We report on the physical properties and electrical characteristics of Y2O3 gate oxides grown on silicon substrates with NH3 plasma treatment by reactive radio-frequency sputtering. The interfacial chemistry of the high-k gate dielectric Y2O3 has been investigated on nitrided and un-nitrided Si using X-ray photoelectron spectroscopy. We found that the Y2O3 gate film having NH3-based interface layer is very effective in reducing equivalent oxide thickness and leakage current as well as improving film qualities. This NH3-nitrided layer is suggested to minimize interfacial YSixOy formation by limiting the amount of Si available to interact with the Y2O3 layer. These Y2O3 gate dielectrics exhibit excellent frequency dependence and weak temperature dependence of leakage current. They also show negligible charge trapping at high electric field stress. (c) 2007 The Electrochemical Society.