We investigate the activation behaviour of magnesium-doped GaN top contact layers and its influence on InGaN/GaN light-emitting diode structures grown by metal organic chemical vapour phase epitaxy (MOVPE). The LED structures were heat treated at 700degreesC up to 16min in an oxygen ambient. Analysis was performed by photoluminescence and cathodoluminescence microscopy measurements (CL) and the results are correlated with capacitance-voltage measurements. It is observed that the donor-acceptor pair band luminescence drastically decreases in the first minute of annealing. Additionally, as a result of annealing, the hole concentration increases by a factor of more than 5. The InGaN-MQW luminescence shows no evident change due to the heat treatment. CL measurements reveal varying disturbed luminescence homogeneity in the lateral dimension for the high- and low-energy side of the InGaN luminescence. Top view CL maps show that the luminescence intensity is distributed inhomogeneously in the as-grown and annealed samples. (C) 2004 Elsevier B.V. All rights reserved.