Physics at Surfaces Andrew Zangwill Book DOI: http://dx.doi.org/10.1017/CB09780511622564 Online ISBN: 9780511622564 Hardback ISBN: 9780521321471 Paperback ISBN: 9780521347525 Cambridge University Press PHYSICS AT SURFACES ANDREW ZANGWILL Georgia Institute of Technology UCAMBRIDGE V UNIVERSITY PRESS CAMBRIDGE UNIVERSITY PRESS Cambridge, New York, Melbourne, Madrid, Cape Town, Singapore, Sao Paulo, Delhi, Dubai, Tokyo, Mexico City Cambridge University Press The Edinburgh Building, Cambridge CB2 8RU, UK Published in the United States of America by Cambridge University Press, New York www.carnbridge.org Information on this tide: www.cambridge.org/9780521347525 © Cambridge University Press 1988 This publication is in copyright. Subject to statutory exception and to the provisions of relevant collective licensing agreements, no reproduction of any part may take place without the written permission of Cambridge University Press. First published 1988 Reprinted 1989, 1990, 1992, 1996 A catalogue record for this publication is available from the British Library Library of Congress Cataloguing in Publication Data Zangwill, Andrew. Physics at surfaces. Bibliography: p. Includes index. r. Surfaces (Physics) 2. Surface chemistry. I. Tide. QC173.4.S94Z36 1988 530.4'1 87-15126 ISBN 978-0-521-32147-1 Hardback ISBN 978-0-521-34752-5 Paperback Cambridge University Press has no responsibility for the persistence or accuracy ofURLs for external or third-party internet websites referred to in this publication, and does not guarantee that any content on such websites is, or will remain, accurate or appropriate. Information regarding prices, travel timetables, and other factual information given in this work is correct at the time of first printing but Cambridge University Press does not guarantee the accuracy of such information thereafter. To the memory of RonaJd D. Parks CONTENTS Preface ix List of acronyms xii 0 Historical sketch Part 1: Clean surfaces 5 1 Thermodynamics 7 2 Chemical analysis 20 3 Crystal structure 28 4 Electronic structure 54 5 Phase transitions 110 6 Elementary excitations 138 7 Optical properties 163 Part 2: Adsorption 183 8 Phy sisorption 185 9 Chemisorption 204 10 Crystal structure 232 11 Phase transitions 257 12 Electronic structure 292 13 Energy transfer 328 14 Kinetics and dynamics 360 15 Surface reactions 400 16 Epitaxy 421 References 433 Index 448 PREFACE The surfaces of bodies are the field of very powerful forces of whose action we know but little. Lord Rayleigh The surface was invented by the devil. Wolfgang Pauli The present volume is a graduate-level introduction to the physics of solid surfaces. It is designed for students of physics, physical chemistry and materials science who are comfortable with modern condensed matter science at the level of, say, Solid State Physics by Ashcroft & Mermin (1976) or Principles of the Theory of Solids by Ziman (1972). In the latter, Ziman points out that scientific knowledge passes from the laboratory to the classroom by a sequence of literary vehicles: original research papers, review articles, monographs and finally textbooks. l believe this book fits well into none of these categories. It is not a textbook - at least not in the traditional sense. The field of surface physics is simply not mature enough to support such an enterprise; too many results are untidy and too many loose ends remain. On the other hand, it is not a review or monograph either. My purpose is neither to set down an established wisdom nor to establish priority among claimants. Indeed, l steadfastly ignore who did what when - except when it is a matter of historical interest. Rather, my interest from the beginning has been to construct a coherent synthesis of an enormous range of material and to present the result in as heuristic and pedagogical a manner as possible. Consequently, I think it is useful to regard the account before you as a travelling companion - a tour guide if you will - through the world of surface physics. It possesses both the virtues and the faults of flesh-and-blood tour guides. This book exists because Ron Parks wanted to learn something about surface physics. To that end, he asked me to prepare a graduate seminar course on the subject for the 1983-4 academic year at the Polytechnic Institute of New York. Physics at Surfaces is an expanded and refined version of lecture notes distributed to the attendees of that course. The notes were intended to fill what I perceived to be a yawning gap in the literature. At the undergraduate level, the slim volumes Surface Physics x Preface by Prutton (1983) and Principles of Surface Chemistry by Somorjai (1972) very ably meet the needs of their intended audiences. At the graduate level, Chemistry in Two Dimensions: Surfaces by Somorjai ( 1981) and Introduction to the Theory ofS olid Surfaces by Garcia-Moliner & Flores (l 979) provide unique perspectives from the point of view of very active researchers in the field. Remarkably, the subject matter presented in these books form two almost disjoint sets! Physics at Surfaces is an attempt to bridge the gap between textbook treatments of condensed matter physics and the primary research literature of surface science. It was necessary, as usual, to choose between depth and breadth of presentation. In opting for the latter, one is challenged to provide unity to a subject which attracts scientists from sub-specialties as diverse as semiconductor device physics, critical phenomena, catalytic chemistry, electron spectroscopy and many-body physics. The choice of topics and logical organization reflect the concerns and prejudices of a condensed matter physicist with a healthy interest in chemical physics. Experiment and theory are intertwined whenever possible although there is little detailed discussion of technique. Explicit references are cited whenever theoretical formulae are quoted without derivation. For experi mental detail, the reader should consult Modern Techniques of Surface Science by Woodruff & Delchar (1986) or Solid State Physics: Surfaces edited by Park & Lagally (l 985). A word about usage. The book is meant to be read as an organic whole. It is heavily self-referential in the sense that I continually revisit concepts and examples introduced in early chapters. It is not a reference work; one cannot look up the work function of Ge(l 11) or the sticking coefficient of N /Cu(l00). In fact, the text contains no data tables (although I do provide 2 an acronym table!). Instead, I stress trends which are presented visually in the figures. I wish to emphasize that careful study of the figures is of especial importance. This is so because the sheer volume of material covered and a desire to limit the cost of the book conspired to produce a rather terse prose style. I owe a considerable debt to the Surface Physics Group at the University of Pennsylvania, circa 1976-80 (T. Gustafsson, E.W. Plummer, J.R. Schrieffer and P. Soven) for my initial introduction to this subject and to many members of the international surface science community for discus sion and correspondence about their work since then. I am grateful to M. denBoer (CUNY), R. Bruinsma (UCLA), L. Roelofs (Haverford) and J. Tully (AT & T) who read and commented on selected chapters. Of course, any vagaries, misconceptions, or outright errors are entirely my responsi- Preface XI bility. Special thanks go to Norton Lang (IBM) and Bill Gadzuk (NBS) for moral support and encouragement at all stages and to my wife Sonia for these and, equally importantly, for patience. Atlanta A. Zangwill January 1987 ACRONYMS AES Auger electron spectroscopy ATR attenuated total reflection cs commensurate solid DOS density of states EDC energy distribution curve EELS electron energy loss spectroscopy ESCA electron spectroscopy for chemical analysis ESD electron stimulated desorption ESDIAD electron stimulated desorption ion angular distribution FIM field ion microscope FWHM full-width at half-maximum FVM Frank-Van der Merwe HEIS high energy ion scattering IRAS infrared absorption spectroscopy IS incommensurate solid KT Kosterlitz-Thouless LOA local density approximation LDOS local density of states LEED low energy electron diffraction LEF laser-excited fluorescence LEIS low energy ion scattering LGW Landau-Ginzburg-Wilson MBE molecular beam epitaxy MBRS molecular beam relaxation spectroscopy MD misfit dislocation MEIS medium energy ion scattering MIGS metal-induced gap states MOCVD metalorganic chemical vapor deposition Acronyms xiii MPI multi-photon ionization NEXAFS near-edge x-ray absorption fine structure NFE nearly-free electron PSD photon stimulated desorption RHEED reflection high energy electron diffraction SBZ surface Brillouin zone SCLS surface core level shift SERS surface-enhanced Raman scattering SEXAFS surface-extended x-ray absorption fine structure SIMS secondary ion mass spectroscopy SK Stranski-Krastanov sos solid-on-solid SP surface polariton STM scanning tunnelling microscopy TPD temperature programmed desorption UHV ultra-high vacuum UPS ultraviolet photoelectron spectroscopy vw Volmer-Weber XPD x-ray photoelectron diffraction XPS x-ray photoelectron spectroscopy