A comparative study of surface reconstruction of wurtzite GaN on (0 0 0 1) 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 conditions The GaN films were grown directly on sapphire substrates using GaN nucleation layers by a Riber 3200 system with a radio-frequency plasma source. In situ reflection high-energy electron diffraction (RHEED) reveals 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 × 2) and (4 × 4), are observed for some of the nucleation conditions. Hall mobility, photoluminescence (PL), X-ray rocking curve data, and transmission electron microscopy (TEM) measurements are used to interpret the observed relationship. The results show that for the conditions investigated, an unreconstructed (1 × 1) surface after growth correlates with improved electrical, optical, and structural properties. The surface reconstructed thin film exhibits a strong columnar growth with inversion domains (IDs). We attribute the degraded characteristics to the presence of a mixture of both polarities in the films with reconstruction.