Epitaxial films of GaN were grown on c-plane sapphire using a high-temperature AlN buffer layer by metalorganic vapour phase epitaxy. The GaN crystal quality was characterized by electron microscopy, atomic force microscopy, cathodoluminescence and high-resolution X-ray diffraction. The best GaN film had a threading dislocation (TD) density of (4.6 +/- 0.2) x 10(7) cm(-2) with 006 and 302 omega-scan full widths at half maximum both less than 180 arcsec. The GaN growth method involved the deposition of facetted GaN islands under conditions of reduced temperature and increased reactor pressure, followed by island coalescence at higher temperature and reduced pressure. The deposition of an in-situ SiNx micromask on the AlN buffer controlled the GaN island density during the initial stages of growth; a longer silane dosing time led to a lower island density. The TD density of the GaN was found to decrease with the silane treatment time of the AlN surface. (C) 2009 Elsevier B.V. All rights reserved.