Nanoliquid Processes for Electronic Devices: Developments of Inorganic Functional Liquid Materials and Their Processing [1st ed.] 978-981-13-2952-4, 978-981-13-2953-1

Table of contents :
Front Matter ....Pages i-xvi
Front Matter ....Pages 1-1
Liquid Process (Tatsuya Shimoda)....Pages 3-11
Front Matter ....Pages 13-13
Guide to Silicon-Based Materials (Tatsuya Shimoda)....Pages 15-19
Liquid Silicon (Tatsuya Shimoda)....Pages 21-34
Thin Film Formation by Coating (Tatsuya Shimoda)....Pages 35-51
Liquid Vapor Deposition Using Liquid Silicon (LVD) (Tatsuya Shimoda)....Pages 53-69
Liquid Silicon Family Materials(1): SiO2, CoSi2, and Al (Tatsuya Shimoda)....Pages 71-91
Liquid Silicon Family Materials(2): SiC (Tatsuya Shimoda)....Pages 93-136
Nano-pattern Formation Using Liquid Silicon (Tatsuya Shimoda)....Pages 137-170
Development of Solar Cells Using Liquid Silicon (Tatsuya Shimoda)....Pages 171-188
Development of Thin-Film Transistors Using Liquid Silicon (Tatsuya Shimoda)....Pages 189-217
Front Matter ....Pages 219-219
Guide to Oxide-Based Materials (Tatsuya Shimoda)....Pages 221-224
Improvement of Solid Through Improved Solutions and Gels (1): Utilization of Reduction Agent and Reduced Atmosphere (Tatsuya Shimoda)....Pages 225-276
Improvement of Solid Through Improved Solutions and Gels (2): The Other Methods (Tatsuya Shimoda)....Pages 277-308
Direct Imprinting of Gel (Nano-rheology Printing) (Tatsuya Shimoda)....Pages 309-374
Novel Materials Proper to Liquid Process (Tatsuya Shimoda)....Pages 375-416
Thin-Film Oxide Transistor by Liquid Process (1): FGT (Ferroelectric Gate Thin-Film Transistor) (Tatsuya Shimoda)....Pages 417-439
Thin-Film Oxide Transistor by Liquid Process (2): UV and Solvothermal Treatments for TFT Fabrication (Tatsuya Shimoda)....Pages 441-505
Thin-Film Oxide Transistor by Liquid Process (3): TFTs with ZrInZnO Channel (Tatsuya Shimoda)....Pages 507-547
Device Fabrication by n-RP (Tatsuya Shimoda)....Pages 549-590

Citation preview

Tatsuya Shimoda

Nanoliquid Processes for Electronic Devices Developments of Inorganic Functional Liquid Materials and Their Processing

Nanoliquid Processes for Electronic Devices

Tatsuya Shimoda

Nanoliquid Processes for Electronic Devices Developments of Inorganic Functional Liquid Materials and Their Processing

Tatsuya Shimoda Japan Advanced Institute of Science and Technology Nomi, Ishikawa, Japan

ISBN 978-981-13-2952-4    ISBN 978-981-13-2953-1 (eBook) https://doi.org/10.1007/978-981-13-2953-1 Library of Congress Control Number: 2018959442 © Springer Nature Singapore Pte Ltd. 2019 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. This Springer imprint is published by the registered company Springer Nature Singapore Pte Ltd. The registered company address is: 152 Beach Road, #21-01/04 Gateway East, Singapore 189721, Singapore

Preface

This book summarizes the research outcomes of the “ERATO Shimoda nano liquid process project” and its extended research, both of which aimed to fabricate nano-­ sized devices directly through creating a novel printing technology. The project was sponsored by the Japan Science and Technology Agency. Because the outcomes were produced by many researchers involved, I have used “we” instead of “I” in the descriptions of the research-related contents. The researchers who led each research project can be found in the corresponding literatures. In the project, Si-based and oxide-based transistors were selected as target nanosized devices. The first work in the project was the development of a series of solution materials together with their proper processes to realize solid films with high performance, followed by research works to create a direct printing method using the developed solutions and to fabricate devices using it. In the case of fabricating a Si-based transistor, i.e., a metal-oxide semiconductor field-effect transistor (MOSFET), the necessary solution materials have been systematically synthesized using cyclopentasilane as a raw material. Specifically, precursor solutions that can be transformed by proper treatment to an i-type, p-type, or n-type Si semiconductor film, a SiO2 insulator film, or a CoSi2 conductor film were developed. We also developed a SiC precursor solution using cyclopentasilane. In the case of oxide transistors, several kinds of insulators, semiconductors, and conductors were developed. In both Si-based and oxide-based materials, the structure of the solution and its solution-to-solid transformation behavior have been intensively studied; an intermediate state from solution to solid, i.e., a gel state in the case of oxide, was investigated in particular detail. Concerning the direct printing, a novel method named nano-rheology printing (n-RP) was developed using nanoimprinting. This method involves the direct imprinting of a precursor gel material that has a plastic deformation ability and can retain its imprinted shape fairly well after post-annealing. Our reasons for adopting an imprinting method will be explained together with its advantages in Chap. 12 in Part I (Introduction of Liquid Process). The n-RP method enabled direct printing of precise patterns in both Si-based and oxide-based materials. In the case of oxide-­ based materials, we found that some gels had an imprinting ability, i.e., plastic v

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deformation ability, at elevated temperatures (below 200 °C) when its structure was an aggregate of tiny clusters. This behavior was fully exploited to fabricate thin-film transistors (TFTs) with a short channel and TFT integrated circuits. In the case of Si-based materials, polysilane derived from cyclopentasilane was found to be imprinted under certain conditions, and patterns with a high aspect ratio were successfully fabricated. With respect to the devices, before we developed transistors, we developed thin-­ film solar cells with a pin structure as a preliminary study of Si-based transistors. In the case of oxide-based devices, a TFT with memory function, i.e., a ferroelectric gate–insulator transistor (FGT), was developed together with a normal switching transistor. Not only transistors but also capacitors were developed, as described in the chapter on oxide-based devices, because we found a novel dielectric material, BiNbO, which had an extremely high relative dielectric constant εr. This book first outlines a liquid process as background information about the technology of the project. To make clear the technological placement of this book, I divide the existing liquid processes into three categories according to the liquid to solid conversion manner and show that the technological category handled by the project belongs to Type 6 in Category 3, as described later. The necessity of creating of a new direct printing method is explained with reference to the limitation of the conventional direct printing technology named “printed electronics (PE).” Nomi, Ishikawa, Japan

Tatsuya Shimoda

Contents

Part I Introduction to Liquid Process 1 Liquid Process..........................................................................................   3 1.1 Liquid and Its Formability..............................................................   3 1.2 Categories of Liquid Process..........................................................   4 1.2.1 First Step: Conversion Way from Liquid to Solid............   4 1.2.2 Second Step: Direct Forming Process..............................   7 Part II Silicon-Based Materials 2 Guide to Silicon-Based Materials...........................................................  15 3 Liquid Silicon...........................................................................................  21 3.1 CPS.................................................................................................  21 3.1.1 Hydrosilanes and CPS......................................................  21 3.1.2 Structures of a CPS Molecule...........................................  23 3.1.3 Electronic Structure of Isolated CPS Molecule................  24 3.1.4 Interaction Between CPS Molecules................................  24 3.2 Silicon Ink.......................................................................................  28 3.2.1 Silicon Ink from CPS........................................................  28 3.2.2 Polymer Structure in Silicon Ink......................................  30 3.2.3 Doped Silicon Inks...........................................................  32 References..................................................................................................  33 4 Thin Film Formation by Coating...........................................................  35 4.1 Coating Process and Molecular Forces...........................................  35 4.2 The Origin of Molecular Forces.....................................................  36 4.2.1 Theory of van der Waals Free Energy..............................  36 4.2.2 Measurement of Refractive Index n..................................  39 4.2.3 Molecular Forces of CPS and Silicon Compounds..........  39 4.3 Coating of Si Ink.............................................................................  42 4.3.1 General Remarks on Si Ink Coating.................................  42 4.3.2 Observations of Liquid Films...........................................  42 vii

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4.3.3 Hamaker Constant and Coating Property.........................  44 4.4 Conversion from Polysilane to Amorphous Si by Pyrolysis...........  45 4.4.1 Film Appearance During Pyrolysis and TG/DTA Analysis of Si Ink.............................................................  45 4.4.2 Raman Scattering Analysis...............................................  46 4.4.3 FT-IR and SIMS Analyses................................................  47 4.4.4 Properties of Amorphous Films........................................  49 References..................................................................................................  50 5 Liquid Vapor Deposition Using Liquid Silicon (LVD)..........................  53 5.1 Formation of I-, N-, and P-Type Silicon Film by LVD...................  54 5.1.1 LVD Method and Experiment...........................................  54 5.1.2 CPS Deposition Process...................................................  55 5.1.3 Film Properties..................................................................  57 5.1.4 Conclusion........................................................................  62 5.2 High-Quality Amorphous Silicon Film with LVD.........................  62 5.2.1 New Equipment for LVD..................................................  62 5.2.2 Film Quality with Processing Temperature......................  63 5.2.3 Film Quality with CPS Supply Speed..............................  65 5.2.4 Electronic Properties of a-Si:H Films...............................  66 5.2.5 Oxygen Contamination in a-Si:H Film.............................  67 5.2.6 Summary...........................................................................  68 References..................................................................................................  68

6 Liquid Silicon Family Materials(1): SiO2, CoSi2, and Al .....................  71 6.1 SiO2 Fabrication from Liquid Silicon.............................................  71 6.1.1 Forming SiO2 Films from Liquid Silicon Material...........  72 6.1.2 The Sole Solution-Processed SiO2 Film for TFTs............  75 6.1.3 Multiuse of Solution-Processed SiO2 Films for TFTs......  77 6.1.4 Conclusions.......................................................................  78 6.2 CoSi2 Fabrication from Liquid Silicon...........................................  78 6.2.1 Metal Silicide from Solution............................................  78 6.2.2 Synthesis of Cobalt Silicide Ink.......................................  79 6.2.3 Formation of CoSi2 Films.................................................  79 6.2.4 TEM Observation.............................................................  80 6.2.5 Comparison of this Process with the Conventional Ones............................................................  81 6.2.6 More Detailed Analyses....................................................  81 6.2.7 Conclusion........................................................................  83 6.3 Al Fabrication Via Solution Process...............................................  83 6.3.1 Triethylamine Alane as a Precursor of Metal Al...............  84 6.3.2 Deposition Process and Reaction......................................  85 6.3.3 Analysis of Film Structure and Al Growth Manner..........  86 6.3.4 Selective Deposition of Al................................................  87 6.3.5 Conclusion........................................................................  90 References..................................................................................................  90

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7 Liquid Silicon Family Materials(2): SiC................................................  93 7.1 SiC Fabrication via Liquid Process................................................  94 7.1.1 Preparation and Characterization of SiC Precursor Polymer.............................................................................  94 7.1.2 a-SiC Film Formation and Analyses of Films..................  98 7.1.3 Polymer Structure............................................................. 102 7.1.4 Polymer-to-Ceramic Conversion...................................... 103 7.1.5 Conclusion........................................................................ 104 7.2 Correlation of Si/C Stoichiometry Between SiC-Ink and a-SiC Film................................................................................ 105 7.2.1 Polymer and Film Preparation.......................................... 105 7.2.2 Correlation Between PSH and a-SiC................................ 106 7.2.3 Structural Properties of an a-SiC Film.............................. 108 7.2.4 Optical and Electrical Properties of an a-SiC Film.......... 111 7.2.5 Conclusion........................................................................ 113 7.3 n-Type a-SiC by Coating................................................................ 114 7.3.1 Polymer and Film Preparation and Their Analyses.......... 114 7.3.2 Polymer Analysis.............................................................. 115 7.3.3 Thin-Film Formation........................................................ 117 7.3.4 Effect of Carbon Content on Film.................................... 118 7.3.5 Effect of Phosphorous Concentration on Film................. 120 7.3.6 Conclusion........................................................................ 123 7.4 P-Type a-SiC via LVD Method....................................................... 123 7.4.1 SiC-Ink Preparation and Film Deposition........................ 123 7.4.2 Ink Analysis...................................................................... 125 7.4.3 Film Analysis.................................................................... 126 7.4.4 Discussion......................................................................... 132 7.4.5 Conclusion........................................................................ 134 References.................................................................................................. 134 8 Nano-pattern Formation Using Liquid Silicon..................................... 137 8.1 Area-Selective Deposition of Silicon Family Materials................. 137 8.1.1 Area-Selective Deposition of Silicon Using the Difference of Molecular Force.................................... 138 8.1.2 Selective Deposition Using the Reactive Difference........ 139 8.2 Beam-Assisted Deposition of Silicon............................................. 140 8.2.1 Free Writing of Silicon by FIB-CVD and Advantage of CPS for a Source Material................... 140 8.2.2 Experimental..................................................................... 141 8.2.3 Deposition of Silicon Patterns.......................................... 141 8.2.4 Characterization of the Deposited Patterns....................... 142 8.2.5 Summary........................................................................... 144 8.3 Direct Imprinting of Silicon Using Liquid Silicon......................... 145 8.3.1 Nanoimprinting and Silicon.............................................. 146 8.3.2 Experimental Section........................................................ 146

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8.3.3 Imprinted Patterns with Mold 1........................................ 150 8.3.4 Influence of Baking Temperature on Imprinting in Mold 2........................................................................... 154 8.3.5 Raman and FTIR Analyses............................................... 157 8.3.6 Solid-Phase Crystallization of Si Nano-patterns.............. 161 8.3.7 Discussion......................................................................... 164 8.3.8 Conclusion........................................................................ 168 References.................................................................................................. 169 9 Development of Solar Cells Using Liquid Silicon................................. 171 9.1 Thin-Film Solar Cells by Coating................................................... 171 9.1.1 Solution Preparation and Film Formation........................ 171 9.1.2 Characteristics of Coated Films and Their Improvement by Hydrogen-Radical Treatment................ 172 9.1.3 Fabrication of Solar Cells and Their Properties................ 173 9.1.4 Conclusion........................................................................ 177 9.2 Thin-Film Solar Cells by LVD....................................................... 178 9.2.1 Solar Cell Fabrication Using LVD.................................... 178 9.2.2 Solar Cell Fabrication Using the Improved LVD............. 181 9.2.3 Conclusion........................................................................ 182 9.3 Application of Liquid Silicon for HBC-Type Solar Cells.............. 182 9.3.1 Experimental Procedure.................................................... 183 9.3.2 Thermal Stability of LVD a-Si Passivation Films............. 184 9.3.3 Storage Stability of c-Si Wafers Passivated with LVD a-Si Films......................................................... 186 9.3.4 Feature of LVD a-Si Passivation Films and Advantage of LVD Method........................................ 186 9.4 Conclusion...................................................................................... 188 References.................................................................................................. 188 10 Development of Thin-Film Transistors Using Liquid Silicon.............. 189 10.1 Poly-Si Thin-Film Transistor (TFT)............................................... 189 10.1.1 Preparation of Liquid Silicon............................................ 190 10.1.2 Poly-Si TFT...................................................................... 190 10.1.3 Ink-Jet Printing of a Channel............................................ 191 10.1.4 Conclusion........................................................................ 193 10.1.5 Experimental Methods...................................................... 193 10.2 Single-Grain Si-TFT....................................................................... 194 10.2.1 Forming Single Grains from Liquid Si............................. 195 10.2.2 Fabrication of Single-Grain TFTs..................................... 198 10.2.3 Single-Grain TFTs on Flexible Substrates........................ 203 10.2.4 Conclusion........................................................................ 208 10.3 TFT on Paper.................................................................................. 208 10.3.1 Poly-Si Film from Polysilane........................................... 209 10.3.2 TFT Fabrication on Paper................................................. 210 10.3.3 Properties of TFT on Paper............................................... 211

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10.3.4 Further Improving as a Conclusion.................................. 215 10.3.5 Experimental Method....................................................... 215 References.................................................................................................. 217 Part III Oxide-Based Materials 11 Guide to Oxide-Based Materials............................................................. 221 12 Improvement of Solid Through Improved Solutions and Gels (1): Utilization of Reduction Agent and Reduced Atmosphere............................................................................................... 225 12.1 Low-Temperature Process of PZT Bulk......................................... 226 12.1.1 Introduction and Experimental......................................... 226 12.1.2 X-Ray Diffraction, TEM, and XPS.................................. 227 12.1.3 XAFS................................................................................ 232 12.1.4 Thermal Analysis.............................................................. 240 12.1.5 Discussion......................................................................... 242 12.1.6 Conclusion........................................................................ 244 12.1.7 Experimental Detail.......................................................... 245 12.2 Low-Temperature Process of PZT Thin Film................................. 247 12.2.1 Low-Temperature Solution Processes for PZT Prior to This Study............................................................ 247 12.2.2 Low-Temperature Process of PZT Thin Film Using Reduced Atmosphere............................................. 247 12.2.3 Process Optimization........................................................ 250 12.2.4 PZT Film Properties......................................................... 250 12.2.5 Conclusion........................................................................ 252 12.2.6 Experimental Methods...................................................... 252 12.3 Ru and RuO Thin Film................................................................... 253 12.3.1 Introduction....................................................................... 253 12.3.2 Thermal Behaviors and Structure of the Precursor........... 254 12.3.3 Effect of Amine Content................................................... 257 12.3.4 Effects of Amine Structure............................................... 260 12.3.5 Properties of the Prepared Ru0 and RuO2 Thin Films......................................................................... 264 12.3.6 Conclusion........................................................................ 266 12.3.7 Experimental Methods...................................................... 267 12.4 Low-Temperature Processed RuO2 by Green Laser Annealing........................................................................................ 268 12.4.1 Introduction....................................................................... 268 12.4.2 Green Laser Irradiation to RuO2 Precursor Films............ 269 12.4.3 Resistivity of the GLA Annealed Films............................ 270 12.4.4 Conclusion........................................................................ 273 12.4.5 Experimental Procedure.................................................... 273 References.................................................................................................. 273

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13 Improvement of Solid Through Improved Solutions and Gels (2): The Other Methods........................................................... 277 13.1 Improvement of Insulator Property of LaZrO by Amelioration of Solution................................................................ 278 13.1.1 Introduction....................................................................... 278 13.1.2 Properties of Films Prepared at Temperatures Between 400 °C and 600 °C............................................. 279 13.1.3 TG/DTA Analysis............................................................. 281 13.1.4 Mass Analysis................................................................... 285 13.1.5 High-Energy XRD Analysis............................................. 286 13.1.6 XAFS Analysis................................................................. 289 13.1.7 Analysis of Elemental Composition for the Annealed Films........................................................... 296 13.1.8 Summary of the Above Analyses...................................... 296 13.1.9 Conclusions....................................................................... 297 13.1.10 Experimental Methods...................................................... 298 13.2 Combustion Synthesized ITO......................................................... 300 13.2.1 SCS-ITO Solutions and Thin-Film Formation................. 300 13.2.2 Solution-Processed TFTs Using SCS-ITO as S&D Electrodes............................................................ 304 13.2.3 Conclusions....................................................................... 306 References.................................................................................................. 307 1 4 Direct Imprinting of Gel (Nano-rheology Printing)............................. 309 14.1 Nano-rheology Printing (n-RP) of ITO.......................................... 310 14.1.1 Introduction to Nano-rheology Printing and Its Feasibility on ITO................................................. 310 14.1.2 Analysis of the Gel Material............................................. 313 14.1.3 Changes in the Gel Film During Nano-rheology Printing............................................................................. 318 14.1.4 Feature of the Nano-rheology Printing............................. 320 14.1.5 Conclusion........................................................................ 320 14.1.6 Experimental Details........................................................ 321 14.2 Evaluating ITO Gels via Cohesive Energy..................................... 323 14.2.1 Preparation of ITO Solution and Thin Films.................... 323 14.2.2 Conventional Methods for Evaluating the State of the Gel.......................................................................... 324 14.2.3 New Methods for Evaluating Cohesive Energy of a Gel............................................................................. 324 14.2.4 Analytical Results Using Conventional Methods............. 327 14.2.5 Evaluated Cohesive Energies of Gels............................... 333 14.2.6 Conclusion........................................................................ 338 14.3 Origin of the Thermal Plasticity of ZrO Gels................................. 338 14.3.1 Introduction....................................................................... 338 14.3.2 Thermal Plasticity Property and Rheology Printing for ZrO Gels........................................................ 339

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14.3.3 Structure of ZrO Gels....................................................... 345 14.3.4 Origin of Thermal Plasticity of Zr-Gels........................... 351 14.3.5 Conclusion........................................................................ 353 14.3.6 Experimental Methods...................................................... 354 14.4 Nano-sized Patterns of RuLaO by n-RP......................................... 355 14.4.1 Conversion from Solutions to Solids................................ 356 14.4.2 Properties of Nano-rheology Printing............................... 362 14.4.3 Analysis of Gels and Solutions......................................... 366 14.4.4 n-RP Mechanism of Ru-La Gel........................................ 369 14.4.5 Conclusion........................................................................ 370 14.4.6 Experimental Methods...................................................... 371 References.................................................................................................. 373 1 5 Novel Materials Proper to Liquid Process............................................. 375 15.1 High Dielectric Constant BiNbO Material (1): Bi:Nb=1:1............ 376 15.1.1 BiNbO Materials for Ceramic Capacitors........................ 376 15.1.2 Electrical Properties of a New BiNbO Material............... 376 15.1.3 Improvement of Solution for a Standard Process of the BNO Films.............................................................. 379 15.1.4 Electric Properties of the Films from the Improved Solution............................................................................. 380 15.1.5 Pyrolysis of the Improved Solutions and Gels.................. 381 15.1.6 Crystalline Identification by XRD and HRTEM.............. 383 15.1.7 Crystallization Pathway of Solution-Processed BNO...... 387 15.1.8 Summary........................................................................... 388 15.1.9 Experimental Details........................................................ 388 15.2 High Dielectric Constant BiNbO Material (2): Nb-Rich Composition.................................................................................... 390 15.2.1 Preparation of BNO Precursor Solution........................... 390 15.2.2 Analysis of Equilibrium Phases Appeared in the Film from N50........................................................ 390 15.2.3 Relation of the Relative Dielectric Constant and XRD Pattern with Nb Content................................... 393 15.2.4 Thermal Analysis of N50, N60, and N67 Solutions......... 394 15.2.5 Relations of the Relative Dielectric Constant and Tan δ with the Annealing Temperature...................... 396 15.2.6 Conclusions....................................................................... 398 15.3 New P-Type Semiconductors.......................................................... 400 15.3.1 Introduction....................................................................... 400 15.3.2 Experimental..................................................................... 401 15.3.3 Ln-Ru(Ir)-O...................................................................... 403 15.3.4 Bi(Pb)-Ru(Ir)-O................................................................ 408 15.3.5 Summary........................................................................... 414 References.................................................................................................. 414

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16 Thin-Film Oxide Transistor by Liquid Process (1): FGT (Ferroelectric Gate Thin-­Film Transistor)............................................. 417 16.1 Pt and PZT Films for FGT.............................................................. 418 16.1.1 Introduction....................................................................... 418 16.1.2 Experimental Procedures.................................................. 419 16.1.3 Optimization of Pt/Ti Films.............................................. 420 16.1.4 Optimization of PZT Films............................................... 425 16.1.5 FGT Device Properties and Performance......................... 427 16.1.6 Conclusion........................................................................ 428 16.2 All-Solution-FGT Example 1......................................................... 429 16.2.1 Introduction....................................................................... 429 16.2.2 Experimental Details........................................................ 430 16.2.3 Structural Properties......................................................... 431 16.2.4 Electrical Properties.......................................................... 433 16.2.5 Conclusion........................................................................ 438 References.................................................................................................. 438 17 Thin-Film Oxide Transistor by Liquid Process (2): UV and Solvothermal Treatments for TFT Fabrication....................... 441 17.1 UV Treatment for TFT.................................................................... 442 17.1.1 Introduction....................................................................... 442 17.1.2 Experimental Details........................................................ 443 17.1.3 Thermal Behavior of In–Ga–Zn–O Solution.................... 445 17.1.4 Effect of UV/O3 Treatment............................................... 445 17.1.5 TFT Performance.............................................................. 448 17.1.6 Summary........................................................................... 450 17.2 UV Treatment for All-Liquid-Processed TFT Example 2.............. 450 17.2.1 Introduction....................................................................... 451 17.2.2 Experimental Details........................................................ 452 17.2.3 Effects of UV/O3 Treatment and TFT Properties.............. 454 17.2.4 Compositional Investigation with Applying UV/O3 Treatment.............................................................. 455 17.2.5 All-Solution-Processed TFT............................................. 457 17.2.6 Conclusion........................................................................ 457 17.3 UV and Solvothermal Treatments for TFTs................................... 459 17.3.1 Introduction....................................................................... 459 17.3.2 Experimental Methods...................................................... 460 17.3.3 Formation of Hybrid Clusters........................................... 462 17.3.4 Light Absorption Analysis of Solutions............................ 463 17.3.5 Low-Temperature UV Annealing of Films and Their Characterization............................................... 463 17.3.6 Low-Temperature Fabrication of Transistors.................... 471 17.3.7 Conclusion........................................................................ 473 17.4 Thermal-UV Treatment.................................................................. 473 17.4.1 Introduction....................................................................... 474

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17.4.2 Experiment Details........................................................... 475 17.4.3 Result and Discussion....................................................... 478 17.4.4 Conclusion........................................................................ 494 17.5 UV Patterning for TFT Fabrication................................................ 495 17.5.1 Introduction....................................................................... 495 17.5.2 UV Irradiation and Re-Dissolving (UV-RD) Method...... 495 17.5.3 Patterning of InO Film [72].............................................. 496 17.5.4 Patterning of Component Films for a TFT....................... 498 17.5.5 TFT Fabrication by the UV-RD Patterning Method......... 499 17.5.6 Conclusion........................................................................ 502 References.................................................................................................. 502

18 Thin-Film Oxide Transistor by Liquid Process (3): TFTs with ZrInZnO Channel................................................................. 507 18.1 ZrInZnO Semiconductor Film........................................................ 508 18.1.1 Introduction....................................................................... 508 18.1.2 Experimental Details........................................................ 509 18.1.3 Film Characteristics.......................................................... 510 18.1.4 TFT Characteristics.......................................................... 514 18.1.5 Conclusion........................................................................ 517 18.2 ZrInZnO TFT with Polysilazane-Based SiO2 Gate Insulator......... 517 18.2.1 Introduction....................................................................... 517 18.2.2 Experimental Details........................................................ 518 18.2.3 Quality of SiO2 Film Made from Polysilazane and Its Leakage Current Mechanism................................ 519 18.2.4 TFT Using ZrInZnO Channel and Polysilazane-­ Derived SiO2..................................................................... 522 18.2.5 Conclusion........................................................................ 525 18.3 ZrInZnO TFT by UV Treatment: All-Liquid-Processed TFT Example 3............................................................................... 525 18.3.1 Introduction....................................................................... 525 18.3.2 Experimental Details........................................................ 526 18.3.3 Preparation of Each Component Layer............................. 526 18.3.4 Fabrication of All-Solution-Processed TFT with High Performance..................................................... 528 18.3.5 Conclusion........................................................................ 530 18.4 All-Liquid-Processed Active-Matrix Backplane for EPD.............. 530 18.4.1 Introduction....................................................................... 530 18.4.2 Oxide Films Used in the AM-BP...................................... 531 18.4.3 Fabrication of Active-Matrix TFT Backplane (AM–TFT–BP)................................................................. 533 18.4.4 Active-Matrix-Driven EPD Panel: Design, Panel Fabrication, and Driving Method............................ 539 18.4.5 Performance of the TFTs and TFT–EPDs........................ 542 18.4.6 Conclusion........................................................................ 543 References.................................................................................................. 544

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1 9 Device Fabrication by n-RP.................................................................... 549 19.1 Fabrication of FGT and TFT by n-RP............................................ 550 19.1.1 The Developed TFTs and Their Solutions........................ 550 19.1.2 Solutions for TFT-1........................................................... 550 19.1.3 Solutions for TFT-2........................................................... 551 19.1.4 TFT Fabrication by Nano-Rheology Printing................... 552 19.1.5 Conclusion........................................................................ 553 19.2 High-Performance TFT by n-RP.................................................... 554 19.2.1 Introduction....................................................................... 554 19.2.2 Problems of the Previously Developed TFT and Their Solutions........................................................... 555 19.2.3 TFT Structure and Fabrication Process............................ 556 19.2.4 Shape, Morphology, and Microstructure of the TFT by n-RP........................................................... 557 19.2.5 Electric Properties of the TFT by n-RP............................ 560 19.2.6 Conclusion........................................................................ 562 19.3 Short Channel TFT by n-RP........................................................... 562 19.3.1 Introduction....................................................................... 562 19.3.2 TFT Structure and Its Fabrication..................................... 563 19.3.3 Synthesis of Metal-Oxide Precursor Solutions................. 564 19.3.4 LRO/Pt Gate Electrode Pattern by the nRP...................... 565 19.3.5 Sub-Micron Channel Length by the nRP.......................... 567 19.3.6 Performance of Submicron Channel Length nRP-Oxide TFTs............................................................... 568 19.3.7 Conclusion........................................................................ 570 19.4 Active-Matrix Backplane by n-RP................................................. 571 19.4.1 Introduction....................................................................... 571 19.4.2 TFT Fabrication Process and Details of n-RP.............................................................................. 572 19.4.3 Solution Preparation and Others....................................... 574 19.4.4 Development of Alignment System for n-RP Process..................................................................... 575 19.4.5 Evaluation of Component Materials of TFT..................... 576 19.4.6 Oxide Gels Patterning by n-RP for TFTs and AM-BP....................................................................... 579 19.4.7 TFT Fabrication Using an Alignment System.................. 584 19.4.8 Fabrication of AM-BP...................................................... 586 19.4.9 Conclusions and Outlook.................................................. 586 References.................................................................................................. 588

Part I

Introduction to Liquid Process

Chapter 1

Liquid Process

Abstract  Liquid process, which uses the nature of liquid to produce products, is first identified and assigned as a technical term. When a liquid process is applied for fabricating devices, there are two cases: one where the liquid process is only used for making films and the other where it is used not only for making films but also for patterning them simultaneously. As the technology of the latter case totally depends on the former, we call the former case the first step and the latter one the second step. The first step can be interpreted as a conversion process from liquid to solid. Here all the conversion methods from liquid to solid, which have been ever known, are addressed and classified and their advantages and disadvantages clarified in terms of device fabrication. Then, direct forming processes as the second step are described related to the printed electronics (PE) technology. Inherent problems of PE when applied for device fabrication are addressed and the objective of the project (ERATO Shimoda nano liquid process project) is clarified. Keywords  Liquid process · Printed electronics · Functional liquid · Solution · Liquid Si · Coffee stain pattern

1.1  Liquid and Its Formability Liquid, one of three states of matter, has a unique character. Even though its density is nearly equal to that of solid, it can flow and change its shape freely. This freedom in shape has been utilized in producing things since ancient times. The examples are numerous. Buildings built with cement and cast metal products are the most familiar examples. The use of concrete is far better than stacking of stones or bricks to construct buildings, bridges, etc. Concrete changed not only building technology but also society itself. Casting, an evolutional method to fabricate metal objects, has a long history and remains one of most useful production methods for metal products. Another example is paint, a colorful coating agent whose fluidic property is exploited for coatings and which can be patterned when necessary.

© Springer Nature Singapore Pte Ltd. 2019 T. Shimoda, Nanoliquid Processes for Electronic Devices, https://doi.org/10.1007/978-981-13-2953-1_1

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1  Liquid Process

Because the fabrication of objects by exploiting the nature of liquids is too universal and ubiquitous to be considered a specific technology, it has never been assigned a specific name. However, naming is necessary to identify the technology and to make further progress in it. Here we name the technology of producing products by exploiting or using the nature of liquids as a “liquid process.” The liquid process thus defined should extend to every kind of product. This book mainly covers applications of the liquid process for fabricating small-sized devices.

1.2  Categories of Liquid Process When a liquid process is used to fabricate devices, two cases exist: one where the liquid process is only used for making films and the other where it is used not only for making films but also for patterning them simultaneously. In the former case, patterning is carried out using a different method, such as photolithography. The latter case enables us to make devices directly through direct patterning of the components of devices. A typical example of the latter case is the “printed electronics” (PE) technology, which involves the application of printing technologies for the fabrication of electronic devices. The technology of the latter case, however, totally depends on the former case. Therefore, the former case is placed as the first step of the liquid process, whereas the latter case is placed as the second step or final step of the liquid process. In this way, the liquid process is largely categorized into two steps: the first step and the second step. The first step is also interpreted as a conversion process from liquid to solid. This conversion process defines the technology of the liquid process. We therefore discuss this topic first to clarify the characteristics of liquid materials and processes used in our liquid process.

1.2.1  First Step: Conversion Way from Liquid to Solid When discussing liquid processes, we refer to the liquid used as the “functional liquid.” This name does not imply that the liquid performs a specific function but rather that a functional solid can be prepared from it. The issue of how to convert a functional liquid into a solid is crucial in the design of a liquid process. This conversion governs the selection of all of the related research items, including the materials, process, and devices in a liquid process. The ever known conversion methods are summarized in Fig. 1.1. Three cases exist: a liquid solidifies while maintaining a constant volume (Case 1), a solid is formed inside a liquid (Case 2), and a solute remains after drying of the solvent followed by a solidification process (Case 3). Case 1 has two types, i.e., Type 1 and Type 2. Type 1 is a phase transformation from a liquid phase to a solid phase. Metals used in a casting technology convert

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from liquid to solid or solid to liquid at their melting point. Glasses and polymers undergo the same transformation at the glass transition temperature. Type 2 is a transition based on a chemical reaction such as cross-linking and condensation. UV curable resin and cement are examples of Type 2 in Case 1. Case 2 also has two types: Type 3 and Type 4. Type 3 is a plating method that produces metal thin films in a plating bath via an electrochemical reaction. Type 4 is a technology or phenomenon in which a solute in a solution deposits onto a solid surface via attractive forces, e.g., molecular forces, and is subsequently solidified by chemical or physical reaction. Deposition of stalactite in limestone caves is an example. Case 3 involves a solution (solute + solvent). It has also two types: Type 5 and Type 6. In Type 5, a solid is formed immediately after evaporation of the solvent without a post-process. This type occurs in solutions of polymers which can be easily solved in a wide range of solvents. Paint is a practical example of Type 5. By contrast, in Type 6, the final solid is formed via an intermediate substance obtained by evaporation of the solvent and is usually converted into a solid by pyrolytic decomposition. This process is one of the common ways to form inorganic materials and metals. In our liquid process described in this book, both Si-based and oxide-­ based materials are formed via this conversion type. Each case or type possesses its own character, and each has both advantages and disadvantages. The advantage of Case 1 is that little volume shrinkage occurs during the transition from liquid to solid; thus, the planned shape and volume of the solid can be achieved. In the case of a metal, however, the large surface tension of metal prevents it from being formed into a small-sized shape. In addition, the relatively high melting points of metals make them difficult to handle in liquid form under normal conditions. Therefore, metal casting has rarely been applied for fabricating small parts with submillimeter dimensions. However, Type 1 has been fully utilized in the formation of glass materials, such as microfluidics devices for micro-sized total analytical systems (μ-TAS), and in texture formation of hard disc drive media.

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1  Liquid Process

Type 2 in Case 1 is a very useful conversion way for micro-sized processing. A resist material used in nanoimprinting is a typical example. UV-hardening resins and thermoset resins have been commercialized. Nanoimprint lithography (NIL), which is now widely used in manufacturing small-sized products, is a good example of a technology involving UV-hardening resins. Whereas the conversion method of Case 1 is very suitable for a size- and shape-oriented formation process because the formed shape is highly similar to that of the planned shape, the selection of compatible materials is limited. As a result, the number of applications for Type 2 functional materials is very small. Case 2 conversion enables the use of technologies such as area-selective plating (Type 3) and area-selective deposition (Type 4). Type 3, area-selective plating, is a technology to plate metal onto a desired positon with a defined area. The damascene process used in semiconductor fabrication is a good example. In this process, the solute is selectively deposited onto a targeted area (pre-patterned area) formed in advance. If catalyst patterns are formed on a substrate in advance, a metal such as aluminum or copper can be plated selectively onto the catalyst area. Otherwise, selective deposition can be achieved through various principles, such as molecular force or electrostatic force. Type 3 and Type 4 are well suited to the fabrication of small-sized patterns because the patterning and formation of solids are carried out at the same time. Some results of Type 3 will be introduced in this book. In Case 3, the liquid used as a raw material is a solution or suspension. A solution is liquid in which one or more solutes is/are dissolved in a solvent or mixture of solvents, which can be molecules, multimer molecules, solvation molecules, clustered molecules, colloid, etc. By contrast, a suspension is a liquid in which the solute is suspended or dispersed in a solvent. The stability of the suspended solute is explained by the well-known Derjaguin–Landau–Verwey–Overbeek (DLVO) theory. Because the solute in Case 3 can take various forms, various materials can be used to prepare solutions or suspensions; thus, the materials selection for solutes and solvents is virtually unlimited. This is a great advantage of Case 3. The first step of the conversion in Case 3 is evaporation of the solvent, leaving behind solids. In the case of a Type 5 product, the remaining solid is the final product. Most organic semiconductors are processed using Type 5 conversion. This type of conversion is especially possible in the case of organic materials, because the molecules themselves in organic substances bear functional groups. Inorganic materials and metals, however, tend to undergo conversion of Type 6. The remaining substance after drying is an intermediate that should subsequently be converted into a solid via pyrolysis. Most materials in this book are formed via Type 6 conversion. We synthesized liquid Si and used it to prepare Si-based materials. Liquid Si was formulated by dissolving polysilane in an organic solvent. Therefore, polysilane remains as an intermediate substance after evaporation of the solvent and is subsequently dehydrogenated via a pyrolysis process to form amorphous Si. In the case of research of oxide-based materials, we formulated a precursor solution by dissolving a metal complex or complexes into a solvent or solvents and then obtained a gel

1.2  Categories of Liquid Process

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as an intermediate substance by evaporating the solvent  or solvents. Subsequent heating decomposed the organic elements in the gel, with introducing oxygen to form a solid oxide. The liquid process, which uses a metal nanoparticle-dispersed ink, belongs to Type 6 because the ink is converted into a metal via an intermediate state. The substance after evaporation of the solvent is an aggregation of metal nanoparticles whose surface is covered by organic ligands. When the aggregated substance (intermediate substance) is heated, the organic elements are decomposed, followed by sintering the metal particles to form the final solid metal.

1.2.2  Second Step: Direct Forming Process The technology of printing is a very interesting prior art when we consider the second step in the liquid process, i.e., the direct formation of solid patterns. In printing, thin-film solid patterns remain on a substrate, such as paper or plastic, after the ink is dried. This printing technology can conceivably be used in applications other than conventional printings. One such idea is to use the printing technology to fabricate electronics parts and devices. The research work to realize this idea started in the mid-1990s and has become a new technology field, i.e., printed electronics (PE). Because this technology enables the formation of various patterns of a desired shape and at a desired position, manufacturing of electronics parts and devices could be simplified with simple equipment, a short processing time, and a minimum amount of wasted materials. PE could be an excellent technology in terms of energy savings, resource conservation, and cost-effective manufacturing because of its cost-reducing advantages in production. However, a far higher technological level is required for PE technology than conventional printing technologies because the printing objects must bear some kind of functionality. For example, the following are emergent problems: instead of color inks, a functional liquid must be developed; the printed substance must exhibit the required properties; stricter size control is required; and sometimes a far smaller size is necessary. In addition to these issues, a new requirement concerning threedimensional shape is a matter of interest in applications of the PE technology because a rectangular shape with a high aspect ratio is often required in electronics parts and devices. This requirement differs substantially from the technology requirements of the conventional printing methods, which are restricted to the twodimensional world. With respect to the current status of the PE technology, conventional printing methods continue to be used in most applications; however, modified versions of these printing methods have been appearing. Figure 1.2 shows the printing principles of relief printing, gravure printing, screen printing, and offset printing. All of these methods involve printing blocks or plates. Unlike these methods, inkjet printing, whose features are shown in Fig. 1.3, is a plateless printing technology. A piezo-type head is exclusively used in PE applications. Each printing method has its own features or distinctions. For example, the

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offset printing method utilizes a solid ink created during its process, whereas the others use only liquid inks. In offset printing, the transfer of the solid pattern from the intermediate roll to the final substrate is an advantageous method, which ­provides high accuracy in dimension and shape. The method of solid pattern transfer is now incorporated into the printing principle of PE and is giving rise to new printing technologies such as inverse relief printing, gravure-offset printing, and screen-­offset printing. The former two printing methods enable smaller size printing than was previously achievable. The printing principles of the inverse relief printing and gravure-offset printing methods are shown in Figs. 1.4 and 1.5, respectively, together with their advantages and disadvantages.

1.2  Categories of Liquid Process

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Why is the use of solid patterns considered a merit in printing? Here, I would like to explain the behavior of ink when a droplet of it with a volume in the picoliter to the nanoliter range is dropped or printed on a solid surface (not on paper). The behavior of the droplet is illustrated in Fig. 1.6. Immediately after being dropped onto a substrate, the ink tends to round its shape by surface tension and becomes a spherical cap at equilibrium when the wetting condition is a partial wetting (Fig. 1.6a); solvent simultaneously evaporates from the ink, which, in turn, deforms the profile of the pattern. Pattern deformation strictly depends on the behavior of a contact line. In the case where the contact line moves according to evaporation of

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solvent, the pattern shrinks while maintaining its spherical cap shape (Fig. 1.6b). However, if the contact line is pinned at the surface, solute is driven from the center to the peripheral area in the spherical cap by the advection of solvent, which is caused by the combination of solvent drying and the surface tension of the ink. As a result, an outskirts-swelled shape is formed (Fig. 1.6c). Because such patterns are often observed when a droplet of spilled coffee is dried, it is referred to as a “coffee stain” pattern. This term is now becoming a technical term in liquid processing. In conclusion, a solid pattern made from liquid ink does not have a sharp edge; it has a round or swelled edge depending on a contact line behavior. These shapes are far from the rectangular ones with a high aspect ratio required for electronics components and devices. The tendency toward round or swelled edges increases with decreasing density of the ink. To avoid the shape degradation of printed patterns, new printing methods incorporating the principle of offset printing have been developed: gravure-offset printing, screen-offset printing, and inverse relief printing. The principle of offset printing, which transfers the patterned solid or semisolid films onto a final substrate, is fully utilized in these printing methods. As a result, the printable size is reduced to a few micrometers in the cases of gravure-offset and inverse relief printings. This size is smaller than that achievable by the conventional printing methods by approximately one order of magnitude. Despite the progress in the PE technology, its miniaturization ability is far inferior to that of the conventional photolithography method used in the semiconductor industry; a large discrepancy of approximately two orders of magnitude exists between them, as shown in Fig. 1.7. When this situation considered, a new direct printing method that we aim to develop should be comparable with current photolithography in terms of miniaturization. For this reason we chose nanoimprinting to

1.2  Categories of Liquid Process

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create a direct printing technology because nanoimprinting technology is applicable to sizes less than 1 nm. The challenge is to develop a method for direct imprinting of functional materials. This approach is a new research theme and is the objective of our project (ERATO Shimoda nano liquid process project).

Part II

Silicon-Based Materials

Chapter 2

Guide to Silicon-Based Materials

Abstract  This chapter is a guidance of the materials and processes related to liquid silicon. Each topic is introduced from chapter to chapter based on the process flow shown in Fig. 2.2. The intestine terms are liquid silicon family materials (LSFMs), which are an assembly of the required materials to fabricate MOSFET base on the liquid process. Most of them can be synthesized by using cyclopentasilane (CPS). Keywords  Liquid silicon · Cyclopentasilane (CPS) · Silicon ink · Polysilane · Liquid vapor deposition In this book, the term of “liquid silicon” collectively refers to liquid-based hydrosilane materials, including cyclopentasilane (CPS), polysilanes derived from CPS, and solutions of polysilane (silicon solution). Silicon films are usually made from silicon solution. The silicon solution, which is formulated for forming films, is referred as “silicon ink.” The starting material for the liquid process of silicon is CPS. MOSFET devices, which are our target devices in the project, require not only intrinsic Si but also both p-type and n-type silicon. In addition to them, SiO2 as an insulator and metal-­silicide as a conductor, which is used in source and drain electrodes, are also required. Fortunately, these materials can be derived from CPS.  However, aluminum is required as a material for interconnects. Because aluminum metal cannot be formed using CPS, we used a liquid material named triethylamine alane (TEA:AlH3NEt3) to form aluminum lines and pads. We categorize these silicon-based and aluminum-­ based liquid materials as belonging to the liquid silicon family materials (LSFMs). Figure 2.1 shows LSFMs together with their synthesizing methods. Because CPS is used as a raw material for all of the silicon-based solutions needed for metal-oxide semiconductor field-effect transistors (MOSFETs), its basic properties are first introduced in Chap. 3. The conversion from CPS to a poly-Si film via silicon ink, a polysilane film, and an amorphous Si film is shown in Fig. 2.2. The rectangular figures correspond to materials, and the conversion process from one material to another is illustrated by a red ellipse figure. The final devices are also shown by the blue rectangular figures with round corners.

© Springer Nature Singapore Pte Ltd. 2019 T. Shimoda, Nanoliquid Processes for Electronic Devices, https://doi.org/10.1007/978-981-13-2953-1_2

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2  Guide to Silicon-Based Materials

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In Chap. 3, I describe the materials and their conversion processes by tracing the production steps shown in Fig. 2.2. After describing the basic properties of CPS, I discuss the formation of the silicon solution and its physical properties. CPS can be polymerized by irradiation with ultraviolet (UV) light so as to form polydihydrosilane (polysilane), which is dissolved into an organic solvent to form an intrinsic silicon solution or silicon ink. During polymerization under UV irradiation, doped silicon solutions of both n-type and p-type can be prepared by doping with phosphorus and boron, respectively. In Chap. 4, the coating of silicon solution to form polysilane films and subsequent conversion of the polysilane films to amorphous Si films via pyrolysis are described. Preparing a uniform film of polysilane on a substrate is not straightforward. Although wetting of the silicon solution on a substrate such as glass, quartz, or silicon is adequate, a film once coated on the substrate tends to break during evaporation of the solvent. As a result, only dotted patterns of polysilane remain on the substrate. This phenomenon of film formation is closely related to molecular forces between the substrate and solute in the solution. Therefore, I describe the molecular force itself and its influence on the formation of silicon films. The observed color change of a coated film during the conversion from polysilane to amorphous Si is interesting. The polysilane film is transparent because of its optical bandgap of approximately 6  eV, whereas that of an amorphous Si film is approximately 1.5 eV. Therefore, the bandgap decreases during heating of the film, leading to absorption of visible light. Strict control of the conversion process from polysilane to amorphous Si is quite important to obtain a semiconductor-grade film with few defects. The amorphous Si film thus obtained can be converted into a poly-

2  Guide to Silicon-Based Materials

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silicon film via solid-phase growth or laser annealing. With respect to devices, thin solar cells and thin-film transistors are fabricated using amorphous Si and poly-Si, respectively. In Chap. 5, I introduce a new Si film deposition method named liquid vapor deposition (LVD) of which path is illustrated at the left-hand side in Fig. 2.2. LVD is a kind of chemical vapor deposition (CVD) method that uses a liquid source such as CPS, polydihydrosilane, or silicon solution. The LVD process is also illustrated in Fig. 2.2. It can be conducted at ambient pressure. We happened to observe a thin film formed on a glass substrate through the deposition of vapor of liquid silicon while conducting a coating experiment of liquid silicon. The film thus deposited was found to be an amorphous Si film with high-density and good electrical properties. Since then, we have been carrying out a series of experiments on LVD, which has some advantages over conventional vacuum-based CVD using monosilane or disilane. We expect this method to add a new value to silicon technologies. In Chap. 6, some of liquid silicon family materials (LSFMs) shown in Fig. 2.1 are introduced as the part 1 of LSFMs. A good insulating SiO2 film can be prepared via oxidation of a polysilane film; thus, the oxidation process is critical to the formation of high-quality SiO2 films. A cobalt disilicide (CoSi2) film was formed using CPS and dicobalt octacarbonyl (Co2(CO)8). This film was developed for a self-­ aligned silicide (salicide) process designed to reduce the contact resistance of the source and drain areas via self-assembly in fabrication of MOSFETs. Instead of a conventional vacuum process, we used a liquid process for a salicide process and deposited excellent CoSi2 films by epitaxial growth. As a member of LSFMs, an aluminum precursor is introduced. We developed a liquid process for Al metal using

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2  Guide to Silicon-Based Materials

triethylamine alane, especially aiming to develop a selective deposition method. This method was based on the conversion method described in Chap. 1 as Type 4 in Case 2 (Fig.  2.1). We synthesized platinum nanoparticles as a reducing agent. Patterns of platinum nanoparticles were deposited prior to the deposition of aluminum. We confirmed that high-quality aluminum metal lines were deposited only on the pre-patterned areas. Next, as the part 2 of the liquid silicon family materials, liquid-processed SiC is introduced in Chap. 7. Although a silicon solution is a mixture of polysilane and an organic solvent, as previously described, the obtained amorphous silicon is hardly contaminated by carbon from the solvent; that is, polysilane never reacts with the selected solvent. Replacing the solvent with a different solvent, however, makes CPS react with the new solvent. We identified solvents that can react with CPS and synthesized SiC inks that we subsequently used to prepare a few kinds of amorphous SiC films. In Chap. 8 I describe developmental works to realize small-sized patterning for device fabrication. Three newly developed methods are introduced. The first one is a method for an area-selective deposition of polysilane patterns in silicon solution, which is based on the principle of Type 4 conversion in Case 2 shown in Fig. 1.1 in Chap. 1. In this method, a pre-patterned substrate with two kinds of areas, one of which absorb polysilane and the other of which do not absorb polysilane, was prepared and subsequently wetted by silicon solution, resulting in selective deposition of polysilane onto the absorbing areas. This method is viable for small-sized patterns up to tens of nanometers. The second method is direct drawing of Si lines using CPS vapor and an electron beam. When we irradiated an electron beam to CPS vapor spreading on the surface of a substrate positioned in a specially designed vacuum chamber, narrow silicon lines were deposited on the substrate, corresponding to the trace of the electron beam. Lines with a width of 100 nm were obtained. The third method is nanoimprinting of silicon. The thermogravimetry–differential thermal analysis (TG–DTA) thermogram for polysilane shows an exothermal decomposition peak at approximately 200 °C, which is lower than the temperature of the solidification peak (300 °C). Utilizing this partial decomposition at 200 °C enabled imprinting of polysilane at approximately this temperature. After demolding the imprinted patterns were annealed at 400 °C for solidification. A large shrinkage of 70–80% in volume occurred during annealing, which means the remaining volume was only 20–30% of the original volume. Interestingly, the aspect ratios of the patterns remained unchanged or even increased after the films were annealed at 400 °C. Moreover the imprinted patterns were hardly oxidized, keeping a pure silicon state. New devices using this method are expected. In the following chapters, I introduce device fabrication using liquid silicon. To make a device, we need several kinds of thin films upon request from the design and specification of the targeted device. Moreover the study parameters such as interface formation and patterning issues should be addressed. In this view device fabrication can be considered as multidiscipline of all the silicon thin-film technologies by liquid process. Two kinds of devices are introduced: solar cells and thin-film

2  Guide to Silicon-Based Materials

19

transistors (TFTs). In Chap. 9, solar cells from liquid silicon are introduced. A solar cell with a p–i–n structure was successfully fabricated by a liquid process using coated films or deposited ones by LVD. As the third topics, liquid silicon was also applied to make heterojunction back contact (HBC) solar cells. That is described in Sect. 9.3. In Chap. 10, a series of TFT developments using liquid silicon is described. TFTs with good properties were fabricated by liquid silicon as a channel material. This is a proof that quality of a Si film from liquid silicon is very competitive with that of solid silicon or from gas. A single-grained TFT whose channel was formed from liquid silicon was demonstrated to exhibit mobility as high as that of MOSFETs on a single crystal wafer. Excimer laser crystallization technique, which has been fully utilized to develop the abovementioned TFTs, was also applied for fabrication of TFTs on plastics or even on paper. TFTs on paper were realized using liquid silicon. That is introduced in Sect. 10.3. The fabrication of small-sized MOSFET by direct printing, which is a final target of this research, remained as a future research subject.

Chapter 3

Liquid Silicon

Abstract  Since Cyclopentasilane (CPS) is used as a raw material for all of the silicon-­based solutions needed for solar cells, thin-film transistors, and ultimately metal-oxide semiconductor field-effect transistors (MOSFETs), its basic properties are first introduced in this chapter. Concerning the production or conversion steps from CPS to Si films shown in Fig. 3.2, I introduce the production steps up to the formation of Si ink (i, p, n) in this chapter. How to synthesize the silicon solution and its physical properties are described. CPS can be polymerized by irradiation with ultraviolet (UV) light so as to form polydihydrosilane (polysilane) and then is dissolved into an organic solvent to form an intrinsic silicon solution or silicon ink. During polymerization under UV irradiation, doped silicon solutions of both n-type and p-type can be prepared by doping with phosphorus and boron, respectively. Keywords  Cyclopentasilane (CPS) · Polydihydrosilane · Si ink · Size-exclusion chromatography-multi-angle laser light scattering (SEC-MALLS) · Specific viscosity

3.1  CPS 3.1.1  Hydrosilanes and CPS It is well known that there are a series of linear hydrosilanes of which chemical formula is expressed as SinH2n+2 among hydrosilane compounds. The compounds having n ≥ 3 are liquid in the room temperature (Table 3.1). The compound which would be a candidate for use in liquid process is a compound with n ≥ 4, because trisilane (n = 3) is so volatile that it cannot be handled in the room temperature. However, linear hydrosilanes are generally very unstable even if n ≥ 4, so they are not adequate to be used in liquid process. Instead, cyclohydrosilanes which have a ring structure having a chemical formula of SinH2n are very hopeful in terms of stability. Among them cyclopentasilane (CPS) with n = 5 and cyclohexasilane (CHS) with n = 6 are the best materials to be used in liquid process because of their high stability and good availability, while either cyclotrisilane (n = 3) or cyclotetrasilane © Springer Nature Singapore Pte Ltd. 2019 T. Shimoda, Nanoliquid Processes for Electronic Devices, https://doi.org/10.1007/978-981-13-2953-1_3

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3  Liquid Silicon

22

Table 3.1  Boiling points of both linear hydrosilanes (SinH2n+2) and cyclohydrosilanes (SinH2n) Molecular formula SiH4 Si2H6 Si3H8 Si4H10 Si5H12 Si6H14

BP (°C) −111.9 −14.5 52.9 108.1 153.2 193.6

Molecular formula Cyclo-Si3H6 Cyclo-Si4H8 Cyclo-Si5H10 Cyclo-Si6H12 Cyclo-Si7H14

BP (°C) N/A N/A 195 202 N/A

Fig. 3.1  The synthesis rout of CPS. [5] (Copyright @ 2014 The Japan Society of Applied Physics)

(n = 4) is not stable to be isolated as single molecules and cyclohydrosilanes with n ≥ 7 are very difficult to be synthesized. CPS and CHS were first synthesized by Hengge et al. in 1973–1978 [1–4]. As for CPS, studies for its structure by a molecular orbital calculation and stability evaluation due to Si–Si σ-conjugated bonding have been reported after that time. We chose CPS as our raw material for liquid silicon considering its robustness in synthesis and availability of information about its physical properties. Following the method by Hengge et al., CPS can be synthesized by the three-step reaction shown in Fig. 3.1. This is called the Kipping method [5]. In our experiment, CPS has been synthesized by the same reaction as follows. Diphenyldichlorosilane was added to a stirred suspension of lithium in tetrahydrofuran, and the resulting mixture was stirred and poured into cold water. The resultant precipitate was collected by filtration, washed with water, dried in vacuum, and recrystallized to give a white powder (decaphenylcyclopentasilane). Its suspension and aluminum chloride in cyclohexane were mixed and stirred at room temperature with bubbling of dry hydrogen chloride gas to give decachlorocyclopentasilane. The remained hydrogen chloride was removed by N2 bubbling from the solution. The solution of decachlorocyclopentasilane in cyclohexane was added dropwise to a suspension of lithium aluminum hydride in diethyl ether at 0 °C, and the resulting mixture was stirred for 12 h and then filtered. The filtrate was concentrated, and the residue was distilled under reduced pressure to afford pure CPS as a colorless liquid. The properties of the CPS are listed in Table 3.2. Note that CPS has a relatively high boiling point, indicating a stable molecule to handle.

3.1 CPS Table 3.2  Properties of CPS

23 Molecular weight (g/mol) Boiling point (°C) Melting point (°C) Refractive index Specific weight Band gap (eV) Surface tension (mN/m) 1 H-NMR (ppm) 29 Si-NMR (ppm)

150 194 −10.5 1.691 0.963 6.5 eV 32.5 3.25 (TMS) 106.9 (TMS)

Fig. 3.2  CPS structure with geometries optimized with B3LYP/cc-pVTZ: planar (D5h), envelope (Cs), and twist (C2) [13]. (Copyright © 2012 Elsevier B.V)

3.1.2  Structures of a CPS Molecule In order to determine the exact structure of the CPS molecule, we have conducted a computational study using ab initio and density-functional methods [6]. The calculations were based on Hartree–Fock theory, Moller–Plesset perturbation theory, and density-functional theory (DFT). The geometrical structures of CPS were optimized within the given symmetry point group. After different structures of Si5H10 with different symmetries were analyzed, three stable structures of CPS were determined by our calculations, namely, planar (D5h), envelope (Cs), and twist (C2), which are shown in Fig. 3.2. The results of our calculations at all theoretical levels employed show that the twist structure is the most stable structure for CPS and that the two other structures of CPS, i.e., planar (D5h) and envelope (Cs), have similar energies at all to the theoretical levels employed (the difference between their energies is on the order of meV or less). The relative energies of each structure are summarized in Table 3.3. These results are in good agreement with the results of previous theoretical studies [7, 8].

3  Liquid Silicon

24 Table 3.3  Energy comparisons between twist structure and others (in meV) Twist Envelope Planar

Hartree-Fock 0 + 0.03 + 60.79

PBE(GGA) 0 + 0.01 + 61.20

B3LYP(HB) 0 + 0.03 + 53.65

MP2 0 + 0.03 + 60.58

Copyright @ 2014 The Japan Society of Applied Physics

Fig. 3.3  The LUMO and the HOMO of a CPS molecule: the red part is the positive part, and the blue part is the negative part. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article) [13]. (Copyright © 2012 Elsevier B.V)

3.1.3  Electronic Structure of Isolated CPS Molecule As in the case of other silane compounds, the delocalization of molecular orbitals (MOs), i.e., conjugation of σ orbitals [9, 10], is an important characteristic of CPS. Figure 3.3 shows the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) of CPS molecule. The LUMO appears similar to the lowest π-bonding orbital of the Si penta-ring, but it shows nodes at the center of Si–H bonds. This can, therefore, be seen as an antibonding orbital from conjugated π-orbitals of the penta-ring and s-orbitals of the hydrogen atoms in a CPS molecule. On the other hand, the HOMO is an antibonding molecular orbital from a σ-orbital of Si atoms with s-orbitals of a specific pair of hydrogen atoms. The shape of the LUMO strongly suggests a mode of interaction between MOs of two molecules in the liquid phase as the mutual delocalization of MOs [11, 12], in which the molecules that constitute the LUMO act as electron acceptors.

3.1.4  Interaction Between CPS Molecules Next, intermolecular interaction between CPS molecules was investigated in order to know the origin of relatively high boiling point of CPS compared with other molecules having same molecular weight (Table 3.4) [13].

3.1 CPS

25

Table 3.4  Melting point (MP) and boiling point (BP) data of certain silane compounds. [13]

Compound n-Si5H12 Iso-Si5H12 Neo-Si5H12 n-Si6H14 Neo-Si6H14 n-Si7H16 Cyclo-Si5H10 Cyclo-Si6H12

MP/°C −72.8 −109.8 −57.8 −47.7 −47.7 −30.1 −10.5 +16.5

BP/°C 153.2 146.2 130.0 193.6 193.6 226.8 194.3 226.0

Copyright © 2012 Elsevier B.V

Fig. 3.4  Typical minimum structures extracted from first-principle molecular dynamics [13]. (Copyright © 2012 Elsevier B.V)

A boiling point is closely related to the cohesive energy of molecules in liquid. In order to investigate the cohesive energy, all the typical configurations of a system that consists of two CPS molecules with minimum energy are searched using first-­ principle molecular dynamic simulations. The obtained configurations are shown in Fig. 3.4. These configurations were fully relaxed for reaching more accurate minimum energy configurations. The binding energies were calculated using Eq. (3.1):

∆E = 2 ⋅ E cps − E2 CPS



(3.1)

in which ECPS is the total energy of an isolated CPS molecule with the twist configuration and E2CPS is the total energy of the system consisting of two CPS molecules.

3  Liquid Silicon

26

Table 3.5  The binding energies [DE (eV)] and the equilibrium distances between the centers of mass of the two CPS molecules [De (Å)] [13]

A B C D

ΔE De ΔE De ΔE De ΔE De

LDA-VWN 6-311G** 0.481 4.19 0.346 4.48 0.208 6.77 0.247 6.11

Aug-cc-pvdz 0.581 4.16 0.356 4.5 0.265 6.78 0.311 6.11

Copyright © 2012 Elsevier B.V

The optimization calculations were carried out using LDA-VWA functional and two basis sets (6-311G** and aug-cc-pvdz) in Gaussian 03 package. The obtained results are summarized in Table 3.5, together with the equilibrium distance between two molecules. It was found that configuration A and configuration B are significantly more stable than the remaining configurations, as seen in Table 3.5. The binding energy and the equilibrium distance between the CPS molecules in configuration A are approximately 0.481 eV and 4.19 A, respectively. One can clearly see that the binding energies strongly depend on the number of Si–H bonds that are oriented toward the centers of the rings of the other CPS molecule. In the case of configuration A, two Si–H bonds of two CPS molecules are oriented toward the center of the ring of each molecule. In the case of configuration B, only one Si–H bond of a CPS molecule is oriented toward the ring of the other CPS molecule. In contrast, no Si–H bond is oriented toward the ring of another CPS molecule in the case of configurations C and D. From this result, we can tell that the Si–H bonds are oriented toward the center of the Si-Si ring of the other CPS molecule plays a crucial role in the interaction between CPS molecules. In order to gain an insight of the interaction between CPS molecules, electronic structures of these configurations were analyzed. The electron density distribution of two CPS molecules approaching each other was calculated by using Eq. (3.2):

∆ρ = ρ 2 cps − ( ρCPS1 + ρCPS2 )



(3.2)

where ρ2cpsis the electron density of the two interacting CPS molecules andρCPS1 + ρCPS2is electron densities of the two isolated CPS molecules. The cross sections of the electron density distribution of two interacting CPS molecules in configurations A, B, C, and D are shown in Fig. 3.5. For configuration A and configuration B, we can see that electron density decreases at the center of the Si–H bonds and increases in the area between the H atom of the Si–H bonds and the center of the penta-rings. The existence of such areas implies that a significant

3.1 CPS

27

Fig. 3.5  Cross section of the deformation of electron density distribution [13]. (Copyright © 2012 Elsevier B.V)

bi-­directional charge transfer between two CPS molecules has occurred. That shape suggests that there appears charge transfer from Si–H σ-bonding orbitals to the LUMO of CPS molecules. We can hence suggest that the interaction between two CPS molecules in which the Si–H bond of one molecule is oriented toward the center of the other CPS molecule constitutes a local bond and the Si–H bond acts as an electron donor to the other CPS molecule. The interaction between CPS molecules can be considered as “hydrogen bond” between Si–H bonds and penta-rings of CPS molecules when considered its stronger interaction energy around 0.5  eV than those of π proton acceptors or electron donors. The fact that the hydrogen bonds are induced by the Si–H bonds that are oriented toward the center of the ring of a CPS molecule can be inferred as the reason for the relatively high boiling points of CPS and cyclohexasilane (CHS) as compared to those of hydrosilanes with the comparable molecular mass. As shown in Table 3.4, the boiling points of n-Si5 H12 and iso-Si5H12 are 153.2 °C and 146.2 °C, respectively, while that of CPS is 194.3 °C. The same thing can be said in CHS. A similar tendency is observed for the melting point data in Table 3.4. This implies that the Si–H bonds oriented toward the center of the CPS ring may play a significant role in the interaction between CPS molecules and also between CHS molecules in their liquid and solid states.

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3  Liquid Silicon

3.2  Silicon Ink 3.2.1  Silicon Ink from CPS Si ink is a mixture of polydihydrosilane and an organic solvent. Cyclooctane is usually used as a solvent. CPS can be converted to polydihydrosilane by ring-opening polymerization, which occurs when UV light is irradiated to CPS. Figure 3.6 shows the optical absorption spectra of CPS just after distillation, CPS kept at 50 °C for 30 min after distillation, and polydihydrosilane. We usually use UV light of 365 nm wavelength; however, CPS just after distillation does not absorb light of 365 nm wavelength. Hence, it is interesting to investigate whether it is polymerized by the UV light that it cannot absorb. (This had been mystery for a long time.) We happened to observe that immediately after distillation, CPS is difficult to polymerize by UV light of 365  nm wavelength. In contrast, after some period kept at room temperature after synthesis, CPS can be polymerized at a much higher rate. We confirmed that a small amount of polydihydrosilane can be formed in CPS by thermal polymerization. As shown in Fig. 3.6, polydihydrosilane absorbs UV light of 365 nm wavelength. Figure 3.6 also shows that CPS kept at 50 °C for 30 min after distillation tends to absorb longer-wavelength UV light than CPS immediately after distillation. These facts suggest that CPS polymerization is a two-step process: (1) polydihydrosilane absorbs UV light, and its energy is transferred to CPS, and (2) the transferred energy is used for the ring-opening polymerization of CPS. This mechanism is just a presumption, and a more detailed study is needed. In order to determine the polymerization behavior, we characterized the molar mass distribution and structure of polydihydrosilane by size-exclusion chromatography (SEC) combined with multi-angle laser light scattering (MALLS) analysis and viscometry (SEC-MALLS-viscometry) [14]. The SEC-MALLS system, viscometer, and refractive index (RI) detector were connected in series by stainless steel tubes, as shown schematically in Fig. 3.7. An injector was set in the glove box. Readings of the excess Rayleigh ratio R(θ) at a scattering angle θ, specific viscosity ηsp, and RI response were acquired every 0.5 s during elution. A Shodex KF-805 column was used with a guard column attached to it. The molar mass (M) and root-­ mean-­square radius (radius of gyration, Rg) were estimated using a Guinier–Zimm Fig. 3.6 Absorption spectra of liquid Si materials: CPS just after distillation, CPS kept at 50 °C for 30 min after distillation, and poly(dihydrosilane). (Copyright @ 2014 The Japan Society of Applied Physics)

3.2  Silicon Ink

29

Fig. 3.7  Schematics of sample preparation and the SEC-MALLS-viscometry system. Within a glove box under an inert atmosphere (O2 concentration 0.5  ppm, dew point 75  °C), poly(dihydrosilane) was synthesized by the UV irradiation of CPS. The photographs indicate the change of the liquids from CPS to poly(dihydrosilane). The sample preparation and SEC-MALLS-­ viscometry measurement were conducted at 25 °C. (Copyright © 2012 Elsevier B.V)

plot. The intrinsic viscosity [η] was determined by calculating ηsp/c directly for each elution volume. Here, c is the concentration of the solute [polydihydrosilane]. For the analysis of a polymer structure, we adopted the sphere model, which is based on the viscosity theory of colloidal particles, to analyze the molar mass dependence of [η] [15]. In this model, [η] is described as



[η ] =

5 N A 4π 3 Rη , 2 M 3

(3.3)

where Rη is the viscosimetric radius and NA is Avogadro’s number. Starting from Eq. (3.3), we describe the scaling behavior of [η] vs M. Figure 3.8(a) shows the RI intensities for four samples synthesized with the UV irradiation times of 0, 30, 60, and 240 min. The sample subjected to no irradiation (0 min) corresponds to CPS, and the inverted peak at an elution volume of 13 mL marks the end of sampling. The RI chromatogram for CPS exhibits a sharp peak at an elution volume of 12 mL. The spectrum for the sample with the 30 min irradiation time exhibits a broad peak at the lower elution volume in addition to the weak CPS peak. For the sample with the 60-min irradiation time, the broad peak at the lower elution volume grows significantly, whereas the CPS peak disappears. This suggests the completion of CPS photopolymerization within 60 min of irradiation. The differential molecular weight fraction of polydihydrosilane is plotted as a function of the molar mass for the four samples in Fig. 3.8(b). The sample with the irradiation time of 0 min has a sharp peak at a molar mass of 150 g/mol, confirming the existence of CPS. Hence, the decrease in the intensity of the peak at 150 g/mol indicates how the molecular weight distribution spreads out when CPS is converted into polydihydrosilane during the first 60 min of irradiation. The molar mass for polydihydrosilane ranges broadly from 102 to 106 g/mol. The molar mass distribution changes significantly for the samples with irradiation times of up to 60 min; however, minor changes in the molar mass distribution are observed for the samples with irradiation times longer than 60 min.

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3  Liquid Silicon

Fig. 3.8 SEC-MALLS result for CPS and poly(dihydrosilane) synthesized with the UV irradiation times of 30, 60, and 240 min dissolved in cyclohexene at 25 °C. (a) RI intensity. (b) Molar mass distribution. (Copyright © 2012 Elsevier B.V)

3.2.2  Polymer Structure in Silicon Ink Here, we explore the structure of polydihydrosilane on the basis of the scaling behaviors of [η] and Rg. Logarithmic plots of [η] vs M and Rg vs M for the sample with the irradiation time of 240 min are shown in Figs. 3.9(a) and 3.9(b), respectively. These plots exhibit a scaling feature, which was fitted by the linear functions described in Eqs. (3.4a) and (3.4b):

[η ] = 0.414 M 0.206 ,

(3.4a)



Rg = 0.033 M 0.410 .

(3.4b)



The scaling relation [η] = KMα is known as the Mark–Houwink–Sakurada equation [16]. The data fitted by Eq. (3.4a) was in good agreement with the scaling relation. The solid line in Fig. 3.9(a) was fitted to [η] between 103 and 3 × 106 g/mol, whereas the solid line in Fig. 3.9(b) was obtained by fitting Rg between 1.099 × 106 and 4.457 × 106 g/mol. Here, we focus on the fitting of [η] vs M and interpret the scaling behavior on the basis of the sphere model [15]. Since [SiH2] (mass Mm = 30.102 g/mol) is considered to be a monomer unit of polydihydrosilane, the degree of polymerization (number of monomer units, N) of a polymer with molar mass M is given by N = M/Mm. Thus, the radius of gyration may be written as

Rg = aN v ,



(3.5)

3.2  Silicon Ink

31

Fig. 3.9  Log–log plots of (a) [η] vs M and (b) Rg vs M for poly(dihydrosilane) with 240 min irradiation time dissolved in cyclohexene at 25 °C. The solid lines in (a) and (b) represent the linear fits of [η] and Rg, respectively. (Copyright © 2012 Elsevier B.V)

where a denotes the “effective length” of [SiH2]. By substituting Eq. (3.5) into Eq. (3.3) and by using the parameter ρ = Rη/Rg, the Mark–Houwink–Sakurada equation of [η] = KMα is obtained, where α = 3ν–1 and the coefficient K is given by K=

10π 3

3

 ρa   ν  NA.  Mm 

(3.6)

According to the present sphere model, the radius of gyration exponent is estimated to be ν = 0.402 for α = 0.206. Since ν = 0.410 in Eq. (3.4b), the agreement between the two results suggests that the sphere model for the viscosity can be appropriately applied to polydihydrosilane with the monomer unit [SiH2]. If the polydihydrosilane has a straight-chain structure, a should be in the range from 0.5 for Flory theta solvent to about 0.8 in a good solvent [16]. However, the experimental value of 0.206 was much smaller than the cutoff value for a straight-chain polymer. Thus, we can presume that polydihydrosilane exists as a branched chain structure. With respect to the polymer shape, ρ = Rη/Rg gives a measure of the compactness of the molecule. By using Eqs. (3.4a) and (3.4b), ρ = 1.09 and ρ = 1.08 were determined for M = 1.099 × 106 and 4.457 × 106 g/mol, respectively. This also suggests that the polydihydrosilane has a particle-like compact shape. For ρ = 1.09, by using Eq. (3.6) and K = 0.414 mL/g in Eq. (3.4a), a was estimated to be 0.145 nm. This value is in good agreement with the bond lengths between Si and H in various molecules [17, 18]. It is concluded that polydihydrosilane has a branched structure with a particle-­ like compact shape in cyclohexene at 25 °C. The existence of a branch point (SiH)

32

3  Liquid Silicon

was also predicted and confirmed with using 1H-NMR and Fourier-transform infrared spectroscopy (FT-IR) [14].

3.2.3  Doped Silicon Inks 3.2.3.1  N-Type Silicon Ink N-type polydihydrosilane was synthesized as follows. White phosphorus was added as a dopant to cycropentasilane (CPS) and then heated at 60 °C to solve the dopant. After that the solution was irradiated by UV light with a wavelength of 405 nm and energy of 300 mW/cm2. Since reaction rate strongly depends on the dopant concentration, it was adjusted by changing the UV irradiation time between 5  min and 120 min. By doing so, doped polysilanes with white phosphorus concentration from 0.01 to 3  wt% were successfully synthesized. N-type silicon inks were made by adding cyclooctane solvent to the synthesized doped polysilanes, the content of which silicon ink ranged from 2 to 10 wt%. A n-type amorphous silicon film can be made by spin-coating a n-type silicon ink onto a substrate and heating the coated film at 390 °C for 30 min. One of the important technical issues in synthesizing n-type silicon ink was formation of dopant material, i.e., white phosphorus. We made it through pyrolytic decomposition of red phosphorus (Sigma-Aldrich Co. LLC.). Bulk red phosphorus was pulverized into powder, and then several gram of the powder was put into a round-shaped glass flask, which was set in a simple apparatus shown in Fig. 3.10 [19]. The red phosphorus was heated by a heating mantle at 430  °C for 6  h. Evaporated gas-phase phosphorus deposited on the inside surface of a glass cylinder around the zone which was cooled down by circulated water from outside. Deposited substance was white phosphorus. It was discharged from the glass cylinder by solving it using carbon disulfide solvent. After filtering it with a 0.2 μm filter, solvent was removed by nitrogen bubbling to obtain solid-phase white phosphorus, which is a transparent material. The white phosphorus thus obtained was used for synthesizing n-doped silicon ink. 3.2.3.2  P-Type Silicon Ink P-type polydihydrosilane was synthesized by adding decaborane to CPS, heating at 80 °C, and irradiation of UV light for polymerization. Like n-type doping described above, the reaction rate of p-type doping also depends on the dopant concentration; it was adjusted by changing the UV irradiation time between 10 min and 60 min. The wavelength and energy of the irradiated light were 365 nm and 15 mW/cm2, respectively. P-type polydihydrosilane having decaborane concentration from 0.01 to 1.5 wt% was synthesized and diluted by CPS to make p-type silicon ink with concentration from 2 to 10 wt%.

References

33

Fig. 3.10  The apparatus in which red phosphorus is transformed to white phosphorus

When p-type silicon ink is converted to an amorphous film, a coated polysilane film has to be packed in a closed container during heating to prevent boron atoms from evaporating from the film and being oxidized. Normally a condition of 390 °C for 30 min is used for fabrication of a p-type amorphous silicon film.

References 1. E. Hengge, G. Bauer, Angew. Chem. Int. Ed. 12, 316 (1973) 2. E. Hengge, in Plenary Lecture at the 5th International Symposium on Organosilicon Chemistry (Karlsruhe, 1978), p. 14 3. E. Hengge, G. Bauer, Angew. Chem. 85, 304 (1973) 4. E. Hengge, G. Bauer, Monatshefte fur Chemie 106, 503 (1975) 5. F.S. Kipping, J.E. Sands, J. Chem. Soc. Trans. 119, 830 (1921) 6. T. Shimoda, T. Masuda, Jpn J. Appl. Phys. 53, 02BA01 (2014) 7. V.S. Mastryukov, M. Hofmann, F. Schaefer III, J. Phys. Chem. A 103, 5581 (1999) 8. C.P. Li, X.J. Li, J.C. Yang, J. Phys. Chem. A 110, 12026 (2006) 9. H.S. Nalwa, Hand book of Photochemistry and photobiology, vol 1 (American scientific, Los Angeles, 2003) 10. J.R.G. Thorne, S.A. Williams, R.M. Hochstrasser, P.J. Fagan, Chem. Phys. 157, 401 (1991) 11. K. Fukui, Science 218, 747 (1982)

34

3  Liquid Silicon

1 2. A.E. Reed, L.A. Curtiss, F. Weinhold, Chem. Rev. 88, 899 (1988) 13. P.T. Lam, A. Sugiyama, T. Masuda, T. Shimoda, N. Otsuka, D.H. Chi, Chem. Phys. 400, 59 (2012) 14. T. Masuda, Y. Matsuki, T. Shimoda, Polymer 53, 2973 (2012) 15. G.R. Strobl, The Physics of Polymers (Springer, Heidelberg, 2007) 16. M.P. Stevens, Polymer Chemistry: an Introduction, 3rd edn. (Oxford University Press, Oxford, 1999) 17. Handbook of Chemistry and Physics 88th ed. (CRC Press, Boca Raton, 2008) 18. P.R. Schleyer, M. Kaupp, F. Hampel, M. Bremer, K. Mislow, J. Am. Chem. Soc. 114, 6791 (1992) 19. T.  Masuda, in Ph. D thesis of Graduate School and Faculty of Information Science and Electrical Engineering “A study on Silicon ink and its Technology for Thin Film Formation”, (Kyushu university, 2014)

Chapter 4

Thin Film Formation by Coating

Abstract  Here, the coating of silicon solution to form polysilane films and subsequent conversion of the polysilane films to amorphous Si films via pyrolysis are described. Preparing a uniform film of polysilane on a substrate is not straightforward. Although wetting of the silicon solution on a substrate such as glass, quartz, or silicon is adequate, a film once coated on the substrate tends to break during evaporation of the solvent. As a result, only dotted patterns of polysilane remain on the substrate. This phenomenon of film formation is closely related to molecular forces between the substrate and solute in the solution. Therefore, the molecular force itself and its influence on the formation of silicon films are described in 4.2 after general description of coating phenomena in 4.1. The observed color change of a coated film during the conversion from polysilane to amorphous Si is interesting. The polysilane film is transparent because of its optical bandgap of approximately 6  eV, whereas that of an amorphous Si film is approximately 1.5 eV. Therefore, the bandgap decreases during heating of the film, leading to the absorption of visible light. Strict control of the conversion process from polysilane to amorphous Si is quite important to obtain a semiconductor-grade film with few defects. The amorphous Si film thus obtained can be converted into a polysilicon film via solid-phase growth or laser annealing. This chapter corresponds to the production steps from Si ink (i, p, n) to amorphous Si film via coating and drying and pyrolysis shown in Fig. 2.2. Keywords  Molecular force · van der Waals interaction · Hamaker constant · Cauchy plot · Polydihydrosilane film

4.1  Coating Process and Molecular Forces Generally speaking, it is necessary to satisfy the two conditions to realize good solid film by coating solution: both wettability of the solution on a substrate and stability of the coated film during drying solvent have to be guaranteed. Everybody can understand no film can be formed without spreading of solution on a substrate, but the spreading of solution which makes a liquid film formation doesn’t always mean © Springer Nature Singapore Pte Ltd. 2019 T. Shimoda, Nanoliquid Processes for Electronic Devices, https://doi.org/10.1007/978-981-13-2953-1_4

35

36

4  Thin Film Formation by Coating

the formation of a good solid film. The total free energy ΔG including surface and interface free energies has to decrease from before to after the coating. The free energy can be evaluated by the surface (or interface) energy γ which is expressed as a sum of a van der Waals (vdW) interaction γvdW and a base–acid interaction γAB. In the following studies, we confirmed the vdW interaction is a dominant molecular one in the cases of CPS and polysilane compounds through evaluating their surface free energies based on the Lifshitz theory. Therefore, it is very useful to calculate vdW energies to evaluate or judge the coating ability of liquid silicon.

4.2  The Origin of Molecular Forces Here we introduce the study which clarified that the molecular forces of CPS and polysilane mainly come from the vdW interaction. The vdW force between macroscopic bodies can be written with a Hamaker constant according to the Lifshitz theory of continuum approximation. In order to calculate the Hamaker constant based on the Lifshitz theory, an accurate full spectrum of dielectric function is required. However, obtaining an accurate full spectrum is very complicated, so some approximation methods such as the Tabor–Winterton approximation (TWA) [1] and the simple spectral method (SSM) [2–4] have been proposed. Comparing these two methods, the TWA is simpler, and the SSM is more accurate especially when adapted to the materials having different ultraviolet absorption frequencies. For this reason, we used the SSM to estimate vdW interaction for CPS and polysilane together with many kinds of solvents. In the calculation of SSM, we aimed to obtain resonant frequency, function of oscillator strength, and static permittivity. Absorption frequency and function of oscillator strength in the ultraviolet region were measured by means of an optical measurement method. Static permittivity, absorption frequency, and function of oscillator strength in the infrared region were mainly referred to the data of the literature [5, 6] and were measured for only some materials including CPS and polysilane.

4.2.1  Theory of van der Waals Free Energy The van der Waals free energy per unit area W132(L) acting between Plate 1 and Plate 2, which are positioned on both sides of Material 3 at a distance of L, is represented as follows, according to the nonretarded Lifshitz theory [7, 8]:



W132 = −

A132 12π L2

(4.1)

4.2  The Origin of Molecular Forces

37

In this equation, A123 is a Hamaker constant in the above configurations, which is represented as follows:



A132 = −

∆ kj =



3kT 2

∞ ’ ∞

∑∑

( ∆13 ∆ 23 )

3

s3

n = 0 s =1

(4.2)

ε k ( iζ n ) − ε j ( iζ n )

ε k ( iζ n ) + ε j ( iζ n )

 2π kT ζn = n  

(4.3)

  

(4.4)

The prime for Σ means that the term for n  =  0 is multiplied by 1/2.  is the Planck constant, T is the absolute temperature, and k is the Boltzmann constant. The dielectric function of material ε(ω) is replaced by ε(iζn), which is obtained by mathematical consideration. Thus, vdW energy is represented by Hamaker constant A. The properties of material are correlated with the Hamaker constant A through ε(iζn) value. Then, a question would be risen. What is ε(iζn)? The answer is that ε(iζn) is related to the absorption spectrum of a material ε”(ω) which represents dispersion of energy, i.e., the origin of vdW energy. In general, dielectric function is represented as follows:

ε (ω ) = ε ′ (ω ) + iε ′′ (ω )

(4.5)



ε”(ω) and ε(iζn) are linked by Kramers–Kronig relation as follows [9]:

ε ( iζ n ) = 1 +

2 π

∞

χε ′′ ( χ )

0

χ2 +ζ 2

∫

dχ

(4.6)

Since it is difficult to obtain ε”(ω) in all the frequency bands, an approximate model of ε(iζn) is represented by Parsegian and Ninham as follows [4, 10]:



ε ( iζ n ) = 1 + ∑ j

dj 1 + ζτ j

+∑ i

fi ωi2 + giζ + ζ 2

(4.7)

ω is the natural frequency, g is the spectrum width, and d and f are a function of strength. The first term 1 is permittivity in vacuum. The first Σ represents rotational relaxation of polar material (Debye oscillator). Though it is important in the case of a material with high polarity such as water, the term can be ignored in the case of nonpolar material. The second Σ represents absorption in the infrared region and in the bands with higher frequency (Lorentz oscillator). The absorptions in the infrared region and in the ultraviolet region are caused mainly by molecular oscillation and

38

4  Thin Film Formation by Coating

electronic oscillation, respectively. In the case of a nonpolar material, Eq. (4.7) is simplified to Eq. (4.8) through ignoring absorption in the microwave range as follows: N

ε ( iζ n ) = 1 + ∑ i =1



Ci =

Ci

1 + (ζ / ωi )

2 fi π ωi

(4.8)

2

(4.9)



Equation (4.8) is known as a Parsegian–Ninham representation. Equation (4.8) is further simplified by assigning absorption frequency ω and function of oscillator strength C for the infrared region and the ultraviolet region that specify ε(iζn) to ε(iζ) [11].

ε ( iζ ) = 1 +

CIR

1 + (ζ / ωIR )

2

+

CUV

1 + (ζ / ωUV )

(4.10)

2



As shown in Eq. (4.4), the number of terms included in the infrared region is far small compared with those in the ultravioet region. In addition, CIR is smaller than CUV in the case of nonpolar substances. Accordingly, an appropriate equation, Eq. (4.11), can be obtained by ignoring the infrared region  in Eq. (4.10), as shown below [3, 12].

ε ( iζ ) = 1 +

CUV

1 + (ζ / ωUV )

(4.11)

2



For most liquids and oxides, the term for the ultraviolet region is significant, and ignoring the infrared region exerts little influence. The infrared term cannot be ignored, however, in cases of substances with strong molecular vibration, such as BaTiO3[11]. In this report, Eq. (4.10) is used, considering the importance of the infrared region. When a material has no absorption in the visual light range, the term ε”(ω) becomes zero and ε(ω) is expressed using refractive index n.

ε (ωvis ) = ε ′ (ωvis ) = n 2

(4.12)



Hough and White used this relation between refractive index and permittivity in order to obtain the two parameters ωUV and CUV that characterize absorption spectra in the ultraviolet region [2, 3]. By applying ζ  =  iω, Eq. (4.11) is represented as follows:

(

) ωω

n2 − 1 = n2 − 1

2

2 UV

+ CUV

(4.13)

4.2  The Origin of Molecular Forces

39

In the case of transparent substances in the visible light range, in the plot with the vertical axis of (n2–1) and the horizontal axis of (n2–1)ω2, (ωUV)−2 is obtained by its gradient, and CUV is obtained by its y-intercept. This is called the Cauchy plot. Thus, as a parameter of the spectra in a wide range of energy, the method of describing dielectric function based on the Lorentz oscillatory model is called SSM. In this study, ωUV and CUV were obtained by Eq. (4.13). By using Eq. (4.10), CIR is expressed as follows:

CIR = ε ( 0 ) − CUV − 1



(4.14)

This is a rough expression of CUV. The values for ωIR and ε(0) can be referred to in the literatures [5, 6]. We used the CUV values thus obtained. Finally, Hamaker constants were obtained according to Eqs. (4.2, 4.3 and 4.4) by applying these values.

4.2.2  Measurement of Refractive Index n Refractive index was measured by means of multiwavelength Abbe refractometers DR-M2 and DR-M4 manufactured by ATAGO. When the wavelength is 589 mm, DR-M2 can measure a refractive index of 1.3000–1.7100 and DR-M4 an index of 1.4700–1.8700. The refractive index was measured at wavelengths of 450, 480, 520, 546, 589, 644, 720, and 1000 μm at a temperature of 20 °C. Static permittivity was measured by means of a surface acoustic wave (SAW) solution sensor that was jointly developed by Japan Radio Co., Ltd., Riso Kagaku Corporation, and SAW&SPR-Tech [13]. Absorption spectra of IR were measured by ALPHA which is the product of Bruker Optics. Since CPS and polysilane are intensively oxidized in air, they were measured in a glove box made by Miwa Seisakusho Co., Ltd. Oxygen concentration was 0.5 ppm or lower, and the dew point was 70 °C or lower.

4.2.3  Molecular Forces of CPS and Silicon Compounds Figure 4.1 shows the Cauchy plots of CPS and polydihydrosilane. These kinds of plots gave the ultraviolet absorption frequencies (ωUV) of several materials including silicon molecules shown in Table 4.1. It is generally said that the saturated compounds had higher absorption frequencies than those of compounds having unsaturated bonds: ωUV tended to become smaller as electrons became delocalized or as conjugation lengths became longer. In this context, it is interesting to see the ωUV of silicon molecules. Polydimethylsiloxane showed a value close to those of saturated compounds. In contrast, the ωUV values of CPS and polydihydrosilane were smaller than those of saturated compounds and rather close to those of the unsaturated compounds. Since Si–Si bonds are proven to be σ-conjugates [14], the reason that the

40

4  Thin Film Formation by Coating

Fig. 4.1  Cauchy plots of CPS and polydihydrosilane. (Copyright © 2009 Elsevier Inc. All rights reserved)

Table 4.1  Structures and ultraviolet absorption frequencies of several compounds

Name/structure Octane

ωUV (rad/s) 1.86 × 1016

Decane

1.85 × 1016

1,4-Hexadiene

1.53 × 1016

1.3-Hexadiene

1.34 × 1016

Decalin

1.84 × 1016

Methylnaphthalene

1.12 × 1016

Polydimethylsiloxane

1.78 × 1016

Si

O

CPS

n

H2 Si

H2Si H2Si

1.13 × 1016

SiH2

SiH2

Polydihydrosilane

H2 Si

Si H2

n

Copyright © 2009 Elsevier Inc. All rights reserved

9.93 × 1015

4.2  The Origin of Molecular Forces

41

Table 4.2  Calculated nonretarded Hamaker constants (unit: kT, T = 293) Material 1 Octane Decalin CPS Polydihydrosilane Polydihydrosilane Polydihydrosilane Polydihydrosilane Polydihydrosilane

Medium Air Air Air Air Decalin CPS Decalin CPS

Material 2 Octane Decalin CPS Polydihydrosilane Polydihydrosilane Polydihydrosilane Quartz Quartz

AHamaker 11.12 15.13 16.46 16.57 1.26 0.07 −0.07 −0.21

Copyright © 2009 Elsevier Inc. All rights reserved

Table 4.3  Comparison of calculated/measured surface tensions (unit: mN/m)

Octane Decalin Polydihydrosilane CPS

Calculated 21.9 29.9 33.0 32.5

Measured 21.4 30.5 32.3 31.6

ωUV values of CPS and polydihydrosilane are relatively small can be explained by energetic stabilization due to σ-conjugates. Now we can calculate the nonretarded Hamaker constants (A132) in materials based on the measured values of ωUV, CUV, ωIR, CIR, and ε(0). Table 4.2 shows the examples of the nonretarded Hamaker constants (A132) in some material combinations. It is found that when air is a medium Hamaker constant, A101 is getting higher compared with when liquids are media. Hamaker constant A101, in turn, can be used to calculated surface energy using the equation below.

γ =

A 101 24π D02

(4.15)

In this equation, γ (mN/m) is a surface tension, and D0(m) is a cutoff distance. Israelachvili proved that the calculated value and the measured value were in agreement when D0 was set to 0.165 nm in an experiment using carbon solvents [15]. Accordingly, this set value of 0.165 nm was used in our calculation as well. Table 4.3 shows the calculated values when D0 was 0.165  nm and the measured values of surface tension. The calculated values and measured values also conformed in the cases of CPS and a silicon compound. The good coincidence between the measured and calculated values of the surface tensions clearly shows that the molecular forces of CPS and polydihydrosilane are mainly expressed by vdW interaction.

42

4  Thin Film Formation by Coating

4.3  Coating of Si Ink 4.3.1  General Remarks on Si Ink Coating The formation of a film having homogeneous thickness is not straightforward. Immediately after Si ink is spin-coated, the solvent starts to evaporate, leaving behind the solute [polydihydrosilane in this case] on a substrate. During this stage, various types of dried solid patterns can be generated depending on the conditions. In some cases, a pattern such as a coffee stain forms, whereas in others, a multi-­ lined or dotted pattern appears. Uniform film formation is not always guaranteed. Whether or not a good film is formed strictly depends on the behavior of a solute substance on a substrate during drying. To clarify the phenomenon, we investigated the stability of polydihydrosilane films on solid substrates by comparison between the observed micrographs at room temperature and the calculated values of the Hamaker constant (AALS) for an air/liquid/solid substrate. To efficiently compare the observations with AALS values, we selected five types of SiO2-based substrates (quartz, glass, and optical glasses A, B, and C) and a TiO2 substrate. Since the evaluation of the Hamaker constant is essential for determining the stability of polymer films, we adopted a simple spectrum method (SSM) [3, 16], which is adequate for a system composed of materials with different resonance frequencies, such as solids and liquids. We measured the optical parameters of several substances by an Abbe refractometry, ellipsometry [17], and FT-IR to determine the optical parameters for the use of the SSM.

4.3.2  Observations of Liquid Films The formation of Si ink is as follows. The wavelength, intensity, and irradiation time were 365 nm, 1 mW/cm2, and 60 min, respectively. Under these conditions, CPS was completely converted to polydihydrosilane with a broad molar mass distribution (MW = 102–106 g/mol). Then it was dissolved in cyclooctane at 1.5 wt %. The film was coated on a solid substrate by spin-coating at 2000 rpm for 30 s. The resultant film thickness was 40 ± 5 nm. All processes were carried out in a glove box, where the oxygen concentration and dew point in the glove box were less than 0.5 ppm and − 75 °C, respectively. First, the coated liquid film was allowed to stabilize overnight at room temperature in the glove box; then, only the film surface was oxidized slowly to freeze the pattern in the film and deactivate it. The resulting film, which was regarded as a polydihydrosilane film covered with a thin SiO2 layer, was taken out into the atmosphere, and the films were observed using an optical microscopy and scanning probe microscopy (SPM). Figure 4.2 shows the micrograph of the polydihydrosilane films coated on six types of substrates [18]. The AALS value calculated by the SSM is noted on each image. Two types of prominent features, i.e., dot arrays and continuous figures, are

4.3  Coating of Si Ink

43

Fig. 4.2  Optical micrographs of poly(dihydrosilane) films on different substrates: (a) quartz, (b) glass, (c, c’) optical glass A, (d) optical glass B, (e) optical glass C, and (f) TiO2 (c) and (c’) images at different points on the same film. The AALS values calculated by the SSM are noted on each micrograph. The length of the bar in each image is 100 μm. (Copyright © 2012 Elsevier B.V) Fig. 4.3  AFM image of a single dot on the glass substrate in Fig. 4.1(b). The height and width of the dot are 1.31 and 18.45 μm, respectively. (Copyright © 2012 Elsevier B.V)

observed in Fig. 4.2. A large number of cyclic dots in the liquid films on the quartz [Fig. 4.2(a)] and glass substrates [Fig. 4.2(b)] indicate film rupture. The separations among the dots in the film on glass were slightly longer than those among the dots in the film on quartz. The film on optical glass A showed two types of patterns: a patch-like configuration of dots in Fig. 4.2(c) and a continuous pattern in Fig. 4.2(c’). In the present case, the two types of patterns coexisted in the same film. The images of the continuous films on optical glasses B and C and TiO2 [Figs. 4.2(d)–(f)] were observed with high reproducibility. The profile of a single dot in the array was measured by SPM.  Figure  4.3 shows a three-dimensional image obtained by atomic force microscopy (AFM) for a typical dot on the glass substrate shown in Fig. 4.2(b).

44

4  Thin Film Formation by Coating

4.3.3  Hamaker Constant and Coating Property We calculated the Hamaker constants based on the SSM method. The stability of the liquid films is determined by the free energy per unit volume G(L) = γSL + γLA + P(L), where L is the film thickness; γSL and γLA are the interfacial free energies of the solid/ liquid and liquid/air boundaries, respectively; and P(L) is the intermolecular interaction. When L is less than approximately 1 μm, the energy term arising from gravity can be ignored. For low-polarity materials, P(L) is dominated by van der Waals energy: P(L) = WALS. The WALS in the nonretarded Lifshitz theory [7, 15] is given by



WALS =

AALS 12π L2

(4.16)

According to this formula, a liquid film with AALS > 0 is stable, and that with AALS  0 showed a continuous figure, while the unstable films with AALS 1000 Ωcm) were used for Fourier-transform infrared spectroscopy (FTIR) measurements, and non-alkali glass substrates (OA-10, Nippon Electric Glass) were used for optical and electrical characterization. CPS was synthesized according to the procedure of Hengge et al. [2, 3] and was used for deposition of i-Si film. Decaborane and white phosphorus were dissolved in CPS at concentrations up to 1 and 3 wt.%, respectively, and were used for deposition of doped a-Si:H films. In order to know vaporization characteristics of CPS, the thermal properties of liquid CPS were examined using thermogravimetry (TG)/differential thermal analysis (DTA) (EXTAR 6200, Seiko Instruments). Approximately 10 mg of CPS was put into a sample pan. To prevent oxidation as much as possible during the measurement, purge gas was purified using a gas purifier (Pureron Japan) to obtain oxygen concentrations  0.24. The shapes of these two plots resemble each other, except for their quantity. The incorporated carbon in the films for X > 0.24 contributes little to the formation of Si–C and C–H2,3 bonds. The NSi–H increases from 4.9 × 1021 cm−3 for the film with X = 0 to 7.1 × 1021 cm−3 for the film with X = 0.67. Clearly, the films contain many hydrogen atoms in the Si–H and C–H2,3 configurations. As shown in Fig. 7.13, the incorporation efficiency of carbon into the films was changed at X = 0.25. However, when hexene (sp2 carbon) is employed as a carbon source, the Si–C bonds in the resultant film increase linearly until X = 0.47, at which point the rate of increase falls. Critical X values of 0.47 for hexene (sp2 carbon) and 0.25 for 1-hexyne (sp carbon) were observed.

108

7  Liquid Silicon Family Materials(2): SiC

Fig. 7.13  Bond density of Si–C (closed circles), Si–H (closed triangles), and C–H (opened circles) in a-SiC films prepared using SiC-inks with various X values. These values were estimated from the total areas of the corresponding peaks appearing in Fig. 7.2 using Eq. (7.1). The integrated intensities of peaks appeared at 760, 2100, and 2850–2950 cm−1 in Fig. 7.2 were employed to estimate the bond densities of Si–C, Si–H, and C–H, respectively. Only for the film with X = 0 (i.e., a-Si film), the peak which appeared at 640 cm−1 in Fig. 7.12 was used for estimating the bond densities of Si–H. (Copyright © 2016 Elsevier B.V. All rights reserved)

Assuming that the C=C bond in hexene reacts with the CPS at a 1:1 molar ratio via hydrosilylation, the number of Si–C bonds in the PSH should increase until the number of moles of C=C equals that of CPS.  An equimolar ratio is achieved at X = 0.50. This value agreed well with the experimental X value of 0.47. Similarly, if the C ≡ C bond in 1-hexyne reacts twice with CPS via hydrosilylation, the number of Si–C bonds in the PSH should increase until the number of moles of 1-hexyne equals half that of CPS, i.e., X = 0.33, which is close to the experimental X value of 0.25. If the hydrosilylation completely progresses, the Si–C bonds in the resultant film should be saturated over the critical X value (0.33). However, it gradually increases even in the film having a higher X value. This result suggests the possibility that hydrosilylation at the critical X value does not sufficiently progress under the reaction conditions of this study. Thus, the temperature and pressure herein should be optimized to promote the hydrosilylation. In summary, the carbon content and the number of Si–C bonds in the resultant film were controlled by the incorporation of sp/sp2 carbon in carbon sources, as well as the composition ratio of carbon and silicon sources in the PSH.  However, as described below, the maximum concentration of the carbon in the films was  0.25. A range of the Cfilm from 0 to 37 at% was obtained. The X dependencies of Cfilm were consistent with those of NSi–C and NC–H in Fig. 7.13. Figure 7.15a, b shows the spectra of Si 2p and C 1 s bands, respectively, for the films prepared using SiC-ink with various X values. Each band was fitted by Gaussian curves, as shown in the plots by solid lines. The chemical shifts of these peaks were corrected using a Si–Si bond energy of 99.5 eV. The bands for an a-Si film are also shown for comparison in the figure as X = 0. All the spectra have similar peak components but with different relative intensities. Regarding Si 2p, the peaks split into two components at binding energies of 99.5 and 100.3 eV because of the coexistence of Si–Si and Si–C bonds, respectively, indicating the incorpora-

110

7  Liquid Silicon Family Materials(2): SiC

Fig. 7.15  XPS bands of the Si 2p and C 1 s for a-SiC films prepared using SiC-inks with various X values. The closed circles and solid lines represent experimental and fitted data, respectively. Gaussian curves were employed for the fitted line. (Copyright © 2016 Elsevier B.V. All rights reserved)

tion of carbon into the silicon network as Si–C bonds for the films with higher X values. The intensity of the Si–C peak at 100.3 eV increases until X = 0.25 and was saturated at X > 0.25. This is consistent with the FTIR result, as shown in Figs. 7.12 and 7.13. As for C 1 s, the peaks also split into two components at binding energies of 282.4 and 283–283.5 eV, which is attributed to C atoms as sp3 C–Si and sp2 C=Si, respectively [38]. No peaks appear at 284–285 eV related to the C–C bonds in the films, despite the presence of many C–C bonds in PSH. This disappearance of C–C bonds in the films is very interesting, and the reason is still unclear. The XPS results suggest that all of the carbon was incorporated as C–Si or C=Si in the film. The C–C bonds in pendant hexyl groups in PSH would be cleaved by thermal decomposition until 400  °C, followed by evaporation as carbon gasses, leaving behind C–Si bonds in the film. The remaining carbon was incorporated into the Si network as C–Si or C=Si bonds through cross-linking at 400 °C. Indeed, many sp3 C–C bonds were detected for the film pyrolyzed at lower temperature [35]. The structural changes that occur during the pyrolysis of PSH are important issues for our material. The binding energies of Si atoms in crystalline silicon as Si–Si and Si–C are 99.0 and 101.4 eV, respectively [39]. Each peak in Fig. 7.15 deviates from the reported values. These shifts result from the change of the environs around the atoms. The films have many hydrogen atoms, as evidenced by the large peaks at 640 and 2850– 2920 cm−1 in Figs. 7.12 and 7.13, in which hydrogen atoms are incorporated in the form of Si–Hn and C–Hn. Since the electronegativity of C and H atoms is larger than that of Si atom, the binding energy of the Si–Si bond in obtained film shifts to higher energy compared with that in crystalline Si. Similarly, the smaller electronegativity of Si and H than that of C results in a shift of the binding energy of the Si–C bond to a lower energy compared with that of crystalline SiC. Based on the FTIR and XPS results, the structure of our a-SiC could be estimated to be as shown in Fig. 7.16. Many hydrogen atoms are incorporated in the form of

7.2 Correlation of Si/C Stoichiometry Between SiC-Ink and a-SiC Film

111

Fig. 7.16  Schematic of the structure of a-SiC film. The structure was described as a disordered a-Si network, in which many hydrogen atoms were incorporated in the form of Si–Hn/C–Hn entities, and carbon atoms are in an sp3/sp2 C–Si/C=Si configuration. (Copyright © 2016 Elsevier B.V. All rights reserved)

Si–Hn/C–Hn entities, and carbon atoms are in an sp3/sp2 C–Si/C=Si configuration. To verify the structure of the films further, Raman spectroscopy was employed [15]. The typical phonon bands of a-Si were observed at 150 and 480 cm−1, which are attributed to first-order scattering by transverse acoustic and transverse optical bands, respectively. The broad energy tail was also observed in the range of 650– 1700 cm−1, corresponding to the mode of tetrahedrally connected Si–C bonds [36]. Moreover, none of the films had a G band at 1540 cm−1 because of the lack of graphitic carbon [28]. The Raman measurement results supported the proposed structure in Fig. 7.16. In the case of conventional polycarbosilane-derived a-SiC, the Si–C bonds come from the –Si–C– backbone, resulting in 50 at% of carbon as the lower limit inevitably. With regard to PSH-derived a-SiC, the most Si–C bonds stem from the branch site where pendant groups bond. Therefore, lower carbon content was easily achieved by changing the number of pendant groups in PSH.

7.2.4  Optical and Electrical Properties of an a-SiC Film Figure 7.17 shows the optical absorption coefficients of the films prepared using SiC-inks with various X values. The data are depicted in a Tauc plot. The intercept to the horizontal axis and the slope of the fitted line gives Eopt = 1.68, 1.86, 2.06, 2.28, and 2.39 eV and B parameter = 7.2 × 105, 6.1 × 105, 4.7 × 105, 4.1 × 105, and 2.6 × 105 eV−1 cm−1, for the films with X = 0, 0.07, 0.25, 0.47, and 0.67, respectively. The B parameter, which is a constant related to the band edge width [40], for obtained film with X = 0 was very close to that previously reported for a-Si film [41], and it decreased with increasing X values.

112

7  Liquid Silicon Family Materials(2): SiC

Fig. 7.17  Tauc plots of the a-SiC films prepared using SiC-ink with various X values. Open circles and thin lines are experimental data and fitted lines, respectively. Eopt values were obtained from each extrapolation line. (Copyright © 2016 Elsevier B.V. All rights reserved)

Table 7.2  Valence band Ev for a-SiC films prepared using SiC-inks with various X values. The data of Ev were measured by photoemission spectroscopy X value EV (eV)

0 −4.90

0.07 −5.03

0.25 −5.26

0.47 −5.51

0.67 −5.57

Copyright © 2016 Elsevier B.V. All rights reserved

Knief et al. reported calculated data of the density of states near the Fermi level of a-SiC, in which the number of states near the valence-band maxima and conduction-­band minima decreases with increasing carbon concentration because of the replacement of Si–Si bonds with stronger Si–C bonds [42]. The results of the abovementioned study suggest that the incorporated carbon (Si–C bonds) induces the shift of Ev and conduction band energy (Ec) toward the lower- and higher-energy sides, respectively. Table 7.2 shows Ev for a-SiC films prepared using SiC-inks with various X values. In these films, the incorporated carbon caused a shift of Ev toward the lower-energy side. Although we don’t make sure the precise position of Ec, with the measurements employed, the energy level of Ec would shift toward the higher-­ energy side. Note that the approximation Ec = (Ev + Eopt) underestimates the Ec values because of tail effects [43]. Figure 7.18 shows the σd (dark conductivity) and σp (photoconductivity) of the a-SiC films at room temperature as a function of the X value. The conductivity for a-Si film is also shown as a reference. When compared with a-Si, our a-SiC exhibits a very sharp drop in σp. The conductivities decrease from σp  =  3.0  ×  10−9 and σd = 3.1 × 10−11 S/cm for the film with X = 0.07 to σp = 8.1 × 10−11 and σd = 1.6 × 10−12 S/cm for the film with X = 0.67. The photosensitivity (σp/σd) of our film is more than two times smaller than that of a conventional film [38, 39]. As a possible explanation for the poor σp, we consider the correlation between the B parameter and σp. The B parameter is mainly related to the tail states and the band edge of extended states, while σp is related to the gap states in addition to the

7.2 Correlation of Si/C Stoichiometry Between SiC-Ink and a-SiC Film

113

Fig. 7.18  Dark conductivity (σd) and photoconductivity (σp) for a-SiC films as a function of X value. The conductivities were measured at room temperature. An AM-1.5G solar simulator with an intensity of 100 mW/cm2 was used to determine the σp. (Copyright © 2016 Elsevier B.V. All rights reserved)

tail states and band edge states [44]. Although the origin of the gap states is different from that of the tail states, Sakata et al. demonstrated that the σp and B parameter are strongly related to each other [45]. The tail states come from the structural fluctuations and the inhomogeneity of the composition. The inhomogeneous nature of a-SiC contains many heterojunction interfaces. Beyer et  al. suggested that the mobility of conduction electrons is reduced by the interface scattering [46]. We interpreted that the existence of a number of CHn entities in a Si network degrades the conductivities. The structural fluctuations and the inhomogeneity of the composition may stem from the variety of hydrogen configurations around C atoms (CH, CH2, and CH3) (Fig. 7.16). In addition, the CH3 groups in a Si network enhance the possibility of forming nearby silicon dangling bonds [47]. The Si network should contain many dangling bonds because the network structure is constructed via dehydrogenation of the –(SiH2)– skeleton. The dangling bonds act as recombination centers for photogenerated carriers. Therefore, there is still room for the improvement of electrical properties in PSH-derived a-SiC film by process optimization and post-hydrogenation. Indeed, reducing the dangling bonds by post-­ hydrogenation process is effective for improving the electrical properties for the film prepared using SiC-ink with X = 0 [48].

7.2.5  Conclusion In the present study, we synthesized PSH with varying Si/C stoichiometry and clarified the correlation of Si/C stoichiometry in PSH and resultant a-SiC films. The carbon content in the films linearly increased until 30 at%, following which the incorporation efficiency of the carbon was inhibited by more than 30 at%. Consequently, PSH provides silicon-rich a-SiC. The excess carbon that did not participate in C–Si configurations was decomposed and was evaporated during pyrolysis. The FTIR and XPS results provided insights into the structural properties of the a-SiC film. The film contained considerable hydrogen atoms in the form of Si– Hn/C–Hn entities. As for electrical properties, the decrease of B parameters for the

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film with higher X values suggests that large structural fluctuations, inhomogeneity, and formation of silicon dangling bonds may be induced by the CHn entities, giving poor conductivities. Further work appears necessary to improve the PSH-­derived a-SiC with the aim of fabrication of a-SiC electronics via the solution route.

7.3  n-Type a-SiC by Coating We synthesize a polymeric precursor for fabricating n-type semiconducting SiC. The polymeric precursor is phosphorus-doped (n-type) polydihydrosilane with pendant hexyl groups (n-PSH). n-PSH transforms to a-SiC or polycrystalline SiC when heated under nitrogen gas at approximately 400 °C or 1000 °C, respectively. The optical, structural, and electrical properties of the a-SiC films were characterized as functions of the mixing ratio of Si, C, and P in the polymer structure [49].

7.3.1  Polymer and Film Preparation and Their Analyses Phosphorus-doped a-SiC films were obtained through pyrolytic transformation of n-PSH. The polymer was synthesized as shown in Scheme 7.2. Cyclopentasilane (CPS) and white phosphorus were synthesized as previously reported [15, 16]. The CPS was n-doped by dissolving into it appropriate amounts of white phosphorus before polymerization [17]. Then the silicon skeleton in the CPS was functionalized by hydrosilylation [18]. The carbon content in n-PSH is defined as the volume ratio (denoted by X) of [1-hexyne]/[CPS + 1-hexyne]. The resultant n-PSH is a transparent, low-viscosity liquid. The molar mass of n-PSH was determined by SEC. FTIR

Scheme 7.2  Synthesis of n-type polydihydrosilane with pendant hexyl groups (n-PSH) from a mixture of CPS, white phosphorus, and 1-hexyne. The photograph is n-PSH. (Copyright © 2016 AIP Advances)

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measurements were carried out for n-PSH and a-SiC films. The pyrolytic transformation from n-PSH to a-SiC was investigated using TG-DTA. A 20–30 vol% solution of n-PSH in cyclooctane was spin-coated at 2000 rpm for 30 s onto a substrate. The n-PSH films were then transformed into a-SiC films by heating them at 400 °C for 15 min on a hot plate. The bond structure of the films was characterized by FTIR. The optical gap Eg was determined from transmittance and reflectance data obtained with FilmTek 3000. The chemical bonding of the films was characterized by XPS. The cross-sectional image of the films was observed by SEM.  The resistivity and dark conductivity were measured using Al-comb electrodes deposited on the a-SiC films. The lateral current between neighboring Al electrodes was measured from 20 °C to 120 °C to make an Arrhenius plot for estimating the activation energy Eact.

7.3.2  Polymer Analysis The viscosity of the mixture of CPS, white phosphorus, and 1-hexyne gradually increases on optical irradiation with heating at 50 °C. The resultant viscous liquid was assumed to be n-PSH and was analyzed by SEC and FTIR. Figure 7.19 shows refractive-index chromatograms of n-PSH with X = 0.2 and of CPS as a reference. The chromatogram of n-PSH exhibits a broad peak at a lower elution volume, whereas that of CPS exhibits a sharp peak at an elution volume of 12 mL. The relative molar mass of the n-PSH to polystyrene was 1000–7000 g/mol. The chemical structure of n-PSH with X = 0.2 and with 2 wt% phosphorus concentration was analyzed based on the FTIR absorbance spectra (Fig.  7.20). As a reference, the spectrum of undoped PSH and polydihydrosilane –(SiH2)n– is also shown. The major bands in the polymer spectra are at 570, 700, 850–900, and 2080 cm−1, which are assigned to the Si–Si wagging, Si–Si rocking, Si–H2 bending, and Si–Hn stretching modes, respectively [13]. The spectra of n-PSH and undoped PSH are essentially the same except for the intensity ratio of split peaks at 550, 700 and 850, and 900 cm−1, which is due to the different molar masses of the two materials. In contrast with the polydihydrosilane, PSH has characteristic peaks related to carbon at 760 and 2850–2950 cm−1, which Fig. 7.19 Refractive-index chromatogram of n-PSH and of CPS as measured by SEC. (Copyright © 2016 The American Ceramic Society)

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Fig. 7.20  FTIR spectra obtained in attenuated-­ total-­reflectance mode for (a) n-PSH, (b) undoped PSH, and (c) polydihydrosilane. (Copyright © 2016 The American Ceramic Society)

Fig. 7.21  TG (bold solid line), DTA (thin solid line), and DTG (dashed line) curves of n-PSH as functions of temperature. The measurements were performed in a nitrogen atmosphere. (Copyright © 2016 The American Ceramic Society)

are attributed to the Si–C stretching and C–H2,3 stretching modes, respectively [14, 15]. The appearance of these new peaks in PSH indicates that pendant hexyl groups are incorporated by hydrosilylation between the silicon skeleton and C ≡ C bonds. Thermal analysis illuminates pyrolysis process of the n-PSH. Figure 7.21 shows the TG, DTG, and DTA signals as functions of temperature. The TG signal indicates a weight reduction of 54% when the n-PSH reaches 400 °C; consequently, 46% of the mass remains as nonvolatile material. The DTA signal shows an endothermic peak at 180 °C due to the evaporation of volatile materials such as nonpolymerized materials and polymer fractions generated by the thermal cleavage of the Si–Si framework [16]. The upward peak at 380 °C is attributed to cross-linking reactions. A Si–Si framework is easily cleaved due to the lower bond energy of Si–Si bonds compared with those of other bonds such as Si–C, Si–H, C–H, and C–C [17]. The polymer transforms into a cross-linked amorphous network due to cleavage and recombination of the Si–Si and Si–C bonds above 180  °C.  The n-PSH is almost completely cross-linked at 380  °C, as evident from the DTA peak at this temperature.

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7.3.3  Thin-Film Formation Figure 7.22 is a cross-sectional SEM image of a spin-coated a-SiC film prepared using n-PSH with X = 0.2. The 250-nm thick film was formed on a glass substrate using 30 vol% n-PSH solution. The upper left-hand side is a photograph of half-area (1 cm × 2 cm) films prepared using n-PSH with X = 0.2. The film with X = 0, that is, phosphorus-doped amorphous silicon (a-Si), is also shown as a reference. These films were prepared using 20 vol% n-PSH solution, leading the thickness of 100 nm. These images reveal the significant flatness of the film surface. The incorporation of carbon into the films and the formation of Si–C bonds were analyzed by XPS. The carbon and oxygen contents were estimated by integrating the intensities of the Si 2p, C 1 s, and O 1 s bands, considering their relative sensitivity factors. The carbon contents of the film prepared using n-PSH with X = 0.1, 0.2, 0.4, and 0.6 were 16, 30, 32, and 36 at.%, respectively, and the oxygen content of all films was  0.2. The carbon incorporation for X > 0.2 contributed little to the formation of Si–C bonds. The concentration NSi–H for all films remained constant at 5–7 × 1021 cm−3, because all samples were processed at the same temperature. Si–H bonds form as a result of the dehydrogenation of n-PSH. In order to know why NSi–C becomes constant for X > 0.2, we must consider the origin of Si–C bonds in n-PSH.  These bonds are generated by hydrosilylation between CPS and the C  ≡  C bond in 1-hexyne (note that 1-hexene also induces hydrosilylation using C=C bonds). However, hexane reacts very poorly with CPS. These results suggest that unsaturated (sp/sp2) carbon is necessary to synthesize n-PSH. Indeed, few Si–C bonds are detected in films prepared with hexane as a carbon source. Heating and light irradiation of hydrosilane result in cleavage of the relatively weak intrachain Si–Si bond, thereby leading to silylene-type radicals [6]. The radicals are ground-state singlets, and their characteristic reactions include insertion into Si–H bonds and p-type addition across C=C and C ≡ C bonds [8, 50]. In the present case, these generated radicals react with unsaturated bonds in 1-hexyne and a Si–H bond in CPS. The reaction between C ≡ C bond of 1-hexyne and CPS by hydrosilylation creates C=C bonds, which are again the origin of the second hydrosilylation. In other words, 1-hexyne reacts twice with CPS due to sp. carbon. Therefore, the Si–C bond

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Fig. 7.25 (a) Optical gap and (b) resistivity of phosphorus-doped a-SiC films as functions of X. The phosphorus concentration was 1.6 × 1021 cm−3. (Copyright © 2016 The American Ceramic Society)

density in n-PSH should increase until the number of moles of 1-hexyne is half that of CPS (i.e., 33:67), which is achieved at X = 0.27. Note that X is a volume ratio and densities of CPS and 1-hexyne are 0.963 and 0.715, respectively. This result is consistent with the critical value of X  =  0.2 shown in Fig.  7.24b. In contrast, when 1-hexene (sp2 carbon) is employed as the carbon source, we confirmed the critical X-value was X = 0.4. Therefore, we assume that the concentration NSi–C saturates for X > 0.2 because at this point almost all reaction sites for Si–C bonds in n-PSH have been consumed by hydrosilylation. The carbon in the film blueshifts the band edge because the optical gap is widened when Si–Si bonds are replaced by higher energy Si–C bonds. Figure 7.25a shows the Eg of phosphorus-doped a-SiC films prepared using n-PSH with X = 0, 0.1, 0.2, 0.4, and 0.6 obtained from the Tauc plot [19]. These measurements were made on an 100-nm-thick film deposited on a glass substrate. The Eg increases with the X until 0.2. For X > 0.2, it remains constant due to the saturation effect of Si–C bonds. Figure 7.25b shows the resistivity of a-SiC films as a function of X. The phosphorus concentration as measured by secondary ion mass spectrometry was 1.6 × 1021 cm−3. The resistivity increases with increasing X and becomes fairly constant at about 109 Ωcm for X > 0.2. The optical and electrical properties of the a-SiC films for X  0.4. The film quality with X > 0.4 should thus decline because of the presence of excess carbon that is not involved in the formation of Si–C bonds. The reason that the value of NSi–C became constant is discussed in the next section. The high values for NC–H indicate that the films contained many carbons, consisting with XPS results, as shown in Fig. 7.34. Notably, the hydrogen in the films is mainly bonded to carbon. NSi–H remained constant at approximately 5 × 1021 cm−3. NSi–H depends solely on the process temperature, indicating that the Si–H bonds are formed as a result of the dehydrogenation of CPS.

Fig. 7.37  Bond density of Si–C (closed triangles), Si–H (closed circles), and C–H (open circles) in the a-SiC:H films prepared using SiC-inks with various X values. (Copyright @ 2014 The Japan Society of Applied Physics)

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7.4.3.4  CBM and VBM Measurement To understand electric properties correctly, the information of the position of energy levels, i.e., conduction band minima (CBM) and valence band maxima (VBM), is very useful. We measured the CBM and VBM by inverse photoemission spectroscopy (IPES) and photoelectron yield spectroscopy (PYS), respectively. This approach is expected to eliminate the tail effect based on the exciton binding energies, giving a precise CBM position and real energy gap Eg. For this purpose, a new series of samples were prepared in the strictly same manner as described above: same raw materials and processes except for the substrates for PYS and IPES measurements. Compositional analysis was conducted by measuring the binding states of the resultant films by XPS, and the optical bandgap Eg was also measured. The results are shown in Fig. 7.39. Figure 7.39a revealed that the incorporated C increased the number of Si–C bonds. Figure 7.39b again showed that the incorporated Si–C bonds widened the Eopt from 1.56 eV (Cfilm = 0) to 2.03 eV (Cfilm = 0.40) as a consequence of the replacement of Si–Si bonds by stronger Si–C bonds. The VBM and CBM were measured by PYS and IPES, respectively. The Fermi level for all of the films was referenced to that of an Au film. The primary feature of PYS is that the VBM is obtained with higher resolution compared to that obtained by conventional XPS. An applied photon energy ranging from 4.0 to 9.0 eV with a resolution of 0.02 eV was employed for the measurements, where a monochromatic D2 light source was used. The photoelectron yield was obtained by dividing the photocurrent by the incident photon rate at each photon energy. Figure 7.40a shows the PYS spectra of films with various Cfilm deposited onto conductive Si substrates. Moreover, the intensity and energy with respect to the vacuum level are plotted on the vertical and horizontal axes, respectively. Linear behavior was observed in the spectra, which means that the spectra can be used to define the VBM by linear extrapolation to zero, as shown schematically by the solid lines and arrows in Fig. 7.40a. As expected, the incorporation of C caused a shift of the edge toward the lower-energy side as a result of gap widening resulting from the replacement of Si–Si bonds by stronger Si–C bonds. In the IPES measurements, bremsstrahlung isochromatic spectroscopy mode with a scanning electron energy from 4 to 15 eV with a resolution of 0.6 eV was employed. The electron gun emits monochromatic electrons to a sample, where they couple to unoccupied electron states. Therefore, IPES enables the direct determination of the CBM [62]. The IPES spectra of a series of films with increasing C contents are shown in Fig. 7.40b, which is depicted in the same format as Fig. 7.40a. The extrapolation of the IPES leading edge as a function of Cfilm shows that the CBM shifted to the higher-energy side upon the incorporation of C. In both the PYS and IPES spectra, broadening of the exponential region as a result of a greater degree of topological/compositional disorder [63] was observed with increasing Cfilm. As evident in Fig.  7.39a, the C incorporated into the films increased the number of Si–C bonds, indicating that the distribution of C atoms in the network is likely compositionally homogeneous. As described above, the a-SiC

7.4 P-Type a-SiC via LVD Method

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Table 7.3  Physical parameters of the investigated a-SiC films as functions of their C content

Cink 0 0.17 0.31 0.54 0.72

Cfilm 0 0.11 0.20 0.32 0.40

Experimental values VBM CBM [eV] [eV] −4.7 −3.1 −4.8 −3.0 −4.9 −2.9 −5.3 −2.4 −5.4 −2.3

Eopt [eV] 1.54 1.70 1.80 1.95 2.02

Eg [eV] (CBM – VBM) 1.6 1.8 2.0 2.9 3.1

Estimated CBM [eV] (VBM + Eopt) −3.2 −3.1 −3.1 −3.4 −3.4

Eg − Eopt [eV] 0.06 0.1 0.2 0.95 1.08

Copyright © 2016 AIP Advances

structure involves many hydrogen atoms which are incorporated in the form of CHn. Therefore, most of the topological/compositional disorder might stem from the variety of hydrogen configurations around C atoms (CH, CH2, and CH3). The measured VBM, CBM, and Eopt values are summarized in Table 7.3. The Eg and CBM values estimated from Eg = (CBM − VBM) and CBM = (VBM + Eopt), respectively, are also included for comparison. Both the measured VBM and CBM values shifted by 0.7 eV when the Cfilm was increased from 0 to 0.4, which led to the widening of the Eg from 1.6 to 3.1 eV and of the Eopt from 1.54 to 2.02 eV. For the amorphous semiconductor materials, the Eg tends to be greater than the Eopt because the former represents the gap state between the mobility edge, whereas the latter represents the gap between exponential tail states [64]. Therefore, the simple equation CBM = (VBM + Eopt) underestimates the true CBM, as shown in Table 7.3. The Eopt increases monotonically with Cfilm, whereas the Eg exhibits an appreciable change at Cfilm = 0.2. The band widenings of Eg and Eopt are consistent with each other at Cfilm  0.2, as listed in Table 7.3 as Eg − Eopt. This result implies the exponential widening of the band tail at Cfilm > 0.2. As aforementioned, our a-SiC film features many hydrogen atoms in the form of CHn entities. In particular, in the case of the film with Cfilm > 0.2, the effect of topological and compositional disorder on the electronic structure should be considered, whereas the incorporated C widened the Eg less effectively in the case of the film with Cfilm  0.4), and decaborane. The polymer is synthesized by the reaction between CPS and the sp2 carbons in cyclohexene. Because the inks with/without decaborane were found to have similar properties by ink analysis, it seems reasonable to conclude that decaborane does not contribute to the polymer structure. It just dissolves in the ink retaining its structure. Taking the polymer structure and the results shown in Figs. 7.36 and 7.37 into account, it can be concluded that the carbon atoms are incorporated as functional groups containing many hydrocarbons in the a-SiC:H films. Therefore, the deposition mechanism is proposed as follows. Following the heating of the SiC-ink, some of the Si–Si bonds in the polymer break into vapors of Si–CmHn and deposited on the substrate, and then a silicon network is formed by further heating, which leads to the incorporate of carbon atoms. According to this model, NSi–C and NC–H are controlled by changing the number of sp2 and sp3 carbons in the source substances. We measured the CBM and VBM by inverse photoemission spectroscopy (IPES) and photoelectron yield spectroscopy (PYS), respectively. The real CBM was larger than the CBM value estimated by the simple equation CBM = VBM + Eopt because of the elimination of tail effects. We clarified that the tail effects were dramatically

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enlarged in the films with higher C contents. Specifically, exponential widening of the band tail inhibited the shift of Eopt in the films with Cfilm > 0.2. Thus, the CBM estimated by the equation CBM  =  VBM  +  Eopt gradually deviates from the real CBM values at higher Cfilm values. These features might be related to the microstructural disorder of the films. Nevertheless, the films had reasonable CBM/VBM values and exhibited good electrical properties [31]. The states could be controlled by changing the composition ratio of the Si and C sources in the SiC-ink.

7.4.5  Conclusion P-type a-SiC:H films using SiC-inks consisting of CPS, cyclohexene, and decaborane were fabricated. The value X, defined as a volume ratio of [cyclohexene]/ [CPS + cyclohexene] in SiC-ink, is an important parameter that affects the carbon content in the a-SiC:H films. The electrical and optical properties of the films prepared using SiC-ink with X  0.4 show lower conductivity than expected because of the incorporation of excess carbon that does not participate in Si–C bond formation. In addition, FT-IR measurements suggest that carbon is incorporated as functional groups containing many hydrocarbons. Therefore, we believe that the electrical properties of these films will be improved through the optimization of the carbon sources. We would like to emphasize that the SiC-inks, the deposition technique, and the film formation mechanism are quite different from those used in the preparation of conventional a-SiC:H films. Nevertheless, the SiC-inks provide p-type a-SiC:H-­ films with Eopt values from 1.56 to 2.11 eV and conductivities from 1.1 × 10−4 to 7.1  ×  10−11 that are prepared using a simple apparatus without any vacuum process.

References 1. S. Yajima, J. Hayashi, M. Omori, Chem. Lett. 4, 931 (1975) 2. Z.-F. Zhang, F. Babonneau, R.M. Laine, Y. Mu, J.F. Harrod, J.A. Rahn, J. Am. Ceram. Soc. 74, 670 (1991) 3. Q. Liu, H.J. Wu, R. Lewis, G.E. Maciel, L.V. Interrante, Chem. Mater. 11, 2038 (1999) 4. R. West, L.D. David, P.I. Djurovich, H. Yu, R. RSinclair, Am. Ceram. Soc. Bull. 62, 899 (1983) 5. Y. Hasegawa, K. Okamura, J. Mater. Sci. 18, 3633 (1983) 6. T.R. Dietrich, S. Chiussi, M. Marek, A. Roth, F.J. Comes, J. Phys. Chem. 95, 9302 (1991) 7. P.P. Gaspar, Reactive Intermediates, vol 1 (Wiley, New York, 1978) 8. Y.N. Tang, Reactive Intermediates, vol 2 (Plenum, New York, 1982) 9. F. Anwari, M.S. Gordon, Isr. J. Chem. 23, 129 (1983) 10. G. Inoue, M. Suzuki, Chem. Phys. Lett. 122, 361 (1985) 11. J.O. Chu, D.B. Beach, J.M. Jasinski, J. Phys. Chem. 91, 5340 (1987) 12. T. Masuda, Y. Matsuki, T. Shimoda, Polymer 53, 2973 (2012)

References

135

1 3. M.H. Brodsky, M. Cardona, J.J. Cuomo, Phys. Rev. B 16, 3556 (1977) 14. H. Murata, H. Matsuura, K. Ohno, T. Sato, J. Mol. Struc. 52, 1 (1979) 15. S.  Liu, S.  Gangopadhyay, G.  Sreenivas, S.S.  Ang, H.A.  Naseem, Phys. Rev. B 55, 13020 (1997) 16. T. Masuda, Y. Matsuki, T. Shimoda, Thin Solid Films 520, 6603 (2012) 17. M.L. Huggins, J. Am. Chem. Soc. 75, 4123 (1953) 18. S. Yajima, Y. Hasegawa, J. Hayashi, M. Iimura, J. Mater. Sci. 13, 2569 (1978) 19. J. Tauc, R. Grigorovici, A. Vancu, Phys. Status Solidi 15, 627 (1966) 20. J. Robertson, E.P. O’Reilly, Phys. Rev. B 35, 2946 (1987) 21. J.I.  Pankove, Semiconductors and Semimetals, “Hydrogeated Amorphous Silicon” Part A (Academic Press, Orlando/London, 1984) 22. H. Wieder, M. Cardona, C.R. Guarnieri, Phys. Stat. Solidi (b) 92, 99 (1979) 23. Y. Katayama, K. Usami, T. Shimada, Philos. Mag. B 43, 283 (1981) 24. Y. Catherine, G. Turban, Thin Solid Films 70, 101 (1980) 25. Y. Tawada, K. Tsuge, M. Kondo, H. Okamoto, Y. Hamakawa, J. Appl. Phys. 53, 5273 (1982) 26. Y.H. Wang, J. Lin, C.H.A. Huan, Mater. Sci. Engineer. B 95, 43 (2002) 27. A. Chehaidar, R. Carles, A. Zwick, C. Meunier, B. Cros, J. Durand, J. Non-Cryst. Solids 169, 37 (1994) 28. F. Tuinstra, J.L. Koenig, J. Chem. Phys. 53, 1126 (1970) 29. D.M. Bhusari, S.T. Kshirsagar, J. Appl. Phys. 73, 1743 (1993) 30. H. Matsumura, T. Uesugi, H. Ihara, Jpn. J. Appl. Phys. 24, L24 (1985) 31. T. Masuda, Z. Shen, H. Takagishi, K. Ohdaira, T. Shimoda, Jpn. J. Appl. Phys. 53, 031304 (2014) 32. A. Sugiyama, T. Shimoda, D.H. Chi, Mol. Phys. 108, 1649 (2010) 33. G. Fritz, J. Grobe, D. Kummer, Carbosilanes, Vol. Volume 7 (Academic Press, 1965) 34. T. Masuda, A. Iwasaka, H. Takagishi, T. Shimoda, Thin Solid Films 612, 284 (2016) 35. T. Masuda, A. Iwasaka, H. Takagishi, T. Shimoda, J. Mater. Chem. C 3, 12212 (2015) 36. T. Friessnegg, M. Boudreau, P. Mascher, A. Knights, S. P. J., and W. Puff. J. Appl. Phys. 84, 786 (1998) 37. A.A.  Langford, M.L.  Fleet, B.P.  Nelson, W.A.  Lanford, N.  Maley, Phys. Rev. B 45, 13367 (1992) 38. A.  Tabata, Y.  Kuno, Y.  Suzuoki, T.  Mizutani, J.  Non-Cryst, Solids 164–166. Part 2, 1043 (1993) 39. J. Schäfer, J. Ristein, S. Miyazaki, L. Ley, Appl. Surface Sci. 123, 11 (1998) 40. N.F. Mott, E.A. Davis, Electronic processes in noncrystalline materials (Oxford University Press, Oxford, 1979) 41. R.S. Sussmann, R. Ogden, Philos. Mag. B 44, 137 (1981) 42. S. Knief, W. von Niessen, J. Non-Cryst. Solids 255, 242 (1999) 43. T. Murakami, T. Masuda, S. Inoue, H. Yano, N. Iwamuro, T. Shimoda, AIP Adv. 6, 055021 (2016) 44. R.J. Loveland, W.E. Spear, A. Al-Sharbaty, J. Non-Cryst. Solids 13, 55 (1973) 45. I. Sakata, Y. Hayashi, M. Yamanaka, H. Karasawa, J. Appl. Phys. 52, 4334 (1981) 46. W. Beyer, H. Wagner, H. Mell, MRS Proc. 49, 189 (2011) 47. A.H. Mahan, P. Raboisson, R. Tsu, Appl. Phys. Lett. 50, 335 (1987) 48. T.  Masuda, N.  Sotani, H.  Hamada, Y.  Matsuki, T.  Shimoda, Appl. Phys. Lett. 100, 253908 (2012) 49. T. Masuda, A. Iwasaka, H. Takagishi, T. Shimoda, G. Soraru, J. Am. Ceram. Soc. 99, 1651 (2016) 50. N. Tokitoh, W. Ando, Reactive Intermediates Chemistry (Wiley, Hoboken, 2005) 51. F. Demichelis, C.F. Pirri, E. Tresso, J. Appl. Phys. 72, 1327 (1992) 52. R.A. Street, Phys. Rev. Lett. 49, 1187 (1982) 53. D. Adler, Phys. Rev. Lett. 41, 1755 (1978) 54. W.B. Jackson, N.M. Amer, Phys. Rev. B 25, 5559 (1982)

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5 5. W. Meyer, H. Neldel, Z. Tech. Phys. (Leipzig) 12, 588 (1937) 56. V. Kirbs, T. Drusedau, H. Fiedler, J. Phys. 2, 7473 (1990) 57. R. Widenhorn, A. Rest, E. Bodegom, J. Appl. Phys. 91, 6524 (2002) 58. N.F. Mott, J. Non-Cryst. Solids 1, 1 (1968) 59. M.J.G. Lee, Phys. Rev. 187, 901 (1969) 60. Y. Tawada, M. Kondo, H. Okamoto, Y. Hamakawa, Sol. Energy Mater. 6, 299 (1982) 61. J.D. Carpenter, B.S. Ault, J. Phys. Chem. 95, 3502 (1991) 62. F.J. Himpsel, T. Fauster, J. Vac. Sci. Technol. A 2, 815 (1984) 63. M. De Seta, S.L. Wang, F. Fumi, F. Evangelisti, Phys. Rev. B 47, 7041 (1993) 64. E.A. Schiff, S. Hegedus, X. Deng, Handbook of Photovoltaic Science and Engineering (Wiley, Chichester, 2003) 65. T.M. Brown, C. Bittencourt, M. Sebastiani, F. Evangelisti, Phys. Rev. B 55, 9904 (1997)

Chapter 8

Nano-pattern Formation Using Liquid Silicon

Abstract  In this chapter, direct patterning methods for device fabrication are introduced. The first one, described in 8.1, is a method for the area-selective deposition of polysilane patterns in silicon solution, which is based on the principle of Type 4 conversion in Case 2 shown in Fig. 1.1 in Chap. 1. Two kinds of selectivity mechanisms are utilized: one is the difference of molecular forces and the other is a reactive difference. The second method, described in 8.2, is a direct drawing of Si lines using CPS vapor and an electron or ion beam. This is a kind of free writing of three-­ dimensional Si patterns named focused-ion-beam chemical vapor deposition (FIB-­ CVD). CPS molecules facilitated the advancement of this method. The third method, described in 8.3, is nanoimprinting of silicon. Well-defined silicon patterns with a high aspect ratio and sharp edges were directly formed by imprinting of liquid silicon. Both dotted and lined patterns whose size ranges from 1 mm to 100 nm were obtained, suggesting that their size could be further reduced. Keywords  Area-selective deposition · Hamaker constant · Polysilane · Focused-­ ion-­beam (FIB)-CVD · Nanoimprinting

8.1  Area-Selective Deposition of Silicon Family Materials As actual examples of formation of solid from liquid, which is based on the principle of Type 4 conversion in Case 2 shown in Fig. 1.1 in Chap. 1, we introduce two experimental results. We utilized this principle for area-selective depositions of silicon family materials, i.e., silicon and cobalt disilicide (CoSi2). The former uses the difference of molecular forces for selectivity, while the latter uses a reaction probability difference between two sites.

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8.1.1  A  rea-Selective Deposition of Silicon Using the Difference of Molecular Force We investigated the possibility of the selective deposition of solute (polysilane) in silicon solution on specific narrow areas in a substrate. Among the molecular forces, the van der Waals force is known as a long distance one which can be utilized as an attractive force causing a selective deposition of solute to a specific position of the substrate in a solution. The van der Waals force between the specific substances which is mediated by a media (solvent in this case) can be evaluated with using Hamaker constant A123. We have evaluated the Hamaker constant based on the Lifsitz theory in which dielectric functions of the component materials are required. We can get to know a dielectric function by simple spectral method (SSM) using both measured absorbing frequency ωUV and oscillator strength CUV.  For details about SSM, please refer to Sect. 4.2. UV light having a wavelength of 365 nm and power of 1 mW/cm2 was irradiated to cyclopentasilane (CPS) for 1 h to synthesize polysilane, which in turn was dissolved in cyclohexene to make a 1.0 vol% solution (silicon solution). In this condition, almost 100% of CPS could be converted to polysilane: no CPS remained in the solution. The substrate which gives selectivity was fabricated by sputtering a TiO2 film on a quartz substrate, followed by the patterning of the TiO2 film into small rectangular patterns by photolithography. The silicon solution was spin-coated on the substrate thus prepared. It was confirmed that solute was deposited only on the TiO2 areas after drying of solvent, which is shown in Fig. 8.1. In the case of large TiO2 patterns, however, small-sized islands of solute are observed between the TiO2 patterns.

Fig. 8.1  Areal selective deposition of silicon on a TiO2 pattered substrate using liquid silicon

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Let’s consider the selective mechanism in this experiment theoretically. The substrate, on which many patterns of TiO2 film fabricated, can be regarded as the one which has a periodical contrast of a Hamaker constant. The Hamaker constants of polysilane to TiO2 and quartz (SiO2) mediated by cyclohexene solvent, of which configuration is denoted as A123, are 2.99 kT and −0.06 kT, respectively. When silicon solution is coated on the substrate, polysilane in the silicon solution ought to be attracted to the TiO2 patterns rather than the quartz surface, depending on the Hamaker constant. The experimental result shown in Fig. 8.1, where polysilane was deposited on the 5 um square TiO2 patterns, supported this theoretical prediction. The compositional analysis with SEM-EDX revealed that the atomic ratios Si:Ti of the substance covering TiO2 patterns were 98.9: 1.1. This result showed a TiO2 pattern was completely covered by silicon. As described above, small-sized islands of solute remained between the TiO2 patterns. Two reasons can be considered as follows. First, the attractive forces from the surrounding TiO2 pattern, which the solute in the central position perceives, are cancelled each other, resulting in a neutral force or very weak one. Second, drying speed of spin-coating is too rapid for the polysilane solute, which is located between the patterns, to move to one of the closest TiO2 pattern. As a result, the solute got left behind between the patterns. In this experiment possibility of a selective formation of silicon patterns was demonstrated by utilizing the difference of van der Waals force in a liquid chamber.

8.1.2  Selective Deposition Using the Reactive Difference The difference of reactivity between one area and the other in a solution can be used for a selective deposition of the solute. We used this principle to form a CoSi2 pattern on the specific position of a Si substrate. The salicide process is a process to form metal-silicide patterns in source, drain, and gate regions selectively by using protecting films, on which silicide reaction does not take place, as masks. To confirm the selectivity in the formation of CoSi2, we coated precursor solution, which was synthesized by the reaction between cyclopentasilane and dicobalt octacarbonyl in a toluene solution, both on a Si substrate with a natural oxidation layer and one without it. Two samples were analyzed by the XRD measurement. The sample without an oxidation layer had peaks related to CoSi2, whereas no peak was observed from the sample having the natural oxidation layer. The cross-sectional TEM images of both samples are shown in Fig.  8.2. Epitaxial growth of a CoSi2 crystal film was confirmed in the sample without the natural oxidation layer by electron-beam diffraction analysis. On the contrary, it was found that the oxidation layer of 5 nm thickness, which was observed between the substrate and coated film, prevented the precursor solution from reacting with the substrate. It is very interesting to know that a very thin oxide layer with a few nanometer thicknesses gave the selectivity of reaction between precursor solution and substrate.

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Fig. 8.2  Cross-sectional TEM images (×1000000) of selective formation of CoSi2 epitaxial films on a surface without natural oxide (a) or one with it (b)

8.2  Beam-Assisted Deposition of Silicon 8.2.1  F  ree Writing of Silicon by FIB-CVD and Advantage of CPS for a Source Material If free writing of three-dimensional Si structures on substrates is feasible, it will contribute significantly to conventional Si nanotechnology techniques, for example, forming a stopgap for an integrated circuit (IC) substrate after the extraction of a transmission electron microscopy (TEM) sample, IC substrate repair, circuit modification of large-scale integration (LSI), and, ultimately, the direct fabrication of Si-based semiconductor devices [1]. Focused-ion-beam chemical vapor deposition (FIB-CVD) and focused-electron-­ beam (FEB)-CVD have received much attention because of their advantages in the fabrication of 3D nanostructures [2, 3]. These processes are well-established methods for local deposition based on the decomposition of adsorbed precursor molecules by ion or electron-beam irradiation. Many materials have been deposited by FIB/FEB-CVD, e.g., C [4], W [5], Fe [6], Cr [7], SiO2 [8, 9], and others [3]. However, as far as we know, there have been few reports of Si deposition [10, 11] and, in particular, no reports on the fabrication of 3D nano-sized Si. One reason is that using monosilane (SiH4) or disilane (Si2H6) as the Si source gas is essential in order to obtain semiconductor Si, and these gases are difficult to handle [10, 11]. Furthermore, the deposition rate of SiH4 and Si2H6 may not be high because such a small molecule has a weak adsorption force with respect to a substrate. Cyclopentasilane (CPS) is comparatively easier to handle than monosilane (SiH4) or disilane (Si2H6). The Si films formed by CPS with liquid processing have satisfactory quality for use in Si-based devices. In this paper, CPS is employed as a new single-source precursor for the deposition of Si by FIB-CVD. We demonstrated the selective deposition of nano-sized Si films directly on substrates by focusing and scanning a Ga ion beam.

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8.2.2  Experimental CPS was sealed hermetically into a stainless steel cylinder with a valve in a glove box, and then, the cylinder was removed from the glove box. The cylinder was then connected to an SMI 2050 FIB system (SII NT) with a stainless steel tube, and a gas purifier filter was connected to the exhaust pipe. Ga FIB-CVD was performed inside an SMI 2050 FIB system by simultaneous introduction of CPS gas and irradiation by the Ga ion beam. The samples were not heated, and the Ga beam was irradiated at room temperature. Selective film growth clearly occurred on the Ga-beam-­irradiated area. After preliminary experiments to verify film deposition using CPS as a Si source gas, the optimum conditions were defined as follows: 30 kV Ga FIB; beam current, 2.3 pA; beam diameter, 10 nm; dwell time, 500 ns; scan step, 18.8 nm; gap between beam nozzle and substrate, 23 μm; and chamber pressure, 1 × 10−3 – 8 × 10−4 Pa.

8.2.3  Deposition of Silicon Patterns Figure 8.3 shows a scanning ion microscope (SIM) image of films deposited on the Si substrate. Samples (i)–(iv) were deposited under current densities ranging from 1.15 to 46.0  pA/μm2. The experimental conditions (current density and setting

Fig. 8.3  SIM images of samples (i)–(iv) deposited on Si substrate by Ga FIB-CVD

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Table 8.1  Experimental conditions and results for samples (i)–(iv) Sample no. (i) (ii) (iii) (iv) Sample no. (i) (ii) (iii) (iv)

Current density (pA/μm2) 1.15 2.3 11.5 46 Deposition time (s) 705 367 78 150

Setting dimension (μm) 2 × 1 × 0.2 2 × 0.5 × 0.2 2 × 0.1 × 0.2 5 × 0.01 × 0.5 Deposition yield (μm3/nC) 0.54 0.36 0.47 0.3

Deposited dimension (μm) 2.1 × 1.1 × 0.38 2.1 × 0.6 × 0.24 2.1 × 0.2 × 0.20 5.1 × 0.1 × 0.20 Deposition rate (Å/min) 324 390 1540 798

Deposited dimension is length × width × thickness (μm)

dimension) and the experimental results (the dimensions of deposited film, deposition time, deposition yield, and deposition rate) are listed in Table 8.1. Although the central part of the films was slightly thinner than the surrounding areas, the deposition was almost uniform. Narrow lines 100 nm in width and having a good shape were achieved after a Ga ion-beam irradiation for few minutes. The results for sample (iv) indicate that the minimum deposition diameter was approximately 100 nm even when the scanning beam diameter was focused to 10 nm and the beam was scanned in 18.8-nm steps. The reason may be the deposition in the vicinity of the irradiated area due to the generation of secondary electrons during beam irradiation. The estimated deposition yields for samples (i)–(iv) were 0.54, 0.36, 0.47, and 0.30 μm3/nC, respectively. The deposition rates were 324, 390, 1540, and 798 Å/ min, which were much higher than those previously reported, 0.3 Å/min [11] and 0.09 Å/min [10]. Figure 8.4 shows SIM images of samples (v) and (vi), on which our project’s name was written in a combination of Japanese characters and English letters approximately 1  μm in size. The kanji characters “下田”and “液体” mean “Shimoda” and “liquid,” and the katakana characters “ナノ”and “プロジェクト” mean “nano” and “project,” respectively. Two samples were written in different fonts: sample (v) in Gothic type and sample (vi) in Ming-cho type; both exhibited relatively good resolution. These results suggest that FIB-CVD using CPS as a Si source gas was relatively effective for fabricating 3D Si nanostructures.

8.2.4  Characterization of the Deposited Patterns To characterize the deposited patterns, large samples were prepared by Ga FIB-­ CVD on quartz substrates to measure Raman spectra and Auger electron spectroscopy (AES) depth profiles. Figure 8.5a shows scanning electron microscope (SEM) images of the checkered patterns [samples (vii) and (viii)] deposited on a quartz substrate. Sample (vii) was the as-deposited sample with a film thickness of 100 nm; sample (viii) was annealed at 800 °C for 60 min and had a film thickness of 200 nm.

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Fig. 8.4  SIM images of samples (v) and (vi) fabricated by Ga FIB-CVD on SiO2/Si substrate

In sample (viii), the surface roughness of the film deteriorated probably because of the evolution of gas during annealing. Figure  8.5b shows the Raman spectra of samples (vii) and (viii). Sample (vii) did not exhibit peaks at 0–800 cm−1, whereas sample (viii) showed a significant peak near 515 cm−1, corresponding to polycrystalline Si. The estimated values of the full width at half maximum and crystalline fraction were 8.8 cm−1 and 62%, respectively, which were compatible with the values obtained from a spin-coated film after annealing under the same conditions. Elemental analysis of the samples was performed by obtaining AES depth profiles of samples (vii) and (viii), as shown in Fig. 8.6. The horizontal axis shows the depth distance converted from the SiO2 etching rate, and the vertical axis shows the atomic concentration (%) of Si, Ga, C, and O. A high concentration of Ga atoms was observed at the surface (~20  nm) of sample (vii). In fact, SIM observations showed Ga seepage of the top surface of the film during Ga-beam irradiation. This may be attributed to the low solubility of Ga in Si. Because a high-concentration layer covered the surface of the deposited film, the Raman peak corresponding to amorphous Si was considered to be undetected for sample (vii). However, the observation of the Raman peak corresponding to amorphous Si is delicate under the experimental conditions of Ga FIB-CVD. Other as-deposited samples obtained by Ga FIB-CVD showed a broad peak near 450 cm−1, which implied the presence of a deposited film that was possibly amorphous Si. In the annealed sample, however, the distribution of Ga atoms was dramatically different: Ga atoms preferentially moved to the center of the film. Although the reason for this change in the composition is unknown, the characteristic Raman peak of poly-silicon was clearly observed owing to the movement of Ga atoms from the surface to the center of the film.

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Fig. 8.5 (a) SEM images of checkered patterns on quartz substrates before and after annealing. Sample (vii) is the as-deposited sample, and sample (viii) was annealed at 800 °C for 60 min. (b) Raman spectra of samples (vii) and (viii)

Ga atom impurities are undesirable if the deposited film is to be used as a semiconductor film. To realize the deposition of uncontaminated semiconductor films, the beam source will have to be changed. FEB-CVD can be considered as a candidate process for obtaining high-quality Si films. As a preliminary experiment, a 150-nm-wide line was deposited by FEB-CVD using CPS; however, the deposition rate of 5.8–223 Å/min was much lower than that of Ga FIB-CVD (324–1540 ­Å/ min), and the deposited film tended to be oxidized owing to the long beam irradiation.

8.2.5  Summary In summary, we selectively deposited Si films by irradiating substrates with CPS gas molecules and a Ga ion beam simultaneously at room temperature. We confirmed the effectiveness of Si deposition by Ga FIB-CVD using CPS, which yielded

8.3  Direct Imprinting of Silicon Using Liquid Silicon

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Fig. 8.6  AES depth profiles of (a) sample (vii) and (b) sample (viii). Atomic concentrations (%) of Si, Ga, C, and O for the samples are indicated by red, blue, green, and yellow dots, respectively

a relatively high resolution and is applicable to the fabrication of 3D Si nanostructures. Those films, however, could not be used in semiconductor devices because they were excessively contaminated with Ga atoms.

8.3  Direct Imprinting of Silicon Using Liquid Silicon Liquid silicon was directly imprinted to form well-defined and fine amorphous silicon patterns [12, 13]. Despite a volume shrinkage as large as 70–80% during whole process, well-defined angular patterns with a high aspect ratio were preserved. We found that the curing step before imprinting was particularly important for the imprinting process, so the optimal temperature was tuned to be 140–180 °C in terms of the film’s deformability and molding properties. The cross-linking of the polymer due to the 1,2-hydrogen shift reaction was induced with the release of a large amount of SiH4 and H2 gases at imprinting temperatures circa between 150 and 250  °C, leading to the solidification of the film. By conducting solid-phase crystallization at 800 °C, the pattern portion was converted into polycrystalline pure silicon, whereas

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the residual film region remained in an amorphous state containing large amounts of oxygen and carbon atoms.

8.3.1  Nanoimprinting and Silicon Nanoimprint lithography (NIL) is used to process thermoplastic or photocurable resins using a mold [14–16], which is attracting attention as a next-generation lithography technique in semiconductor processing because it is simple and provides a resolution on the order of sub-nanometers [17, 18]. However, this technique is only an alternative technique to conventional lithography. Therefore, the etching process remains an essential step in the fabrication of silicon microstructures. To overcome this problem, a direct nanoimprint lithography using functional materials would be very attractive to reduce the process cost. As examples of functional materials for the direct NIL, sol–gel materials, nano-particle-based solutions, spin-on-­ glass (SOG) resists, and silicon itself have been utilized, a summarized article about the direct NIL was found in the review paper written by K.-J. Byeon and H. Lee [19]. Among them silicon is very attractive because of its absolute importance for a semiconductor industry. Chou et  al. have reported a laser-assisted direct imprint (LADI) method for obtaining the micropattern of a silicon film without lithographic and etching processes [20]. In this method, a single excimer laser pulse melts a thin surface layer of silicon, and a mold is embossed on the resulting liquid layer. This method is excellent in principle but suffers from several practical limitations. In contrast, here, we offer a more versatile method of using liquid silicon as an imprinting material. We describe a technique for the simple and highly accurate direct imprinting and formation of a silicon pattern onto a substrate by combining liquid silicon and the nanoimprint method.

8.3.2  Experimental Section 8.3.2.1  Liquid Silicon Preparation After cyclopentasilane (CPS) was purified by distillation, polydihydrosilane (liquid silicon) was obtained by irradiating CPS with 313-nm wavelength light for 600 s followed by irradiation with 365-nm light for 1000 s. Size exclusion chromatography revealed that the molecular weight of the obtained liquid silicon ranged broadly from 100 to 1,000,000 g mol−1, with an average of 6000 g/mol [21]. The viscosity of the just-synthesized liquid silicon was 30 mNs/m2 and increased to 50 mNs/m2 after 5 h.

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8.3.2.2  Determination of Imprinting Conditions The imprinting condition was determined by referring to the thermal desorption spectrometry (TDS) results, which were previously measured [22], and the thermogravimetry (TG) and differential thermal analysis (DTA) results for polydihydrosilane. According to the previously reported TDS results of the spin-coated film with increasing temperature, the generation of small amounts of silane and H2 gases was first observed at around 200 °C. After that the main generation peak of silane gases with low-molecular-weight components such as SiH2 and SiH3 was observed at 220–280 °C, followed by intensive generation of H2 gases at 300–380 °C. The reason why the generation of silane gases occurs intensively at lower temperature compared with that of H2 gases is that the Si–Si bond energy (224 kJ mol−1) is weaker than that of the Si–H bond energy (318 kJ mol−1). Figure 8.7 shows the TG-DTA results for polydihydrosilane. The TG curve indicates that the weight of polydihydrosilane started to decrease at approximately 50  °C and that 55% of the weight was lost by the time the temperature reached 400 °C. The plot of the first derivative of the TG curve (DTG) exhibited peaks at 100, 200, and 300 °C. The first peak at 100 °C is attributed to the desorption of small-molecule hydrosilanes. The peak at 200  °C likely represents the partial decomposition of the fragments in polydihydrosilane; thus, we expected that the partial solidification of silicon occurred at this point. Such transitions are important considerations in the design of an imprinting procedure. The peak at 290 °C corresponds to the authentic decomposition of liquid silicon to form solid amorphous silicon. Therefore, a large weight loss was observed at approximately this temperature. In the temperature range from 350 to 450 °C, the weight loss was very small, which means that stable amorphous silicon formed when the temperature exceeded that of the final consolidation peak. In addition to the aforementioned measurements, we conducted preliminary experiments to define the parameters of the imprinting procedure, especially the starting and finishing temperatures. After several trials, we confirmed that imprinting a liquid silicon film after baking at approximately 150–180  °C results in a Fig. 8.7  TG (blue line), DTA (black line), and DTG (red line) traces of PS as functions of heating temperature. (Reproduced with permission from Ref. 26. Copyright 2016 American Chemical Society)

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Fig. 8.8  Schematic of the nanoimprinting process: (a) Liq-Si was spin-coated on a substrate; (b) the film was baked at Tb = 20–220 °C for 5 min; (c) the mold was pressed into the film at 10 MPa for 10 min while maintaining the temperature at Tc; (d) the temperature was increased to 220 °C, and then the applied pressure and temperature were maintained for 10  min; (e) the mold was released after cooling; and (f) the imprinted patterns were post-annealed at 380 °C for 10 min to complete the conversion to a-Si. (Copyright © 2016 American Chemical Society)

better-­quality film compared with imprinting from room temperature. In an imprinting procedure where imprinting starts at 150–160 °C and is followed by a temperature increase to 220–230 °C and subsequent demolding, liquid silicon patterns were observed to be reasonably well preserved. 8.3.2.3  Imprinting Procedure The fabrication of silicon patterns using nanoimprinting included the following steps (Fig. 8.8): (a) the liquid silicon film was formed with drop casting or spin-­ coating onto a substrate with dimensions of 2 × 2 cm2; (b) the film was baked at Tb = 20–220 °C for 1–5 min on a hot stage (Tb: baking temperature); (c) the mold was pressed into the film at 7–10 MPa using a specially designed Toshiba machine; (d) the temperature increased from Tb to 220 °C with maintaining the pressure, and then the temperature and applied pressure were then maintained for 10–15 min; (e) the film was cooled to below Tb while maintaining the pressure followed by demolding; and (f) the film was post-annealed at 380–400 °C for 10 min to complete the conversion to amorphous silicon (a-Si). The liquid silicon film provided good mold-­ releasing properties during the demolding process and did not compromise the adhesion of the mold to the substrate below 220 °C.

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Table 8.2  Used hole molds including rectangular prism-shaped cavities and cylindrical-shaped ones, which were fabricated in tetraethyl orthosilicate (TEOS) deposited films on a silicon substrate by lithography and etching processes No. B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 B11 B12

Shape Square Square Square Square Square Square Circle Circle Circle Circle Circle Circle

Designed size (μm) 1.0 × 1.0 1.0 × 1.0 1.0 × 1.0 0.6 × 0.6 0.6 × 0.6 0.6 × 0.6 Dia. = 0.4 Dia. = 0.4 Dia. = 0.4 Dia. = 0.32 Dia. = 0.32 Dia. = 0.32

Pitch (μm) 5.0 3.0 2.5 3.0 1.8 1.5 2.0 1.2 1.0 1.6 0.96 0.8

Actual size (μm) 0.99 × 0.98 0.99 × 0.99 1.01 × 0.99 0.62 × 0.61 0.64 × 0.62 0.64 × 0.63 Dia. = 0.43 Dia. = 0.43 Dia. = 0.44 Dia. = 0.33 Dia. = 0.34 Dia. = 0.35

Actual volume (nm3) 4.37 × 108 4.41 × 108 4.50 × 108 1.70 × 108 1.79 × 108 1.81 × 108 6.53 × 107 6.53 × 107 6.84 × 107 3.85 × 107 4.08 × 107 4.33 × 107

The actual size was confirmed by the observation of a cross-sectional SEM image. All molds have the same depth of 450 nm. Copyright © 2016, Royal Society of Chemistry

8.3.2.4  Mold Preparation In the experiments, two molds with a dimension of 1 × 1 cm2 were fabricated: Mold 1 and Mold 2. Mold 1 is a mold including line-and-space patterns and hole patterns, while Mold 2 includes only hole patterns. A SiO2 film was formed by the plasma enhanced-chemical vapor deposition (PE-CVD) using tetraethyl orthosilicate (TEOS) on a silicon substrate, and lithography and etching processes were performed to produce a mold. In Mold 1, the 12-hole patterns (B1–B12) were fabricated by changing the size and spacing of the holes, and the 8 line-and-space (hereafter referred as L&S) patterns (D1–D8) were also fabricated to have dimensions of L (line)  =  S (space) = 180 nm to 1 μm. All the patterns had the same depth of 450 nm. Using cross-sectional scanning electron microscopy (SEM), we measured the sizes of the patterns. Table 8.2 lists the measured shape, size, and spacing of each hole pattern together with the designed ones, and Table 8.3 shows the measured line width and spacing of each L&S pattern together with the designed width. On the other hand, only two patterns were produced in Mold 2: square-shaped holes with a designed diameter of 1000 nm and depth of 500 nm and circular holes with a designed diameter of 160 nm and depth of 500 nm. Actual dimensions of the patterns were measured by the SEM. The estimated sizes are presented in Table 8.4, which were slightly different from the designed size. A self-assembled monolayer of a fluorosilane release agent (heptadecafluoro-­ 1,1,2,2-tetrahydrodecyl trimethoxysilane) was deposited on a mold surface by a vapor-phase reaction [23]. The contact angles of liquid silicon to the FAS17/SiO2 substrate, and the SiO2 substrate were 50° and 10°, respectively.

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150

Table 8.3  The used L&S molds which were fabricated using tetraethyl orthosilicate (TEOS) deposited films on a silicon substrate by lithography and etching processes No. D1 D2 D3 D4 D5 D6 D7 D8

Designed size Line (μm) 1.0 0.60 0.40 0.32 0.28 0.24 0.20 0.18

Space(μm) 1.0 0.60 0.40 0.32 0.28 0.24 0.20 0.18

Actual size Line (μm) 1.13 0.69 0.46 0.35 0.29 0.24 0.21 Nd

Space(μm) 0.95 0.56 0.40 0.33 0.29 0.26 0.23 Nd

The actual size was confirmed by the observation of a cross-sectional SEM image. All molds have the same depth of 450 nm. Copyright © 2016, Royal Society of Chemistry Table 8.4  Dimensions of a hole pattern in the molda Side length or diameter of lower base (nm) Side length or diameter of upper base (nm) Depth (nm) Volume (cm3)

Square-shaped hole 1000

Circular hole 155

1100

165

550 6.1 × 10−13

515 4.1 × 10−14

Copyright © 2016 American Chemical Society

8.3.3  Imprinted Patterns with Mold 1 Let’s see the results of Mold 1. Each pattern imprinted with Mold 1 was given the same notation as that of the mold used. For example, the patterns formed using the B5 and D5 molds were denoted as the B5 dotted pattern and the D5 L&S pattern, respectively. First, all the patterned areas were observed using an optical microscope, and it was confirmed that good imprinting was successfully achieved over wide areas greater than 100 × 100 mm2. Figure 8.9a, b show optical microscopy images of the dotted patterns formed using the B1 (side length = 0.99 μm and pitch 5 μm) and B3 (side length = 0.99 μm and pitch 2.5 μm) molds, respectively, while Fig. 8.9c, d show optical microscopy images of the L&S patterns formed using the D1 (L&S = 1.0 μm) and D2 (L&S = 0.6 μm) molds, respectively. Figure 8.9e, f show dotted patterns formed using the B9 (dia. = 320 nm hole and pitch = 1.6 μm) and B12 (dia. = 320 nm hole and pitch = 800 nm) molds, respectively, as examples of the smallest patterns. Figure 8.10a shows a cross-sectional SEM image of the B12 dotted pattern. Dots with a cylindrical shape and a diameter of 200 nm were clearly formed. Figure 8.10b shows an enlarged image of the B10 dots formed using the B10 mold (dia. = 320 nm and pitch = 960 nm), which were of the same size as those formed using the B12

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Fig. 8.9  Optical microscopy images of imprinted silicon patterns to confirm the imprinting feasibility of liquid silicon. (a) Dotted patterns of B1. (b) Dotted patterns of B3. (c) Lined and spaced patterns of D1. (d) Lined and spaced patterns of D2. (e) Dotted patterns of B9. (f) Smallest dotted patterns of B12. (Copyright © 2016, Royal Society of Chemistry)

Fig. 8.10  Cross-sectional SEM images of dotted and lined and spaced patterns to confirm the shapes of imprinted amorphous silicon patterns. (a) Cross-sectional SEM image of the dotted pattern formed by the B12 hole mold (dia = 0.32 μm). (b) An enlarged image of the dotted patterns formed using the B12 mold (dia = 0.32 μm). (c) Lined and spaced patterns formed using the D7 mold (L&S = 0.20 μm). (d) Extended figure of (c). All the patterns have the well-edged shapes and increase perpendicularly from a flat surface without any skirts around their bottoms. (Copyright © 2016, Royal Society of Chemistry)

mold. In this dotted pattern, the amorphous silicon cylinders rose perpendicularly from a flat surface without any skirts around their bottoms, the tops of the dots were flat surfaces with right angle edges, and the dots had very smooth side surfaces. In Fig. 8.10c, the D7 L&S pattern fabricated using the D7mold (L&S = 220 nm) is shown. It can be seen in this figure that well-edged lined patterns with high aspect ratios were clearly formed. Figure 8.11a show SEM images of the dotted patterns for B1, B6, and B10. It was confirmed that the pitch of each mold and its corresponding pattern coincided very well. The estimated dimensions of the lateral and height directions are summarized in Table  8.5. Each value is the average obtained for three patterns. The

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Fig. 8.11  SEM images of dotted patterns for the spacing, areas, and heights. Upper and lower images are top and side views, respectively, (a) dotted patterns of B1. (b) B6. (c) B10. (Copyright © 2016, Royal Society of Chemistry) Table 8.5  Amorphous silicon-dotted patterns formed by using hole molds

B1 B2 B3 B4 B5 B6 B7 B8 B9 B10 Average

Square Square Square Square Square Square Circle Circle Circle Circle –

a-Si Pattern size Area size Height (μm) (μm) 0.72 × 0.69 0.34 0.69 × 0.67 – 0.61 × 0.61 – 0.37 × 0.36 – 0.36 × 0.38 – 0.40 × 0.39 0.33 Dia. = 0.24 – Dia. = 0.27 0.32 Dia. = 0.26 – Dia. = 0.22 0.34 – 0.33

Volume (nm3) 1.7 × 108 1.5 × 108 1.2 × 108 4.4 × 107 4.5 × 107 5.3 × 107 1.5 × 107 1.8 × 107 1.8 × 107 1.3 × 107 –

Aspect ratio 0.48 – – – – 0.84 – 1.2 – 1.6 –

Shrinkage ratio Lateral Height direction direction 0.72 0.76 0.69 – 0.61 – 0.59 – 0.59 – 0.62 0.73 0.56 – 0.63 0.71 0.59 – 0.67 0.76 0.63 0.74

Volume ratio 0.39 0.34 0.27 0.26 0.25 0.28 0.23 0.28 0.25 0.34 0.29

Areas of all the patterns were measured by SEM images, while the heights of the selected patterns were measured by cross-sectional SEM. The shrinkage ratio defined by the pattern/mold was calculated and listed. Copyright © 2016, Royal Society of Chemistry

height values were all between 320 and 340 nm. Thus, the dot heights were similar for all the patterns, irrespective of the mold size. The area and height data were then used to calculate the volume of each pattern. Note that the actual height values were only obtained for the four patterns B1, B6, B8, and B10. The height values for the remaining patterns were each assumed to be 330 nm. The calculated volumes for the patterns are listed in Table 8.5.

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153

Next, the volumes of the formed dotted patterns were compared with those of the corresponding molds. These results are also presented in Table 8.5 as shrinkage rate values. Three types of shrinkage rates were defined as follows: Lateral shrinkage ratio  =  [area of the dotted pattern/cross-sectional area of the mold]0.5; Height shrinkage ratio = height of the dotted pattern/depth of the mold (= 450 nm); Volume shrinkage ratio = volume of the dotted pattern/volume of the mold. The lateral shrinkage ratio exceeded 0.65 for B1, B2, and B10 but was less than 0.6 for the other patterns. Accordingly, the average was 0.63. In contrast, the height shrinkage values ranged from 0.71 to 0.76 with an average of 0.74. Clearly, the shrinkage was greater in the lateral direction than in the height direction. As a result, the calculated volume shrinkage ranged from 0.25 to 0.39 with an average value of 0.29. Therefore, the liquid silicon, once filled into the dotted molds, shrank to approximately 30% of the original mold volume after final annealing in the case of a dotted pattern. The aspect ratios of the B1, B6, B8, and B10 dotted patterns are also listed in Table 8.5. The highest value of 1.6 was obtained for the B12 dotted pattern. The results for the L&S patterns were then evaluated. Cross-sectional SEM images of all the L&S patterns are shown in Fig. 8.12. Here again, the magnitude of all the images is the same. As can be seen in the figure, well-defined rectangular patterns were obtained for the largest to the smallest molds. The width and height values determined using these SEM images are listed in Table 8.6. It was once again confirmed that the pitch of each mold precisely coincided with that of the corresponding dotted patterns. The highest-aspect ratio of 3.1 is obtained for the D8 pattern. Because no difference was observed in the pitch values for the molds and corresponding patterns, it was assumed that the lined patterns did not shrink along

Fig. 8.12  Cross-sectional SEM images of lined and spaced patterns used for measuring the spacings, areas, and heights. (The image of the D1 pattern was missed to take.) The length of the scale bar in each image is 500 nm. (Copyright © 2016, Royal Society of Chemistry)

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Table 8.6  Amorphous silicon-dotted patterns formed by using line and space molds Mold size Width Space (μm) (μm) D1 1.13 0.95 D2 0.69 0.56 D3 0.46 0.40 D4 0.35 0.33 D5 0.29 0.29 D6 0.24 0.26 D7 0.21 0.23 D8 0.18 0.18 Average – –

a-Si Pattern size Width Height (μm) (μm) – – 0.23 0.21 0.17 0.22 0.13 0.18 0.11 0.19 0.10 0.19 0.10 0.26 0.07 0.22 – 0.20

Aspect ratio – 0.91 1.3 1.4 1.7 1.9 2.6 3.1

Shrinkage ratio Width Height direction direction – 0.33 0.46 0.36 0.49 0.37 0.40 0.38 0.41 0.42 0.42 0.48 0.58 0.39 0.49 0.38 0.45

Volume ratio – 0.15 0.18 0.15 0.16 0.18 0.27 0.19 0.17

Cross sections of all the patterns were measured using SEM images. The shrinkage ratio defined by the pattern/mold was calculated and listed. As for the volume shrinkage ratio, it was calculated by multiplying the shrinkage ratio in a width direction with that in a height direction, because no shrinkage was observed in a linear direction. Copyright © 2016, Royal Society of Chemistry Table 8.7  Summary of the average shrinkage ratios for the dotted and L&S patterns Dot pattern L&S pattern

Lateral direction 0.63 0.38

Height direction 0.74 0.45

Volume 0.29 0.17

Copyright © 2016, Royal Society of Chemistry

the line direction. Based on this assumption, both the linear and volume shrinkage ratios were calculated. The lateral shrinkage ratios ranged from 0.33 to 0.43 with an average value of 0.38, while the height shrinkage rates ranged from 0.41 to 0.43 with an average value of 0.45. Once again, greater shrinkage occurred in the lateral direction than in the height direction. The volume shrinkage ratios were then calculated and found to range from 0.15 to 0.20 with an average value of 0.17. Table 8.7 summarizes the average values for the shrinkage ratios. Based on these results, it can be concluded that the shrinkage was greater for the L&S patterns than for the dotted patterns, and shrinkage in the lateral direction was greater than that in the height direction for both pattern types.

8.3.4  I nfluence of Baking Temperature on Imprinting in Mold 2 In the preliminary study for the determination of imprinting conditions, we confirmed that the baking condition before imprinting is a critical to ensure a good imprinting. In other word, the deformability of the polysilane film could be tuned to

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Fig. 8.13  Optical micrographs of as-imprinted patterns that were embossed at different Tb values: (a) 100 °C, (b) 140 °C, (c) 180 °C, and (d) 220 °C. (e) SEM image of (c). (Copyright © 2016, Royal Society of Chemistry)

obtain good patterning. So, small degree of cross-linkage of polysilane by heating (baking) would be very effective to tune the deformability. Figure 8.13a–d shows photographs of as-imprinted square-shaped polysilane patterns with using Mold 2, which were embossed at Tb = 100, 140, 180, and 220 °C for 5 min, while Fig. 8.13e shows the SEM image corresponding to Fig. 8.13c. The patterns imprinted at Tb   390 °C (data not shown). Therefore, the upper limit of the fabrication temperature for the p-i-n structure with a satisfactory interface is 390 °C. Although a low Tp is preferable to minimize the impurity diffusion, the lower limit of the fabrication temperature for obtaining a-Si:H layers was 360 °C. The film oxidizes easily in the air at Tp 300  nm) with high uniformity and no cracks using the spin-cast method under the conditions employed at this time. Therefore, it is necessary when fabricating high-performance solution-processed solar cells to understand the liquid engineering principles for coating a thick film on a substrate and also to employ suitable coating techniques for a large substrate, such as an inkjet or slit-coating method. In addition to improving the quality of thick i-Si layer, there are three problems still unsolved, which are as follows: (1) reduce the concentration of carbon, oxygen, and nitrogen (of the order of 1019–1021 cm−3) causing the donor-like state; [5] (2) decrease the Schottky-barrier height at the p-Si/ZnO interface, which results in a large deviation from ideal J–V curves; and (3) remove native silicon oxide layers between each silicon layer, which acted as resistive components in the cells. We believe that increasing g can be achieved by overcoming these issues as well as preparing a high-quality thick i-Si layer.

9.1.4  Conclusion In conclusion, we have fabricated a-Si:H p-i-n structure by employing a solution-­ based process using doped and non-doped polydihydrosilane solutions. Further, we have demonstrated the effectiveness of hydrogen-radical treatment for improving

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the conductivity of a-Si:H layers by a reduction in spin density. The solution-­ processed a-Si:H solar cells are shown to have the efficiency (η) of 0.31–0.51% under AM-1.5G (100 mW/cm2) illumination. Even though the efficiancy is low, the formation of the solution-processed silicon p-i-n structure indicates that the solution-­ based process might be a promising technology for future application to silicon devices. We also believe that the characteristics of our solar cells will be improved by optimization of processes and introducing the light-trapping structure.

9.2  Thin-Film Solar Cells by LVD Amorphous silicon films fabricated using the LVD (liquid vapor deposition) method, which was described in Chap. 5, were applied for fabricating solar cells. As a first trial, one layer in a cell was fabricated by a LVD film, while the other two were deposited by Hot-wire (HW) CVD.  Next we tried to make solar cells using an improved LVD method that was introduced in Sect. 5.2 in Chap. 5. Two layers were deposited by the improved LVD in this time. The developed solar cell showed a relatively high performance.

9.2.1  Solar Cell Fabrication Using LVD [6] 9.2.1.1  Cell Fabrication Four cells were fabricated of which production methods are summarized in Table 9.3. For the deposition of hydrogenated amorphous (a-SiH) films, both LVD and hot-wire (HW) CVD were used. All the Si films were deposited by HWCVD in cells 1 and 3. The n-Si layer in the n-i-p structure and the p-Si layer in the p-i-n structure were deposited by LVD in cells 2 and 4, respectively. Properties of the films used are shown in Tables 9.4 and 9.5. An Al-doped ZnO (AZO) film with a sheet resistivity of 50 Ω/sq and a thickness of 200  nm was sputtered onto a flat glass substrate (OA-10). Then, a-Si:H films were deposited by HWCVD with order n-i-p or p-i-n with respective layer thickTable 9.3  Stacking order and deposition method for each layer in a-Si:H solar cells. The films were deposited on a glass substrate Sample Cell 1 Cell 2

AZO Sputter

n-Si HWCVD Thermal-CVD

i-Si HWCVD HWCVD

p-Si HWCVD HWCVD

ITO Sputter

Sample AZO p-Si i-Si Cell 3 Sputter HWCVD HWCVD Cell 4 HWCVD Thermal-CVD Copyright © 2015 Elsevier B.V. All rights reserved

n-Si HWCVD HWCVD

ITO Sputter

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Table 9.4  Flow rate of reactant gases in HWCVD and optical gap Eg, dark conductivity σ, and photoconductivity σp for the resultant a-Si:H

p-Si n-Si i-Si

Gas flow (sccm) H2 SiH4 – 10 – 10 10 10

B2H6 5 – –

PH3 – 20 –

Eg (eV) 1.76 1.61 1.79

σ (S/cm) 6.8 × 10−6 2.9 × 10−3 1.3 × 10−11

σp (S/cm) – – 1.3 × 10−5

Copyright © 2015 Elsevier B.V. All rights reserved

Table 9.5  Film properties of a-Si:H. CH is hydrogen content measured by FTIR and SIMS

p-Si n-Si i-Si

CH (at. %) by FTIR 6.1 4.0 6.7

by SIMS 10.4 3.5 7.1

Eact (eV)

Eg (eV)

σ (S/cm)

0.28 0.15 0.81

1.56 1.67 1.65

1.4 × 10−4 9.8 × 10−3 1.5 × 10–11

Eg is the optical gap estimated from Tauc plots. Eact and σ are activation energy and dark conductivity, respectively. The dopant concentrations for p- and n-Si were 3.3 × 1021 and 8.5 × 1020 cm−3, respectively

nesses of 20, 100, and 20 nm. In processing, particular layers in some of these solar cells were prepared by LVD using liquid CPS, as shown in Table 9.3. 4 μl of CPS was used for the source. For the detailed process about LVD, please refer to Sect. 5.1 in Chap. 5. Next, indium tin oxide (ITO) of 100-nm thickness and an area of 1 cm × 1 cm was sputtered onto the n-i-p and p-i-n structures. Finally, some part of the Si film was removed by CF4 etching to allow for direct electrical contact to the AZO electrode layer. The current–voltage (J–V) characteristics of the cells deposited on a glass substrate (OA-10) were measured under 100 mW/cm2 illumination. The HWCVD used was nearly identical to that described in the literature [3]. The apparatus was made of stainless steel with a width of 40 cm, depth of 40 cm, and height of 35 cm. A tungsten wire with a diameter of 0.5 mm and a length of 200 cm was used as the catalyzer. The wire was placed between the substrate holder and gas inlet inside the chamber. The distance between the substrate holder and the wire was kept at 8 cm. After putting in the substrate and evacuating the chamber, the wire was heated at 1850 °C by resistive heating. The temperature of the wire was estimated based on the temperature dependence of electric resistivity of the wire. The substrate was also heated at 300  °C by a heater mounted at the back of the holder. Reactant gas was introduced into the chamber from the opposite side of the substrate holder across the tungsten wire. The flow rate of the gas was adjusted using flow meters. The flow rate conditions are listed in Table 9.4. The values of optical gap Eg, dark conductivity σd, and photoconductivity σp for the resultant films are also listed.

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9.2.1.2  Cell Properties Figure 9.6a–b show the J–V characteristics of the Glass/AZO/n-i-p/ITO and Glass/ AZO/p-i-n/ITO solar cells, respectively. The photovoltaic parameters, such as open-­ circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and energy conversion efficiency (η) of the cells, are also shown in Fig. 9.6. Comparing the curves of cells 1 and 2 and the curves of cells 3 and 4, we can see they are very similar with each other, indicating the doped a-Si:H films prepared by LVD were comparable to those prepared by HWCVD, in terms of forming an operational solar cell. Eg in the LVD-grown p-Si layer was 0.20 eV narrower than that in the p-Si grown by HWCVD. Cells 3′ and 4′ correspond to cells 3 and 4 with light incident from the p-Si side. A comparison of cells 3 and 4 with cells 3′ and 4′ ­indicates that the LVD-grown p-Si is undesirable as a window material because of its narrower Eg. Jsc for cell 4′ was 68% of that for cell 4, because more incident light was absorbed in the window layer of the LVD-grown p-Si. Therefore, a solution-­processed amorphous SiC film which is a wide bandgap p-type material would be a candidate.

Fig. 9.6  J–V characteristics of the solar cells with 100 mW/cm2 illumination. The photovoltaic parameters for the cells are listed on the right-hand side. (a) a-Si:H films fabricated in order n-i-p, and the incident light was from the p-Si side. All the Si was fabricated by HWCVD in cell 1, and only the n-Si layer was fabricated by thermal CVD in cell 2. (b) a-Si:H films fabricated in order p-i-n. All the Si was fabricated by HWCVD in cell 3, and only the p-Si layer was fabricated by thermal CVD in cell 4. The incident light was from the n-Si side for cells 3 and 4, and cells 3′ and 4′ denote illumination from the p-Si side. (Copyright © 2015 Elsevier B.V. All rights reserved)

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9.2.2  Solar Cell Fabrication Using the Improved LVD [7] The a-Si:H film developed by using the improved LVD, which has two parallel substrates for film deposition (see Sect. 5.2 in Chap. 5), was applied into an amorphous silicon solar cell. The structure of the cells is shown in Fig. 9.7a. Different from the above experiment, two layers among three were deposited with using solution materials. The cell was fabricated on a glass substrate (OA-10GF) with 400-nm zinc oxide (2% Al2O3 dopant) and a 100-nm AgAu alloy-reflective layer (5 atomic% Au) and a 5-nm Ti-adhesive layer. The cell was fabricated as n–i–p structure, with the p-Si layer serving as a window layer. Among the three silicon layers, n-Si was fabricated by the solution process with a white phosphorus (P4) dopant. The intrinsic layer is deposited by using the improved LVD with a processing temperature of 360 °C and a CPS supply temperature of 115 °C with a thickness of 150 nm, followed by a post-hydrogen-radical treatment. After removing the oxidized surface by 1% HF solution, p-Si and top ITO electrodes (1.0 mm2) were deposited by HWCVD and sputtering, respectively. The photo and dark I–V curves are displayed in Fig. 9.7. The cell shows a 4.3% conversion rate, with a Jsc of 10.4 mAcm−2, a Voc of 0.68 V, and a fill factor of 0.60. The EQE of the cell was calculated from the spectral

Fig.9.7 (a) Schematic device design for solar cells. (b) Characteristic photo I–V curves for the solar cell under AM1.5 irradiation. (c) The dark I–V curve and (d) the external quantum efficiency (EQE) of the amorphous silicon solar cell. (Copyright © 2015, Royal Society of Chemistry)

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response of the devices within the 300–800-nm wavelengths. It shows a maximum peak of 65%, at 500 nm (2.6 eV) of the spectrum. This result clearly demonstrates that there is no major spectrum-specific loss mechanism limiting the performance of the cell prepared by LVD.

9.2.3  Conclusion The LVD method is a simpler process than our previous spin-coating process. It works under atmospheric pressure and resolves two critical problems associated with spin-coating: the high void and the levels of oxygen concentration. We deposited doped a-Si:H films by LVD at a temperature of 370 °C in nitrogen gas at atmospheric pressure. We clarified that it is easy to dope the a-Si:H film either n- or p-type by dissolving appropriate amounts of white phosphorus or decaborane, respectively, in the liquid CPS before vaporization. The solar cell adopting either n- or p-type a-Si:H film for its constituent layer was successfully operated. By using liquid improved LVD apparatus which has two parallel substrates for film deposition, radicals were generated on the substrate at low temperatures, which yielded the formation of a high-quality film on the opposite substrate. When using this technique for intrinsic layer preparation in solar cells, we achieved the same level as PECVD on a flat substrate. Without plasma damage, it has potential applications in amorphous silicon solar cells, HIT solar cells, thin-film transistors, and other devices.

9.3  Application of Liquid Silicon for HBC-Type Solar Cells In this section we introduce the application of liquid silicon for a-Si heterojunction solar cell (SHJ-SC) which is one of the commercial solar cells in the marketplace. Among SHJ-SCs a heterojunction back contact solar cell (HBC-SC) is ideal because of its high-conversion efficiency [8–10]. Doped liquid Si could be printed to make pattered p and n-type a-Si films for heterojunction back-contacts. But the problem is that the annealing temperature around 400 °C after printing is required. It would degrade the property of a-Si passivation films when they are deposited by conventional plasma-enhanced chemical vapor deposition (PECVD) because of a typical deposition temperature of it is about 200 °C or less [11]. If the liquid-source vapor deposition (LVD) method were used for a passivation film, however, better stability for it would be expected. Since wafers are heated at 360  °C or more during the deposition, a-Si films formed may have high-thermal stability against post-­annealing at 400  °C.  Here we report the stability of a-Si films formed by LVD using CPS vapor against post-deposition annealing, air exposure, and 1-sun light soaking.

9.3  Application of Liquid Silicon for HBC-Type Solar Cells

183

9.3.1  Experimental Procedure The deposition chamber is used for the LVD, of which schematic view is shown in Fig. 9.8 (refer to Fig. 5.7 in Chap. 5 for more detail). We prepared floating-zone-­ grown, n-type, double-side mirror-polished, (100)-oriented c-Si wafers with a resistivity of 1–5 Ωcm, a thickness of 290 μm, and a bulk minority carrier lifetime of >10 ms and cleaved them into 20 × 20 mm2-sized pieces. We set three c-Si pieces in the deposition chamber parallel to each other at an interval of approximately 2 mm after the removal of native oxide in 3% HF diluted with deionized water. The substrates were heated at 360–400 °C during a-Si deposition. CPS was vaporized by heating at 85 °C and introduced into the deposition chamber. 15–20 nm-thick a-Si films were formed on the c-Si substrates after 75  min deposition. We then performed the additional annealing of the substrates in the deposition chamber at 200 °C for 1 h to improve the quality of a-Si/c-Si interfaces. To evaluate the passivation ability of the a-Si films, the effective minority carrier lifetime (τeff) of the samples was measured by microwave photoconductivity decay (μ-PCD) after taking them out of the chamber. The τeff values shown below are maximum values in a 20  ×  20  mm2 sample area. Furthermore, to investigate the thermal stability of the passivation quality of the films, additional post-deposition annealing was performed in a tubular furnace at 360–400 °C under nitrogen atmosphere. The τeff measurement and 30  min furnace annealing were repeated alternately. We also evaluated the thermal stability of a c-Si wafer passivated with PECVD a-Si films deposited at 150 °C under a pressure of 7 Pa and a SiH4 flow rate of 20 sccm using a plasma power and a frequency of 20 W and 27 MHz, respectively, for comparison. To confirm the stability of LVD a-Si passivation films against air exposure, c-Si wafers passivated with LVD a-Si films formed at 360–400 °C were exposed to air under 64%RH at 23 °C for up to 1 week, during which the samples were wrapped in aluminum foils to exclude the effect of degradation by light soaking. We measured the τeff of these samples for every 12 or 24 h. For the investigation of the light-­ induced degradation of LVD a-Si films, we laminated c-Si wafers passivated with Fig. 9.8  The schematic view of a LVD chamber used in this study

184

9  Development of Solar Cells Using Liquid Silicon

Fig. 9.9  Schematic of a laminated c-Si wafer passivated with LVD a-Si films used for light soaking. (Copyright © 2016 The Japan Society of Applied Physics)

LVD a-Si films using glass, ethylene vinyl acetate (EVA) copolymer and a back sheet, by which we can avoid the effect of air exposure. A laminated sample structure is schematically shown in Fig. 9.9. 1-sun light was illuminated to the laminated samples for up to 2700 min. We deposited a-Si films also on glass substrates under the same conditions mentioned above and measured the thickness of the a-Si films by spectroscopic ellipsometry, by which the emergence of the epitaxial growth of Si can be examined.

9.3.2  Thermal Stability of LVD a-Si Passivation Films Figure 9.10a shows the τeff of a c-Si wafer passivated with PECVD a-Si films as a function of the duration of post-annealing at 380 °C. A rapid reduction in τeff was confirmed after 30 min annealing. This is probably due to the desorption of hydrogen atoms from a-Si films and a-Si/c-Si interfaces and the resulting deterioration of the interface quality. Figure 9.10b–d shows the τeff values of c-Si wafers passivated with LVD a-Si films as a function of the duration of post-annealing at 360, 380, and 400  °C, respectively. A markedly high τeff of up to ∼350  μs is obtained for c-Si wafers passivated with LVD i-a-Si films before post-deposition annealing. These values are smaller than those reported by Mews et al. prepared by using a-Si films formed by the spin-coating and annealing of Si ink [12]. However, it should be emphasized that Mews et  al. have used considerably thick (>50  nm) a-Si films, while our LVD a-Si films of  2. Alternatively, for samples annealed in oxygen, the intensity and FWHM of XRD lines of RuO2 did not depend on the amount of MEA. The slight resistivity decrease with MEA addition up to MEA/Ru = 2 can be related to slightly slowed decomposition of the precursor with MEA (Fig.  12.20). A slowed and gradual decomposition up to higher temperatures may allow the RuO2 film to be more uniform and denser.

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12  Improvement of Solid Through Improved Solutions and Gels (1): Utilization…

12.3.4  Effects of Amine Structure The effects of amine structures (including alkanolamines, alkyl amines, and amino acids) on the resistivity of the Ru0 and RuO2 thin films and the thermal decomposition of the Ru precursor were investigated. The resistivity values of the thin films prepared from various amine-added Ru solutions (amine/Ru = 2 equiv.) and annealed in nitrogen or oxygen at 500  °C for 10  min are shown in Table  12.4. Under an annealing atmosphere of nitrogen, the resistivity apparently depends on the structure of the added amine. Two characteristic features of the change in resistivity were observed. (1) Amines having carboxylic acid (amino amine) or alcohol (alkanolamine) groups resulted in resistivity up to one order lower than alkyl amines. (2) The primary amines led to lower resistivity than the secondary and tertiary amines. In the case of oxygen annealing, the resistivity was much less differentiated among the structures of amines due to less affected thermal decomposition by MEA. The surface morphology of the films after annealing in nitrogen was studied by AFM (Fig. 12.27). Samples with lower resistivity have smaller grain sizes and lower surface roughness, i.e., the dependence of the surface morphology on the structures of amines is the same as that of the resistivity. The surface roughness values of the films prepared from solutions containing triethanolamine (TEA), dimethylaminoethanol (DMAE), and n-butylamine (Bu-A) are much larger than that of MEA, 4-amino-1-butanol (BuOH–A), and b-alanine (β-Ala). The influence of the structure of the added amine can be understood after consideration of the coordinating behaviors of different amines. Alkanolamines and amino acids, such as MEA, BuOH-A, and b-Ala, are multidentate ligands, coordinating to metal atoms at multiple points, and therefore well stabilizing the coordinated precursor structure. An example of MEA coordination through both the amino and hydroxyl groups is discussed in Ref.[66]. In addition, a primary amine should have a higher coordinating ability than the secondary and tertiary amines because of its less shielded amino end that allows better coordination to metal atoms. A stably coordinated Ru precursor underwent a reductive and slowed decomposition in nitrogen, as shown in the thermal analysis data, allowing sufficient reduction of Ru4+ to form Ru0 films. As a result, among all the studied solutions, those with MEA, BuOH-A, and β-Ala led to films of the lowest resistivity after annealing in nitrogen. The photographs of the samples annealed in nitrogen from solutions with these amines are shown in Fig. 12.28. The Ru0 samples (annealed in nitrogen) with high conductivity, as mentioned above, possess metallic luster, while those with lower conductivity have a color closer to the RuO2 films (annealed in oxygen). Therefore, amines with higher coordination ability also show higher reducing ability. The XRD patterns did not reveal an unambiguous difference among the samples, all showing Ru0 as the dominating phase and some having weak RuO2 peaks. Some oxide phase (e.g., intergranular thin oxide) may not have well crystallized and could not be detected by XRD.

Additive amines Name None Monoethanolamine Diethanolamine Triethanolamine Dimethylaminoethanol 3-Amino-1-propanol 4-Amino-1-butanol Propylamine n-Butylamine 2-Ethylhexanamine β-alanine

Amino alcohol Amino alcohol Amino alcohol Amino alcohol Amino alcohol Amino alcohol Alkyl amine Alkyl amine Alkyl amine Amino acid

Structure

Primary Secondary Tertiary Tertiary Primary Primary Primary Primary Primary Primary

Boiling point (°C) – 171 217 335 133 186 206 48 78 169 –

500 °C × 10 min in N2 Thickness (nm) Resistivity (Ωcm) 28.1 2.5 × 10−4 25.3 2.1 × 10−5 35.1 3.8 × 10−5 38.8 4.4 × 10−5 30.3 7.0 × 10−5 27.1 3.1 × 10−5 23.6 2.3 × 10−5 22.9 2.1 × 10−4 31.6 2.3 × 10−4 38.4 2.0 × 10−4 24.6 2.1 × 10−5

500 °C × 10 min in O2 Thickness (nm) Resistivity (Ωes) 60.7 1.2 × 10−3 60.1 4.3 × 10−4 73.5 6.1 × 10−4 91.1 6.5 × 10−4 68.6 7.0 × 10−4 74.9 6.7 × 10−4 77.5 7.1 × 10−4 48.4 8.4 × 10−4 59.4 8.4 × 10−4 45.5 9.0 × 10−4 76.7 5.5 × 10−4

Table 12.4  Ru (under nitrogen) and RuO2 (under oxygen) thin films prepared from precursor solutions with various amines. Copyright © 2015, Royal Society of Chemistry

12.3 Ru and RuO Thin Film 261

262

12  Improvement of Solid Through Improved Solutions and Gels (1): Utilization…

D

d

D

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ZD^͗ϯ͘ϴϲŶŵ

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ɴͲůĂ

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Fig. 12.27  AFM images of thin films prepared from precursor solutions with various amines at 500 °C for 10 min under a nitrogen atmosphere. (Copyright © 2015, Royal Society of Chemistry)

1

2 0($

7($

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Fig. 12.28  Photographs of samples prepared using different amines. (Copyright © 2015, Royal Society of Chemistry)

To obtain further insight into the coordination structure of amine–ruthenium complexes, high-energy X-ray diffraction measurements were performed for the solution and their derived gels after drying. The total structure factors, S(Q), were derived from the measured raw data, which were then Fourier transformed to total correlation functions, T(r), and pair distribution functions, G(r) (Fig.  12.29). The peaks indicated by arrows in Fig. 12.29a were assigned according to the structural data for related ruthenium complexes [69, 71], and RuO2.26 Peak 1 arises from the C–C and C–O correlations, which are mainly contributed by the solvent, while peaks

12.3 Ru and RuO Thin Film

263

Fig. 12.29 (a) Total correlation functions, T(r), and (b) pair distribution functions, G(r), of the solutions and their 150 °C-dried gels, with the addition of different amines in the solutions. The starting material, ruthenium nitrosyl acetate, was measured as a reference. (Copyright © 2015, Royal Society of Chemistry)

2 and 3 are assigned to Ru–N correlations (in Ru–NO) and Ru–O(N) correlations (N in amine coordinated to Ru), respectively. Peak 4 can be attributed to O to O(N) correlation (O=C-O in the solvent and O–Ru–O(N) in the complex), and both peaks 5 and 6 are assigned to Ru to Ru correlations. Peak 6 is considered to be the correlation from the Ru to Ru associated with Ru–O–Ru in high-nuclearity (four or more ruthenium atoms) complexes, the presence of which is suggested by the results of mass spectrometric analysis presented above. The Ru–O–Ru core of such complexes may bear a RuO2-like structure which has a Ru to Ru distance (with a multiplicity of eight) at the position of peak 6 in addition to another one (with a multiplicity of two) at the position of peak 5 [72]. Peak 5 is also considered to be contributed by the Ru to Ru correlations associated with Ru–O–Ru in lower-nuclearity complexes. A prominent effect of amines is observed in the dried gels. The gel without amine addition has a spectrum close to that of the starting material, ruthenium nitrosyl acetate. With the addition of amine, the correlation peaks of the next nearest neighboring Ru to Ru decreased (peaks 5 and 6), particularly for the one with a larger distance centered at 3.5–3.6 angstroms (peak 6). The magnitude of decrease in peak intensity corresponds to the coordination ability of the amines and the conductivity (as well as color) of the final films as presented above, i.e., MEA > DMAE>Bu-A. This suggests that, with amine coordination, the number of ruthenium atoms in the complex was reduced. Particularly, complexes with four or more ruthenium atoms were significantly reduced (peak 6). Correspondingly, in the pair distribution function of the gels (Fig. 12.29), the periodicity disappeared with stronger amine coordination. The periodicity indicates the correlation between complex clusters, and the periodic distance (~1 nm) is a measure of the cluster size. The simultaneous decrease in peak 6 in Fig. 12.29a and periodicity in Fig. 12.29b indicates the destruction of the multinuclear structure by amine coordination. This is consistent with our observation that, with increasing MEA addition, the high-molecular peaks in the mass spectra decreased. In other words, stable coordination of amine molecules prevented the formation of a large Ru–O–Ru core structure. The smaller Ru–O–Ru core as well as

264

12  Improvement of Solid Through Improved Solutions and Gels (1): Utilization…

the increased reducing moiety provided by the coordinated amine around led to effective reduction of the core in pyrolysis. This is the working mechanism of amines to inhibit RuO2 formation and facilitate Ru0 formation. In the solution, the effect of amine on the structure is not as apparent as in the gel. This can be attributed to the presence of a large amount of solvent (propionic acid), which itself is a strong coordinator and suppressed the coordination of amine. The complex cluster sizes, as indicated by the periodicity in the pair distribution function (Fig. 12.29b), are all ≈0.5 nm for the solutions with and without different types of amines. The increase of cluster sizes upon drying suggests that the clusters underwent further agglomeration/condensation to form larger ones. For samples with amines of high coordination ability such as MEA, the complexes underwent less agglomeration/condensation or, instead, even partially dissociated to smaller structures stabilized by strongly coordinated amine molecules around, resulting in the decrease of peak 6 in Fig. 12.29a.

12.3.5  Properties of the Prepared Ru0 and RuO2 Thin Films The properties of the Ru0 and RuO2 thin films prepared from Ru(NO)(OAC)3 with 2 equiv. MEA were examined in more detail. In addition to a nitrogen atmosphere, the use of vacuum in annealing also resulted in Ru0 films. It is notable that Ru0 thin films were formed in nitrogen or vacuum in the absence of a reducing atmosphere (e.g., hydrogen). In this system, organic components in the amine-coordinated structure have acted as reducing agents to form metallic Ru. The surface morphology of the films was studied by AFM and SEM, which revealed that the flatness of the film surface after nitrogen annealing was much higher than that after vacuum and oxygen annealing. The root mean square roughness (RMS) values of the films annealed in nitrogen (Ru0 film) and oxygen (RuO2 film) were 0.89 and 7.25 nm, respectively [65]. The film annealed in vacuum (Ru0 film) possessed the highest RMS value (11.7  nm). It is probable that this large RMS value is caused by the breakage of the film through shrinkage along the surface to form voids. We measured the resistivity of the thin films as a function of annealing temperature and atmosphere (Fig. 12.30). The resistivity of an oxygen-annealed thin film decreased gradually with increased annealing temperature. Under the nitrogen atmosphere, the resistivity decreased rapidly at 300 °C. The resistivities of the thin films prepared at 500  °C for 10  min under nitrogen, vacuum, and oxygen were 2.1 × 10−5, 8.9 × 10−5, and 4.3 × 10−4 Ω cm, respectively; that is, the resistivity after nitrogen annealing was much lower than that after vacuum and oxygen annealing. The conductivity of these films was similar to that of vacuum-processed Ru0 and RuO2 thin films (1.9 × 10−5 and 3.5 × 10−4 Ωcm, respectively) [50, 73]. Their nanometer thickness (~25 nm, Table 12.4) coupled with such high conductivity indicates high quality of the films. In nitrogen, the Ru0 film started to form at 300 °C, with increasing amount and crystallinity at higher temperatures [65]. In contrast, it was formed in vacuum even

265

12.3 Ru and RuO Thin Film

(

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7HPSHUDWXUH㼻& Fig. 12.30  Resistivity of the thin films prepared by a solution process from the precursor solution (Ru(NO)(OAc)3–MEA) under various annealing temperature and atmospheric conditions. (Copyright © 2015, Royal Society of Chemistry) Table 12.5  Elemental composition of the Ru (under nitrogen) and RuO2 (under oxygen) thin films, in atomic% (values in parentheses are the atomic ratios to Ru)a (Copyright © 2015, Royal Society of Chemistry) No. Ru1 Ru2 RuO a

Coating times/ annealing gas 1/N2 2/N2 2/O2

Ru 61.0 (1.00) 93.3 (1.00) 31.6 (1.00)

O 37.0 (0.61) n.d. 63.8 (2.02)

C 1.5 (0.03) 3.7 (0.04) 1.7 (0.05)

H n.d. 2.9 (0.03) 2.5 (0.08)

Cl 0.5 (0.008) n.d. 0.4 (0.01)

n.d. = below the detection limit and not detectable

at a low temperature of 200 °C, indicating that thermal decomposition of the Ru precursor took place at a rather low temperature. The resistivity of films annealed in vacuum (at 400 °C, 1.8 × 10−4 Ωcm) is much higher than that of films in nitrogen above 400 °C (at 400 °C, 2.7 × 10−5 Ωcm). The films annealed in vacuum had voids which were not observed in the films annealed in nitrogen. The voids must be caused by volume shrinkage during thermal decomposition and crystallization in vacuum. Because the quality of Ru0 thin films depends on the thermal decomposition behavior, which is in turn determined by the amine ligands, further optimization of the precursor may allow high-quality films to be produced at 200  °C or lower temperatures. Elemental composition of the films obtained from the solution with MEA/Ru = 2 was determined, and the results are presented in Table 12.5. The O/Ru molar ratio of the RuO2 film is 2.02, consistent with the stoichiometric ratio of 2, while the ratio of the Ru0 films is 0.61 (one time spin-coated) or less than the detection limit (1cm2 V−1 s−1 0.3 V/dec 0 V >105

To obtain oxide patterns with the desired properties by the n-RP process, several parameters such as initial film thickness, drying temperature (Tdry), imprinting temperature, imprinting pressure, and maximum temperature during imprinting (Tmax) were optimized for each layer. We have formulated the starting precursor materials so as to give them thermoplastic properties during thermal imprinting. The RuO gel

19.4 Active-Matrix Backplane by n-RP

581

derived from a nitrosyl acetate complex had thermoplastic property. For the cases of InO, ITO, and InGaO, gels having thermoplastic properties were obtained from acetylacetonate complexes. SiO2 and SQ gels were found to have thermoplastic properties with appropriately n-RP conditions. All kinds of the printed patterns, each of which dimension is a 100 μm by 100 μm square, are shown in Fig. 19.23. Going through the optimization of patterning conditions, we recognized that Tdry and Tmax were two critical parameters for making well-defined patterns. These temperatures were decided according to thermal analysis data measured by

Fig. 19.23 (a) RuO source/drain electrode patterns, (b) ITO source/drain patterns, (c) InO channel patterns, (d) InGaO channel patterns, (e) SiO2 channel stopper patterns, and (f) SQ channel stopper patterns. (Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

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19  Device Fabrication by n-RP

Fig. 19.24  TG-DTA traces for (a) RuO, (b) ITO, (c) InO, (d) InGaO, (e) SiO2, and (f) SQ. (Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

thermogravitometry-differential thermal analysis (TG-DTA). The TG-DTA results of all the precursor solutions are shown in Fig. 19.24. The precursor solutions have some features in common; all the solutions show two mass loss steps, with the first step occurring in the range of 50–150  °C and the second step occurring over 250 °C. The first step is caused by evaporation of solvent because this step is accompanied by endothermic peaks in the DTA curves. The second step is due to pyrolysis of the oxide precursors and is accompanied by an exothermic reaction. We can identify that the precursor solution transforms into a gel state and realized thermoplastic property between the first and second steps. We set a drying temperature Tdry at the temperature over the end of the first step in most cases. In the case where Tdry was located at close to the second step, however, the precursor gel did not exhibit a thermoplastic property. In the discussion of Chap. 14, it was concluded that the gels had no thermoplastic properties when the metal–oxygen networks in the gel was formed.

19.4 Active-Matrix Backplane by n-RP

583

Fig. 19.25  Examples of destroyed patterns of ITO when Tmax was set too high. (a) Tmax = 200 °C and (b) Tmax = 220 °C. (Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Fig. 19.26  Enlarged TG trace for the ITO precursor solution

20

15

100% @100 qC

TG(%)

83% @150 qC 75% @225 qC

10

5

0

100

200

300

400

500

Temperature (°C)

Once such metal–oxygen networks are formed, the gel becomes hard enough to be processed by n-RP. Regarding Tmax in a n-RP process, first, Tmax should be higher than Tdry; otherwise, the oxide gel showed no plasticity during imprinting. Note that the oxide gels had a critical temperature above which they became soft (Tc); the oxide gels exhibited no deformation when Tmax was too far below Tc. However, higher temperature is not always better, and gel patterns were destroyed when Tmax was set too far above Tc. We found that ITO gel patterns were destroyed when imprinted at Tmax = 200 °C and 220 °C, as shown in Fig. 19.25a, b, respectively, which are both higher than the Tc of 180 °C for an ITO gel. The fingers and holes are observed in the ITO patterns, and the patterns appear to become larger when the temperature is getting higher. The pattern destruction can be a direct result of the thermal decomposition of the oxide gel that was observed by TG-DTA. An enlarged TG trace for the ITO solution is shown in Fig.  19.26. This trace indicates that the mass gradually decreased as the temperature increased. The mass decreased by 75% between 100 and 225 °C. The mass loss means that organic substances in the metal complexes, together with a small amount of residual solvent in the precursor gel, desorbed or decomposed to become gases, which escaped gradu-

584

19  Device Fabrication by n-RP

ally from the gel as drying temperature increased. The gel film was pressed and sealed inside the mold during the n-RP process. Judging from the results of TG-DTA and the environmental conditions during the n-RP, it is reasonable to assume that the gases generated by drying the gel at high temperature cannot escape to the outside of the mold, leading to the destruction of the soft gel pattern. In our experience, the proper Tdry was less than 150 °C, and Tmax was set in the range of 150–200 °C.

19.4.7  TFT Fabrication Using an Alignment System After making sure an alignment deviation is less than 5 μm, we proceeded to fabricate TFTs with the parameters described in the above. Initially, we designed a pixel TEG to investigate the process ability of the oxide materials derived from specially designed precursor solutions and to confirm whether they could be used for TFTs. The TEG design is shown in Fig. 19.27a. Both the channel length and width were 20 μm. Using our alignment system and the optimized n-RP conditions, we successfully fabricated two types of TEG (type 1 and type 2) patterns using n-RP. A typical optical microscope image of a TEG is shown in Fig. 19.27b, and information about the constituent layers in types 1 and 2 are tabulated in Table 19.6. The measured transfer curves for types 1 and 2 TFTs are presented in Fig. 19.28a, b, respectively. The electrical characteristics of type 1 were as follows: on/off ratio, 102; subthreshold swing, 2.95 V/decade; field-effect mobility, 0.004 cm2 V−1 s−1; and threshold voltage, −0.19  V.  The electrical characteristics of type 2 were as follows: on/off ratio, 103; subthreshold swing, 3.09 V/decade; field-effect mobility, 0.08cm2 V−1 s−1; and threshold voltage, 7.22 V.

Fig. 19.27  Top-view images of the TEG design and the fabricated TFT. (a) TEG design (gate, gate electrode; source/drain, source and drain electrodes; Ch, channel; and CS, channel stopper). (b) A typical optical microscope image of a TEG fabricated by n-RP. (Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

19.4 Active-Matrix Backplane by n-RP

585

Table 19.6  Used materials in TEG TFT Type1 RuO LaZrO InO SiO2 RuO

Gate electrode Gate insulator Channel Channel stopper SiO2 Source and drain RuO

10-5

Type2 RuO SiO2 InGaO SiO2 ITO

10-5

(a)

10-6

Pattern shape Convex Convex Convex Concave Convex

(b)

10

-6

I(A)

I(A)

10-7 10-8

10-7 10-8

10-9 10-10 -10

10-9 -5

0

5 VG (V)

10

15

-15

-10

-5

0 VG (V)

5

10

15

Fig. 19.28  Transfer characteristics of TFTs fabricated by n-RP. (a) Type 1 and (b) type 2. (Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

The reason for the low performance of these TFTs could be attributed to the poor properties of both the insulator/channel and source–drain/channel interfaces. At the insulator/channel interface, the rough surface of the LaZrO film could seriously affect the channel layer. The source–drain/channel interfaces in types 1 and 2 TFTs were InO/RuO and InGaO/ITO, respectively. The on/off ratios and field-effect mobilities of type 2 TFTs were higher than those of type 1 TFTs. In the type 1 TFT, the InO/RuO interface can be considered to have a large contact resistance because RuO and Ru metal have p-type carriers, whereas InO has n-type carriers. However, InO-based oxides were used as channel and electrodes layers; therefore, the material affinity at the interface in the type 2 TFT was better than that in the type 1 TFT. Finally, cross-sectional observation by a transmission electron microscopy (TEM) was performed to confirm whether a clear layer structure was achieved. A cross-sectional image of the TFT is shown in Fig. 19.29, in which each layer in the TFT structure can be clearly observed. These results show that the selected materials and the developed alignment system satisfy the requirements of TFT fabrication. In conclusion, we succeeded in fabricating operable oxide TFTs, in which all the layers were patterned using the n-RP printing method.

586

19  Device Fabrication by n-RP

Fig. 19.29  Cross-sectional TEM image of the TFT in the type 2 TEG (source/drain, source and drain electrodes; CS channel stopper, Ch channel, Ins insulator, and gate gate electrode). (Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

19.4.8  Fabrication of AM-BP As the final work in this section, two types of AM-BP (TFT array) were fabricated: AM-BP 1 and AM-BP 2. The materials used in these AM-BPs are tabulated in Table 19.7. In this experiment, we employed platinum (Pt) as a gate electrode because the gate electrode requires a high carrier density and good conductivity to operate an AM-BP. Optical microscope and atomic force microscope images of these AM-BPs are shown in Figs. 19.30 and 19.31, which show the AM-BPs with well-defined patterns and tight alignment. Representative electronic properties of a TFT in the AM-BP 2 are shown in Fig. 19.31d. The electrical characteristics are as follows: on/ off ratio, 104; subthreshold swing, 1.6 V/decade; and threshold voltage, 5.4 V.

19.4.9  Conclusions and Outlook Here we reported on the fabrication of all-solution-processed oxide AM-BPs using a thermal nanoimprint method, termed n-RP, with a tight alignment system. We developed an alignment system for n-RP using a UV-cured resin. The UV-cured resin was the proper glue to fix a mold to the substrate because the adhesive properties of the UV-cured resin remained stable even when the imprinting temperature was increased to over 150 °C. A tight alignment system capable of alignment within 5 μm was realized, although the method is simple. We prepared oxide gels having thermoplastic properties and optimized the n-RP conditions to endow the gels with

19.4 Active-Matrix Backplane by n-RP

587

Table 19.7  Material of each layer used in TFT arrays Gate electrode Gate insulator Channel Channel stopper Source and drain

TFT- Array1 Pt SiO2 InGaO SQ ITO

TFT-Array2 Pt LaZrO InGaO SQ ITO

Pattern shape Convex Convex Convex Concave Convex

Fig. 19.30  Images of TFT-array 1 fabricated by n-RP. (a) Photograph, (b) optical microscope image, (c) AFM image of the TFT stacking structure and (d) schematic diagram of (c). (Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

good process ability for making oxide patterns. The drying temperature of the gel films and the maximum temperature during the n-RP process were important factors; therefore, these temperatures were optimized based on the thermal behavior of the gels, as analyzed by TG-DTA measurements. Next, TEGs were fabricated to confirm the process conditions for the constituent layers and to realize the operation of TFTs. TEGs with all layers prepared by n-RP

588

19  Device Fabrication by n-RP

Fig. 19.31  Image and properties of TFT-array 2 fabricated by n-RP. (a, b) Optical microscope images, (c) schematic diagram, and (d) transfer characteristics of a representative TFT. (Copyright © 2016 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

were successfully developed using the new alignment system, and their TFT operation was confirmed. Based on these results, a AM-BP was developed, in which alignment to within 5 μm was realized over a 6 mm × 6 mm area. We observed the electric field effect of the TFTs in this AM-BP.  At this stage it is difficult to conclude that electrical characteristics of our AM-BP are suitable for the operation of an active-matrix backplane for a display. It is notable, however, that the AM-BPs printed by n-RP can be considered to be a significant progress in the field of printed electronics.

References 1. T.  Kaneda, D.  Hirose, T.  Miyasako, P.T.  Tue, Y.  Murakami, S.  Kohara, J.  Li, T.  Mitani, E. Tokumitsu, T. Shimoda, J. Mater. Chem. C 2(1), 40 (2014) 2. T. Miyasako, B.N.Q. Trinh, M. Onoue, T. Kaneda, P.T. Tue, E. Tokumitsu, T. Shimoda, Appl. Phys. Lett. 97, 173509 (2010)

References

589

3. J. Li, H. Kameda, B.N.Q. Trinh, T. Miyasako, P.T. Tue, E. Tokumitsu, T. Mitani, T. Shimoda, Appl. Phys. Lett. 97, 102905 (2010) 4. P.T.  Tue, T.  Miyasako, J.  Li, H.T.C.  Tu, S.  Inoue, E.  Tokumitsu, T.  Shimoda, IEEE Trans. Electron Devices 60, 320 (2013) 5. T. Shimoda, J. Li, P. T. Tue, H. Tsukada, Japan patent application no. 2013-194038, (2013) 6. A.C. Arias, S.E. Ready, R. Lujan, W.S. Wong, K.E. Paul, A. Salleo, M.L. Chabinyc, R. Apte, A. Robert, Y.W. Street, P. Liu, B. Ong, Appl. Phys. Lett. 85(15), 3304 (2004) 7. K.E. Paul, W.S. Wong, S.E. Ready, R.A. Street, Appl. Phys. Lett. 83(10), 2070 (2003) 8. T. Shimoda, Y. Matsuki, M. Furusawa, T. Aoki, I. Yudasaka, H. Tanaka, H. Iwasawa, D. Wang, M. Miyasaka, Y. Takeuchi, Nature 440(7085), 783 (2006) 9. H.  Sirringhaus, T.  Kawase, R.H.  Friend, T.  Shimoda, M.  Inbasekaran, W.  Wu, E.P.  Woo, Science 290(5499), 2123 (2000) 10. J.Z. Wang, Z.H. Zheng, H.W. Li, W.T.S. Huck, H. Sirringhaus, Nat. Mater. 3(3), 171 (2004) 11. P.  Beecher, P.  Servati, A.  Rozhin, A.  Colli, V.  Scardaci, S.  Pisana, T.  Hasan, A.J.  Flewitt, J. Robertson, G.W. Hsieh, F.M. Li, A. Nathan, A.C. Ferrari, W.I. Milne, J. Appl. Phys. 102(4), 043710 (2007) 12. D.H. Lee, Y.J. Chang, G.S. Herman, C.H. Chang, Adv. Mater. 19(6), 843 (2007) 13. Y. Nakamura, S. Matsumoto, S. Arae, Y. Sone, Y. Hirano, Ricoh Tech. Rep. 39, 07 (2014) 14. D. Kim, Y. Jeong, K. Song, S.-K. Park, G. Cao, J. Moon, Langmuir 25(18), 11149 (2009) 15. M. Janeta, L. John, J. Ejfler, S. Szafert, RSC Adv. 5(88), 72340 (2015) 16. J. Li, E. Tokumitsu, M. Koyano, T. Mitani, T. Shimoda, Appl. Phys. Lett. 101(13), 132104 (2012) 17. K. Nagahara, D. Hirose, J. Li, J. Mihara, T. Shimoda, Ceram. Int. 42(6), 7730 (2016) 18. P.T. Tue, K. Fukada, T. Shimoda, High-performance oxide thin film transistor fully fabricated by a direct rheology-imprinting. Appl. Phys. Lett. 111, 223504 (2017) 19. Y. Murakami, J. Li, D. Hirose, S. Kohara, T. Shimoda, J. Mater. Chem. C 3(17), 4490 (2015) 20. P.T. Tue, S. Inoue, Y. Takamura, T. Shimoda, Appl. Phys. A 122(6), 1 (2016) 21. P.T.  Tue, T.  Miyasako, J.  Li, H.T.C.  Tu, S.  Inoue, E.  Tokumitsu, T.  Shimoda, IEEE Trans. Electron Devices 60(1), 320 (2013) 22. O. F Göbel, M. Nedelcu, U. Steiner, Adv. Funct. Mater. 17(7), 1131 (2007) 23. R. Ganesan, J. Dumond, M.S.M. Saifullah, S.H. Lim, H. Hussain, H.Y. Low, ACS Nano 6(2), 1494 (2012) 24. S.-H.K.  Park, C.-S.  Hwang, D.-H.  Cho, S.M.  Yoon, S.  Yang, C.  Byun, M.  Ryu, J.-I.  Lee, O.S. Kwon, W.-S. Cheong, H.Y. Chu, K.I. Cho, SID Symp. Dig. Tech. Pap. 40(1), 276 (2009) 25. J. Li, P. Zhu, P. T. Tue, S. Inoue, T. Shimoda, submitted to the J. Mater. Chem. C 26. W. Xu, D. Liu, H. Wang, L. Ye, Q. Miao, J.-B. Xu, Appl. Phys. Lett. 104(17), 173504 (2014) 27. S.Y. Lee, Trans. Electr. Electron. Mater. 16(3), 03 (2015) 28. J.K. Jeong, H.W. Yang, J.H. Jeong, Y.-G. Mo, H.D. Kim, Appl. Phys. Lett. 93(12), 123508 (2008) 29. Phan, Tue; Li, Jinwang; Shimoda, Tatsuya, “Nano-Rheology Printing of Sub-0.2 μm Channel Length Oxide Thin-Film Transistors” to be published in Nano Futures 30. P.T. Tue, S. Inoue, Y. Takamura, T. Shimoda, Combustion synthesized indium-tin-oxide (ITO) thin film for source/drain electrodes in all solution-processed oxide thin-film transistors. Appl. Phys. A 122(6), 1–8 (2016). https://doi.org/10.1007/s00339-016-0156-y 31. Y.  Murakami, J.  Li, T.  Shimoda, Highly conductive ruthenium oxide thin films by a low-­ temperature solution process and green laser annealing. Mater. Lett. 152, 121–124 (2015). https://doi.org/10.1016/j.matlet.2015.03.084 32. C. R. Kagan, P. Andry, Thin film transistors, CRC Press, ISBN-10:0824709594, (2003) 33. P.T. Tue, K. Fukada, T. Shimoda, High-performance oxide thin film transistor fully fabricated by a direct rheology-imprinting. Appl. Phys. Lett. 111, 223504 (2017) 34. D. Hirose, H. Koyama, K. Fukada, Y. Murakami, K. Satou, S. Inoue, T. Shimoda, All-solution-­ printed oxide thin-film transistors by direct thermal nanoimprinting for use in active-matrix arrays. Phys. Status Solidi A 214, 1–15 (2016). https://doi.org/10.1002/pssa.201600397.

590

19  Device Fabrication by n-RP

35. Y. Murakami, P. T. Tue, H. Tsukada, J. Li, T. Shimoda, Preparation of ruthenium metal and ruthenium oxide thin films by a low-temperature solution process, Proc. 20th Int. Disp. Workshops (IDW), (2013), pp. 1573–1576 36. D. Hirose, T. Shimoda, Evaluating the state of indium-tin oxide gels via estimation of their cohesive energy, Jpn. J. Appl. Phys. 53, 02BC01-1-02BC01-7 (2014) 37. J.H. Park, W.J. Choi, S.S. Chae, J.Y. Oh, S.J. Lee, K.M. Song, H.K. Baik, Structural and electrical properties of solution-processed gallium-doped indium oxide thin-film transistors. Jpn. J. Appl. Phys. 50, 080202 (2011) 38. D. Kim, C.Y. Koo, K. Song, Y. Jeong, J. Moon, Compositional influence on sol-gel-derived amorphous oxide semiconductor thin film transistors. Appl. Phys. Lett. 95(0), 252103 (2009)