Freestanding GaN layers of various thicknesses grown by HVPE have been studied by time-resolved spectroscopy combined with structural and electrical measurements. We have observed an increase of the PL lifetime with increasing layer thickness, however, a saturation of the recombination times has been detected for the GaN layers thicker than 400 µm. We explain the observed thickness-dependent behavior of the decay times by competition of two nonradiative mechanisms, namely, for layers with thickness less than 400 µm the main nonradiative channel is related to the structural defects, while in thicker layers the recombination decay time is limited by impurities and/or vacancies. © 2007 Elsevier Ltd. All rights reserved.