Two-step growth of high quality GaN using V/III ratio variation in the initial growth stage

The effect of V/III ratio in the initial stage of high temperature GaN growth using a metalorganic chemical vapor deposition system was investigated. Nucleation layer growth and coalescence process in the initial growth stage of GaN were in situ characterized by normal incidence reflectance measurement and surface morphology was ex situ characterized by atomic force microscope (AFM). Structural and optical properties were also characterized by a high-resolution X-ray diffraction and photoluminescence, respectively and correlated with AFM images. As the initial V/III ratios at high temperature GaN growth were lowered, in situ reflectance measurements showed low and long reflectance peaks. It is attributed to the enhanced three-dimensional growth mode, which result in rough surface morphology and larger island size in the initial stage of GaN growth. The larger island size means that the density of nuclei is reduced and the mixed or edge dislocations are suppressed during coalescence process. After the initial growth of high temperature GaN at a low V/III ratio, GaN layers were grown at a high V/III ratio to enhance the lateral growth. Such a two-step growth method of high temperature GaN using a V/III ratio variation led to high-quality GaN films with well-aligned growth steps, low dislocation density and intense band edge emission.