PROPERTIES of Silicon Carbide

PROPERTIES OF Silicon Carbide Edited by GARY L HARRIS Materials Science Research Center of Excellence Howard university, Washington DC, USA Lr Published by: INSPEC, the Institution of Electrical Engineers, London, United Kingdom © 1995: INSPEC, the Institution of Electrical Engineers Apart from any fair dealing for the purposes of research or private study, or criticism or review, as permitted under the Copyright, Designs and Patents Act, 1988, this publication may be reproduced, stored or transmitted, in any forms or by any means, only with the prior permission in writing of the publishers, or in the case of reprographic reproduction in accordance with the terms of licences issued by the Copyright Licensing Agency. Inquiries concerning reproduction outside those terms should be sent to the publishers at the undermentioned address: Institution of Electrical Engineers Michael Faraday House, Six Hills Way, Stevenage, Herts. SG1 2AY, United Kingdom While the editor and the publishers believe that the information and guidance given in this work is correct, all parties must rely upon their own skill and judgment when making use of it. Neither the editor nor the publishers assume any liability to anyone for any loss or damage caused by any error or omission in the work, whether such error or omission is the result of negligence or any other cause. Any and all such liability is disclaimed. The moral right of the authors to be identified as authors of this work has been asserted by them in accordance with the Copyright, Designs and Patents Act 1988. British Library Cataloguing in Publication Data A CIP catalogue record for this book is available from the British Library ISBN 0 85296 870 1 Printed in England by Short Run Press Ltd., Exeter Introduction Semiconductor technology can be traced back to at least 150-200 years ago, when scientists and engineers living on the western shores of Lake Victoria produced carbon steel made from iron crystals rather than by 'the sintering of solid particles' [I]. Since that time, silicon and silicon-based materials have played a major role in the development of modern semiconductor device technology. The earliest pioneering work in SiC as a semiconductor material can be attributed to the Marconi company in 1907 [2]. They observed SiC electroluminescence for the first time. For almost 50 years, the work by the Marconi company appeared to go unnoticed. In 1955, LeIy [3] developed a sublimation process using a growth cavity that produced rather pure platelets of SiC. The individual crystals were randomly sized and primarily hexagonal in shape. The process provided no control over the nucleation and orientation of these platelets. Since that time the interest in SiC as a semiconductor material can best be described as a roller-coaster ride with many peaks and valleys. The first international conference on SiC was held in Boston, Massachusetts in 1959. Over 500 scientists attended the conference and 46 papers were presented. In 1968 and 1973, two other International Conferences were held, the last in Miami Beach. For almost ten years, the SiC effort in the USA, and around the world, went through another valley, except in the then USSR where the research effort remained fairly constant. This valley, or lack of interest, has been 'blamed on the lack of progress in crystal growth' [4]. Nishino [5] and Powell [6] made a large contribution to the renewed interest in SiC with the development of the heteroepitaxial growth of SiC on Si. This effort was then duplicated by Sasaki et al [7], Liaw and Davis [8] and Harris et al [9]. These recent results have led to three SiC Workshops hosted by North Carolina State University (1986, 1987,1988), four International SiC Conferences co-sponsored by Howard University and Santa Clara University, yearly SiC workshops in Japan, an international high temperature semiconductor conference, the formation of several SiC companies, SiC-based products, new SiC solid solutions, and an overall renewed interest in this material. Research on SiC in Japan has also been on the same roller-coaster. The work in the 1960s in Japan was centred around basic solid-state physics and crystal growth. The research in the 1970s almost completely died. In the 1980s, SiC made a comeback due largely to the project sponsored by the Ministry of International Trade and Industry (MITI) on 'Hardened ICs for Extreme Conditions', headed by the Electrotechnical Laboratory. In 1984, the Society of SiC was organized which has now become the Society of SiC and Related Wide Bandgap Semiconductors. The society has an annual meeting and the Institute of Space and Astronautical Science (ISAS) has organized an annual High Temperature Electronics meeting. Recent topics in SiC research in Japan are micropipes in wafers, control of polytype in epitaxy, surface analysis of epitaxial layers, application for power devices, device simulation, and others [10]. As a crude measure of SiC activity, we have counted the number of articles in the Engineering Abstracts, Japanese Journal of Applied Physics, Applied Physics Letters, Materials Letters, Journal of Materials Science, Soviet Physics of Semiconductors, Soviet Physics Technical Letters, etc. for the years 1987 to 1994 (the 1994 figure is a projection based on one half of the year). The information is summarized in FIGURE 1. This data is a new unevaluated count of the number of publications dealing with silicon carbide. It provides a qualitative idea of the rate at which research in this area has grown. In 1987, only a few researchers and laboratories generated most of the published works but, in recent years, the activity at these laboratories has increased and many new research teams have become involved in SiC research throughout the US, Europe, Asia and Japan. This data does include the research activities in the former USSR where SiC activity has been very constant and at a very high level (several major research teams throughout the country). The relative research effort of semiconductors such as Si, Ge, III-V compounds and II-VI compounds is several orders of magnitude higher. However, their level of funding is also several orders of magnitude higher. The SiC effort has provided a larger 'bang for the buck'. This, I hope, will become evident after reviewing this book. ** 1994 data is a projection ** FIGURE 1 Silicon carbide publications (1987-1994) The only chemically stable form of silicon and carbon is silicon carbide. The crystalline structure of SiC can be considered to consist of the close-packed stacking of double layers of Si and C atoms. One set of atoms (Si or C) is shifted along the main axis of symmetry by a quarter of the distance between the nearest similar layers. The bonding of silicon and carbon atoms is 88% covalent and 12% ionic with a distance between the Si and C atoms of 1.89 A. Each Si or C atom is surrounded by four C or Si atoms in strong tetrahedral sp3-bonds. The stacking of the double layers follows one of three possible relative positions. They are arbitrarily labelled A, B and C. One unique aspect of SiC is the stacking sequences of these three double layers which is the source of SiCs large number of crystallographic forms called polytypes. Polytypism is a one-dimensional polymorphism that is a result of the stacking sequence. The only cubic form of SiC is called beta silicon carbide (P-SiC) which has a stacking sequence of ABCABCABC... Another convenient way of viewing these polytypes is the Ramsdell notation which is a number followed by a letter. The number represents the number of double layers in the stacking sequence and the letter represents crystal structure. For example, we have 3C for cubic or p-SiC. All hexagonal (H) and rhombohedral (R) types are commonly referred to as a-SiC with 6H the most common a-type (a more detailed discussion is given in Chapter 4). If the double layers have the same adjoining position, they are hexagonal. For instance, 2H with the ABABAB stacking sequence is one hundred percent hexagonal or its h (hexagonal fraction) equals one. The crystal structure of SiC corresponds to either cubic ZnS, the same as Si and GaAs, or hexagonal ZnS wurtzite (the colour of 3C-SiC is yellow and cc-SiC is colourless like CdS). The crystal lattices are usually given in the standard hexagonal or cubic forms. It is evident that substitutional impurities can occupy different sites depending on the polytype. Most of the polytypes, except 2H, are metastable. However, 3C does transform to 6H at temperatures above 20000C and other polytypes can transform at temperatures as low as 4000C [11,12]. Numerous electronic and optoelectronic applications have been proposed based on SiCs basic electronic and optical properties. There are five primary applications of SiC-based material: 1) micro-structures, 2) optoelectronic devices, 3) high temperature electronics, 4) radiation hard electronics and 5) high power/high frequency devices. Micro-structure applications include X-ray masks and micro-machined structures such as speaker diaphragms and special micro-application tools. Optoelectronic applications would include substrates for the nitride family of devices, light emitting diodes, and UV detectors. Because of SiCs large bandgap almost all devices fabricated on SiC can be considered for high temperature applications. Nuclear reactor electronics, military systems and deep space electronics survivability are greatly improved with SiC. SiCs thermal conductivity and high field mobility will allow increased power density and high frequency operation. Many of these applications can be summarized in the 'Fruits of SiC Tree' in FIGURE 2. Operable SiC devices have been fabricated successfully with 4H-SiC, 6H-SiC and 3C-SiC. For example, blue LEDs (light emitting diodes) were reported in 1977 that operated at elevated temperatures for virtually an unlimited lifetime with an overall efficiency of 2x 10'5.

Load more