We report the effect of Mg on the properties of both Ga-face and N-face GaN grown by plasma-assisted molecular-beam epitaxy. We demonstrate a homogeneous incorporation of Mg in the epilayers, with the Mg content being about one order of magnitude lower in N-face GaN compared to Ga-face material. In the case of Ga-face GaN, segregating Mg inhibits the formation of the self-regulated Ga film which is used as a surfactant for the growth of undoped and Si-doped GaN layers. The growth window of GaN:Mg is hence significantly reduced. In contrast, Mg doping of N-face GaN favors the synthesis of 3C-GaN, which becomes the dominant polytype for Mg concentrations higher than 3 × 1018 cm–3. High-resolution transmission electron microscopy images reveal the cubic ordering of the GaN nitride layer, with the [111] axis perpendicular to the substrate. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)