A comparative study of surface reconstruction of wurtzite GaN on (0001) sapphire by RF plasma-assisted molecular beam epitaxy

We present a comprehensive study of the electrical, optical. and structural properties of wurtzite GaN films grown under various initial growth condit ions The GaN films were grown directly on sapphire substrates using GaN nuc leation layers by a Riber 3200 system with a radio-frequency plasma source. In situ reflection high-energy electron diffraction (RHEED) reveals a stro ng correlation between nucleation conditions, including the nitridation ste p, and the final surface reconstruction of the GaN thin film. Well-defined reconstruction patterns, primarily (2 x 2) and (4 x 4), are observed for so me of the nucleation conditions. Hall mobility, photoluminescence (PL), X-r ay rocking curve data, and transmission electron microscopy (TEM) measureme nts are used to interpret the observed relationship. The results show that for the conditions investigated, an unreconstructed (1 x 1) surface after g rowth correlates with improved electrical. optical. and structural properti es. The surface reconstructed thin film exhibits a strong columnar growth w ith inversion domains (IDs). We attribute the degraded characteristics to t he presence of a mixture of both polarities in the films with reconstructio n. (C) 2001 Published by Elsevier Science B.V.