Complex and incommensurate ordering in Al _0.72 Ga _0.28N thin films grown by plasma-assisted molecular beam epitaxy

Structures with incommensurate ordering along the [0001] direction are observed in wurtzite Al0.72 Ga0.28 N alloys grown by plasma-assisted molecular beam epitaxy on c -plane sapphire. Films grown in environments with group-III/N ratios greater than 1 exhibit ordered superlattice structures that are incommensurate with the wurtzite crystal lattice. In contrast, films grown under nitrogen-rich conditions exhibit ordered structures with a periodicity of four cation-N monolayers. The increasing complexity of the ordering with increasing Ga-rich growth environment suggests that the ordering is related to the presence of a Ga overlayer believed to exist on the surface of the growing film.