We report on the polarity control of the crystal polarity of AlN layers grown on sapphire substrates by oxygen doping during AlN nucleation. Oxygen concentration amongst AlN nuclei as well as the resulting impact on surface morphology and crystalline quality was investigated using isotopic oxygen as a dopant. We observed that the density of AlN nuclei increased with increasing oxygen concentration, and that AlN films with Al-polar can be obtained with an oxygen concentration of over 10(21) cm(-3) in the AlN nucleation layer. The results agree well with the surface morphologies and (10 (1) over bar2) full width at half maximum values of the X-ray omega-rocking curve of AlN overgrowth layers, indicating that the proposed method with the optimized oxygen concentration can be used to control the polarity and obtain AlN films with a smooth surface and good crystalline quality. (C) 2013 Elsevier B.V. All rights reserved