Dislocation reduction mechanism in low-nucleation-density GaN growth using AlN templates

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 is 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. We have studied these initial growth stages and showed that reducing the density of GaN islands contributes to the reduction in dislocation density. However, an excessive reduction in island density leads to the worsening of film surface morphology, with the appearance of pits and spiral features. We have observed that dislocation density is related to the crystal tilts of GaN nucleation islands and that pits and spirals are 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.