In this study we investigated the incorporation of the dopants Si, Mg, and Fe and the unintentional impurities O and C in GaN films grown on m-plane (10 (1) over bar0), a-plane (11 (2) over bar0) as well as semi-polar (10 (1) over bar1), (10 (1) over bar(1) over bar), (11 (2) over bar2), and (11 (2) over bar(2) over bar) bulk GaN substrates by metalorganic chemical vapor deposition. GaN layer stacks were grown under various growth conditions and analyzed by secondary ion mass spectroscopy. Whereas the dopant incorporation was little affected by the crystallographic orientation, differences were observed in the doping profiles, in particular for Mg and Fe doping. Significant variations were observed in the impurity incorporation. While N-rich surfaces exhibited a high affinity towards O incorporation in general, C incorporation trends were strongly dependent on the specific growth conditions. (c) 2009 Elsevier B.V. All rights reserved.