The boule-like growth of GaN in a vertical AIXTRON HVPE reactor was studied. Extrinsic factors like properties of the starting substrate and fundamental growth parameters especially the vapor gas composition at the surface have crucial impact on the formation of inverse pyramidal defects. The partial pressure of GaCl strongly affects defect formation, in-plane strain, and crystalline quality. Optimized growth conditions resulted in growth rates of 300-500 mu m/h. GaN layers with thicknesses of 2.6 and of 5.8 mm were grown at rates above 300 mu m/h. The threading dislocation density reduces with an inverse proportionality to the GaN layer thickness. Thus, it is demonstrated that growth rates above 300 mu m/h are promising for GaN boule growth. (C) 2010 Elsevier B.V. All rights reserved.