High p-type conductivity of GaN - commonly achieved via doping with Mg - is a challenge to be faced on the way to the realization of a GaN-based diluted magnetic semiconductor showing ferromagnetism at room temperature. In this work we present a study of GaN: Mg films grown in a metalorganic vapor phase epitaxy process where the Mg atoms are incorporated in a delta-doping fashion. The effect of a post-growth annealing step on the diffusivity of magnesium is studied by means of atomic force microscopy and photoluminescence measurements in the ultraviolet regime. A comparison of the presented results with transmission electron microscopy studies leads to the conclusion that a considerable amount of Mg is transported towards the surface where it is thermally removed during the annealing step. (c) 2008 Elsevier B.V. All rights reserved.