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 H-2 or N-2 ambient at temperatures more than 1000 degrees C. 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. (c) 2004 Published by Elsevier B.V.