Silicon carbide wafers contain domains with varying sizes and degrees of tilt. The present research has shown that this microstructure is mimicked in GaN films deposited on AlN-containing buffer layers and that it masks most variations in the FWHM of the X-ray rocking curves of the former. The shape and FWHM in the GaN curves are determined both by domain tilting and dislocation broadening; the latter was dominant in areas of reduced tilt. Analyses of the on- and off-axis X-ray data acquired from these regions of lower tilt revealed the marked effect of the higher density of edge dislocations on broadening. This effect decreased with increasing GaN thickness due to dislocation annihilation. The densities of edge dislocations in GaN films deposited at 1010degreesC on pitted, less pitted and very smooth AlN layers of the same thickness grown at 1010degreesC, 1130degreesC and 1220degreesC, respectively, were lowest and highest in those films grown on the last two respective layers. Additional studies showed that GaN films grow on Al0.2Ga0.8N layers via step-flow and possess a lower edge dislocation density than films grown via the Stranski-Krastanov mode on AlN because of the reduced misfit and the absence of boundaries between coalesced islands. (C) 2003 Elsevier B.V. All rights reserved.