P-N JUNCTIONS IN INTERMETALLIC SEMICONDUCTORS Being the text of a thesis submitted to the University of London in the Faculty of Engineering for the degree of Doctor of Philosophy by Herbert Douglas Barber B.Sc.(Eng), M.Sc. Electrical Engineering Dept. Imperial College University of London 1965 2. Abstract A technique has been developed to prepare shallow penetration alloyed p-n junctions utilizing atomic or ionic hydrogen to achieve low temperature wetting. The electrical characteristics of diodes prepared by this technique on the intermetallic compound, indium antimonide, have been measured as a function of doping density, temperature, crystallographic orientation, dislocation density, surface treatment and metallurgical processing. The diode charac- teristics have been interpretted in terms of the properties of the base crystals and the junction regrowth. An analytic expression for the current-voltage relationship of a diode in conditions of medium level injection has been developed. When combined with existing expressions for conditions of high and low level injection, the theory accurately describes the characteristics of InSb p+n diodes. Evidence has been obtained from measurements of space charge generation currents for a trapping level in InSb at 0.133 ev _Lt having a temperature dependence of -1.55 x 10 ev/ K. Metallurgical investigations have been shown that facets form on fylla and k100- surfaces in InSb and on f.:1113- surfaces only in germanium. It has also been observed that Ai1113- surfaces dissolve more rapidly during alloying than do Be113- surfaces. Models for C11 3- andkail surfaces are presented and used to explain these observations as well as metallurgical observations made by other workers. 3. Acknowledgements I am indebted to Dr.J.C.Anderson and to Professor J.Lamb for providing the facilities required for this work and for supporting it throughout. The support and interest of Dr.E.L.Heasell is also gratefully acknowledge. I would like to thank the members of the Materials Laboratory for their co-operation during the course of this work. In particular I would like to thank Dr.S.C.Choo and Mr.J.Hollis for valuable discussions on recombination in InSb, Dr.P.M.Gundry and Mr.P.Norgate for helpful discussions on the metallurgical and chemical aspects of the work, and Mr.J.Conradi and Mr.A.C.Papadakis who did a large part of the work in determining the In-InSb and Sn-InSb liquidus curves. Thanks are also due to members of the Transistor Laboratory for their help in te design of the low noise preamplifier and the broad band capacitor, and to the technical staff of the department for their assistance in constructing the many pieces of apparatus used in the work. It is also a pleasure to acknowledge the help of Mr.A.Willoughby who supplied the strained sample of InSb and information on unpublished results of his work on dislocations. I am indebted to Dr.J.D.Mullin of the Royal Radar Establishment for providing most of the single crystals of InSb and to Dr.K.F.Hulme, Dr.D.T.Hurle and Dr.J.B.Mullin for their interest and stimulating discussions. I am also indebted to Dr.G.K.Teal of Texas Instruments who kindly provided me with a copy of an 4. unpublished report on some as-9ects of the investigations of InSb devices at the T.I.Laboratory in Dallas Texas. The development of the H100 etch used in the present work was a result of their investigation. This work would not have been possible without the encourage- ment and understanding of my wife and the financial support of the Board of Trade in the form of an Athlone Fellowship and the National Research Council of Canada in the form of a NATO Special Scholarship. 5. INDEX Title 1 Abstract 2 Acknowledgements 3 CHAPTER 1. INTRODUCTION 1.1 Purpose of the Study 9 1.2 Properties of Indium Antimonide 11 1.2.1 Physical and Metallurgical Properties 11 1.2.2 Polar Properties 15 1.2.3 Electronic and Optical Properties 20 1.3 Diffusion in III - V Compounds 23 1.4 p-n Junctions in III - V Compounds 27 1.4.1 The development of p-n Junctions in III-V Compounds 27 1.4.2 Junctions in InSb 31 CHAPTER 2. THEORETICAL COrISIDERATIONS 2.1 Direct Current Characteristics of p-n Junctions 35 2.1.1 Formulation of p-n Junction Theory 35 2.1.2 Low Level Diffusion Currents 41 2.1.3 High Level Diffusion Currents 49 2.1.4 Recombination Current in Forward Dias 52 2.1.5 Complete Forward Bias Current Dependence 54 2.1.6 Reverse Currents 63 2.1.7 Reverse Breakdown 65 6. 2.2 Transient Response of p-n Junctions 68 2.2.1 Capacity 68 2.2.2 Step Response and Recombination 68 CHAPTER 3. EXPERIMENTAL APPARATUS AND PROCEDURES 3.1 Crystal Preparation and Evaluation 71 3.2 Development of Alloying Techniques 73 3.2.1 Methods of Junction Evaluation 73 3.2.2 Alloying Difficulties in InSb 74 3.2.3 Radiation Wetting and Controlled Alloying 78 3.3 Diode Fabrication 84 3.4 Electrical Measurements 86 3.4.1 Cryostat 86 3.4.2 Direct Current Measurements 89 3.4.3 Capacity Measurements 92 3.4.4 Pulse Measurements 94 3.5 Area Measurements 97 CHAPTER 4. CRYSTALLOGRAPHIC EFFECTS ON ALLOYING 4.1 Alloying on A t211..1- and B [1111 Surfaces 100 4.2 Alloying on -(10). and •:110} Surfaces 111 4.2.1 Experiments on InSb 111 4.2.2 Experiments on Germanium 115 4,3 Surface Models 119 4.3,1 (:111.1 Surfaces 120 4.3.2 -(100:Y Surfaces 122 7. 4.4 Crystal Growth and Dissoliation 125 4.4.1 Introductory Considerations 125 4.4.2 Growth on fliq Surfaces 128 4.4.5 Growth on {:100} Surfaces 131 4.4.4 Growth on [110} Surfaces 137 4.4.5 Imnlications on Metallurgical Phenomena 138 CHAPTER 5. INDIUM ANTIMONIDE p+n DIODES 5.1 Properties of Materials Used • 148 5.1.1 Intrinsic Carrier Concentration 148 5.1.2 Properties of the Base Crystals 152 5.1.3 Diffusion Potential and Properties of the Regrowth 158 5.2 Abrupt Junction and Space Charge Region Approximations 163 5.3 Direct Current Characteristics 168 5.3.1 Forward Conduction 168 5.3.2 Reverse Conduction and Breakdown 178 5.4 Transient Measurements 199 5.4.1 Capacity Measurements 199 5.4.2 Pulsed Lifetime Measurements 202 5.4.3 Plasma Effects 205 5.5 Surface Effects 209 5.5.1 Influence of Etching and Ambients 209 5.5.2 Anomalous Hardening Effect 216 5.6 Effects of High Dislocation Densities 219 5.7 Preliminary Work on n+p Diodes 224 5.7.1 Sn-InSb Liquidus 225 5.7.2 Sn-InSb Junctions 229 8. CHAPTER 6. CONCLUSION 231 APPENDIX A - Diffusion and Thermal Conversion 235 A.1. Materials and Techniques 235 A.2. Thermal Conversion 236 A.3. Zinc and Cadmium Diffusion 238 APPENDIX B - Theory of Diffusion Currents 241 B.1.General Formulation 21+1 B.2.Low Level Case 241 B.3.High Level Case 243 APPENDIX C - High Injection Step Response 244 APPENDIX D Shot Tower APPENDIX E - Capillary Alloying 248 APPENDIX F - Junction Area Correction 253 References 251+ Photograph of electrical apparatus 265 Reprints of publications CHAPTER1 9. Introduction 1.1 Purpose of the Study. The metallurgy, electrical properties and methods of fabrication of p-n junctions in elemental semiconductors have been the subject of vast amounts of research since the discovery of transistor action(1) in 1948. Extremely rapid advances in semiconductor physics and devices have arisen from the reciprocal stimulation of pure research and technology. As a result a wealth of information is available which has made it possible to design devices to exacting standards in these materials. Without these developments many of the spectacular advances in other fields of research and endeavour would have been very seriously hampered, if not impossible. Every material possesses different properties. Since design is the process of combining and compromising the properties of the components to approximate the desired properties of the device, it is not necessary to argue the value of a choice of materials for any application. It is for this reason that increasing resources are being channelled into the search for new materials and for an understanding of their properties. New families of semiconductors have been and are being discovered as a result. One of these families, first recognized as semiconductors by Welker(2) in 1952, is the family of IIIb-Vb (hereafter called III-V) compounds. Since 10. 1952 these compounds have received increasing attention and although many of the properties of these materials rointed to unique and useful applications the technology and understanding, first of material preparation and then of device fabrication has caused much slower development than that achieved in elemental semiconductors. The preparation of one of these compounds, indium antimonide, has been extensively studied(3) and is not hampered by the severe difficulties, such as the high vapor pressure of one component, encountered in other Ill-V compounds. Indium antimonide is therefore available in single crystals of comparable perfection and purity to the elemental semiconductors, silicon and germanium. It is largely for this reason that indium antimonide was chosen for this work. The purpose of this work was to begin a study of p-n junctions which would yield fundamental and technological information for device design in indium antimonide, similar to that already available for silicon and germanium, This is, of course, a vast project and hence the following limited object- ives were envisaged: (a)The necessary techniques for p-n junction fabrication in indium antimonide were to be developed and understood.