Physical Properties of Amorphous Materials Institute far Amarphous Studies Series Series editors David Adler Mussuchusetts Institute of Technology Curnbridge, Mussuchusetts and Brian B. Schwartz Institute for Arnorphous Studies Bloornfield HilIs, Michigun und Brooklyn College of the City University of New York Brooklyn, New York PHYSICAL PROPERTIES OF AMORPHOUS MATERIALS Edited by David Adler, Brian B. Schwartz, and Martin C. Steele A Continuation Order Plan is available for this series. A continuation order will bring delivery of each new volume immediately up~n publication. Volumes are billed only upon actual shipment. For further information please contact the publisher. Physical Properties of Arnorphous Materials Edited by David Adler Massachusetts Institute of Technology Carnbridge, Massachusetts Brian B. Schwartz Institute for Arnorphous Studies Bloornfield Hills, Michigan and Brooklyn Col1ege of the City University of New York Brooklyn, New York and Martin C. Steele Institute for Arnorphous Studies Bloornfield Hills, Michigan Springer Science+Business Media, LLC Library of Congress Cataloging in Publication Data Physical properties of amorphous materials. (Institute for amorphous studies series) Includes bibliographical references and index. 1. Amorphous substances. I. Adler, David. II. Schwartz, Brian B., 1938- . III. Steele, Martin C. (Martin Carl), 1919- . IV. Series. QC176.8.A44P49 1985 530.411 84-26370 ISBN 978-1-4899-2262-5 ISBN 978-1-4899-2260-1 (eBook) DOI 10.1007/978-1-4899-2260-1 Proceedings of a lecture series on Fundamentals of Amorphous Materials and Devices, held during the period 1982-1983 at the Institute for Amorphous Studies, Bloomfield Hills, Michigan © 1985 Springer Science+Business Media New York Originally published by Plenum Press, New York in 1985. Softcover reprint ofthe hardcover 1st edition 1985 All rights reserved No part of this book may be reproduced, stored in a retrieval system, or transmitted, inany form or by any means, electronic, mechanical, photocopying, microfilming, recording, or otherwise, without written permission from the Publisher PREFACE The Institute for Amorphous Studies was founded in 1982 as the international center for the investigation of amorphous mate rials. It has since played an important role in promoting the und er standing of disordered matter in general. An Institute lecture series on "Fundamentals of Amorphous Materials and Devices" was held during 1982-83 with distinguished speakers from universities and industry. These events were free and open to the public ,and were attended by many representatives of the scientific community. The lectures themselves were highly successful inasmuch as they provided not only formal instruction but also an opportunity for vigorous and stimulating debate. That last element could not be captured within the pages of a book but the lectures concentrated I on the latest advances in the field which is why their essential I contents are he re reproduced in collective form. Together they constitute an interdisciplinary status report of the field. The speakers brought many different viewpoints and a variety of back ground experiences io bear on the problems involved but though I language and conventions vary the essential unity of the concerns I is very clear as indeed are the ultimate benefits of the many-sided I approach. The impact of amorphous materials on the worlds of science and technology has been enormous covering such diverse applica I tions as solar energy image processing energy storage computer I I I and telecommunication technology thermoelectric energy conver I sion, and new materials synthesis. If the subject is to reach its full fruition its role in educational programs will have to be com I mensurate. To an astonishing extent, the teaching of solid state physics is still restrictively tied to crystalline concepts, and the time has come for a change and a balance. Both will undoubtedly come but reform and reorientation take time. It is one of the Institute' spurposes to accelerate the process. v vi PREFACE We reconfirm our belief and confidence that amorphous materials will serve humankind in a variety of important ways ,and we recognize that the scientists who are now in the fore front of this field are in effect social as well as technological pioneers. My special thanks are due to the contributors to this volume to I whom I wish continued success in the realms of new scientific ideas and technological achievement. S • R. Ovshinsky Troy Michigan I CONTENTS Introduction 1 D. Adler PART ONE: GENERAL PROPERTIES Chemistry and Physics of Covalent Amorphous Semiconductors D. Adler I. Introduction ..•...••....•..•.••••.••... 5 11. Solid State Theory A. General Approach ..•.•..•...•...••. 6 B. Structure .....•.....•.....•..•.... 8 C. Adiabatic Approximation and Vibrational Structure ••...••..•• 13 D. Electronic Structure ..•.•••..•...• 14 111. Crystalline Solids A. Conventional Viewpoint .•.•••.••... 19 B. Alternative Viewpoint •••••••••.••• 22 IV. Effects of Disorder ••.....•..•.••...... 25 V. Experimental Tests of the CFO Model .... 33 VI. Defects in Amorphous So lids ...•........ 40 VII. Amorphous Silicon Alloys A. Defects .••....•.••.•...•..••.•.... 45 B. Ground-State Electronic Structure. 56 C. Transport Properties .•.••••.•...•• 65 D. Nonequilibrium Transport ••....•••• 69 E. Metastable Effects ...•.••.••.••••• 71 F. Effects of Fluorine •••••••••..•••• 73 vii viii CONTENTS VIII. Amorphous Chalcogenide Alloys A. Defects ••.••••••••••••...••••..••• 77 B. Ground-State Electronic Structure •• 82 C. Transport Properties .••••••••••••• 85 D. Nonequilibrium Transport .•••.••••• 87 E. Swi tching ......................... 93 F. Metastable Effects •••••••••••••••. 94 IX. Conclusions ............................ 95 Acknowledgements .•••.•••••..•..•.•••.•. 96 References .•••••••.•.•••..•••..•••.•••. 96 Fundamentals of Amorphous Materials S.R. Ovshinsky I. Indroduction .............•............. 105 II. Chemical Considerations .•.••..••••.•..• 109 III. Thermodynamic Considerations ••.•.••••.• 112 IV. Chemistry as a Design Tool •.••....••••• 116 V. Mechanical Properties 120 VI. Phonons ................................ 121 VII. Materials Synthesis for Device Applications ........................ 122 VIII. Chemical Modification ••.•••••••.•••••.• 135 IX. Conclusion ............................. 142 Acknowledgments •••••••••.•.••••••.•••.• 145 Re ferences ............................. 146 PART TWO: STRUCTURE The Constraint of Discord D. Weaire I. Introduction 157 II. A New Way of Looking ••••••••••.•••••••• 159 III. Order and Disorder in Two Dimensions ..• 162 IV. Soap Cells ............................. 162 Acknow1edgements •••.••••••.••••.••..••• 168 References ............................. 168 CONTENTS ix Structural Studies of Amorphous Materials A. Bienenstock I. Introduction A. Decription of Crystalline Systems •• 171 B. Description of Arnorphous Systems 172 11. Radial Distribution Functions A. Amorphous Silicon and Germanium 173 B. Amorphous Compounds with Tetrahedral Coordination •••••••. 175 C. Elemental Amorphous Semiconductors. 176 D. Simple Metallic Glasses •••••.••••• 177 111. POlyatomic Systems ••.•••••.••••.••••••• 178 IV. Synchrotron Radiation •••••••••••••••••• 186 A. Extended X-ray Absorption Fine Structure •••••••••••••••••• 190 B. Anomalous X-ray Scattering •••••••• 194 C. Differential Anomalous Scattering •• 196 V. Future Synchrotron Radiation Research on the Structure of Amorphous Materials .•••••••••••••••••.•••• 198 Acknowledgements •••••••••••••••••.••• 199 References ........................... 199 EXAFS of Disordered Systems E.A. Stern I. Introduction ••.•••••••••.•••••••••••••• 201 II. EXAFS 203 III. EXAFS Versus Diffuse Scattering for Disordered Systems ••••••••••••••• 209 IV. Application of EXAFS to Amorphous Ge •.• 212 V. Summary and Conclusions •.••.••••••••••. 218 References 219 Mössbauer Spectroscopy -- ARewarding Probe of Morphological Structure of Semiconducting Glasses P. Boolchand I. Introduction 221 11. Mössbauer Spectroscopy ••••......•...... 224 A. Probe of Ge Sites in Network Glasses .• 227 B. Probe of Chalcogen Sites in Network Glas ses •••.••.••.••...•. 232