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We investigate the effects of the Ⅴ/Ⅲ ratio of a low-temperature GaN buffer layer on the growth of the overlaying nonpolar a-plane GaN film grown on r-plane sapphire by metal-organic chemical-vapor deposition (MOCVD).With other experimental conditions Keeping fixed,the low-temperature GaN buffer layers are grown under various Ⅴ/Ⅲ ratios of 1000,3000,6000 and 9000,respectively.The characteristics of the a-plane GaN films are analyzed by scanning electron microscopy,high resolution x-ray diffraction,Raman spectrum,and low temperature photoluminescence.The results show that the Ⅴ/Ⅲ ratio of the buffer layer has significant effects on the crystal quality of the a-plane GaN film,and a Ⅴ/Ⅲ ratio of 6000 is found to be the most suitable condition to achieve pit-free flat GaN surface.