Predominantly cubic phase GaN (c-GaN) micro-crystals have been fabricated on GaAs substrates by radio-frequency plasma-assisted molecular beam epitaxy (RF-MBE) using relatively thin low-temperature-grown GaN buffer layer, which promotes the generation of Ga droplets acting as the nuclei for the three-dimensional (3D) growth. These crystals were found to be highly luminescent, and their optical quality was much better than that of the two-dimensionally (2D) grown c-GaN region as investigated by micro-cathodoluminescence (micro-CL). Micro-Raman analysis revealed that there is an apparent difference in the structural perfection between micro-crystals and the 2D Grown region. These results suggest that defects or local strain, originating from the large difference in lattice constants and thermal expansion coefficients between GaN and GaAs, could be avoided by the transition from 2D to 3D growth mode with the use of this nucleation process, as is the case with a droplet epitaxy technique. © 2005 Elsevier B.V. All rights reserved.