Influence of moisture and oxygen on the formation of cubic phase GaN in halide vapor phase epitaxial growth

Halide vapor phase epitaxy (HVPE) was performed for the growth of GaN films. The hexagonal phase alpha-GaN films were grown on (0001) and (<11(2)over bar 0>) sapphire substrate, using the Ga/HCl/NH3/He system. Reflection high energy electron diffraction (RHEED) and scanning electron microscopy (SEM) were used for the study of phase orientation and surface morphology of the resulting films. Luminescence was assessed by photoluminescence (PL) and local cathodoluminescence (CL) measurements at room temperature. It was observed that the oriented islands of cubic gallium nitride (beta-GaN) were formed on the surface of as-grown alpha-GaN films. Their growth occurred during: postgrowth, when the temperature in the deposition zone decreased. It was shown that the growth of the beta-GaN islands was caused by the presence of trace moisture and oxygen remained in the gas phase. Apart from the partial pressures of H2O vapor and oxygen the growth conditions of alpha-GaN films showed a significant effect on the density of the resulting islands. Possible reasons for the formation of beta-GaN followed by subsequent growth are discussed.