GaN layers have been grown on patterned c-plane (0 0 0 1) sapphire and (0 0 0 1) GaN Metal Organic Vapour-Phase Epitaxy (MOVPE)/sapphire substrates by Hydride Vapour-Phase Epitaxy (HVPE) using the epitaxial lateral overgrowth (ELO) process in a single HVPE reactor. The crystal growth mechanisms via the lateral and vertical extensions were analysed with respect to the theoretical results based on the phenomenological model. The best experimental conditions supporting a high lateral extension were deduced. Coalescence of growth structures was favoured and high-quality uniform GaN layers about 10 [an thick were successfully grown directly on low-cost sapphire substrates. The properties of GaN layers were systematically investigated by scanning electron microscopy (SEM). GaN films exhibited a mirror-like surface without defects through the thickness. (C) 2007 Elsevier B.V. All rights reserved.