We deposited pure m-plane GaN (1 (1) over bar 0 0) layers on LiAlO2 (1 0 0) Substrates by MOVPE using Mg-doped InGaN buffer layers of various thickness. These sealing layers are grown under nitrogen ambient and help to improve the film coalescence while reducing the oxygen background doping of the GaN films. We show that for an increasing thickness of the InGaN:Mg buffer layer, the highly strained GaN layer slightly relaxes. An improved surface morphology can be achieved for thicker InGaN:Mg buffers while the defect density clearly increases above a certain thickness, as it can be seen in X-ray rocking curve measurements. In temperature-dependent photoluminescence spectroscopy, we observe not only peaks due to DAP transitions at an energy of around 3.3 eV, but also a defect-related peak at 3.43 eV as well as a peak at 3.49 eV connected to free and bound excitonic transitions. It is interesting to note that for an increasing thickness of the InGaN:Mg buffer we detect a higher intensity of the excitonic transition while the defect-related signal seen at 3.43 eV is getting more pronounced. It might be possible that during the film relaxation basal plane stacking faults are introduced into the layer. (C) 2008 Elsevier B.V. All rights reserved.