The nitridation of Al2O3 (0001) substrate surfaces by radio-frequency nitrogen plasma has been investigated. A 1.5-nm-thick surface nitride layer occurred for 100 min nitridation at high substrate temperature, while the nitridation appeared to be limited to a surface atomic plane at low temperature. In-plane lattice constant relaxation was observed in both cases. A high nitridation temperature resulted into a Ga face and a low temperature to N-face polarity of overgrown GaN films. However, low temperature nitridation and an AlN buffer layer also produced a Ga-face polarity. The results are consistent with low formation energy of AlN/sapphire films with Ga-face polarity. © 2002 American Institute of Physics.