Epitaxial growth of gallium nitride thin films on A -Plane sapphire by molecular beam epitaxy

In this article, we propose a crystallographic model to describe epitaxy of GaN on (112̄0) sapphire (A plane). The (11̄02) cleavage plane in sapphire is shown to extend to the GaN lattice as the (112̄0) plane, facilitating the formation of cleaved facets. It is shown that, although the lattice mismatch is much smaller than in the case of epitaxy on (0001), the difference in the planar symmetry in this case results in high-strained bonds near the interface. The use of nitridation and a low temperature buffer is therefore necessary. A systematic study of GaN growth on the A-plane sapphire by plasma-assisted molecular beam epitaxy was carried out to study the effects of plasma nitridation of the substrate and the growth of a low temperature GaN buffer on the structure and optoelectronic properties of the films. Transmission electron microscopy (TEM) studies indicate that films grown on substrates which were not nitridated prior to growth have a significant fraction of zinc-blende domains and poor orientation relationship with the substrate. On the contrary, nitridation leads to films with superior structural and optoelectronic properties. The low temperature GaN buffer, grown on nitridated substrates, was found to also have a pronounced effect on the optoelectronic properties of the GaN films, especially in those with low carrier concentrations. The correlation between TEM and photoluminescence studies suggests that the transition at 3.27 eV can be attributed to the cubic domains in the films.