At present., a major part of commercially available group III nitride optoelectronic devices is grown on sapphire substrates by metalorganic vapour phase epitaxy. Besides classical techniques involving selective overgrowth of patterned substrates, GaN epitaxial layers with dislocation densities in the order of 10(7) cm(-2) (Appl. Phys. Lett. 78 (2001) 1976; MRS Internet J. Nitride Semicond. Res. 7 (2002) 8) can be fabricated on sapphire by epitaxial overgrowth of three-dimensional (3D) GaN islands. In this work, we prove the independence of this technique from the initial substrate surface state if the island formation is induced by a silane/ammonia treatment. Subsequent layer coalescence can be accelerated by high overgrowth temperatures and high group V/III precursor ratios on the expense of the dislocation density. Once optimised, nucleation conditions are operated for the growth of GaN-on-sapphire, the dislocation density is not further reduced by repeated 3D nucleation steps. With growing epilayer thickness, the intensity of cathodo- and photoluminescence spectra increases while excitonic line widths narrow. The latter is a quantitative measure for the decreasing dislocation density. According to both luminescence experiments, 3D nucleated GaN-on-sapphire layers are under increasing compressive strain for film thicknesses above 3 mum. (C) 2003 Elsevier B.V. All rights reserved.