MATERIALS SCIENCE AND TECHNOLOGY EDITOR A. S. NOWICK Henry Krumb School of Mines Columbia University New York, New York A. S. Nowick and B. S. Berry, ANELASTIC RELAXATION IN CRYSTALLINE SOLIDS, 1972 E. A. Nesbitt and J. H. Wernick, RARE EARTH PERMANENT MAGNETS, 1973 W. E. Wallace, RARE EARTH INTERMETALLICS, 1973 J. C. Phillips, BONDS AND BANDS IN SEMICONDUCTORS, 1973 J. H. Richardson and R. V. Peterson (editors), SYSTEMATIC MATERIALS ANALYSIS, VOLUMES I, II, AND III, 1974; IV, 1978 A.J. Freeman and J. B. Darby, Jr. (editors), THE ACTINIDES: ELECTRONIC STRUC­ TURE AND RELATED PROPERTIES, VOLUMES I AND II, 1974 A. S. Nowick and J. J. Burton (editors), DIFFUSION IN SOLIDS: RECENT DEVELOP­ MENTS, 1975 Λ W. Matthews (editor), EPITAXIAL GROWTH, PARTS A AND B, 1975 y. M. Blakely (editor), SURFACE PHYSICS OF MATERIALS, VOLUMES I AND II, 1975 G. A. Chadwick and D. A. Smith (editors), GRAIN BOUNDARY STRUCTURE AND PROPERTIES, 1975 John W. Hastie, HIGH TEMPERATURE VAPORS: SCIENCE AND TECHNOLOGY, 1975 John K. Tien and George S. Ansell (editors), ALLOY AND MICROSTRUCTURAL DESIGN, 1976 Μ. T. Sprackling, THE PLASTIC DEFORMATION OF SIMPLE IONIC CRYSTALS, 1976 James J. Burton and Robert L. Garten (editors), ADVANCED MATERIALS IN CATALYSIS, 1977 Gerald Burns, INTRODUCTION TO GROUP THEORY WITH APPLICATIONS, 1977 L. H. Schwartz and J. B. Cohen, DIFFRACTION FROM MATERIALS, 1977 Zenji Nishiyama, MARTENSITIC TRANSFORMATION, 1978 Paul Hagenmuller and W. van Gool (editors), SOLID ELECTROLYTES: GENERAL PRINCIPLES, CHARACTERIZATION, MATERIALS, APPLICATIONS, 1978 G. G. Libowitz and M. S. Whittingham, MATERIALS SCIENCE IN ENERGY TECH­ NOLOGY, 1978 Otto Buck, John K. Tien, and Harris L. Marcus (editors), ELECTRON AND POSI­ TRON SPECTROSCOPIES IN MATERIALS SCIENCE AND ENGINEERING, 1979 Lawrence L. Kazmerski (editor), POLYCRYSTALLINE AND AMORPHOUS THIN FILMS AND DEVICES, 1980 In Preparation Manfred von Heimendahl, ELECTRON MICROSCOPY OF MATERIALS: AN INTRO­ DUCTION Polycrystalline and Amorphous Thin Films and Devices Edited by LAWRENCE L. KAZMERSKI Photovoltaics Branch Solar Energy Research Institute Golden, Colorado ACADEMIC PRESS A Subsidiary of Harcourt Brace Jovanovich, Publishers New York London Toronto Sydney San Francisco COPYRIGHT © 1980, BY ACADEMIC PRESS, INC. ALL RIGHTS RESERVED. NO PART OF THIS PUBLICATION MAY BE REPRODUCED OR TRANSMITTED IN ANY FORM OR BY ANY MEANS, ELECTRONIC OR MECHANICAL, INCLUDING PHOTOCOPY, RECORDING, OR ANY INFORMATION STORAGE AND RETRIEVAL SYSTEM, WITHOUT PERMISSION IN WRITING FROM THE PUBLISHER. ACADEMIC PRESS, INC. 111 Fifth Avenue, New York, New York 10003 United Kingdom Edition published by ACADEMIC PRESS, INC. (LONDON) LTD. 24/28 Oval Road, London NW1 7DX Library of Congress Cataloging in Publication Data Main entry under title: Polycrystalline and amorphous thin films and devices. (Materials science and technology series) Bibliography: p. Includes index. 1. Thin films. 2. Thin film devices. 3. Semi­ conductor films. 4. Photoelectronic devices. 5. Optical films. 6. Protective coatings. I. Kazmerski, Lawrence L. TK7871.15.F5P647 621.38Γ73 79-8860 ISBN 0-12-403880-8 PRINTED IN THE UNITED STATES OF AMERICA 80 81 82 83 9 8 7 6 5 4 3 2 1 To my mother, my wife Kathleen, and to Keira and Timothy List of Contributors Numbers in parentheses indicate the pages on which the authors' contributions begin. ALLEN M. BARNETT* (229), Institute of Energy Conversion, University of Delaware, Newark, Delaware 19711 PATRICK J. CALL (257), Solar Energy Research Institute, Golden, Colorado 80401 D. E. CARLSON (175), RCA Laboratories, Princeton, New Jersey 08540 A. H. CLARK (135), University of Maine at Orono, Orono, Maine 04469 DAVID EMIN (17), Division 5151, Sandia National Laboratories, Albuquerque, New Mexico 87185 LEWIS M. FRAAS (153), Chevron Research Company, Richmond, California 94802 LAWRENCE L. KAZMERSKI (59), Photovoltaics Branch, Solar Energy Research Institute, Golden, Colorado 80401 BRENTON L. MATTES (1), University of Michigan, Ann Arbor, Michigan 48109 JOHN D. MEAKIN (229), Institute of Energy Conversion, University of Dela­ ware, Newark, Delaware 19711 R. A. MICKELSEN (209), The Boeing Aerospace Company, Seattle, Washing­ ton 98124 ALLEN ROTHWARF* (229), Institute of Energy Conversion, University of Delaware, Newark, Delaware 19711 KENNETH ZANIO (153), Hughes Research Laboratories, Malibu, California 90265 •Present address: Electrical Engineering Department, University of Delaware, Newark, Delaware 19711 + Present address: Electrical Engineering Department, Drexel University, Philadelphia, Pennsylvania 19104 xi Preface Considerable efforts have been expended in trying to understand and predict the electronic, optical, and physical properties of thin material layers. Remarkable progress has been made in these areas, interrelating the basic characteristics and in some instances, applying the results in the realization of active and passive device technologies. Thin films of various materials are currently in use as protective and optical coatings, selective membranes, and thermal transfer layers. Metal, semiconductor, and insula­ tor thin films are used extensively in the electronics industry, with applica­ tions ranging from submicron area very large-scale integration (VLSI) memory units to large-areal (comprising many square miles) energy conver­ sion devices. Although a number of excellent reviews and books on this subject are found in the literature (see the bibliography at the end of the book), most of them focus on the behavior of metal and insulator thin films, dealing with the semiconductor film in a more superficial manner. Semiconductor thin film R&D activity is evolving dramatically, following intense interest in low-cost, large-scale applications. However, many of the major contributions in this expanding field remain segmented in the litera­ ture. It is the purpose of this book to consolidate this information in a single source that can provide a general basis for understanding polycrystal­ line and amorphous semiconductor thin films and devices. This book is organized into two parts. The first (Chapters 1-5) deals with the basic properties—growth, structure, electrical and optical mechanisms —encountered in amorphous and polycrystalline thin semiconductor films. The second part (Chapters 6-9) covers the applications and problems of these layers in active semiconductor devices and passive technologies. The authors were chosen primarily on the basis of their research activities in the selected topical areas, and they represent university, industrial, and na­ tional research laboratories. xni xiv PREFACE The book begins with an introduction to the several mechanisms that suggest a hierarchy in the growth and structure (solid, liquid, and vapor phases) of amorphous and polycrystalline films [Brenton L. Mattes, Univer­ sity of Michigan]. This is followed by a synopsis of the electrical and optical properties of amorphous thin films [Da/vid Emin, Sandia National Laboratories]. This chapter discusses the structure of covalently bonded amorphous semiconductors, provides various models of localization, con­ trasts the amorphous semiconductor during and after illumination, and considers the important issues confronting research in this field. The electrical properties of polycrystalline semiconductor thin films are reviewed in Chapter 3 [Lawrence L. Kazmerski, Solar Energy Research Institute]. This treatment focuses on the transport phenomena in elemental and compound semiconductor films, with some special emphasis placed on grain boundary contributions. The optical properties of these films, primar­ ily dealing with the optical constants relating to electronic structure, are covered in Chapter 4 [Alton H. Clark, University of Maine]. The discussion of basic mechanisms topics concludes with Chapter 5, which details the electronic structure of grain boundaries in polycrystalline semiconductors [Lewis M. Fraas, Chevron Research Company, and Kenneth Zanio, Hughes Research Laboratories]. A key discussion of the methods of defect modification leading to grain boundary passivation is included. The appli­ cations topics begin with a thorough examination of active amorphous thin-film devices [David E. Carlson, RCA Laboratories]. A review of amorphous device technology, experimental methods, recent developments in thin-film devices based on hydrogenated amorphous silicon, and predic­ tions of future directions for this technology are covered. Devices based on polycrystalline semiconductor thin films are summarized in Chapter 7 [Reid A. Mickelsen, The Boeing Aerospace Company]. Thin-film transistors, diodes, photoconductors, and luminescent films form the basis for this chapter. A future large-scale, large-area device, the thin-film solar cell, is overviewed in Chapter 8 [Allen Rothwarf, John D. Meakin, and Allen M. Barnett, Institute of Energy Conversion, University of Delaware]. The present research situation for these promising devices is discussed, and forecasts of future technologies are presented. Finally, the important area that addresses the applications of passive thin films, including their func­ tions, materials selection, and taxonomy, is treated [Patrick J. Call, Solar Energy Research Institute]. Topics include optical films, protective coat­ ings, corrosion, high and low energy surfaces, and selective membranes. The understanding of the basic mechanisms that control the processes and properties of thin semiconductor films continues to evolve. The appli­ cations of these films expand as more ideas are generated. Device improve­ ment and diversity persist. The future of the semiconductor thin film, Preface xv however, depends on the ingenuity and expertise of those involved in the research and development activities. It is the sincere hope of the editor and authors that this book can especially serve that group and aid in the future deployment of technologies based on amorphous and polycrystalline thin films. The editor expresses his sincere appreciation to Sigurd Wagner for his suggestions and encouragement in organizing the book. The valuable assistance of Peter Sheldon and Phillip J. Ireland in editing and reviewing the manuscripts is gratefully acknowledged. Finally, the editor wishes to recognize and to thank Susan Sczepanski who provided (with great dili­ gence) many of the figures in the book, and Betsy Fay-Saxon who prepared (with great patience) the manuscripts into their final formats. Growth and Structure of Amorphous and Polycrystalline 1 Thin Films BRENTON L. MATTES University of Michigan Ann Arbor, Michigan 1.1 Introduction 1 1.2 Review of Amorphous and Polycrystalline States 2 1. Amorphous State 2 2. Polycrystalline State 4 1.3 Quenching Process Interactions 4 1.4 Growth of Amorphous and Polycrystalline Thin Films 5 1.5 Cluster Model Applied to Si and Amorphous Si:Η Alloys 6 1. Clusters 6 2. Cluster Hierarchy 9 1.6 Cluster Model Applied to III-V Compounds 13 References 14 1.1 INTRODUCTION The mechanisms involved in the formation of crystalline or noncrystal­ line states by condensation from vapor and liquid phases primarily depend on the time that atoms or clusters of atoms interact to form bonds in metastable and stable structures. Crystallization is the long-range ordering of atoms in a periodic solid-phase lattice near equilibrium conditions. There are many comprehensive treatments on the nucleation and growth of crystalline solids [1-7] and thin films [8-12]. Basically, the theories assume adatom attachment and/or phenomenological thermodynamic and Arrhenius relationships [6,7,11,12], Although theories on crystallization appear to be at hand, there are few theoretical treatments on the formation 1 c POLYCRYSTALLINE AND AMORPHOUS Copyright 1980 by Academic Press, Inc. THIN FILMS AND DEVICES All rights of reproduction in any form reserved ISBN 0-12-403880-8 2 BRENTON L. MATTES of the amorphous solid state. Only qualitative thermodynamic and ener­ getic driving forces have been proposed [7,13,14]. However, there are many detailed studies on structural models [15-26], deposition processes [27-31], and physical properties [16,27,28,33-39]. This review will not necessarily fill the gap but is intended to introduce several mechanisms that seem to suggest a hierarchy in the growth and structure of condensed states of the solid, liquid, and vapor phases. 1.2 REVIEW OF AMORPHOUS AND POLYCRYSTALLINE STATES Amorphous and, in general, polycrystalline thin films possess no un­ ique directionality or axis on a macroscopic plane. Microscopically, there may be some debate as to whether an amorphous state with only short- range order is a random dense packing of atoms [13,17,24] or microcrys- tallites (clusters 10-15 A in diameter) [15,16,23,24], as either may exist in the liquid phase. Polycrystalline materials are crystalline but with random grain size, shape, and orientational packing. 1. Amorphous State The amorphous state appears to require bonding anisotropics asso­ ciated with the polymorphisms of elemental solids [40] and, in addition, atom size differences for alloy and compound solids [41]. The most general process used to form amorphous materials is to quench from the liquid phase. This in general only works for the elements S, Se, P, As, and Β (Fig. 1.1, region I) [40] and for metallic glasses* such as Pdo Si 2 [21,42]. Other 8 0 elemental amorphous materials are obtained by a variety of vapor, electro­ lytic, and sputter processes that deposit materials onto substrates at differ­ ent temperatures. In addition, ion implantation can produce an amorphous state by creating locally vapor-quenched pockets in a surface layer of crystalline material [24]. As polymorphism decreases (Fig. 1.1, region II), the substrate tempera­ tures must be below — 400°C for C, Si, and Ge, and < 77 Κ for Ni, Fe, Bi, Sb, and Te. Elements in region III can only be amorphous when the substrates are at ~4 K. In region IV, not only are very low substrate temperatures required, but stabilizing factors such as contamination, very thin films, and favorable substrate interactions are necessary. Only Na, K, Cs, and Rb with nearly isotropic bonding (region V) have not exhibited an amorphous state. * Glasses are amorphous solids that might be described as being composed of two or more compounds with differing cluster sizes and/or cluster formation temperatures.