The chemistry of oxygen atoms at the surface of an AlGaN layer for Al0.35Ga0.65N/GaN heterostructures was investigated by scanning photoemission microscopy (SPEM) using synchrotron radiation. SPEM imaging and space-resolved photoemission spectroscopy showed that the oxygen atoms were preferentially incorporated into AlGaN rather than GaN due to the high reactivity of Al with oxygen. In situ annealing at 1000 degreesC could lead to the outdiffusion of oxygen impurities from the bulk AlGaN, resulting in a significant increase in the intensity of Al-O bonds at the AlGaN surface. Therefore, it is suggested that the unintentional doping of oxygen impurities in AlGaN could yield a heavily doped n-type AlGaN layer, resulting in a drastic reduction in effective Schottky barrier heights of metal contacts on AlGaN/GaN heterostructures. (C) 2004 The Electrochemical Society.