It was recalled that conventional thin-film growth was a 2-step process that started with a low-temperature buffer layer (also known as the wetting or nucleation layer), followed by the growth of high-temperature GaN islands that eventually coalesced and planarized to form a basic template. These initial growth stages were studied and it was shown that reducing the density of GaN islands contributed to the reduction in dislocation density. However, an excessive reduction in island density led to worsening of the film surface morphology, with the appearance of pits and spiral features. It was observed that the dislocation density was related to the crystal tilts of GaN nucleation islands and that pits and spirals were generated from the screw-type dislocation bundles formed at low GaN island densities. The alternative use of AlN templates (instead of conventional GaN buffer layers) consequently improves the c-axis crystal orientation and reduces the threading dislocation density in the GaN films while simultaneously providing a high-quality surface morphology.

Dislocation Reduction Mechanism in Low-Nucleation-Density GaN Growth Using AlN Templates. D.Morita, A.Fujioka, T.Mukai, M.Fukui: Japanese Journal of Applied Physics, 2007, 46[5A], 2895-900