Undoped GaN epilayers were grown on sapphire (0 0 01) substrates using different H-2/N-2 carrier gas ratios (0%, 50%, 70%, 80%, and 100% N-2 content in the mixtures) in metal organic vapor phase epitaxy (MOVPE). Growth was observed in situ by reflectometry. Additionally experiments were carried out in which growth was stopped in the high-temperature growth step for the same carrier gas mixtures. The morphology development was correlated with the structural and electrical characteristics of the layers. (C) 2007 Elsevier B.V. All rights reserved.