ScAlMgO 4 : an Oxide Substrate for GaN Epitaxy
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Internet Journal o f
Nitride S emiconductor Research
Volume 1, Article 1
ScAlMgO4: an Oxide Substrate for GaN Epitaxy E. S. Hellman, C. D. Brandle, L. F. Schneemeyer, D. Wiesmann, I. Brener, T. Siegrist, G. W. Berkstresser, D. N. E. Buchanan, E. H. Hartford AT&T Bell Laboratories This article was received on September 14, 1995 and accepted on January 16, 1996.
Abstract We report the use of ScAlMgO4 as a substrate for the epitaxial growth of wurzitic GaN. The low misfit (+1.8%) allows coherent epitaxy of GaN, as observed by RHEED. The congruent melting of ScAlMgO4 makes Czochralski growth possible, suggesting that large, high quality substrates can be realized. Boules about 17mm in diameter are reported. We have used nitrogen-plasma molecular beam epitaxy to grow GaN epitaxial films onto ScAlMgO4 substrates. Band-gap photoluminescence has been observed from some of these films, depending primarily on the deposition conditions. A 3x3 superstructure has been observed by RHEED on the GaN surfaces. Structural analysis by x-ray diffraction indicates very good in-plane alignment of the GaN films. We also report thermal expansion measurements for ScAlMgO4.
1. Introduction 1.1. Sapphire: The Standard of Comparison The availability of an appropriate substrate for epitaxial growth is crucial to the practical application of new semiconductor materials. The factors determining appropriateness include crystallographic, physical, chemical, and economic parameters. For the wide band-gap semiconductor GaN, the most appropriate substrate for practical applications such as light-emitting-diodes (LED’s) has proven to be c-plane sapphire. [1] This fact might be surprising in view of some inappropriate crystallographic parameters, but not so surprising considering its physical, chemical and economic parameters. Sapphire is strong, hard, and inert. It is available in large, inexpensive, high quality wafers from a number of vendors. However, for potential use of GaN in blue lasers, these virtues may prove to be insufficient to counteract the one drawback of sapphire: its huge lattice mismatch with GaN. The -13% misfit [a] results in a very large dislocation density in GaN epitaxial films on sapphire [2] . It is at present not understood why LED’s work as well as they do in the presence of such a large dislocation density. For epitaxial growth, 13% is almost the largest misfit that can be tolerated while still getting a well-aligned film. The surprisingly high quality of GaN films that have been obtained on sapphire may be due to an interfacial AlN reaction layer. [3, [3]
1.2. Alternatives to Sapphire A variety of alternatives to sapphire have been investigated over the years, [4] including Si, GaAs, NaCl, GaP, InP, W, TiO2, SiC, ZnO, MgAl2O4 (spinel), and MgO. Only the last four can be considered possible improvements over sapphire. 1.2.1. 6H-SiC The mismatch is only -3.1% to GaN and -1.0% to AlN. SiC has the additional advantage, for many applications, of being conductive. Although excellent results have been obtained with epitaxia
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