We investigated the optimum conditions for the homoepitaxial growth of AIN on freestanding AIN (0001) substrates (off angle: 0.6-0.9 degrees) by metalorganic vapor phase epitaxy. The crystallinity, surface morphology, and full width at half maximum as obtained from X-ray rocking curves of the homo-epitaxially grown AIN layer were strongly dependent on the growth conditions. When AIN layers are grown on an AIN substrate using a relatively low V/III ratio of 80, which is commonly used for AIN layer growth on a sapphire substrate, a high density of hillocks appeared at the AIN surface. In addition, the crystallinity of these hillocks was significantly worse than that of the substrate. By optimization of the growth temperature and Vila ratio, homoepitaxial AIN layers with good surface flatness and high crystallinity were realized. (C) 2013 Elsevier B.V. All rights reserved.