In total, 120 mu m thick GaN layers without cracks have been grown on 2 in sapphire substrates by hydride vapor phase epitaxy. This has been achieved by optimization of the flow patterns in the reactor based on 3D process modelling, choice of the growth parameters especially the carrier gas composition and the usage of suitable GaN/sapphire templates. An important finding is that an H-2 content of around 50% in the N-2 carrier yields the lowest crack density. (c) 2005 Elsevier B.V. All rights reserved.