Electrochemistry of Silicon and Its Oxide

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1 Electrochemistry of Silicon and Its Oxide

2 Electrochemistry of Silicon and Its Oxide Xiaoge Gregory Zhang Cominco Ltd. Mississauga, Ontario, Canada and McMaster University Hamilton, Ontario, Canada KLUWER ACADEMIC PUBLISHERS NEW YORK, BOSTON, DORDRECHT, LONDON, MOSCOW

3 ebook ISBN: Print ISBN: Kluwer Academic Publishers New York, Boston, Dordrecht, London, Moscow Print 2001 Kluwer Academic/Plenum Publishers New York All rights reserved No part of this ebook may be reproduced or transmitted in any form or by any means, electronic, mechanical, recording, or otherwise, without written consent from the Publisher Created in the United States of America Visit Kluwer Online at: and Kluwer's ebookstore at:

4 To the people who assisted, encouraged, and inspired me

5 There is an expanding frontier of ignorance Richard P. Feynman

6 Foreword It may be argued that silicon, carbon, hydrogen, oxygen, and iron are among the most important elements on our planet, because of their involvement in geological, biological, and technological processes and phenomena. All of these elements have been studied exhaustively, and voluminous material is available on their properties. Included in this material are numerous accounts of their electrochemical properties, ranging from reviews to extensive monographs to encyclopedic discourses. This is certainly true for C, H, O, and Fe, but it is true to a much lesser extent for Si, except for the specific topic of semiconductor electrochemistry. Indeed, given the importance of the electrochemical processing of silicon and the use of silicon in electrochemical devices (e.g., sensors and photoelectrochemical cells), the lack of a comprehensive account of the electrochemistry of silicon in aqueous solution at the fundamental level is surprising and somewhat troubling. It is troubling in the sense that the non-photoelectrochemistry of silicon seems to have fallen through the cracks, with the result that some of the electrochemical properties of this element are not as well known as might be warranted by its importance in a modern technological society. Dr. Zhang s book, Electrochemical Properties of Silicon and Its Oxide, will go a long way toward addressing this shortcoming. As with his earlier book on the electrochemistry of zinc, the present book provides a comprehensive account of the electrochemistry of silicon in aqueous solution. The topics covered are mostly fundamental in nature and include comprehensive accounts of the silicon/electrolyte interface, anodic oxidation, etching, photoeffects, cathodic reactions and redox couples, porous silicon, and photoelectrochemistry. The book starts with a discussion of basic semiconductor electrochemistry that sets the stage for the discussion of specific phenomena (e.g., anodic oxidation) in later chapters. Careful attention has been paid to the accurate definition of terms and to identifying what is unique about silicon with regard to its electrochemical properties. The level of the book renders it suitable as an introduction to the field of silicon electrochemistry, with its value being greatly enhanced by inclusion of material that is not related to photoeffects, which has been covered extensively elsewhere. As with any book, the material that is presented reflects the interests of the author and this book is no exception. However, Dr. Zhang has stepped back and taken a new look at a field that for so long has been dominated by one particular aspect of the subject. Accordingly, it is my fervent hope that the book will spark a renewed interest in the general electrochemistry of one of the most important and remarkable of elements, silicon. State College, Pennsylvania D.D. Macdonald ix

7 Preface The importance of electrochemistry in silicon technology has spurred intense research activity in the last five decades, resulting in a tremendous amount of experimental data and theoretical information on the silicon/electrolyte interfaces. This book is a comprehensive compilation and digestion of this body of information. The aim of this book is to serve as a centralized information source for anyone who is interested in the surface and electrochemical properties of silicon and its oxides. It will be most useful to the scientists who study the surface phenomena of silicon and to the engineers who design and fabricate silicon-based electronic devices. It can also serve as a general reference for researchers working in the fields of semiconductor electrochemistry and surface science, in which silicon is commonly used as a model material. In addition, it can be useful to the geologists who study rock water interactions and to the people who work on etching and surface treatment of glasses. My fascination with the electrode behavior of silicon began some fourteen years ago when I was a postdoctoral research fellow at MIT investigating the formation mechanism of porous silicon. 2,8,9 The decision to write this book came shortly after the publication of my first book entitled Corrosion and Electrochemistry of Zinc in I recognized then: From the viewpoint of material users and researchers, it is most beneficial that all the relevant information, theoretical and practical on all aspects of electrochemistry of one element be systematically organized in a single source. Compiling and digesting the results from all discrete studies provide a collective and holistic perspective in evaluating the validity and difference in the data and theories resulting from individual studies, and allow a more complete characterization of each specific phenomenon. Also, writing a book of this nature is itself a research process, through which new data and new insights that are not clear in individual studies, can be generated. As a result, about 100 new tables, figures and illustrations of synthesized information are generated and presented here for the first time. Two general considerations have been taken in the selection of the information from the literature. The first is the emphasis on the information pertaining to silicon as a single crystalline material and not that pertaining to alloys and structures made or transformed from silicon. This emphasis is essential in warranting a sufficient focus and space for such an extensively studied material as silicon. The second is the emphasis on details and specificity of data because the diverse electrochemical phenomena observed on silicon electrodes are governed by specific conditions and are differentiated only in detail. In practice, it is the condition-phenomenon specific information xi

8 xii PREFACE rather than the generalized information that is most useful to the people who work on concrete problems. Each phenomenon is dealt with by a systematic characterization first based on a collective body of experimental data followed by a comprehensive discussion on the underlying mechanisms. It is believed that while the experimental data generally remains true and therefore valuable over time, theories and mathematical formulations tend to vary with individual studies, and their usefulness, with a few exceptions, are limited to its time. This is because the former is the measurement of facts whereas the latter is the interpretation of the measurement. In discussing mechanisms, the focus is on physical schemes rather than mathematical formulations. This is because many details of the phenomena observed in the complex system of silicon/electrolyte interface are still not understood and mathematical formulations are not really meaningful without a clear understanding of the physical schemes. Thus, for each phenomenon, the concepts and theories on the physical schemes proposed in different studies are compared against the collective body of data. Generalization is provided when a coherence exists in the data and the theories. When it does not exist, effort is made to provide a comprehensive analysis with new hypotheses that is more consistent with the collective body of data from a global perspective. Generally, for a complex system a collective view is more accurate and more complete than individual ones. The information in the book is organized in nine chapters comprising roughly of two parts: The first part, consisting of Chapters 1 to 4, deals with the conditions of silicon/electrolyte interface and second part, of Chapters 5 to 8, deals with the phenomena of silicon electrodes. The first chapter provides an overview on the basic concepts and theories of semiconductor electrochemistry to serve as a reference base on which the electrochemical properties of silicon are described. Chapter 2 covers the information on the surface conditions of silicon and the characteristics of silicon/electrolyte interfaces. The information related to properties of silicon oxides in general and to those of anodic oxides in specific are presented in Chapters 3 and 4. Chapter 5 deals with the anodic behavior of silicon electrodes with respect to the chemical nature of the reactions and the kinetic processes. Chapter 6 summarizes research findings on the various cathodic reactions and the reactions of redox couples. Chapter 7 is devoted to the etching of silicon by compiling the data on etch rate and morphology of etched surfaces, and by discussing the etching mechanisms in different solutions. Chapter 8, the largest chapter, deals with the formation and morphology of porous silicon, and the complex mechanistic aspects, utilizing all information presented in the earlier chapters. The last chapter, Chapter 9, provides a summary on the book from six different respects that connect the information in different chapters, and an indication on the areas of interest for future research. ACKNOWLEDGMENTS A book such as this is the result of labor for many years, during which I have received assistance, encouragement, and inspiration in various forms from many people.

9 PREFACE xiii I wish, first of all, to express my immense gratitude to my wife Li and my son Kevin for their understanding and support in my undertaking such a time and energyconsuming endeavor. I am specially grateful to D.D. Macdonald of the Pennsylvania State University for his reviewing the manuscript and writing a foreword for this book. My sincere thank is due to a number of people who reviewed the different chapters: J. Bardwell of The National Research Council of Canada, T.D. Burleigh of University of Pittsburgh, J.J. Kelly of University of Utrecht, and E.S. Kooij of University of Twente. I would also like to acknowledge M. Christopherson and H. Föll of Christian-Albrechts University for providing photographs on porous silicon. I would like particularly to thank K. Howell of Kluwer Academic/Plenum Publishers, the editor of this book; his enthusiastic engagement and critical reading of the entire book have been especially important. I am also grateful to X. Gu, J. Jin, M.C. Zhang, and A.Q. Xing who helped in various tasks such as literature search, copying, and typing and so on, which are invisible but essential in the development of a book. I would also take this opportunity to express my gratitude to S.L. Smith of University of California at Davis, and R.M. Latanision and S.D. Senturia of MIT for providing me the opportunity many years ago to study the electrochemistry of silicon, from which I gained the knowledge and developed a lasting fascination with silicon. Finally, I wish that this book can be a tribute to the authors, whose research work contributed to the huge knowledge base on silicon that made this book possible. Xiaoge Gregory Zhang Toronto, Canada xgzhang@interlog.com

10 Contents LIST OF SYMBOLS xxiii CHAPTER 1. Basic Theories of Semiconductor Electrochemistry Introduction 1 Energetics of Semiconductor/Electrolyte Interface Potential and Charge Distribution in Space Charge Layer Kinetics of Charge Transfer Photoeffects Energy Levels in Semiconductor Energy Levels in Electrolyte Distribution of Energy Levels in Electrolyte Energy Levels at Semiconductor/Electrolyte Interface Carrier Density in Space Charge Region Depletion Layer Accumulation Layer and Inversion Layer Helmholtz Double Layer Surface States Fermi Level Pinning Equivalent Circuit and Capacitance of Semiconductor/Electrolyte Interface Flatband Potentials Basic Theories Limitations of the Basic Theories Limiting Current Breakdown Potential Distribution Current Multiplication Photocurrent Photopotential Efficiency ofenergy Conversion xv

11 xvi CONTENTS Surface Recombination Open-Circuit Potential ExperimentalTechniques CHAPTER 2. Silicon/Electrolyte Interface Basic Properties of Silicon Thermodynamic Stability in Aqueous Solutions Surface Adsorption Hydrogen Termination Mechanistic Aspects Fluoride Termination Adsorption of Metal and Organic Impurities 2.4. Native Oxide In Air In Water and Solutions 2.5. Hydrophobic and Hydrophilic Surfaces 2.6. Surface States 2.7. Flatband Potentials Effect of ph Effect of Surface Condition Effect of Surface States Band Diagrams 2.8. Open-Circuit Potentials Effect of Various Factors Corrosion Current CHAPTER 3. Anodic Oxide Introduction Types of Oxides 3.. Thermal Oxide Chemical Vapor Deposition Liquid-Phase Deposition Native Oxide and Anodic Oxide Use of Oxides in Device Fabrication 3.3. Formation of Anodic Oxides Growth Mechanisms General Effect of Solution Composition Effect of Silicon Substrate Effect of Polarization Conditions Effect of Illumination Electroluminescence

12 CONTENTS xvii Reactions 105 Ionic Transport within Oxide 106 Growth on n Si 108 Electroluminescence 109 An Overall Growth Model 110 Growth Kinetics 112 Thermal Oxides 112 Anodic Oxides Properties Physical and Chemical Properties Interface Electrical Properties Thermal Oxides Anodic Oxides CHAPTER 4. Etching of Oxides Introduction General Thermal Oxide Quartz and Fused Silica Deposited Oxides Anodic Oxides Etching Mechanisms Reactions In Nonfluoride Solutions In HF-Based Solutions Rate Equations Effect of Oxide Structure CHAPTER 5. Anodic Behavior Introduction Current Potential Relationship 5.. Fluoride Solutions Effect of Solution Composition Alkaline Solutions Photoeffect Quantum Yield and Surface Recombination Effective Dissolution Valence Hydrogen Evolution Limiting Current Impedance of Interface Layers Tafel Slope and Distribution of Potential

13 xviii CONTENTS Passivation Tafel Slope Potential Distribution Occurrence Passivation in Alkaline Solutions Passive Films Current Oscillation Amplitude and Frequency Oscillation of Anodic Oxide Thickness and Properties Mechanisms A New Model Participation of Bands and Rate-Limiting Processes Reaction Mechanisms Turner Memming Model Later Modifications Model to Account for Electron Injection into the Current Band Modification for Hydrogen Termination Consideration of Chemical versus Electrochemical Reaction Individual Steps in the Transfer of Valence Electrons Models for the Reaction Mechanisms in Alkaline Solutions An Overall Reaction Scheme Elemental Steps Reaction Paths CHAPTER Cathodic Behavior and Redox Couples Introduction Hydrogen Evolution 6.. Kinetics Surface Transformation Metal Deposition Kinetics Morphology Deposition of Silicon Redox Couples Individual Redox Couples Other Redox Species Electroluminescence Associated with Redox Reactions

14 CONTENTS xix Open-Circuit Photovoltage 268 Surface Modification Metallic Deposits Polymer Coatings Nonaqueous Solutions CHAPTER 7. Etching of Silicon Introduction General Fluoride Solutions Absence of Oxidants Effect of Effect of Effect of Other Oxidants Alkaline Solutions KOH Solutions Etching Mechanism Other Inorganic Solutions Solutions Hydrazine Organic Solutions EDP Solntions Ethanolamine Tetramethyl Ammonium Hydroxide (TMAH) Etch Rate Reduction of Heavily Doped Materials Anisotropic Etching Sensitivity of Etch Rates to Crystal Orientation Mechanisms Rate-Limiting Process Passivation Models Surface Reaction Kinetics-Based Models Mechanism of Anisotropic Etching Basic Features of Anisotropically Etched Surfaces Surface Roughness Microroughness Macroroughness Crystallographic Characters and Formation of Hillocks Origins of Roughness Applications Cleaning RCA Cleaning Defect Etching

15 xx CONTENTS Material Removal 347 Uniform Material Removal 347 Selective Material Removal 349 CHAPTER 8. Porous Silicon Introduction Formation of Porous Silicon Characteristics of i V Curves Conditions for PS Formation and Electrochemical Polishing Effective Dissolution Valence and Hydrogen Evolution Growth Rate of Porous Silicon Mass Transport Chemical Dissolution during PS Formation 8.3 Morphology General Diameter and Interpore Spacing Effect of Doping Effect of Potential Primary and Branched Pores Pore Arrays Variation from Surface to Bulk Interpore Spacing Distribution of Pore Diameter Pore Density Pore Orientation and Shape Pore Branching Interface between PS and Silicon Depth Variation Transitional Layer Two-Layer PS Fill of Pores Density and Specific Surface Area Composition Crystallographic Structure Summary PS Formed at OCP PS Formed under Special Conditions Formation Mechanisms Historical Development Discovery of PS and the Initial Model Macropores on n-si and the Barrier Breakdown Model Characterization of PS and Growth Kinetics

16 CONTENTS xxi Depletion Layer and Field Intensification Model Carrier Diffusion Model Formation Condition of PS Quantum Confinement Model Surface Curvature Model Formation of Uniformly Spaced Pore Array Formation of Two-Layer PS on Illuminated n-si Theories on the Macro PS Formed on Lowly Doped p-si Miscellaneous Hypotheses Integration of Models Current Burst Theory Advances in the Understanding of Electrochemical Reactions Summary Analysis of the Mechanistic Aspects Involved in PS Formation Effect of Radius of Curvature Potential Drop in the Substrate Anisotropic Effect Reactions on the Surfaces of Silicon and Silicon Oxide Distribution of Reactions and Their Rates on Pore Bottoms Dissolution of PS Potential Drops in Different Phases of the Current Path Relativity of the Dimensions and Events Pore Diameter and Interpore Spacing Varitation of Morphology from Surface to Bulk. Initiation of Pores Summary 8.7. Properties and Applications CHAPTER 9. Summaries and General Remarks 9.1. Complexity 9.2. Surface Condition 9.3. Oxide Film 9.4. Sensitivity to Curvature 9.5. Sensitivity to Lattice Structure 9.6. Relativity 9.7. Future Research Interests REFERENCES 453 INDEX 499

17 List of Symbols Symbols Definition Section lattice constant activities of oxidized and reduced species capture coefficients for electrons and holes at the surface Tafel slope capacitance concentration of electrolyte surface states capacitance thickness of oxide film densities of occupied and empty states in the electrolyte normalizing factors intrinsic Debye length diffusion coefficient of electrons diffusion coefficient of holes electron charge energy level field energy at which there is zero surface state charge breakdown field energy levels of the oxidized and reduced species conduction and valence band edges at the surface redox potential corrosion potential width of band gap surface field activation energy frequency of current oscillation Fermi-Dirac distribution function Maxwell-Boltzmann distribution function non-equilibrium occupancy of surface levels density of allowed states rate of photo carrier generation degeneracy of the energy level anodic and cathodic currents , , xxiii

18 xxiv LIST OF SYMBOLS Symbols Definition Section, exchange current densities limiting current due to hole diffusion in the bulk photo currents generated in the double layer and in the bulk photo current conduction band and valence band currents anodic currents via the conduction band and valence band cathodic currents via conduction band and valence band exchange current densities via conduction band and valence band ionic and electronic currents hole diffusion current passivation current net current density corrosion current density current densities at pore bottom, pore tip and pore wall light intensity peak and valley current values of I V curve in HF flux of electrons and holes from/into the conduction and valence bands light frequency Boltzmann s constant rate constant equilibrium constants hole diffusion length atomic weight molar concentration per liter effective dissolution valence atomic density effective density of states in the valance band acceptor and donor concentrations densities of empty states and occupied states intrinsic carrier density anionic concentration captured electron density in the surface states surface electron and hole concentrations at equilibrium electron concentration at equilibrium hole concentration at equilibrium density of holes in the valence band captured holes in the surface states charge charge in the Helmholtz double layer charge in the solution radius of curvature rate of particle movement etch rate radius of atoms rate of dissolution, etching, etc , , 7.2

19 LIST OF SYMBOLS xxv Symbols Definition Section rate of recombination via surface states recombination rates of electrons and holes via surface states surface recombination velocity time oxide thickness electrode potential volume flatbandpotential possivation potential melting temperature applied potential excess voltage open circuit photo potential potential across the space charge layer at equilibrium measured potential potential drop, overpotential width of space change layer quantum efficiency relative permittivity permittivity of vacuum Schotteky barrier height standard electrode potential reorientation energy number of holes involved in reaction surface free energy life time of hole chemical potentials of electrons and holes mobility of electrons and holes ionic mobility fraction of surface coverage angle of the side pore from the main pore density resistivity power efficiency of photo cell anodic overpotential concentration overpotential in electrolyte space charge layer over potential efficiency of hydrogen evolution transition coefficient velocity of electrons in oxide film electron affinity partitioning coefficient of potential partitioning coefficient of potential change American Society of Testing and Materials buffered HF solution current efficiency ,

20 xxvi LIST OF SYMBOLS Symbols Definition Section chemical vapor deposition etching solution mixed with ethylenediamine, pyrocatechol and water isopropyl alcohol liquid phase deposition metal-oxide-semiconductor N-Methylacetamine Acetonitrile Acetonitrile porous silicon low pressure chemical vapor deposition potential of zero charge open circuit potential potential of saturated calomel electrode standard potential of hydrogen electrode tetramethyl ammonium hydroxide, etching solution rate determining step number of rotations per minute ultra violet light

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