The morphology and microstructural evolution of a nucleation layer were analyzed by using high-resolution transmission electron microscopy. Low-temperature nucleation of GaN on (00•1) sapphire was investigated. Depositions were made for 20, 40, 60, 120 and 180s at 560C by metal-organic chemical vapour deposition. It was shown that the shortest deposition times give rise to the formation of cubic islands. Subsequently, the density and the size of the nucleated islands increased and they start to transform into wurtzite from the interface with the substrate. From the start, the nuclei contain misfit dislocations. At these early growth stages, the relaxation state changed from one island to another; this probably underlies the subsequent mosaic growth of the high-temperature-active GaN layersDefects and Nucleation of GaN Layers on (0001) Sapphire. F.Degave, P.Ruterana, G.Nouet, J.H.Je, C.C.Kim: Journal of Physics - Condensed Matter, 2002, 14[48], 13019-23