The impact of AlN nucleation layers on the strain evolution in a subsequent GaN layer was investigated by in-situ wafer curvature measurement. It is shown that growth temperature and thickness of the AlN nucleation layer strongly influence the built-in strain and crystalline quality of GaN. A two-step AlN growth procedure leads to a decrease of compressive strain as well as the reduction of the total dislocation density (6-9 x 10(8) cm(-2)) in the overgrown GaN layer. TEM analysis reveals different relaxation mechanisms of GaN in v-pits and on flat surfaces of AlN. With a two-step AlN nucleation layer a low wafer bow can be achieved together with a low dislocation density. (C) 2013 Elsevier B.V. All rights reserved.