CRYSTALLINE-STRUCTURE CHANGES IN GAN FILMS GROWN AT DIFFERENT TEMPERATURES

Thin GaN films were grown on (0001) sapphire at various temperatures b etween 520 and 1050 degrees C using metalorganic chemical vapor deposi tion (MOCVD). Optical properties and crystalline structures of the fil ms were investigated by means of photoluminescence, Raman scattering, and X-ray diffraction. A noticeable structure transition occurred at a round 750 degrees C with higher growth temperatures favoring the hexag onal structure and lower ones the cubic. Defect formation was also see n to be temperature dependent. The yellow luminescence which was clear ly observed in our 700-850 degrees C films was attributable to the cub ic and hexagonal structure mixing. The drastic reduction of yellow lum inescence and the substantial enhancement of near band edge emission i n the 950-1050 degrees C films indicated that this temperature range i s optimum for growing high quality wurtzite films.