Two-step epitaxy, involving low-temperature GaN nucleation layers and high-temperature GaN overgrowths deposited onto sapphire (00▪1) by metalorganic chemical vapour deposition, was investigated by means of transmission electron microscopy and atomic force microscopy. The results indicated that the nucleation layers consisted of faceted crystalline islands which exhibited a spread of rotations about the (00▪1) axis. These islands underwent a metamorphosis during annealing, and became rounded. Lateral growth during the high-temperature deposition of GaN occurred via the attachment of atomic species to steps that were associated with the rounded islands; thus resulting in faceted flat-topped islands. Growth also occurred preferentially in certain regions which then evolved into so-called growth patches. The latter then grew vertically, and laterally, over the underlying sub-grains. The coalescence of these patches produced a continuous GaN layer. It was shown that most threading dislocations did not form during the coalescence of islands, as had previously been assumed. Two other possible sources of threading dislocations were identified: these were growth faults in nucleation layers, and point defects. Partial dislocations which were associated with faults in the nucleation layers were the main source of basal-plane dislocations, which could develop into threading dislocations via self-glide and climb. It was also suggested that a mosaic structure which was observed in fully-grown GaN layers was due to elastic interactions between threading dislocations, which glided and climbed to form sub-grain boundaries.Gallium Nitride Epitaxy on (0001) Sapphire. V.Narayanan, K.Lorenz, W.Kim. S.Mahajan: Philosophical Magazine A, 2002, 82[5], 885-912