Effect of treatments of sapphire substrate on growth of GaN film

We have studied the surface of sapphire substrate as the first hetero-interface for the growth of wrutzite GaN film. The surface of sapphire substrate was thermally roughened by annealing in H2 or N2 ambient at temperatures more than 1000 8C. This surface plays a decisive role of nuclei site of low-temperature (LT) buffer layer for obtaining GaN film with smooth surface having Ga-face (+c) polarity. By changing the nitridation temperatures of the sapphire substrate, the polarity of GaN film can be controlled from +c to N-face ( c) polarity corresponding to smooth and hexagonal-facetted surface morphology of GaN film, respectively. This indicates that the role of LT-buffer layer might be to prevent the surface of sapphire from unintentional nitridation, which is likely to result in c GaN film by lowering the substrate temperature