GaN crystals were grown on graphite and sapphire substrates at 990-1050 degrees C by reaction of Ga2O with flowing NH3. Ga2O gas was produced at a constant rate (1.3 wt% min(-1)) by reaction of Ga2O3 with carbon at 1000-1060 degrees C. The effect of NH3 concentration (3-100 vol%) and the nature of the substrate on the morphology and orientation of the GaN crystals were determined by scanning electron microscopy, transmission electron microscopy, X-ray diffraction and selected area electron diffraction. It was found that sheet and plate-like crystals grew at different orientations to the substrate with different NH3 concentrations and substrates. (c) 2006 Elsevier Ltd. All rights reserved.