Crystalline structure changes in GaN films grown at different temperatures

Thin GaN films were grown on (0001) sapphire at various temperatures between 520 and 1050 °C using metallorganic chemical vapor deposition (MOCVD). Optical properties and crystalline structures of the films were investigated by means of photoluminescence, Raman scattering, and X-ray diffraction. A noticeable structure transition occurred at around 750 °C with higher growth temperatures favoring the hexagonal structure and lower ones the cubic. Defect formation was also seen to be temperature dependent. The yellow luminescence which was clearly observed in our 700-850 °C films was attributable to the cubic and hexagonal structure mixing. The drastic reduction of yellow luminescence and the substantial enhancement of near band edge emission in the 950-1050 °C films indicated that this temperature range is optimum for growing high quality wurtzite films.