The influence of hydrogen and nitrogen carrier gases used during the preparation of the nucleation layer on the structural and electrical properties of GaN layers has been investigated. The GaN were grown on sapphire substrates using metal organic chemical vapor deposition. The nucleation layer morphology strongly depends on the carrier gas affecting the electrical properties of GaN epitaxial films through changes of the ratio of edge to mixed and screw-type threading dislocations. X-ray diffractometry, X-ray reflectometry, atomic force microscopy, and defect selective etching were employed to study the structural properties of both the nucleation layer and the GaN epilayers deposited on top of this. It is found that the density of edge-type dislocations determines the resistivity of GaN epilayers and that one key factor for varying the density of these dislocations is the morphology of the nucleation layer, i.e. the morphology of the GaN epilayer can be controlled by the type of carrier gas used in the preparation of the nucleation layer. The electrical resistivity of our GaN epilayers is typically about 0.5Omegacm and more than 3 x 10(4) Omega cm with nucleation layers grown using hydrogen and nitrogen carrier gases, respectively. (C) 2004 Elsevier B.V. All rights reserved.