 αL= 50.0 αT = 15.4
65 < αL <
370 αL< 300 αS = 7–80 αS = 10–40

Porosity (%) 51

Frequency (MHz) 1000

Substrate resistivity (Ω cm) 0.001–0.005


1000 2000 30–60 30–80

0.001–0.005 0.025 (maximum) 10–20 0.1–0.2

65 45–69 30–75 50 56

attenuation in 50% and 56% porous silicon films made from substrates of resistivity 10–20 Ω cm and 0.1–0.2 Ω cm, respectively (Cho et al. 1996). Attenuation in the 50% film varied from 7 dB/cm to 80 dB/cm over the range 30–60 MHz, with a frequency dependence of f4.25, while the 56% film showed a f1.05 dependence, with the attenuation changing from
10 dB/cm to 40 dB/cm over the range 30–80 MHz. This large difference in attenuation was said to be an indicator of different microstructures for the two films. Recent work (Aliev and Goller 2015) argues that viscous (Akheiser) damping is the dominant attenuation mechanism for
1 GHz-bulk acoustic waves in mesoporous silicon made from 0.001–0.005 Ω cm p-type substrates. Application of an expression for the porosity dependence of this type of damping in porous silicon gave attenuation coefficients of
9 dB/cm and
15 dB/cm for [100]-propagating
1 GHz-shear acoustic waves in layers of porosity 51% and 65%, respectively. The corresponding attenuation coefficients for longitudinal waves were found to be
30 dB/cm and
50 dB/cm and may be directly compared with preliminary values of
65 dB/cm to
370 dB/cm measured by acoustic transmission spectroscopy for 45%–69% mesoporous layers made from like substrates (Thomas 2011). These values may also be compared with that obtained from performance characterization experiments on bulk acoustic wave resonators with an underlying porous silicon layer where the attenuation coefficient for 2 GHz-longitudinal acoustic waves was estimated to be < 300 dB/cm for porosities in the range 30%–75% (Aliev et al. 2012).

Phonon-to-Fracton Crossover The vibrational spectrum of structures exhibiting fractal character is predicted to crossover from ordinary phonon to fracton (elastic excitations of fractal networks) excitation regimes at some characteristic length. This phonon-to-fracton crossover has been suggested as a possible explanation for artefacts seen in Brillouin spectra of certain porous silicon films. In particular, the absence of bulk acoustic phonon peaks

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and the unusually broad surface and film acoustic mode peaks observed in Brillouin spectra of high-porosity films formed from p-type crystalline silicon were taken to be manifestations of a crossover of the vibrational spectrum from translational invariance to self-similarity (Beghi et al. 1997, Lockwood et al. 1999).

Multilayered Films and Related Structures Acoustic characterization studies on multilayered porous silicon structures have focused on understanding sound propagation in binary periodic porous silicon superlattices. Theoretical work in 2005 (Kiuchi et al. 2005) and 2007 (Reinhardt and Snow 2007) predicted that porous silicon superlattices could act as one-dimensional phononic crystals, the acoustic analogues of photonic crystals. Such behavior was subsequently observed in Brillouin scattering (Parsons and Andrews 2009) and acoustic transmission spectroscopy (Aliev et al. 2010) experiments on porous silicon superlattices with modulation wavelengths on the order of the corresponding acoustic wavelength (i.e., 100 nm and 1 μm, respectively). In the former study, evidence for zone folding of longitudinal acoustic phonons and a hypersonic bandgap was found, while in the latter,
50 dB stop bands for GHz-frequency longitudinal acoustic waves were observed in transmission spectra. In a follow-up Brillouin scattering study (Parsons and Andrews 2012), phonon dispersion curves along the modulation axis were mapped for superlattices with modulation wavelengths
100 nm. Zone-folded bulk longitudinal acoustic modes and a surface-localized mode with a frequency near the upper edge of a phononic band gap centered at 16 GHz were observed. Optical reflectance measurements on the same samples revealed photonic band gaps, showing that the superlattices are one-dimensional hypersonic phononic-photonic (so-called phoxonic) crystals. Acoustic wave propagation at oblique angles to the modulation axis has also been studied in porous silicon superlattices. Brillouin scattering experiments on superlattices with constituent layer porosity ratios close to unity revealed zone-folded longitudinal acoustic phonons with frequencies strongly dependent on propagation direction and folding order of the mode branch (Parsons and Andrews 2014). A Rayleigh surface mode and an apparent pseudo-surface mode, both of which are dispersive, were also observed. Elastic properties of porous silicon superlattices with modulation wavelengths small relative to acoustic phonon wavelengths have also been explored by Brillouin spectroscopy (Polomska-Harlick and Andrews 2012, Andrews et al. 2009). The motivation for these experiments was to test the applicability of an effective elastic medium model (Grimsditch and Nizzoli 1986) to these superlattices, which are unconventional in the sense that they are formed by electrochemical etching of a single material rather than by alternating deposition of two different materials. Partial agreement between measured and calculated elastic moduli suggested that the model holds promise as a means for accurately predicting the elastic properties of porous silicon superlattices.

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Acoustic wave phenomena in multilayered porous silicon structures possessing a higher degree of complexity than that of superlattices have also been investigated. Acoustic transmission spectroscopy work on a structure consisting of a 2.7 μm-thick 57% porous layer sandwiched between acoustic Bragg mirrors with
2.9 μm periodicity and constituent layer porosities of 40% and 57%, revealed a transmission maximum at 1 GHz within a stop band ranging from 0.8 to 1.2 GHz, thereby demonstrating the first porous silicon acoustic microcavity (Aliev et al. 2010). Porous silicon-based hypersonic rugate filters with a sinusoidal variation in acoustic impedance have also been fabricated and their functionality verified by acoustic transmission spectroscopy (Thomas et al. 2010). The filters displayed
40 dB-deep first-order transmission stop bands for longitudinal acoustic waves and strong suppression of higher-order bands.

Summary The studies summarized above have resulted in important advances in our understanding of acoustic wave behavior in porous silicon and, from a broader scientific perspective, of classical wave behavior in porous and artificially structured media in general. Nevertheless, opportunities for further study abound. In particular, in-depth investigations of phonon attenuation and phonon-to-fracton crossover in this intriguing and technologically important material system would complement existing work and add to our knowledge of acoustic wave behavior in porous and fractal networks. Moreover, acoustic wave behavior in more elaborate porous silicon-based structures is a topic of current research interest and points to possible use in new device applications.

References Aliev GN, Goller B (2014) Quasi-periodic Fibonacci and periodic one-dimensional hypersonic phononic crystals of porous silicon: experiment and simulation. J Appl Phys 116:094903 Aliev GN, Goller B (2015) Hypersonic phononic stopbands at small angles of wave incidence in porous silicon multilayers. J Phys D Appl Phys 48:325501 Aliev GN, Goller B, Kovalev D, Snow PA (2009) Porosity dependence of the acoustic longitudinal velocity in heavily doped p++ porous silicon layers. Phys Status Solidi C 6(7):1670–1674 Aliev GN, Goller B, Kovalev D, Snow PA (2010) Hypersonic acoustic mirrors and microcavities in porous silicon. Appl Phys Lett 96:124101 Aliev GN, Goller B, Snow PA (2011) Elastic properties of porous silicon studied by acoustic transmission spectroscopy. J Appl Phys 110:043534 Aliev GN, Goller B, Snow PA, Heinrich H, Yuan B, Aigner R (2012) Porous silicon bulk-acousticwave resonator with integrated transducer. Nanoscale Res Lett 7(1):378 Andrews GT, Clouter MJ (2001) Influence of HF etching on the velocity of surface acoustic phonons in air-aged porous silicon layers. Semicond Sci Technol 16:679–681 Andrews GT, Zuk J, Kiefte H, Clouter MJ, Nossarzewska-Orlowska E (1996) Elastic characterization of a supported porous silicon layer by Brillouin scattering. Appl Phys Lett 69(9):1217–1219

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Andrews GT, Clouter MJ, Zuk J (2004) Brillouin light scattering study of surface acoustic phonons in p+ porous silicon layers. Semicond Sci Technol 19:1306–1310 Andrews GT, Polomska AM, Vazsonyi E, Volk J (2007) Brillouin light scattering from porous silicon films and multilayers. Phys Status Solidi A 204:1372–1377 Barla K, Herino R, Bomchil G, Pfister JC, Freund A (1984) Determination of lattice parameter and elastic properties of porous silicon by x-ray diffraction. J Cryst Growth 68:727–732 Beghi MG, Bottani CE, Ghislotti G, Amato G, Boarino L (1997) Brillouin scattering of porous silicon. Thin Solid Films 297:110–113 Bellet D, Lamagnère P, Vincent A, Bréchet Y (1996) Nanoindentation investigation of the Young’s modulus of porous silicon. J Appl Phys 80(7):3772–3776 Boumaiza Y, Hadjoub Z, Doghmane A, Deboub L (1999) Porosity effects on different measured acoustic parameters of porous silicon. J Mater Sci Lett 18:295–297 Cho H, Sato H, Takemoto M, Sato A, Yamanaka K (1996) Surface acoustic wave velocity and attenuation dispersion measurement by phase velocity scanning of laser interference fringes. Jpn J Appl Phys Part 1 35(5B):3062–3065 Da Fonseca RJM, Saurel JM, Depaux G, Foucaran A, Massone E, Taliercio T, Lefebvre P (1994) Elastic characterization of porous silicon by acoustic microscopy. Superlattice Microst 16(1): 21–23 Da Fonseca RJM, Saurel JM, Foucaran A, Camassel J, Massone E, Taliercio T, Boumaiza Y (1995a) Acoustic investigation of porous silicon layers. J Mater Sci 30:35–39 Da Fonseca RJM, Saurel JM, Foucaran A, Massone E, Taliercio T, Camassel J (1995b) Acoustic microscopy investigation of porous silicon. Thin Solid Films 255:155–158 Fan HJ, Kuok MH, Ng SC, Boukherroub R, Lockwood DJ (2001) Determination of coverage in passivated porous silicon by Brillouin spectroscopy. Appl Phys Lett 79(27):4521–4523 Fan HJ, Kuok MH, Ng SC, Boukherroub R, Baribeau JM, Fraser JW, Lockwood DJ (2002a) Brillouin spectroscopy of acoustic modes in porous silicon films. Phys Rev B 65: 165330 Fan HJ, Kuok MH, Ng SC, Boukherroub R, Baribeau JM, Lockwood DJ (2002b) A Brillouin scattering study of the effect of chemical passivation on the elastic properties of porous silicon. Semicond Sci Technol 17:692–695 Fan HJ, Kuok MH, Ng SC, Lim HS, Liu NN, Boukherroub R, Lockwood DJ (2003) Effects of natural and electrochemical oxidation processes on acoustic waves in porous silicon films. J Appl Phys 94(2):1243–1247 Fang Z, Hu M, Zhang W, Zhang X, Yang H (2009) Mechanical properties of porous silicon by depth-sensing nanoindentation techniques. Thin Solid Films 517:2930–2935 Grimsditch M, Nizzoli F (1986) Effective elastic constants of superlattices of any symmetry. Phys Rev B 33:5891–5892 Kiuchi A, Gelloz B, Kojima A, Koshida N (2005) Possible operation of periodically layered nanocrystalline porous silicon as an acoustic band crystal device. Mater Res Soc Symp Proc 832:207 Lockwood DJ, Kuok MH, Ng SC, Rang ZL (1999) Surface and guided acoustic phonons in porous silicon. Phys Rev B 60(12):8878–8882 Oisten MK, Bergstrom PL (2009) A Young’s modulus study of n- and p-type porous silicon. Phys Status Solidi A 206(6):1278–1281 Parsons LC, Andrews GT (2009) Observation of hypersonic phononic crystal effects in porous silicon superlattices. Appl Phys Lett 95:241909 Parsons LC, Andrews GT (2012) Brillouin scattering from porous silicon-based optical Bragg mirrors. J Appl Phys 111(12):123521 Parsons LC, Andrews GT (2014) Off-axis phonon and photon propagation in porous silicon superlattices studied by Brillouin spectroscopy and optical reflectance. J Appl Phys 116:033510 Phani KK, Niyogi SK, Maitra AK, Roychaudhury M (1986) Strength and elastic modulus of a porous brittle solid. J Mater Sci 21:4335–4341

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Polomska AM (2010) Elastic properties of porous silicon superlattices. Ph.D. thesis, Memorial University of Newfoundland Polomska AM, Andrews GT (2009) Elastic constants of porous silicon superlattices. Phys Status Solidi C 6:1665–1669 Polomska-Harlick AM, Andrews GT (2012) Systematic Brillouin light scattering study of the elastic properties of porous silicon superlattices. J Phys D Appl Phys 45:075302 Reinhardt A, Snow PA (2007) Theoretical study of acoustic band-gap structures made of porous silicon. Phys Status Solidi C 204(5):1528–1535 Thomas L (2011) Porous silicon multilayers for gigahertz bulk acoustic wave devices. Ph.D. thesis, University of Bath Thomas L, Aliev GN, Snow PA (2010) Hypersonic rugate filters based on porous silicon. Appl Phys Lett 97:173503

Characterization of Porous Silicon by Infrared Spectroscopy Yukio H. Ogata

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Sample Preparation and Instrumentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Hydrogen-Terminated Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Oxidation of Porous Silicon . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . IR Measurement Using Methods Other Than the Transmission . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Conclusion . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

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Abstract

The surface of electrochemically etched porous silicon is passivated with hydrogen just after preparation. The surface is gradually oxidized under ambient atmosphere, and the rate depends upon the ambient condition. The chemical and physical changes affect the properties of porous silicon-based devices. Proper understanding of the surface is important, and infrared (IR) spectroscopy is an effective and easy tool for monitoring and/or characterizing the surface state. Silicon is almost transparent to IR light, and hence the convenient transmission measurement is applicable to films and membranes of porous silicon. The measurement technique is first described, and then assignments of absorption bands in the spectra are given for the hydrogen-terminated and oxidized surface. The prevention of oxidation and the functionalization of porous silicon surface are important for many practical uses, where IR measurements can be used to monitor the surface. In addition, methods other than the transmission mode are briefly introduced.

Yukio H. Ogata: deceased. Y. H. Ogata (*) Institute of Advanced Energy, Kyoto University, Uji, Kyoto, Japan e-mail: [email protected] # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_48

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Keywords

Ab initio molecular orbital calculation · Attenuated total reflection (ATR) · Back-bond oxidation · Fourier transform IR (FTIR) · Hydrogen termination · Infrared spectroscopy (IR) · Multiple internal reflection infrared spectroscopy (MIR-FTIR)

Introduction A silicon surface is stabilized with hydrogen termination with the hydrofluoric acid (HF) treatment or during the porosification in solution containing fluoride (Trucks et al. 1990; Searson and Zhang 1990; Gerischer et al. 1993). The surface states, SiHx, influence the properties. The passivated surface undergoes oxidation on putting it in an environment, where some oxidants are present; the oxidation rate depends upon the type of oxidant or the oxidizing ability. The oxidation also affects the properties. Silicon surface is often modified with organic species in order to stabilize the surface against its oxidation and more to give new functionality (Sailor 2011). Understanding of these chemical states of the silicon surface is indispensable for the study on porous silicon. There are many analytical methods to access the chemical information. Among them, infrared spectroscopy (IR) is a powerful tool to analyze the atomic bonding in a molecule (Günzler and Gremlich 2002; Tolstoy et al. 2003; Settle 1997; Stuart 2004) and is widely used. In this chapter, only basic IR response of porous silicon itself is described, but the technique is widely utilized in such as characterization/quantification of materials loaded in pores like proteins or drug molecules, special inhomogeneity, oxide nature, free carrier concentrations, and degree of derivatization. The reader may find some examples in the other chapters.

Sample Preparation and Instrumentation IR measurement is usually based on the transmission mode. It is the easiest way of the measurement. Silicon is almost transparent against infrared light, whereas it absorbs visible light to some extent. Moderately or lightly doped silicon enables the transmission measurement of a porous silicon layer. Porous silicon has a large surface area (Herino et al. 1987), and the transmission IR provides a good quality of the spectrum. A silicon wafer after dipping in dilute HF can be used as the reference for the IR measurement. Highly doped silicon or degenerate silicon (n+ or p+ Si) exhibits some absorption, especially at high-frequency region due to the high impurity concentration. Even in the case, the transmission IR becomes possible if the layer is detached from the substrate by applying high current after formation of a porous silicon layer and the free-standing porous silicon layer is measured. These days, almost all IR measurement uses Fourier transform IR (FTIR), which is superior to the dispersive IR in the sensitivity, sampling time, and resolution.

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Measurable wavenumber region depends upon the detector used: the most popular detectors are TGS (triglycine sulfate), 350–7,800 cm 1, and semiconductor-type detector MCT (HgCdTe), the low detection limit around 650 cm 1.

Hydrogen-Terminated Porous Silicon Silicon is tetravalent, and the crystal has a tetrahedral structure. Hydrogen molecules can bond with silicon in the crystal as SiHx (x = 1–3). An example of the spectra prepared from p-type silicon (100) is shown in Fig. 1a. Three absorption bands are clearly observed in three frequency regions: 2,090–2,150 cm 1, ~920 cm 1, and 620–700 cm 1. A broad band in the 1,000–1,300 cm 1 region is sometimes found in some spectra. This is caused by post-oxidation during the sample drying procedure. The peak assignment had been performed comparing with the spectra of the related materials, amorphous hydrogenated silicon (a-Si:H) (Lucovsky et al. 1979; Knights et al. 1978) and HF-treated silicon (Burrows et al. 1988; Chabal and Raghavachari 1984). Porous silicon and a-Si:H resemble with each other except the crystallinity. Their FTIR spectra are basically similar. HF-treated silicon is of course monocrystalline and measured by the reflection method. The spectrum is exactly that of porous silicon. The only difference between them is the azimuth. The monocrystalline surface is uni-oriented, and the orientation gives different spectra between p and s polarizations when the incident beam is polarized. Pores in microporous silicon are randomly oriented, and the polarized beam gives similar spectra. On the one hand, mesoporous silicon with a few branching and macroporous silicon are expected to show the difference when using differently polarized incident beam. The assignment of the absorption bands appearing in porous silicon is investigated experimentally and theoretically (Unagami 1980a; Gupta et al. 1988, 1991; Kato et al. 1988; Ito et al. 1990; Ogata et al. 1995a, 1998). The theoretical or

a

b p-Si (100) PS oxidized

p-Si (100) PS as-prepared

SiHxstr

Absorbance

Absorbance

SiH2, SiH deform SiH2 scis

Meas.

Si-O-Si

Si-O-Si

OnSi-Hx Cal.

2500

2000

1500

1000

Wavenumber / cm–1

500

2500

2000

1500

1000

Wavenumber / cm

500

–1

Fig. 1 IR spectra of porous silicon prepared from p-type silicon (100): (a) as prepared and the results of vibrational analysis by the molecular orbital calculation and (b) oxidized in dry air at 333 K for 50 days

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Table 1 Absorption bands appearing in as-prepared porous silicon Wavenumber (cm 1) 2,142 2,108 2,087 916 667 626

Assignment SiH3 stretching SiH2 stretching SiH stretching SiH2 scissors bending SiH2 wagging SiH bending

Remark

Dimer (SiH2)n bending also participates

References Kato et al. (1988) and Ogata et al. (1995a) Kato et al. (1988) and Ogata et al. (1995a) Kato et al. (1988) and Ogata et al. (1995a) Gupta et al. (1988) and Ogata et al. (1995a) Ogata et al. (1998) Ogata et al. (1998)

computational analyses such as ab initio molecular orbital calculation can be the powerful tool for the assignment. An assignment of the vibration modes is given in Table 1. The exact values of wavenumbers vary depending upon the measurements, and hence, the small variations should not be reproachable.

Oxidation of Porous Silicon Silicon is a very less noble element, and it is prone to be oxidized under ambient atmosphere. The oxidation can be easily followed by FTIR (Kato et al. 1988; Gupta et al. 1991; Unagami 1980b; Borghesi et al. 1994; Ogata et al. 1995b; Lucovsky 1979). Figure 1b gives an IR spectrum undergoing oxidation (Ogata et al. 1995b). Many absorption bands appear due to the oxidation, and SiHx-related bands still remain in some cases. The oxidation rate depends on the surrounding environment. The sample put in dry air at ambient temperature is fairly stable against oxidation, while the oxidation proceeds quite fast in saturated humidity atmosphere and at high temperature (Ogata et al. 1995b). The possible assignment to the vibrational modes is given in Table 2. The features are classified into three frequency regions: (a) a broad absorption around 3,600 cm 1 resulting from the O–H stretching vibrations, (b) absorptions slightly higher than the SiHx stretching (2,080–2,150 cm 1) attributed to the back-bond oxidation, and (c) a strongest absorption at 1,000–1,200 cm 1 and many absorptions lower than 1,000 cm 1 caused by Si–O–Si and deformational vibrations related to OnSiHx. Theoretical calculation helps the assignment (Kato et al. 1988; Ogata et al. 1995b; Lucovsky 1979; Lucovsky et al. 1983) but partially. The assignment of vibrational modes becomes complicated because many species are possible and the unoxidized SiHx states remain. Some vibrations of OnSiHx, especially the deformational modes appearing in the frequency lower than 1,000 cm 1, are difficult to be assigned with the computational analysis because of the presence of many modes and the coupling. Some experimental verification is always necessary for the assignment.

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Table 2 Absorption bands appearing in oxidized porous silicon (except SiHx relating vibrations) Wavenumber (cm 1) 3,660 3,600 2,256

Assignment O–H stretching O–H stretching O3Si–H stretching

2,200

O2Si–H2 stretching

2,160

OSi–H3 stretching

1,050

Si–O–Si stretching OnSiHx deformation

870

800 708 470

OnSiHx deformation OnSiHx deformation Si–O–Si out-ofplane rocking

Remark Sharp peak from an isolated hydroxyl group Broad absorption from physisorbed water

Broad absorption with a shoulder at 984 cm 1 Si–O–Si symmetric stretching is also possible in the region 700–900 cm 1

References

Gupta et al. (1991), Borghesi et a. (1994) and Ogata et al. (1995b) Gupta et al. (1991), Borghesi et al. (1994) and Ogata et al. (1995b) Gupta et al. (1991), Borghesi et al. (1994) and Ogata et al. (1995b) Gupta et al. (1988) and Lucovsky et al. (1983) Gupta et al. (1991) and Lucovsky et al. (1983) Gupta et al. (1991) and Lucovsky et al. (1983) Gupta et al. (1991) and Lucovsky et al. (1983) Lucovsky et al. (1983)

IR Measurement Using Methods Other Than the Transmission The surface of porous silicon is often modified with some materials in order to stabilize it against oxidation or to give a new functionality (Sailor 2011; Buriak 2002; Boukherroub et al. 2002; Salonen et al. 2002). Modification with organic materials is followed by FTIR. Porous silicon has a huge surface area, and the high surface area enables a very high sensitivity of the adsorbed molecules on the surface. It must be noted that the surface is sometimes involuntarily oxidized. The FTIR can be performed using a porous layer formed on a silicon substrate with the transmission mode and also the pulverized sample. For the latter, the pulverized sample is mixed with KBr and then pressurized to make a pellet. The diffused light is analyzed (diffuse reflectance FTIR). The absorption bands appear in the same frequencies as the transmission spectroscopy, but the intensity is different because of the difference in the optical path. The qualitative comparison with the transmission spectroscopy needs the conversion with the use of the Kubelka-Munk function.

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Table 3 Porous silicon-related absorption bands other than Si–H and Si–O Wavenumber (cm 1) 2,850–2,950

870

812 770 680 610

Assignment Adventitious carbon contamination from ambient air Si–Nx bending

Si–Fx stretching Si–CHx stretching or rocking Si–C stretching Si–Si lattice vibration

Remark Appearing in aged sample

References Ogata et al. (1995b)

Observed using ATR-FTIR Very weak

James et al. (2010)

TO + TA combination

Ogata et al. (1995a) Salonen et al. (2002) and Canaria et al. (2002) Canaria et al. (2002) Ogata et al. (1995a) and Johnson and Loudon (1964)

The attenuated total reflection (ATR) is used to analyze the state of the very surface collecting the evanescent light. This method uses a tight contact of the sample with a prism with high refractive index such as ZnSe and KRS-5 (mixed crystal of TlI and TlBr). The use of silicon replacing the prism for the ATR enables the multiple internal reflection infrared spectroscopy (MIR-FTIR) (Rao et al. 1991; Kimura et al. 2001). Two edges of a silicon sample are beveled with 45 and polished. IR beam comes into the sample from the beveled edge and goes out to an FTIR analyzer from the other edge after multiple reflections at the both surfaces. The top surface is exposed to an electrolyte, and the spectrum provides the in situ and almost instantaneous information of the surface. The method has been effectively utilized for the investigation of temporal change on silicon or porous silicon, such as the early stage of the porosification (Kimura et al. 2001) and the surface change during the potential or current oscillation during anodic polarization (Chazalviel et al. 1998; Kimura et al. 2003). Finally, it may be useful to give some IR absorption bands found in porous silicon other than listed in Tables 1 and 2 (Table 3).

Conclusion IR is a powerful and easy-to-use technique to obtain the surface chemical state of porous silicon. The convenience results from the transparency of silicon for IR light and the high surface area. The basic features begin from the knowledge of the bondings to hydrogen, Si–H, and to oxygen Si–O. The model calculations sometimes provide useful information in the assignment. It is true for the stretching vibrations, which are isolated and appear in the frequency region of 2,050–2,300 cm 1, while vibrational modes appearing at the low frequencies attributed to the deformational modes are often coupled and crowded, and hence the theoretical analysis is often difficult. Additional experimental work often helps the assignment.

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The assignment still remains some discussion, but the major understanding had been achieved in the last century.

References Borghesi A, Guizzetti G, Sassella A et al (1994) Induction-model analysis of Si–H stretching mode in porous silicon. Solid State Commun 89:615–618 Boukherroub R, Wojtyk J, Wayner D et al (2002) Thermal hydrosilylation of undecylenic acid with porous silicon. J Electrochem Soc 149:H59–H63 Buriak J (2002) Organometallic chemistry on silicon and germanium surfaces. Chem Rev 102:1271–1308 Burrows V, Chabal Y, Higashi G et al (1988) Infrared-spectroscopy of Si(111) surfaces after HF treatment – hydrogen termination and surface-morphology. Appl Phys Lett 53:998–1000 Canaria C, Lees I, Wun A et al (2002) Characterization of the carbon–silicon stretch in methylated porous silicon – observation of an anomalous isotope shift in the FTIR spectrum. Inorg Chem Commun 5:560–564 Chabal Y, Raghavachari K (1984) Surface infrared study of Si(100)-(2x1)H. Phys Rev Lett 53:282–285 Chazalviel J, da Fonseca C, Ozanam F (1998) In situ infrared study of the oscillating anodic dissolution of silicon in fluoride electrolytes. J Electrochem Soc 145:964–973 Gerischer H, Allongue P, Kieling V (1993) The mechanism of the anodic-oxidation of silicon in acidic fluoride solutions revisited. Ber Bunsen-Ges Phys Chem 97:753–756 Günzler H, Gremlich H-U (2002) IR spectroscopy: an introduction. Wiley-VCH, Weinheim Gupta P, Colvin V, George S (1988) Hydrogen desorption-kinetics from monohydride and dihydride species on silicon surfaces. Phys Rev B 37:8234–8243 Gupta P, Dillon A, Bracker A et al (1991) FTIR studies of H2O and D2O decomposition on porous silicon surfaces. Surf Sci 245:360–372 Herino R, Bomchil G, Barla K et al (1987) Porosity and pore-size distributions of porous silicon layers. J Electrochem Soc 134:1994–2000 Ito T, Yasumatsu T, Watabe H et al (1990) Effects of hydrogen-atoms on passivation and growth of microcrystalline Si. MRS Symp Proc 164:205–210 James TD, Parish G, Musca CA et al (2010) N2-Based thermal passivation of porous silicon to achieve long-term optical stability. Electrochem Solid-State Lett 13:H428–H431 Johnson F, Loudon R (1964) Critical-point analysis of phonon spectra of diamond silicon and germanium. Proc R Soc Lond A 281:274–290 Kato Y, Ito T, Hiraki A (1988) Initial oxidation process of anodized porous silicon with hydrogenatoms chemisorbed on the inner surface. Jpn J Appl Phys 27:L1406–L1409 Kimura Y, Kondo Y, Niwano M (2001) Initial stages of porous Si formation on Si surfaces investigated by infrared spectroscopy. Appl Surf Sci 175:157–162 Kimura Y, Nemoto J, Shinohara M et al (2003) In-situ observation of chemical states of a Si electrode surface during a galvanostatic oscillation in fluoride electrolytes using infrared absorption spectroscopy. Phys Status Solidi A 197:577–581 Knights J, Lucovsky G, Nemanich R (1978) Hydrogen-bonding in silicon-hydrogen alloys. Phil Mag B 37:467–475 Lucovsky G (1979) Chemical effects on the frequencies of Si–H vibrations in amorphous solids. Solid State Commun 29:571–576 Lucovsky G, Nemanich R, Knights J (1979) Structural interpretation of the vibrational-spectra of a-Si:H alloys. Phys Rev B 19:2064–2073 Lucovsky G, Yang J, Chao S et al (1983) Oxygen-bonding environments in glow-discharge deposited amorphous silicon–hydrogen alloy-films. Phys Rev B 28:3225–3233

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Ogata Y, Niki H, Sakka T et al (1995a) Hydrogen in porous silicon – vibrational analysis of SiHx species. J Electrochem Soc 142:195–201 Ogata Y, Niki H, Sakka T et al (1995b) Oxidation of porous silicon under water-vapor environment. J Electrochem Soc 142:1595–1601 Ogata Y, Kato F, Tsuboi T et al (1998) Changes in the environment of hydrogen in porous silicon with thermal annealing. J Electrochem Soc 145:2439–2444 Rao A, Ozanam F, Chazalviel J (1991) Insitu Fourier-transform electromodulated infrared study of porous silicon formation – evidence for solvent effects on the vibrational linewidths. J Electrochem Soc 138:153–159 Sailor MJ (2011) Porous silicon in practice. Wiley-VCH, Weinheim Salonen J, Laine E, Niinisto L (2002) Thermal carbonization of porous silicon surface by acetylene. J Appl Phys 91:456–461 Searson P, Zhang X (1990) The anodic-dissolution of silicon in HF solutions. J Electrochem Soc 137:2539–2546 Settle F (1997) Handbook of instrumental techniques for analytical chemistry. Prentice Hall, Upper Saddle River Stuart BH (2004) Infrared spectroscopy: fundamentals and applications. Wiley, Hoboken Tolstoy VP, Chernyshova I, Skryshevsky VA (2003) Handbook of infrared spectroscopy of ultra thin films. Wiley-VCH, New York Trucks G, Raghavachari K, Higashi G et al (1990) Mechanism of HF etching of silicon surfaces – a theoretical understanding of hydrogen passivation. Phys Rev Lett 65:504–507 Unagami T (1980a) Formation mechanism of porous silicon layer by anodization in HF solution. J Electrochem Soc 127:476–483 Unagami T (1980b) Oxidation of porous silicon and properties of its oxide film. Jpn J Appl Phys 19:231–241

Cell Culture on Porous Silicon Nicolas H. Voelcker and Suet P. Low

Contents Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Early Studies . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Surface Modification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Topography . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Gradients . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . Summary . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . .

714 714 715 718 722 725 726

Abstract

Cell culture is a powerful in vitro characterization technique to optimize the properties of a biomaterial for in vivo biomedical use by conversely revealing potential sources of cytotoxicity. A comprehensive literature survey of the range of cell types cultured on porous silicon is given, together with a discussion of how surface chemistry, topography, and porosity gradients affect cell behavior. Keywords

Cell culture · Electrografting · Hydrosilylation · Mesenchymal cells (MSC) · Neuroblastoma cells · Oxidation · Topography · Viability assays

N. H. Voelcker (*) ARC Centre of Excellence in Convergent Bio-Nano Science and Technology, Future Industries Institute, University of South Australia, Adelaide, SA, Australia e-mail: [email protected] S. P. Low Mawson Institute, University of South Australia, Adelaide, SA, Australia # Springer International Publishing AG, part of Springer Nature 2018 L. Canham (ed.), Handbook of Porous Silicon, https://doi.org/10.1007/978-3-319-71381-6_50

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Introduction Cell culture is often utilized to determine the biocompatibility of materials and precedes or even replaces in vivo animal and human testing. The behaviour of cells such as attachment, proliferation, morphological changes, metabolic changes, cytotoxicity, protein expression, and RNA expression are all important factors that have to be taken into account when designing a biomaterial (Freshney 2005; Masters 2000). In this regard, many materials are being investigated for their suitability for the culture of cells or even to study cellular interactions. Porous silicon is a popular choice for biosensor, bio-microelectromechanical systems (bioMEMS), biomaterials and tissue engineering applications. The porous structure, degradability, electrical conductivity, overall biocompatibility (see chapter ▶ “Biocompatibility of Porous Silicon”), and ease of surface modification make this a fascinating platform to investigate cell culture interactions. For example, porous silicon disks are being developed for delivery of therapeutic ocular cells (Fig. 1).

Early Studies A variety of mammalian cells have been successfully cultured onto porous silicon surfaces. The first publications on this topic by Bayliss et al. demonstrated that attachment of Chinese hamster ovary (CHO) cells proceeded on porous silicon surfaces to a similar extent as on bulk silicon (Bayliss et al. 1997a, b). This was also confirmed with the neuronal cell line B50 (Bayliss et al. 2000). Cell viability in these studies was determined using two colorimetric assays, the MTT based on enzymatic reduction of a tetrazolium salt to a purple formazan and the neutral red uptake assay. B50 and CHO cells were cultured on bulk silicon, porous silicon, glass, and polycrystalline silicon. Both viability assays suggested that the neuronal cells showed preference for porous silicon above the other surfaces, while CHO cells showed the lowest viability on the porous silicon surface (Bayliss et al. 1999, 2000). The surfaces of the porous silicon used in these early studies were not modified postetching, and it was not until a study utilized porous silicon surfaces with an oxide

Fig. 1 Cells being cultured on porous silicon particles that have been compressed into a disk form as part of a cell delivery platform (Low 2008)

Cell Culture on Porous Silicon

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layer for cell culture that surface chemistry was found to play a crucial factor (Chin et al. 2001). Rat hepatocytes were cultured onto ozone-oxidized porous silicon that was further modified by fetal bovine serum and collagen type I coating. Here, the hepatocytes showed a preference for the collagen-coated surface (Chin et al. 2001). Viability assays such as MTT, XTT, MTS, or Alamar Blue are commonly used to determine the suitability of a material as a support for the attachment and growth of cells. These assays are based on the reduction of the tetrazolium dyes by cellular enzymes to formazan dyes with characteristic color. In 2006, it has come to light that porous silicon, even with an oxide layer, interferes with these assays by reducing tetrazolium dyes (Laaksonen et al. 2007; Low et al. 2006). Passivating the surface against hydrolytic attack reduces but does not completely remove the interfering behavior (Laaksonen et al. 2007). Dye uptake viability assays such as neutral red which make use of the ability of viable cells to incorporate the dye in the lysosomes were found to be not compatible with porous silicon either, since the neutral red dye can also ingress into the porous layer (Low et al. 2006). These findings suggest that viability assays for cells in contact with porous silicon need to be carefully evaluated for compatibility.

Surface Modification Surface modification of porous silicon has been used to protect the surface against hydrolytic attack in aqueous medium and stabilize or slow down surface degradation. It can also be used to promote or prevent mammalian cell adhesion (Low et al. 2006; Faucheux et al. 2004). The changes in surface chemistry have long been known to affect the attachment and proliferation of anchorage-dependent mammalian cells on materials featuring otherwise almost identical topography, where cell attachment can be inhibited on very hydrophobic or hydrophilic surfaces (Groth and Altankov 1996; Yanagisawa et al. 1989). This has been mainly attributed to the amount of serum proteins (containing attachment factors) that is pre-adsorbed to the surface (Faucheux et al. 2004), which in turn can mediate cell attachment (Webb et al. 2000). Freshly etched porous silicon (Si–H) is rather hydrophobic, whereas ozoneoxidized surface (Si–OH) is very hydrophilic. Attachment of proteins in cell culture medium has been known to bind to moderately hydrophilic surfaces, leading to greater cell attachment on those surfaces (Webb et al. 1998). Water contact angles, qualitatively describing surface wettability for freshly etched and surface-modified porous silicon surfaces, are shown in Table 1. Arguably, the simplest method to stabilize the porous silicon surface is oxidation. A popular technique is to use ozone to rapidly generate a Si–OH capped surface with a thin oxide layer. Alternatively, thermal treatment in air (400–800  C) is used to generate thicker oxide layers (Pap et al. 2004). Surface hydroxyl groups can be further reacted with silanes, which can further stabilize the surface against hydrolytic attack, as well as provide a means of attaching functional groups to the surface. Alternatively, in hydrosilylation initiated through thermal or UV pathways, the Si–H

716 Table 1 Sessile drop water contact angle measurements for unmodified and surfacemodified porous silicon etched under the same conditions (Low et al. 2006)

N. H. Voelcker and S. P. Low Surface modification of porous silicon Freshly etched Amino silanized Collagen coated Polyethylene glycol silanized Fetal bovine serum coated Ozone oxidized

Contact angle >99 56 32 26 10