Morphology and Interface chemistry of the Initial Growth of GAN and ALN on 6h-SIC and Sapphire
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MORPHOLOGY AND INTERFACE CHEMISTRY OF THE INITIAL GROWTH OF GAN AND ALN ON (x-SIC AND SAPPHIRE Z. SITAR, L. S. SMITH, M. J. PAISLEY, AND R. F. DAVIS Department of Materials Science and Engineering, Campus Box 7919 North Carolina State University, Raleigh, NC 27695-7919 ABSTRACT The morphology and interface chemistry occurring during the initial stages of growth of GaN and AIN layers has been obtained. Films were grown using gas source MBE equipment containing an ECR plasma source to activate molecular nitrogen. The experiments consisted of sequential depositions of about one monolayer thick films and XPS analysis. Evidence for silicon nitride formation on the SiC surface was obtained from the studies of both the Si oxidation states and the substrate peak intensity dependence on film thickness. The growth of GaN on sapphire appeared to occur via the Stranski-Krastanov mode, while the growth on SiC showed characteristics of three-dimensional growth. AIN grew in a layer-by-layer mode on both substrates. I. INTRODUCTION Successful commercialization of infrared and selected visible light-emitting optoelectronic devices simultaneously stimulated research in the wide-bandgap semiconductors for similar devices operable in the blue and ultra-violet regions of the spectrum. Much attention has been given to the III-V nitrides, particularly GaN and AIN, which posses direct bandgaps of 3.45 and 6.28 eV, respectively. Since they form a continuous range of solid solutions [1-3] and also superlattices [4, 5] they are suitable for bandgap engineering in the range of 3.45-6.28 eV. Single crystal wafers of these nitrides are not available, thus they must be grown heteroepitaxially. Sapphire has been the most commonly used substrate, despite the huge lattice mismatch. The quality of heteroepitaxially grown films depends on two key issues: the lattice mismatch between the substrate and the film and the values of their respective surface energies. Both influence film growth in the very early stages and, as such, have a significant influence on the overall quality of the heteroepitaxial films. Three different growth regimes are distinguished. The first one is layer-by-layer (or twodimensional) growth, in which the material is deposited in sequential monolayers of coverage. This type of growth is observed, when the substrate surface energy, Os, exceeds the sum of the surface energies of the overgrowth, 0o, and the interfacial energy, Gi (i.e. Os > yo + yi ). In other words, the overgrowth "wets" the substrate. The second regime is referred to as StranskiKrastanov growth in which the first few monolayers completely cover the surface of the substrate, while the subsequent layers form islands of deposited material. This type of growth satisfies the same equation as the layer-by-layer growth, but islanding occurs due to high strain energy which arises due to the lattice mismatch. The third regime is three dimensional growth, in which the material immediately forms islands on the surface. This type of growth occurs when s-< Oyo+ ji. Surface en
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