A free-standing GaN substrate, which was over 5cm in size and had a low dislocation density, was prepared by hydride vapor-phase epitaxy; using GaAs (111)A as the starting substrate. A SiO2 mask with round openings was formed directly on the GaAs (111)A substrate. A thick GaN layer was then grown which had numerous large hexagonal inverse-pyramidal pits, constructed mainly of {11•2} facets, on the surface. Upon removing the GaAs substrate, and polishing, a free-standing GaN substrate which was about 500μm in thickness was obtained. The dislocation distribution was studied by means of transmission electron microscopic, cathodoluminescence and etch-pit density measurements. This showed that limited areas, with dislocation densities of up to 109/cm2, existed in areas with dislocation densities as low as 2 x 105/cm2. This dislocation distribution was caused by {11•2} facet growth in hexagonal pits. The dislocations were concentrated at the centers of hexagonal pits, and large areas with a low dislocation density were therefore formed. A new mechanism was proposed for reducing dislocation densities.

Growth and Characterization of Free-Standing GaN Substrates. K.Motoki, T.Okahisa, S.Nakahata, N.Matsumoto, H.Kimura, H.Kasai, K.Takemoto, K.Uematsu, M.Ueno, Y.Kumagai, A.Koukitu, H.Seki: Journal of Crystal Growth, 2002, 237-239, 912-21