Si-doped GaN epitaxial layers were grown on 0 degrees and 1 degrees tilted sapphire Substrates by metalorganic chemical vapor deposition (MOVCD). It was found that GaN epitaxial layers on tilted substrates were grown with step-flow mode while those on untilted substrates were grown with spiral growth mode. It was also found that number of compensating acceptor, N-COM, equals 1.5 x 10(17) and 8.7 x 10(16) cm(-3) for GaN epitaxial layers grown on 0 degrees and 1 degrees tilted sapphire substrates, respectively. Furthermore, it was found that there existed two values of activation energy of 15.2 and 23 meV for the GaN epitaxial layer grown on untilted sapphire substrate. In contrast, only the activation energy of 15.2 meV was found from the GaN epitaxial layer grown on I' tilted sapphire substrate. (c) 2005 Elsevier B.V. All rights reserved.