SPRINGER SERIES IN SURFACE SCIENCES 43 Springer-Verlag Berlin Heidelberg GmbH ONLINE LIBRARY Physics and Astronomy springeronline.com SPRINGER SERIES IN SURFACE SCIENCES Series Editors: G. Ertl, H. Liith and D.L. Mills This series covers the whole spectrum of surface sciences, including structure and dynamics of clean and adsorbate-covered surfaces, thin films, basic surface effects, analytical methods and also the physics and chemistry of interfaces. Written by leading researchers in the field, the books are intended primarily for researchers in academia and industry and for graduate students. 38 Progress in Transmission Electron Microscopy 1 Concepts and Techniques Editors: X.-F. Zhang, Z. Zhang 39 Progress in Transmission Electron Microscopy 2 Applications in Materials Science Editors: X.-F. Zhang, Z. Zhang 40 Giant Magneto-Resistance Devices By E. Hirota, H. Sakakima, and K. Inomata 41 The Physics of Ultra-High-Density Magnetic Recording Editors: M.L. Plumer, J. van Ek, and D. Weller 42 Islands, Mounds and Atoms Patterns and Processes in Crystal Growth Far from Equilibrium By T. Michely and J. Krug 43 Electr'-'lie Properties of Semiconductor Interfaces ByW.~. "onch Volumes 1-37 are listed at the end of the book Winfried Monch Electronic Properties of Semiconductor Interfaces With 146 Figures and 17 Tables Springer Professor em. Dr. Winfried Monch Universitiit Duisburg-Essen Fakultiit fiir Naturwissenschaften Laboratorium fiir Festkiirperphysik 47048 Duisburg, Germany [email protected] Series Editors: Professor Dr. Gerhard Ertl Fritz-Haber-Institute der Max-Planck-Gesellschaft, Faradayweg 4-6, 14195 Berlin, Germany Professor Dr. Hans Liith Institut fiir Schicht -und lonentechnik Forschungszentrum Jiilich GmbH, 52425 Jiilich, Germany Professor Douglas L. Mills, Ph.D. Department of Physics, University of California, Irvine, CA 92717, USA Library of Congress Cataloging-in-Publication Data: Miinch, Winfried. Electronic properties of semiconductor interfaces / Winfried Miinch. p. cm. - (Springer series in surface sciences ; 43) Includes bibliographical refer ences and index. 1. Diodes, Schottky-barrier. 2. Semiconductors-Junctions. 1. ISBN 978-3-642-05778-6 ISBN 978-3-662-06945-5 (eBook) DOI 10.1007/978-3-662-06945-5 Tide. II. Series. TK7871.89.S35M66 2003 621.3815'2-dc22 2003066401 ISSN 0931-5195 ISBN 978-3-642-05778-6 This work is subject to copyright. Ali rights are reserved, whether the whole or part of ilie material is concerned, specifically the rights of translation, reprinting, reuse of illustrations, recitation, broadcasting, reproduction on microfilm or in any other way, and storage in data banks. Duplication of iliis publication or parts thereof is permitted only under the provisions of the German Copyright Law of September 9, 1965, in its current version, and permission for use must always be obtained from Springer-Verlag Berlin Heidelberg GmbH. Violations are liable for prosecution under the German Copyright Law. springeronline.com © Springer-Verlag Berlin Heidelberg 2004 Originally published by Springer-Verlag Berlin Heidelberg New York in 2004 Softcover reprint of ilie hardcover 1s t edition 2004 The use of general descriptive names, registered names, trademarks, etc. in this publication does not imply, even in the absence of a specific statement, that such names are exempt from the relevant protective laws and regulations and therefore free for general use. Typesetting: Data prepared by the auilior using a Springer Word macro package Finallayout: LeTeX, Leipzig Cover concept: eStudio Calamar Steinen Cover production: design & production GmbH, Heidelberg Printed on acid-free paper SPIN: 10959480 57/3141/ba -5 4321 o To Gisela with many thanks for your patience and encouragement Preface Semiconductor interfaces are a most instructive example of the intimate intercon nections between fundamental and applied science. Braun's discovery of the recti fying properties of metal-semiconductor contacts marks the beginning of semi conductor science in 1874. Although Schottky's explanation of this phenomenon in 1938 was possible only after the quantum-theory of solids had been formulated cuprous oxide and selenium rectifiers were industrially produced in large quanti ties much earlier. Ironically, Poganski's work revealed the technical selenium rec tifiers to consist of n-CdSe/p-Se semiconductor heterostructures rather than to be Cd/p-Se metal-semiconductor contacts as initially inferred. Finally, it took an other 60 years before experimental data and theoretical concepts of the band lineup at both metal-semiconductor contacts and semiconductor heterostructures could be brought into harmony. On the theoretical side, Heine's concept of inter face-induced gap states, which he published in 1965, plays the most prominent role. As "new" semiconductors, such as silicon carbide and the III-nitrides, emerged, the interplay between fundamental research and device-oriented devel opment continues and keeps the fascinating field of semiconductor interfaces in vivid motion. My thanks go to the many of my former students who have produced a real wealth of experimental data on metal-semiconductor interfaces. Their data were the basis not only of our intense internal discussions but served also for the many and sometimes controversial but in the end fruitful arguments with members and heads of other research groups around the world. In addition, I would like to thank Professor Axel Lorke, my successor, for providing me with the possibility to con tinue my scientific work. It is my great pleasure to thank quite a number of col leagues of mine who trustfully provided me with valuable experimental data, mostly effective barrier heights and ideality factors of Schottky diodes, which were hidden in their files but unfortunately not contained in their publications. Furthermore, I have to thank Professors Inder P. Batra, Marvin L. Cohen, and Max G. Lagally, who gave permission to reprint figures which appeared in articles of theirs. Millheim an der Ruhr, October 2003 Winfried Monch Contents 1. Introduction ..................................................................................................... ! 1.1 Metal-Semiconductor Contacts ............................................................... 1 1.1.1 Rectification of Metal-Semiconductor Contacts ..................... 1 1.1.2 Explanation of Rectification by Depletion Layers ................... 4 1.1.3 Metal-Induced Gap States ........................................................ 7 1.1.4 Rules, Correlations, and Models ............................................... 9 1.1.5 MIG S-and-Electronegativity Theory ...................................... 14 1.2 Semiconductor Heterostructures and the IFIGS Concept... ................... IS 2. Depletion Layer ............................................................................................. 21 2.1 The Schottky Barrier. ............................................................................. 21 2.2 Capacitance of Schottky Barriers .......................................................... 25 2.3 Image-Force or Schottky Effect... .......................................................... 27 3. Determination of Barrier Heights and Offsets ........................................... 33 3.1 Current Transport across Schottky Contacts ......................................... 33 3.2 Effective Barrier Height ofldeal Schottky Contacts ............................. 38 3.3 Effective Richardson Constant... ........................................................... 39 3.4 !IV Characteristics of Real Schottky Contacts ...................................... .42 3.5 Barrier Heights of Real Schottky Contacts 1: !IV Characteristics..... 4 7 3.6 Ballistic-Electron-Emission Microscopy ............................................... 57 3. 7 Barrier Heights of Real Schottky Contacts 2: BEEM IcoiVVtip Characteristics .............................................................. 61 3.8 Barrier Heights of Real Schottky Contacts 3: Internal Photoemission Yield Spectroscopy .......................................... 63 3.9 Core-Level Photoemission Spectroscopy .............................................. 67 3.9.1 Chemical Shifts and Interface Band-Bending Determination by PES ............................................................. 67 3.9.2 Growth Modes of Metals on Semiconductors ......................... 70 3.10 Barrier Heights of Real Schottky Diodes 4: XPS .................................. 74 3.11 Metal-Induced Core-Level Shifts and the Intrinsic Interface Electric-Dipole Layer. ............................................................ 75 3.12 Determination of Band Offsets at Heterostructures ............................... 79 3.12.1 Internal Photoemission Yield Spectroscopy ........................... 79 3.12.2 Core-Level X-Ray Photoemission Spectroscopy .................... 81 X Contents 4. Laterally Inhomogeneous Schottky Contacts ............................................ 83 4.1 Potential Distribution ............................................................................. 83 4.2 Current Transport in Schottky Contacts with One Circular Patch ........ 91 4.3 Current Transport in Schottky Contacts with Many Circular Patches ................................................................................................... 93 4.4 Test of the Patch Concept with Modified Contacts ............................... 97 4.5 //Vand BEEM Test of the Patch Concept... .......................................... 98 4.6 Origin of Lateral Barrier-Height Inhomogeneities .............................. l03 4.6.1 Natural or Intrinsic Nonuniformities .................................... 103 4.6.2 Extrinsic Nonuniformities ..................................................... 10 4 5. The IFIGS-and-Eiectronegativity Theory ................................................ ! 07 5.1 Band-Structure Lineup and Electronegativity: A Chemical Approach ......................................................................... 10 7 5.1.1 Barrier Heights of Schottky Contacts ................................... 107 5.1.2 Interface Dipoles at Semiconductor Heterostructures .......... 109 5.2 Barrier Heights and Interface Dipoles: A Phenomenological Approach ............................................................................................. Ill 5.3 Interface-Induced Gap States ............................................................... ll5 5.3.1 Metal-Induced Gap States ..................................................... ll5 5.3 .2 Semiconductor Heterostructures ........................................... 118 5.3.3 Valence-Band Offsets at Semiconductor Heterostructures ... ll9 5.4 Virtual Gap States: One-Dimensional Model... ................................... ll9 5.5 Virtual Gap States: Three-Dimensional Model... ................................ 122 5.5.1 Mean-Value !f-Points and the Dielectric Band Gap .............. 122 5.5.2 Branch-Point Energy in the ViGS Continuum ...................... l25 5.5.3 Decay Length of the ViGS at their Branch Point... .............. 129 5.6 Slope Parameter of Barrier Heights ..................................................... 131 6. The MIGS-and-Eiectronegativity Concept: Experiment and Theory ... l35 6.1 Schottky Contacts ................................................................................ 135 6.1.1 Introductory Remarks ........................................................... 135 6.1.2 Silicon Schottky contacts ...................................................... 136 6.1.3 GaAs Schottky Contacts ....................................................... 139 6.1.4 GaP Schottky Contacts ......................................................... 140 6.1.5 GaN Schottky Contacts ......................................................... 141 6.1.6 SiC Schottky Contacts .......................................................... 142 6.1. 7 ZnSe Schottky Contacts ........................................................ 146 6.1.8 Schottky Contacts on Layered Ga-Chalcogenides ............... 146 6.1.9 CuinSe Schottky Contacts ................................................... 148 2 6.1.10 Schottky Contacts on Ternary Ill-Y Alloys ......................... 149 6.1.11 Metal-Insulator Contacts ...................................................... 155 6.1.12 Direct Observations ofMIGS ............................................... l59 6.2 Heterostructures ................................................................................... 160 6.2.1 Introductory Remarks ........................................................... 160 6.2.2 Non-Polar Heterostructures .................................................. 162 Contents XI 6.2.3 Lattice-Matched Ternary and Quaternary III-V Alloys ....... l63 6.2.4 I-III-Vl Chalcopyrites ......................................................... 168 2 6.2.5 Layered Serniconductors ....................................................... l70 6.2.6 Metamorphic Heterostructures .............................................. 172 6.2. 7 Semiconductor-Insulator Interfaces ..................................... 173 6.2.8 Langer-Heinrich Rule .......................................................... 176 7. First-Principles Calculations of Barrier Heights and Valence- Band Offsets ................................................................................................ 181 7.1 Introductory Remarks .......................................................................... 181 7.2 Schottky Barrier Heights ..................................................................... 181 7.3 Valence-Band Offsets at Heterostructures .......................................... 186 7.3.1 Isovalent and Lattice-Matched AlGaAs/GaAs ..................... 186 7.3.2 SiC Heterostructures ............................................................. l87 8. Temperature and Pressure Effects ........................................................... 189 8.1 Temperature Effects ............................................................................ 189 8.2 Pressure Effects ................................................................................... 193 8 .2 .1 Schottky Barrier Heights ....................................................... 193 8.2.2 Valence-Band Offsets ........................................................... 196 8.3 Pseudomorphic Heterostructures ......................................................... l97 9. Barrier Heights and Extrinsic Interface Defects ..................................... 203 9.1 Defect-Induced Changes ofBarrier Heights ....................................... 203 9.2 Application to p-GaP(110) Schottky Contacts .................................... 206 9.3 Unified Defect Model. ......................................................................... 208 10. Extrinsic Interface Dipoles ......................................................................... 209 10.1 Interface Doping of Schottky Contacts ............................................... 209 10.2 Interface Structure ............................................................................... 214 10 .2.1 MetaVSi(111 )-(7x7)i Contacts .............................................. 214 10 .2.2 Epitaxial Silicide/Silicon Interfaces ...................................... 217 10 .2.3 Polar Heterostructures ........................................................... 224 11. Ohmic Contacts ........................................................................................... 227 Appendix .............................................................................................................. 231 References ........................................................................................................... .235 Subject Index. ..................................................................................................... .257 1. Introduction 1.1 Metal-Semiconductor Contacts 1.1.1 Rectification of Metal-Semiconductor Contacts Semiconductor science began in 1874. At the end of this year, Ferdinand Braun described his discovery of the rectifying properties of metal-semiconductor con tacts in an article entitled Ober die Stromleitung durch Schwefelmetalle.1 This publication deals with an interface-controlled device and, in this, it is an excellent example of the intimate interaction between fundamental research of bulk, surface, as well as interface properties and device physics that has been a characteristic of semiconductor physics since then. The expression Halbleiter or semi-conductor was, most probably, first used by Koenigsberger and Weiss in 1911. "Bei einer groBen Anzahl nati.irlicher und ki.instlicher Schwefelmetalle und sehr verschiedenen Sti.icken, sowohl Krystallen von so vollkommener Ausbildung, wie ich i.iberhaupt bekommen konnte, als derben Sti.icken habe ich gefunden, daB der Widerstand derselben verschieden war mit Richtung, lntensiUit und Dauer des Stromes. Die Unterschiede betragen bis zu 30pCt. des ganzen Werthes. Ich habe 000 benutzt Quecksilbercontact, stark gegen gepreBte Kupfer-, Platin- und Silberdrahte und endlich bei einem Sti.ick eine bereits vorhandene Fassung mit dicken Neusil berbi.igeln."2 Braun verified this summary of his observations with two tabulated series of experimental data. One displays the current flow through a metal resistor and a chalcopyrite sample, while the other one compares the current through the chalcopyrite sample when the applied voltage and by this the direction of the cur rent is reversed. Figure 1.1 shows these data of Braun. The chalcopyrite crystal clearly exhibits unipolar conduction, as the rectifying behavior was called then 1 On the Current Transport in Metal Suljides 2 "In a large number of natural and synthetic sulfides and with very different samples, in crystals as perfectly shaped as I could get them as well as rough pieces, I have found that their resistance varied by up to 30%, depending on direction, intensity, and duration of the current. I have used mercury contacts, wires of copper, platinum, and silver, which 000 were heavily pressed against the samples, and an already existing mounting made of German silver with another piece." W. Mönch, Electronic Properties of Semiconductor Interfaces © Springer-Verlag Berlin Heidelberg 2004