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High quality GaN films with thickness up to 160um were prepared by HVPE on O-polarity ZnO bulk substrate,and ZnO was removed afterwards to get a free-standing GaN.The morphologies of the GaN films were investigated by scanning electron microscopy.Raman measurements of the free-standing GaN showed E2 (High) peak at 567.1 cm-1 with FWHM 2.9 cm-1,E2 (Low) peak at 143.5 cm-1,A1 (LO) peak at 737.3 cm-1.The room temperature photoluminescence and cathodoluminescence measurements showed that a sharp peak in the vicinity of the band edge at 366nm with high intensity.Effects of growth temperature on GaN layer on ZnO substrate were discussed.