Study on grain boundary defects in GaN film grown by MOCVD

Full text: Microstructures of h-GaN film by means of MOCVD on (0001) sapphire substrate has been studied by focused ion beam milling (FIB), field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Two different types of grain boundary defects are observed. For the first type, the grain boundaries are surrounding around the pyramid-shaped grains. These defects form from the mismatch of the crystal lattices between the GaN film and the sapphire substrate. The grain boundary defects penetrate the GaN epilayer and the average grain size increases with increasing layer thickness up to about 500nm from the interface to the top surface of the GaN film. For the second type, the grain boundaries are surrounding around the voids forming from the impurities in the film. Both types of grain boundaries have a distribution with a hexagonal symmetry that may be the origin of the anisotropic transport properties observed previously in the GaN films. A model for the growth mechanism of the h-GaN film are proposed based on the results obtained