A characterization study of heteroepitaxial grown m-plane GaN on m-plane sapphire substrates by MOCVD was undertaken. Using X-ray diffraction and photoluminescence, the growth characteristics and epi-layer properties of m-GaN layers were investigated with special emphasis on the role of AIN buffer layers in preventing unintentional nitridation prior to GaN deposition. Substrate nitridation was found to lead to undesirable crystallographic orientations. In-plane lattice parameters of m-GaN obtained from X-ray reciprocal space mapping indicate anisotropic residual strain is present in these layers even under optimized growth conditions. Compressive and tensile strains were observed along either [0001] or [11 (2) over bar0] directions, depending on AIN buffer layer conditions and the presence of extended structural defects. In addition, extended structural defects commonly observed in GaN showed a significant effect on stacking fault related luminescence in m-GaN. Published by Elsevier B.V.