The morphological evolution of a-GaN deposited by metalorganic chemical vapor deposition (MOCVD) on rsapphire is studied.The influences of Ⅴ/Ⅲ ratio and growth temperature on surface morphology are investigated.Ⅴ-pits and stripes are observed on the surface of a-GaN grown at 1050 ℃ and 1100℃,respectively.The overall orientation and geometry of Ⅴ-pits are uniform and independent on the Ⅴ/Ⅲ molar ratio in the samples grown at 1050 ℃,while in the samples grown at 1100 ℃,the areas of stripes decrease with the adding of Ⅴ/Ⅲ ratio.We deduce the origin of Ⅴ-pits and stripes by annealing the buffer layers at different temperatures.Because of the existence of inclined (10(-1)1) facets,Ⅴ-pits are formed at 1050℃.The (10(-1)1) plane is an N terminated surface,which is metastable at higher temperature,so stripes instead of V-pits are observed at 1100℃.Raman spectra suggest that the growth temperature of the first layer in the two-step process greatly affects the strain of the films.Hence,to improve the growth temperature of the first layer in the two-step method may be an effective way to obtain high quality a-GaN film on r-sapphire.