Abstract
Based on the results of microstructural characterization of GaN films by transmission electron microscopy, the roles of the substrate and the buffer layer in epitaxial growth have been reviewed in terms of crystal orientation relationship, lattice stress and lattice strain. In case of a large lattice-mis-match system where there are large differences in lattice parameters and crystal structure between the substrate and the growing crystal, the lattice strain at the hetero-interface should be considered in a different way from that in a "small lattice-mismatch" system. A buffer layer has some effects in addition to those of the substrate. Buffer layers of AlN and GaN are thought to have a similar effect on a sapphire substrate since they are in a category of large lattice-mismatch system, but they might have different effects in case of a small lattice-mismatch system as on a GaN-AlN template layer.
