Oxygen doping of c-plane GaN was investigated by metalorganic chemical vapor deposition (MOCVD), using O2 as the oxygen precursor. The oxygen incorporation into GaN was strongly affected by the surface orientation of the GaN film. At an O2 partial pressure of 10 Pa, growth on a smooth (0001) GaN surface resulted in an oxygen concentration of 8 × 1016 cm−3. However, the oxygen incorporation increased drastically to concentrations up to 3 × 1019 cm−3, when the growth was performed on rough surfaces containing other crystal planes than (0001) GaN. Such surfaces include (0001) GaN with hexagonal pits, or the hexagonal islands formed during the initial stage of growth of GaN films on sapphire substrate. Furthermore, above a certain critical O2 partial pressure, which depended on the growth temperature, oxygen itself caused a perturbation of the smooth GaN growth on (0001) surfaces, leading to the formation of hexagonal pits during growth. The highest oxygen concentration attained during smooth growth was 3.5 × 1017 cm−3. The results imply, that oxygen is not a suitable dopant for the growth of uniform n+ (0001) GaN layers. (© 2003 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)