Abstract
We have investigated the variation of low-temperature (LT) GaN buffer layer on sapphire substrate for the growth of GaN film by metalorganic chemical vapor deposition, changing the deposition conditions such as substrate treatment, V/III ratio, thickness and ambient during the annealing. The dependence of the buffer layer on the conditions is qualitatively discussed for the deposition of GaN film with better quality. Another function of the buffer layer except for the nucleation site and relaxation of lattice-mismatch has been found by taking the approaches to the control and evaluation of the polarity in the buffer layer and GaN film. Decreasing the temperature down to 500-600℃ for the deposition of buffer layer after cleaning sapphire substrate in H_2 ambient at 1000℃ would suppress the unintentional fluctuation at the interface of the substrate resulting in the generation of nucleation site with N-face polarity. We have considered that the key point of LT-buffer layer would convert the polarity from N-face GaN film with hexagonal-facetted surface to Ga-face GaN film with smooth surface by lowering the temperature.
