Abstract
Samples of GaN(0001) epitaxial films on sapphire Al2O3(0001) with different thicknesses of GaN buffer layers were characterized by x-ray diffraction method in both in-plane and plane-normal directions. The results show that all the epitaxial films are of good quality with the GaN[101̄0] ∥Al2O3[112̄0] and GaN[12̄10]∥Al2O3[11̄00]. This arrangement of crystal orientation can be attributed to the chemical potential overriding the lattice spacing mismatch. The x-ray results also indicate that the crystal coherence lengths in the in-plane direction are smaller than those measured in the plane-normal direction, i.e., a columnar-like structure normal to the film is observed. The rocking curve widths in the in-plane direction are also larger than those measured in the plane-normal direction. In-plane measurement of rocking curve and coherence length are essential physical quantities directly related to the electron mobility which was measured predominantly in the in-plane direction. The best epitaxial structure is the one grown with 10 nm buffer layer. © 1996 American Institute of Physics.