S

“This book contains a collection of the most recent studies written by highly recognized ilicon and authors in the field. The book is valuable especially for young scientists seeking inspiration from the most fascinating discoveries in the field. The book can also serve an excellent reference for experts.” Prof. Vassilios Vargiamidis Concordia University, Montreal, Canada

Nanoscale materials are showing great promise in various electronic, optoelectronic, and energy applications. Silicon (Si) has especially captured great attention as the leading material for microelectronic and nanoscale device applications. Recently, various silicides have garnered special attention for their pivotal role in Si device engineering and for the vast potential they possess in fields such as thermoelectricity and magnetism. The fundamental understanding of Si and silicide material processes at nanoscale plays a key role in achieving device structures and performance that meet S real-world requirements and, therefore, demands investigation and exploration of nanoscale device applications. This book comprises the theoretical and experimental ilicide analysis of various properties of silicon nanocrystals, research methods and techniques to prepare them, and some of their promising applications.

Yu Huang is a faculty member in the Department of Materials Sciences and Engineering at the University of California, Los Angeles (UCLA), USA. She received her PhD in physical chemistry from Harvard University, USA. Her research focuses on the fundamental principles governing nanoscale material synthesis and assembly at the molecular N edited by level, which can be utilized to design nanostructures and nanodevices

with unique functions and properties to address critical challenges in anowires Yu Huang electronics, energy science, and biomedicine. She has received several recognitions King-Ning Tu including MRS student award, the Grant Prize Winner of Collegiate Inventors’ Competition, the IUPAC Young Chemist Prize, Lawrence Postdoctoral Fellowship, MIT Technology Review World’s Top 100 Young Innovator Award, NASA Nanotech Brief Nano 50 Innovator award, the Kavli Fellowship, the Sloan Fellowship, the PECASE, DARPA Young Faculty Award and, the NIH Director’s New Innovator Award.

King-Ning Tu received his PhD in applied physics from Harvard University in 1968 and was associated with IBM T. J. Watson Research Center for 25 years before joining the UCLA, USA, in 1993. He is distinguished professor in the Department of Materials Science and SILICON AND Engineering and the Department of Electrical Engineering at the UCLA. He has over 500 journal publications with citations over 18,000 and h-factor of 74. He received the TMS John Bardeen Award in 2013. He has ILICIDE

Huang S co-authored the textbook Electronic Thin Film Science and authored the books Solder

Joint Technology: Materials, Properties, and Reliability and Electronic Thin-Film Reliability. Tu His research interests are focused on metal–silicon reactions, solder joint reactions, NANOWIRES point-contact reactions in nanowires, polarity effect of electromigration on interfacial reactions, and kinetic theories of interfacial reactions. applications, fabrication, and properties

V159 ISBN 978-981-4303-46-0

SILICON AND SILICIDE NANOWIRES

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edited by Yu Huang King-Ning Tu

SILICON AND SILICIDE NANOWIRES applications, fabrication, editors and properties Preben Maegaard Anna Krenz Wolfgang Palz

The Rise of Modern Wind Energy Wind Power for the World Published by Pan Stanford Publishing Pte. Ltd. Penthouse Level, Suntec Tower 3 8 Temasek Boulevard Singapore 038988

Email: [email protected] Web: www.panstanford.com

British Library Cataloguing-in-Publication Data A catalogue record for this book is available from the British Library.

Silicon and Silicide Nanowires: Applications, Fabrication, and Properties Copyright © 2014 by Pan Stanford Publishing Pte. Ltd. All rights reserved. This book, or parts thereof, may not be reproduced in any form or by any means, electronic or mechanical, including photocopying, recording or any information storage and retrieval system now known or to be invented, without written permission from the publisher.

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ISBN 978-981-4303-46-0 (Hardcover) ISBN 978-981-4303-47-7 (eBook)

Printed in the USA Contents

Preface xv

1. In situ Observations of Vapor–Liquid–Solid Growth of Silicon Nanowires 1 S. Kodambaka 1.1 Introduction 1 1.2 Experimental 4 1.3 Silicon Nanowire Nucleation Kinetics 6 1.4 Silicon Nanowire Growth Kinetics 11 1.5 Summary and Outlook 14

2. Growth of Germanium, Silicon, and Ge–Si Heterostructured Nanowires 23 Shadi A. Dayeh and S. Thomas Picraux 2.1 Introduction 23 2.2 The VLS Growth Mechanism 24 2.3 Size Effects in Nanowire Growth 30 2.4 Temperature Effects on Nanowire Growth 36 2.5 Pressure Effects on Nanowire Growth 38

Nanowire Growth 40 2.72.6 Defects Dopant Precursorduring VLS Influence Growth of on Semiconductor Nanowires 42 2.8 Ge Core/Si Shell Heterostructured Nanowires 47 2.9 Unique Opportunities for Bandgap Engineering in Semiconductor Nanowires 50 2.10 Conclusions 52

3. Transition Metal Silicide Nanowires: Synthetic Methods and Applications 59 Jeremy M. Higgins, Andrew L. Schmitt, and Song Jin 3.1 Introduction 59 3.2 Formation of Bulk and Thin-Film Metal Silicides in Diffusion Couples 66 3.2.1 Basic Description 67 vi Contents

3.2.2 Diffusion, Thermodynamics, and Nucleation in Silicide Reactive Phase Formation 67 3.2.2.1 Diffusion and the dominant diffusing species 67 3.2.2.2 Thermodynamics of silicide reactions in binary diffusion couples 69 3.2.2.3 Basics of nucleation 70 3.2.3 Kinetics of Silicide Layer Growth 70 3.2.3.1 Nucleation-controlled kinetics 71 3.2.3.2 Diffusion-controlled kinetics 71 3.2.3.3 Reaction rate–controlled kinetics 73 3.2.3.4 Bulk versus thin-film diffusion couples 74 3.2.4 Phase Formation 77 3.2.4.1 Walser–Bene first phase rule 77 3.2.4.2 Effective heat of formation approach 78 3.2.5 Modern Developments 79 3.3 Silicide Nanowire Growth Techniques 80 3.3.1 Silicidation of Silicon Nanowires 81 3.3.2 Delivery of Silicon to Metal Films 84 3.3.3 Reactions of Transition Metal Sources with Silicon Substrates 86 3.3.3.1 Metal vapor 86 3.3.3.2 Metal halides 87 3.3.4 Simultaneous Metal and Silicon Delivery 88 3.3.4.1 Chemical vapor transport 88 3.3.4.2 Chemical vapor deposition 90 3.3.5 Vapor-Phase Technique Comparison 94 3.4 Applications of Silicide Nanowires 100 3.4.1 Nanoelectronics 100 3.4.2 Nanoscale Field Emitters 102 3.4.3 Spintronics 102 3.4.4 Thermoelectrics 103 3.4.5 Solar Energy Conversion 104 3.5 Conclusion 105 Contents vii

4. Metal Silicide Nanowires: Growth and Properties 121 L. J. Chen and W. W. Wu 4.1 Introduction 121 4.2 Epitaxial Growth of Silicide Nanowires on Si Substrate 122 4.2.1 Epitaxial NiSi2 Nanowires 123 2 Nanowires with Length Tunability 126 4.2.2 4.2.3 EpitaxialGrowth of α-FeSi High-Density Titanium Silicide Nanowires in a Single Direction on a Silicon Surface 130 4.3 Growth of Free-Standing Silicide Nanowires and Their Properties 133 4.3.1 Growth of Single-Crystal Nickel Silicide Nanowires with Excellent Electrical Transport and Field-Emission Properties 133 4.3.1.1 Well-aligned epitaxial Ni31Si12 nanowire arrays 134 4.3.1.2 Growth of free-standing single-crystal NiSi2 nanowires 139 4.3.2 Cobalt Silicide Nanostructures: Synthesis, Electron Transport, and Field-Emission Properties 145 4.3.3 Synthesis and Properties of the Low- Resistivity TiSi2 Nanowires Grown with Metal Fluoride Precursor 151 4.3.4 Ti5Si4 Nanobats with Excellent Field-Emission Properties 157 4.4 Formation of Silicide/Si/Silicide Nano- Heterostructures from Si Nanowires 163 4.4.1 Controlled Growth of Atomic-Scale Si Layer with Huge Strain in the Nano- Heterostructure NiSi/Si/NiSi through Point-Contact Reaction between Nanowires of Si and Ni and Reactive Epitaxial Growth 163 4.4.2 Repeating Events of Nucleation in Epitaxial Growth of Nano CoSi2 and NiSi in Nanowires of Si 170 viii Contents

4.4.3 Reactions between Si Nanowires and Pt Pads 173 4.4.3.1 Formation of PtSi nanowire and PtSi/Si/PtSi nanoheterostructures 173 4.4.3.2 Epitaxial relationship of PtSi formation within a silicon nanowire 175 4.4.3.3 PtSi/i–Si/PtSi nanowire heterostructures as high- performance p-channel enhancement mode transistors 177 4.5 Conclusions 178

5. Formation of Epitaxial Silicide in Silicon Nanowires 187 Yi-Chia Chou, Kuo-Chang Lu, and King-Ning Tu 5.1 Introduction 187 5.1.1 Overview of Contacts in Microelectronics and Nanoelectronics 187 5.1.2 Introduction to Contacts in Nanoscale Electronics 188 5.1.2.1 Transition-metal silicides 188 5.1.2.2 One-dimensional nanostructures 189 5.1.2.3 Si-based nanocircuits in Si nanowires 191 5.1.3 Introduction to Solid-State Phase Transformations 192 5.2 Introduction to Solid-State Phase Transformation in Thin Film 195 5.2.1 Thin Film Metal Silicide Formation 195 5.2.1.1 Phase sequence 195 5.2.1.2 Growth kinetics 197 5.2.2 Examples of Silicides Formation on Si Wafers 199 5.2.2.1 Ni silicides formation 199 5.2.2.2 Co silicides formation 200 5.2.3 Summary 201 5.2.3.1 Metal-rich silicides 202 Contents ix

5.2.3.2 Monosilicides 202 5.2.3.3 Disilicides 202 5.3 Nanoscale Silicide Formation by Point Contact Reaction between Ni/Co and Si Nanowires 206 5.3.1 Introduction 206 5.3.2 Experimental Methods 207 5.3.3 Point Contact Reactions between Nanowires of Si and Co 208 5.3.3.1 CoSi formation by the supply of Co nanodots into Si nanowires 208 5.3.3.2 Epitaxial growth of CoSi2 in Si nanowires 210 5.3.4 Point Contact Reactions between Nanowires of Si and Ni 214 5.3.4.1 Formation of NiSi contacts within Si nanowires and NiSi/Si/NiSi nanowire heterostructures as building blocks for field-effect transistors 214 5.3.4.2 Epitaxial relationship between NiSi and Si and atomically sharp interfaces 216 5.3.4.3 Kinetic analysis of reactive epitaxial growth of nano-NiSi/Si/NiSi 217 5.3.4.4 Fabrication of 2 nm to 200 nm highly strained Si in dimension controlled NiSi/Si/NiSi heterostructures 220 5.3.5 Comparison of Co and Ni Silicides in Si Nanowires 221 5.3.6 Summary 222 5.4 Homogeneous Nucleation of Nanoscale Silicide Formation 223 5.4.1 Introduction 223 5.4.2 Results and Discussions 224 5.4.2.1 Stepwise growth and repeating events of nucleation 224 x Contents

5.4.2.2 Supply limit reaction 226 5.4.2.3 Homogeneous nucleation: experimental observations 227 5.4.2.4 Homogeneous nucleation: correlation between experiments and theory 230 5.4.2.5 Homogeneous nucleation: supersaturation 233 5.4.3 Summary 234 5.5 Conclusion 235

6. Interaction between Inverse Kirkendall Effect and Kirkendall Effect in Nanoshells and Nanowires 245 A. M. Gusak and T. V. Zaporozhets 6.1 Introduction 245 6.2 Basic Notions 251 6.2.1 Kirkendall Shift and Frenkel–Kirkendall Voiding in Bulk Samples 251 6.2.2 Inverse Kirkendall Effect 255 6.2.3 Gibbs–Thomson Effect for Vacancies (Elementary) 256 6.2.4 Gibbs–Thomson Effect for Basic Components 258 6.3 Instability of Hollow Nanostructures 259 6.3.1 Shrinking of Pure Hollow Shells 261 6.3.1.1 Model 262 6.3.2 Shrinking of Chemical Compound Hollow Shells 266 6.3.2.1 Basic equations 267 6.3.2.2 Main assumptions 268 6.3.3 Instability of Binary (Solid Solution) Hollow Shells 273 6.3.3.1 Boundary conditions 275 6.3.4 Energy Barrier—Does it Really Suppress the Shrinking? 282 6.3.2 Conclusions to Section 6.3 290 6.4 Formation of Hollow Shells 292 6.4.1 Formation of IMC Hollow Shells 292 Contents xi

6.4.1.1 A simple case of the competition between “Kirkendall-driven” and “curvature-driven” effects 292 6.4.1.2 General case 294 6.4.2 Formation of Binary Solution Hollow Shells 302 6.4.3 Formation of a Spherical Nano-Shell in Monte Carlo Simulation 304 6.4.4 Conclusions to Section 6.4 305 6.5 Cross-Over from Formation to Collapse 307 6.5.1 Phenomenological Model 307 6.5.2 Monte Carlo Simulation 316 6.5.2.1 Shrinking and segregation kinetics in Monte Carlo simulation 316 6.5.3 Conclusions to Section 6.5 317

7. Electrical Transport Properties of Doped Silicon Nanowires 325 Aya Seike and Iwao Ohdomari 7.1 Introduction 325 7.2 Fabrication Processes and Electrical Measurements 328 7.2.1 Device Fabrication 328 7.2.2 Methods of Electrical Characterization 330 7.3 Introduction of Strain into Nanowire Channels by Oxidation, and Evaluation of Stress within Individual Nanowires 331 7.3.1 Stress Induced during Oxidation Using Pattern-Dependent Oxidation (PADOX) Theory 331 7.3.2 Three-Dimensional Molecular Dynamics Simulations of Stress Distributions in Nanowires 333 7.3.3 Evaluation of Induced Strain inside Si Nanowires by UV Raman Spectroscopy 334 7.4 Electrical Characterization of Nanowire FETs 336 7.4.1 Effects of Stress on Carrier Transport in Nanowire FETs 336 xii Contents

7.4.2 Electrical Characterization 337 7.4.2.1 Potential distribution inside the nanowire channels 337 7.4.2.2 I–V characteristics of nanowire FETs 338 7.4.2.3 Size dependence of transconductance on nanowire size 340 7.5 Summary 340

8. Silicon Nanowires and Related Nanostructures as Lithium-Ion Battery Anodes 343 Liangbing Hu, Lifeng Cui, Seung Sae Hong, James McDonough, and Yi Cui 8.1 Lithium-Ion Batteries and Different Types of Anodes 343 8.2 Advantages and Challenges of Silicon Anodes 346 8.3 Thin Film Silicon Anodes and Microsized Particles 350 8.4 Vapor–Liquid–Solid (VLS)-Grown SiNWs as High-Capacity Anode 354 8.5 Surface Characterization and Electrochemical Analysis of the Solid–Electrolyte Interphase (SEI) on Silicon Nanowires 357 8.6 Si Core–Shell Structures for Anodes 361 8.7 Other Si Nanostructures 367 8.8 Solution-Processed Si Nanostructures 371 8.9 Some Fundamental Aspects 373 8.10 Remaining Challenges and Commercialization 380

9. Porous Silicon Nanowires 389 Yongquan Qu and Xiangfeng Duan 9.1 Introduction 389 9.2 Synthesis of Porous Silicon Nanowires 390 9.2.1 One-Step Chemical Etching 391 9.2.2 Two-Step Chemical Etching 393 9.2.2.1 Effect of [H2O2] 395 9.2.3 Effect of Doping Levels of Silicon Wafers 397 Contents xiii

9.2.4 Mechanism of Formation of Porous Silicon Nanowires 397 9.3 Properties of Porous Silicon Nanowire 401 9.3.1 Optical Properties 401 9.3.2 Electrical Properties 402 9.3.3 Porosity of the Porous Silicon Nanowires 403 9.4 Applications of Porous Silicon Nanowire 404 9.4.1 Photocatalysis 404 9.4.2 Platform for Drug Delivery 406 9.4.3 Lithium-Ion Battery 407 9.5 Conclusion 409

10. Nanoscale Contact Engineering for Si Nanowire Devices 413 Yung-Chen Lin and Yu Huang 10.1 Scope of the Chapter 413 10.2 Introduction 414 10.2.1 The Challenges of Modern Transistor for Contact Engineering 414 10.2.2 NW Transistor and Silicided NW Transistor 414 10.2.3 The Properties and Applications of Metal Silicides 415 10.3 Synthetic Approaches to Nanoscale Silicides 416 10.4 Contact Formation through Solid-State Reaction 420 10.4.1 Introduction of Silicide/Si Heterostructure by Solid-State Reaction 420 10.4.2 The Growth of Silicide NWs by Solid-State Reaction 421 10.4.3 Forming Silicide/Si NW Heterostructure by Solid-State Reaction 422 10.5 Silicide Growth Mechanism 422 10.5.1 The Growth Phases of Nickel Silicides in the NW Structure 422 10.5.2 Growth-Limiting Steps in the Nickel Silicide System at Nanoscale 425 xiv Contents

10.5.3 Nucleation-Controlled Growth or Interfacial-Limited Growth of Nickel Silicide 427 10.5.4 Stress-Limited Growth of Nickel Silicide Phases 428 10.5.5 Diffusion-Limited Growth of Nickel Silicide 431 10.6 New Technical Approaches or Structures for Low-Contact Resistance FET and Short-Channel Device 433 10.6.1 The Challenging for the Low-Device Junction Resistance 433 10.6.2 Comparison of Junction FET, Junctionless FET, and Metal Heterojunction FET 434 10.7 Electronic Properties of Silicide NWs and Silicide/Si/Silicide Heterostructures 437 10.7.1 The Resistivity of Silicide Materials 437 10.7.2 Low-Resistivity Contacts: Ohmic Contacts 437 10.7.3 Conductive Contacts and Beyond: Magnetic Contacts and Schottky Contacts 440 10.7.4 High-Mobility Field-Effect Transistor and Short-Channel Device 445 10.8 Conclusion 445 Index 453 Contents xv

Preface

Nanoscience and nanotechnology are a recent development. At the moment, the research on nanoscale materials science for nanotechnology is ubiquitous. Much has been accomplished in the processing of nanoscale materials, such as growth of silicon nanowires, as well as in the study of physical and device properties of silicon nanowires. Although the research has not reached a stage where nanotechnology can be called mature and mass production of nanodevices can be done, it is time to have a critical review of what has been done and what is the direction and potential of nanotechnology research in the near future. This is the purpose of this book, and it is limited to the processing, properties, and applications of nanowires

example, is the large-scale integration of nanowires. We can handle ofa few silicon pieces and of silicides. nanowires One easily, of the but difficulties it is not at toall betrivial overcome, if we have for to handle a million of them. Nevertheless, maybe it is realistic to

number of nanowires, such as in biosensor devices. focus first on those applications in which we just need to use a small chapter by Kodambaka reviews the in situ transmission electron microscopy This review (TEM) volume observations has a collection of vapor–liquid–solid of ten chapters. (VLS) The growth first of silicon nanowires. The direct observations of dynamic growth phenomena of Si nanowires help identify the key parameters in controlling the morphological and structural evolution of nanowires. The second chapter by Dayeh and Picraux reviews the growth of coaxial as well as radial direction growth of Ge/Si heterostructured growth, defect formation during growth, and the growth mechanism nanowires. The specific aspects of size effect of Ge on Si nanowire small size on VLS growth and the critical radius for suppressing the growth.are covered on the basis of thermodynamics, especially the effect of The third chapter by Higgins, Schmitt, and Jin is a review on the synthesis of metal silicide nanowires. Due to the complex multiphase behavior of silicide formation, the synthesis of silicide nanowires via xvi Preface

chemical methods is rather complicated. Several synthetic strategies have been developed to overcome this challenge. This chapter highlights the strategies of current approaches, the future synthetic challenges, and a review of the emerging applications. The fourth chapter by Chen and Wu is to reviews the formation of nano-silicide phases on Si substrates as well as in Si nanowires. The microstructures and the epitaxial interfaces between Si and a large number of silicides of near-noble and transition metals have been studied by high-resolution transmission electron microscopy (HRTEM), and lattice images are presented and reviewed. Controlled growth of nano-heterostructures of silicide/Si/silicide such as NiSi/ Si/NiSi and PtSi/Si/PtSi are given.

contact reaction–induced epitaxial growth of Ni and Co silicides in Si The nanowires. fifth chapter In this by chapter, Chou, eventsLu, and of repeatingTu reviews homogeneous the point- nucleation have been observed. Historically, on analyzing repeating homogeneous nucleation, it was found that it cannot explain the rapid rate of bulk single-crystal growth during the melting process. Thus, F. C. Frank’s model of spiral crystal growth around a screw dislocation was invented. This model was used to observe repeating homogeneous nucleation in nanowires, and it enabled to directly compare theoretical and experimental studies on nucleation. The sixth chapter by Gusak and Zaporozhers is a review on the

in hollow nanoshells and nanotubes. Owing to the two surfaces in interaction between inverse Kirkendall effect and Kirkendall effect

a hollow nanoshell and nanotube, a vacancy flux pre-exists in the Thenanostructure kinetic analysis and it presented induces the in inversethis chapter Kirkendall is an exampleeffect to ofaffect the currentinterdiffusion active and,study in of turn, kinetic the stability processes of the in nanoscale nanoshell andmaterials. nanotube. The seventh chapter by Seike and Ohdomari is a review on electrical transport properties of doped Si nanowires under strain. It presents a comprehensive study directed toward a better understanding of electron and hole transport in n-type and p

complementary-type strained metal-oxide-semiconductorSi nanowire metal-oxide-semiconductor (nano-CMOS) circuits field- andeffect systems, transistors for (MOSFETs). example, in It 32 is nm aimed process for applications node, and hopefullyin nano- to extend it into the 10 nm process node regime. In this nanoscale regime, stress plays the most important role in carrier transport. Preface xvii

on channel transconductance is discussed. Thus,The the eighth effect chapter of stress by and Hu, strain Cui, Hong, on carrier McDonough, transport, and especially Cui is a review on silicon nanowires and related nanostructures to serve as Li-ion battery anodes. Owing to its highest-known theoretical capacitance (~4200 mAh/g), Si holds great potential as anode material for Li-ion batteries. Yet, its high volume expansion in absorbing Li ions is a reliability concern. Hence, nanosize anode materials, such as Si-based carbon composites, are reviewed in this chapter. The improved electrochemical performance of Si-based anodes for applications is discussed. The ninth chapter by Qu and Duan is a review on porous silicon - plex surface states, the porous Si nanowires show strong photo- and nanowires.electro-luminescence Due to deep in visiblequantum light confinement range and thuseffect have and/or potential com applications in Si-based optoelectronics, bio-imaging, and biosen- sors. The recent progress on the synthesis, characterization, proper- ties, and pore-formation mechanism are summarized. The tenth chapter by Lin and Huang reviews nanoscale contact engineering for Si nanowire devices. The one-dimensional nanostructure, such as nanotubes and nanowires, are attractive building blocks for nanoelectronics. Making reliable electrical silicide contacts to individual silicon nanowire devices is one of the key factors that determine the nanodevice performance and reliability. This chapter reviews the detailed synthetic approaches, silicide growth kinetics, electronic properties, and device applications of nanoscale silicide/silicon/silicide heterostructures formed by solid- state reactions. As editors, we hope that this book would serve as a stepping stone for readers to reach a higher level of scientific understanding and an advanced field of device applications. Yu Huang King-Ning Tu Autumn 2013