A detailed double crystal x-ray diffractometry study of epitaxial AlN thin films grown on sapphire, silicon and silicon carbide substrates was carried out to compare the structure, residual stress and defect concentration in these thin films. The structure of AlN is wurtzite with a small distortion in lattice parameters. This results in a small residual stress of the order of 109 dynes/cm2 in the film and can be accounted for from the difference in thermal expansion coefficients between the film and substrate. Both the x-ray and transmission electron microscopy measurements indicate a low defect density in the AlN thin film grown on 6H-SiC substrate which could be attributed to the small difference in lattice parameters between AlN and 6H-SiC. © 1995 American Institute of Physics.