We have used a range of X-ray diffraction techniques to study the structural properties of As-doped GaN samples grown on sapphire substrates by plasma-assisted molecular beam epitaxy (PA-MBE). The X-ray methods employed include simple omega-2theta scans, 2D reciprocal space maps and pole figures. We have estimated the ratio of cubic to hexagonal GaN as a function of As flux and sample thickness. We show that the cubic content is only weakly dependent on the arsenic flux in contrast to the intensity of the photoluminescence peak at 3.2 eV, previously associated with cubic-phase GaN material. This suggests that the photo luminescence at 3.2 eV does not arise mainly from direct recombination in cubic phase material, but is probably associated with a donor-acceptor pair transition. By studying As-doped GaN films of different thicknesses, we are able to show that the cubic material is predominantly at the substrate-epilayer interface and that in thicker samples, the growth of hexagonal phase material is dominant. As a result of this study, we can conclude that in As-doped GaN samples grown at 800degreesC on sapphire by PA-MBE, arsenic does not appear to stimulate the growth of pure cubic phase material. (C) 2002 Elsevier Science B.V. All rights reserved.