In heteroepitaxy stress, either intrinsic or thermal, is a commonly observed phenomena. We have investigated in detail stress during metal organic chemical vapor-phase epitaxy of GaN on sapphire and silicon in situ by a curvature measurement technique. While some well-known sources as strained layers do result in tensely or compressively stressed films, other sources of stress are not as well known as, e.g., intrinsic tensile stress from initial 3D islands growth and subsequent gap closure between the islands. Doping with Si is a source of strong tensile stress which we observe to be strongly dependent on the dislocation density and doping concentration. We propose that Si induces tensile stress by promoting the movement of edge type dislocations and by this reducing the correlated lattice planes. For the first time, a combination of in situ curvature and true temperature measurement directly shows the impact of wafer curvature on the wafer surface temperature. (C) 2004 Elsevier B.V. All rights reserved.