The influence of AIN nucleation layer (NL) growth conditions on the quality of GaN layer deposited on (0 0 0 1) sapphire by organometallic chemical vapor phase epitaxy (OMVPE) has been investigated by X-ray diffraction, atomic force microscopy and transmission electron microscopy. Growth pressure, temperature and time were varied in this study. Results indicate that there exists an optimal thickness of the NL is required for optimal growth. Both thin and thick NLs are not conducive to the growth of high-quality GaN layers. Arguments have been developed to rationalize these observations. (C) 2002 Elsevier Science B.V. All rights reserved.