We investigate the effects of heat-treatment under NH3 ambient on the formation of Ga-O at the interface between GaAs and high-k dielectric materials deposited by atomic layer deposition. Compared with the heat-treatment under N-2 ambient, the monochromatic X-ray photoelectron spectroscopy analysis reveal that the formation of Ga-O is greatly suppressed under NH3 ambient for HfO2 on GaAs. However, the same experiments for Al2O3 on GaAs show that the effect is negligible. We examine the different reaction mechanisms of the NH3 nitridation processes for two different high-k dielectric materials on GaAs. (C) 2009 The Electrochemical Society. [DOI: 10.1149/1.3193534] All rights reserved.