In this study, we investigated the relationship between surface morphology and oxygen concentration in GaN layers synthesized from Ga2O vapor and NH3. The surface morphology of GaN layers changed from rough (with large numbers of pits) to (0 0 0 1) smooth planes with the increase in NH3 concentration. SIMS mapping analysis revealed that the oxygen concentration was on the order of 10(17) atoms/cm(3), the lowest level obtained in GaN layers synthesized from Ga2O vapor and NH3, at the smooth (0 0 0 1) surfaces. By contrast, high oxygen concentration over 10(20) atoms/cm(3) was detected at the pitted areas. We concluded that GaN films with low oxygen contamination can be obtained by suppressing pit formation and promoting the smoothness of the (0 0 0 1) surface, even when using Ga2O as the Ga source. (C) 2011 Elsevier B.V. All rights reserved.