In order to achieve large area semipolar GaN layers with high crystal quality, we have etched trenches into n-plane and r-plane sapphire wafers exposing c-plane-like side-walls, from which GaN stripes can be grown by metalorganic vapor phase epitaxy mainly in c-direction, forming semipolar {10 (1) over bar1} or {11 (2) over bar2} surfaces after coalescence. Here, we describe how to improve such layers by optimizing the side-facet orientation and by including a SiN nanomask interlayer in situ into the growth process, eventually resulting in a basal plane stacking fault density below 5 x 10(3) cm(-1). Moreover, doping experiments have revealed a substantially lower Mg incorporation efficiency on the {11 (2) over bar2} surface as compared to the c-plane, whereas Si does not show such differences. (C) 2014 Elsevier B.V. All rights reserved.