Unintentional n-type doping is commonly observed to occur during the growth of nominally undoped GaN on sapphire. Scanning capacitance microscopy reveals that a layer of GaN adjacent to the GaN/sapphire interface is n-type, whilst the remainder of the epilayer appears insulating. If two-dimensional GaN growth is commenced immediately after the growth of the nucleation layer, then the unintentionally doped layer is not present and the material is highly resistive. However, such samples have a high threading dislocation density (ca. 5 x 10(9) cm(-2)). To reduce the threading dislocation density, the GaN nucleation layer is annealed and growth is then continued at a low V:III ratio to promote three-dimensional island formation. These islands are eventually coalesced using an increased VAII ratio and temperature. Longer coalescence times lead to lower threading dislocation densities. The width of the unintentionally n-doped layer is found to increase as the coalescence time increases, whilst the carrier concentration in the layer is not observed to change. The dopant in the unintentionally conductive layer is shown, using secondary ion mass spectrometry, to be oxygen. (C) 2008 Elsevier B.V. All rights reserved.