In this Study the in Situ deposition of SiNx masks by metalorganic vapor phase epitaxy (MOVPE) has been optimized to achieve c-plane oriented GaN layers oil sapphire with a dislocation density <2 x 10(8) cm(-2). The defect termination was found to be most efficient if the SiNx is located after the growth of 100 nm GaN, whereas deposited directly on the AIN nucleation it was less efficient but yielded highly compressively strained layers indicated by a donor bound exciton peak position of 3.493 eV in photoluminescence (13 K). Furthermore we observed by in situ reflectometry that a higher deposition temperature during the silane treatment was strongly increasing the surface roughening yielding a faster coalescence during the GaN overgrowth but finally influencing the defect termination negatively. in terms of lateral overgrowth a high V/III ratio (2D growth mode) was most efficient in terms of defect reduction, whereas a 3D-2D-process at lower V/III ratio yielded Much faster overgrowth but influenced the defect termination negatively. (C) 2008 Elsevier B.V. All rights reserved.