Achieving a high output power and a high energy efficiency in ultraviolet light-emitting devices requires a reduction in the dislocation density of an underlying GaN template. Conventional thin film growth was a two-step process that starts 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 coalesce and planarize to form the basic template for device structures. 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 a 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 improved the c-axis crystal orientation and reduced 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