A comprehensive study was made of the electrical, optical and structural properties of wurtzite GaN films grown under various initial growth conditions The GaN films were grown directly on sapphire substrates using GaN nucleation layers and a radio-frequency plasma source. In situ reflection high-energy electron diffraction revealed a strong correlation between nucleation conditions, including the nitridation step, and the final surface reconstruction of the GaN thin film. Well-defined reconstruction patterns, primarily (2 x 2) and (4 x 4), were observed for some of the nucleation conditions. Hall mobility, photoluminescence, X-ray rocking curve data, and transmission electron microscopy measurements were used to interpret the observed relationship. The results showed that for the conditions investigated, an unreconstructed (1 x 1) surface after growth correlated with improved electrical, optical, and structural properties. The surface reconstructed thin film exhibited a strong columnar growth with inversion domains. The degraded characteristics were attributed to the presence of a mixture of both polarities in the films with reconstruction.

A Comparative Study of Surface Reconstruction of Wurtzite GaN on (0001) Sapphire by RF Plasma-Assisted Molecular Beam Epitaxy. K.K.Lee, W.A.Doolittle, T.H.Kim, A.S.Brown, G.S.May, S.R.Stock, Z.R.Dai, Z.L.Wang: Journal of Crystal Growth, 2001, 231[1-2], 8-16