GaN layers were grown by gas-source-molecular beam epitaxy on sapphire substrates using ammonia as a nitrogen source. The nitridation of an Al2O3 surface prior to the GaN growth was followed in situ by reflection high-energy electron diffraction. A strong variation of the surface lattice parameter was observed corresponding to the formation of an AW relaxed layer. The nucleation of GaN on such a nitridated surface is facilitated, as checked by atomic force microscopy. Optimization of the nitridation procedure is achieved by investigating the photoluminescence properties of GaN thin films grown for different nitridation times. It is found that the band-edge and the yellow-band luminescences are strongly dependent on the nitridated starting surface. Finally, the optimized nitridation process is used to grow high-quality GaN epitaxial layers. (C) 1996 American Institute of Physics.