Microstructure of GaN epitaxial films grown on (0001) sapphire by chemical beam epitaxy as related to buffer growth conditions

GaN buffer layers of about 20 nm thickness have been deposited on (0001) sapphire between 500 and 600 degreesC. The best morphology was found to correspond to a deposition made at 560 degreesC Next. different annealing steps were made between 800 and 920 degreesC in ammonia or UHV. It is revealed that a partial desorption of GaN occurs when the buffer is heated up to 900 degreesC. In addition. three GaN epilayers of more than 1 mum thickness were grown on selected buffer layers after annealing. It is shown that the best results are obtained for a GaN epilayer grown on a buffer layer annealed at 900 degreesC in UHV. in agreement with AFM and optical measurements.