The dislocations in epitaxial AIN film directly grown on (0 0 0 1) sapphire have been investigated by transmission electron microscope and X-ray diffraction (XRD), including direct comparison with conventional GaN film grown on sapphire substrates. Unlike GaN, the threading dislocations (TDs) of AIN are almost entirely of pure-edge character, and the screw dislocation density is much less than that of GaN film, leading to a very narrow full-width-at-half-maximum of similar to 70 arcsec in (0 0 0 2) XRD of AIN film. The origin of screw TDs in GaN is found to be primarily related to the highly faulted layer at the interface region, which is introduced by using a low temperature (LT) nucleation layer. The high temperature AIN growth without any LT initial layer leads to the dramatic reduction of screw dislocations in AIN. Simultaneously, the AIN growth is dominated by two-dimensional step-flow mode while the island growth that occurs for GaN film on sapphire is suppressed. Moreover, we obtained a very low dark current density of 1.1 x 10(-9) A/ cm(2) at a 5 V bias on AIN metal-semiconductor-metal diodes, which indicates a lower defect density and good material quality of our AIN thin film. (c) 2005 Elsevier B.V. All rights reserved.