We have investigated the effects of annealing temperature on the physical and electrical properties of the HfSixOy /HfO2 thin film for high-k gate oxides in a metal-oxide-semiconductor device. The oxidation and subsequent postoxidation N-2 annealing at 500degreesC of Hf deposited directly on the Si substrate results in the HfSixOy /HfO2 stack layer with excellent electrical properties. For instance, we observe a negligible hysteresis window, an excellent equivalent oxide thickness (1.2 nm), and a low leakage current density (less than or equal to4 x 10(-5) A/cm(2) at 2 V after compensating the flatband voltage shift). However, the formation of an interfacial SiOx layer enhances as annealing temperature increases. Based on current observation, we suggest that annealing temperature must be carefully controlled to obtain the excellent electrical properties of HfO2 /HfSixOy high-k gate oxides. (C) 2004 American Vacuum Society.