Theoretical and experimental studies of surface processes in the course of molecular-beam epitaxy of gallium nitride
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ICATION, TREATMENT, AND TESTING OF MATERIALS AND STRUCTURES
Theoretical and Experimental Studies of Surface Processes in the Course of Molecular-Beam Epitaxy of Gallium Nitride I. A. Bobrovnikovaa^, I. V. Ivonina, V. A. Novikova, and V. V. Preobrazhenskiib aTomsk
State University, Tomsk, 634050 Russia ^e-mail: [email protected] bInstitute of Semiconductor Physics, Siberian Branch, Russian Academy of Sciences, Novosibirsk, 630090 Russia Submitted May 20, 2008; accepted for publication May 30, 2008
Abstract—The method of atomic-force microscopy has been used to experimentally study the effect of growth conditions on the structure of the surface of epitaxial GaN layers grown by molecular-beam epitaxy. Quantitative values of the density, height, and width of growth centers in relation to the conditions of epitaxy are obtained; the average length of the diffusion path of particles limiting the GaN growth rate have been estimated; and the activation energies and the surface-diffusion coefficients for these particles have been calculated. The equilibrium composition of adsorbed layers at the GaN (0001) surface in a wide range of deposition temperatures and pressures of gallium and nitrogen has been calculated with account taken of the following components: gallium atoms, nitrogen atoms, and NH molecules. On the basis of the comparison of experimental data on the structure of the GaN surface with results of calculations concerning the composition of adsorbed layers on the growth surface, it was assumed that the growth of GaN layers is limited by supply of gallium. PACS numbers: 68.35.Dv, 68.37.Ps, 68.47.Fg, 68.55.Ln DOI: 10.1134/S1063782609030270
1. INTRODUCTION
2. EXPERIMENTAL
Nitrides of Group III elements are widely used in modern solid-state electronics; these nitrides are widegap semiconductors with direct optical transitions. Due to their unique properties, these compounds can be used in fabrication of optoelectronic devices operating in the blue–green and ultraviolet spectral regions and also at high temperatures and frequencies and also in aggressive media [1–4]. Epitaxial films of nitrides find the most important applications. However, a large mismatch between lattice parameters of the epitaxial layer and those of materials of substrates used (sapphire, silicon, SiC) often induce difficulties into the obtainment of epitaxial films of these compounds. In this context, it is very important to study the structure of the growth surface, initial growth stages, surface processes in the adsorption layer, and growth mechanisms in the case of epitaxy of nitrides of Group III elements. The aim of this study was to identify the mechanism and limiting stages of growth of gallium nitride in the case of molecular-beam epitaxy (MBE). To this end, we experimentally studied the surface of epitaxial GaN layers using atomic-force microscopy (AFM) and also calculated the composition of adsorption layers on the growth surface in the case of MBE of GaN on the GaN (0001) substrates. We used this approach previously in studies of mecha
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