X-ray crystallographic study of GaN epitaxial films on Al2O3(0001) substrates with GaN buffer layers

Samples of GnN(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 Me of good quality with the GaN[<10(1)over bar 0>] parallel to Al2O3[<11(2)over bar 0>] and GaN[<1(2)over bar 10>]parallel to Al2O3[<1(1)over bar 00>]. This arrangement of crystal orientation can be attributed to the chemical potential overriding the lattice spacing mismatch. The x-my 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. (C) 1996 American Institute of Physics.