The defect structure of GaN film grown on sapphire by plasma-assisted molecular beam epitaxy (PAMBE) depends on the growth temperature and thickness of the aluminum nitride (AlN) buffer layer. High-resolution X-ray diffraction was used to measure symmetric (0 0 0 2) and asymmetric (1 0 (1) over bar 2) rocking curve (omega-scans) broadening, which allowed the estimation of screw threading dislocation (TD) and edge TD densities, respectively. For GaN grown on lower-temperature buffer, the density of screw TD was increased while the density of edge TD was decreased. Further examinations revealed that the edge TD was closely related to stress in GaN film and the screw TD was controlled by AlN surface roughness. Since the GaN defect was dominated by edge TD, the total TD was also effectively suppressed with the use of lower-temperature buffer with appropriate thickness. (C) 2009 Elsevier B.V. All rights reserved.