Cubic GaN was grown on GaAs(0 0 1). The GaN layers were found to exhibit residual strain with a measured a-lattice parameter of 4.4990 angstrom. The GaN layers were grown on GaAs(1 1 0) forming cubic crystal structure with an a-lattice parameter of 44947 angstrom for the LT-GaN buffer layers and a hexagonal structure with an a-lattice parameter of 2.7463 angstrom and a c-lattice parameter of 5.5882 angstrom for the main GaN layers. Comparisons between the two GaAs substrate orientations. GaN on GaAs(1 1 0) showed a smoother surface (R-rms of 5.369 nm) than that on GaAs(0 0 1) (R-rms of 9.776 nm). It was shown by a TEM investigation that semi-polar GaN layers with low defect densities could be grown by MOCVD on GaAs (1 1 0) substrates. The TEM results indicated GaN [1 1 (2) over bar 0]//GaAs[1 1 0] and GaN (1 0 (1) over bar 3)//GaAs (1 1 0) crystal orientations. (C) 2014 Elsevier B.V. All rights reserved.