A two-step pressure process was recently developed for depositing AlN layers on sapphire substrates by MOCVD. Together with optimum substrate nitridation, the process resulted in AlN layers that had FWHM values of similar to 350 and similar to 550 arcsec for the (0002) and (1012) rocking curves, respectively. While the films show excellent X-ray FWHM values, they often have poor morphology as well as small inversion domains spread throughout the film surface. A systematic study was initiated to understand the effect of substrate nitridation and the initial process conditions on the surface morphology and the structural properties. Using atomic force microscopy, transmission electron microscopy, and X-ray diffraction, it was found that the nitridation conditions as well as the initial deposition conditions result in a mixed polarity surface coverage of the substrate. It is possible to force the growth of predominately Al- or N-polarity AlN layers by choosing different initial growth conditions. (C) 2003 Elsevier B.V. All rights reserved.