Capacitance-voltage characteristics of Ru/HfxSi1-xOy/Si gate stacks with HfxSi1- xOy composition Hf/Si = 75/25 and Hf/Si = 50/50 were analyzed. The characteristics revealed a high density of negative fixed charges N-ox approximate to -5 x 10(12) cm(-2) in the oxide film. We have observed that the value of the Ru work function remained constant after forming gas annealing at temperatures between 430 and 510 degrees C, respectively, while fixed charges were reduced at temperatures above 470 degrees C. We speculated that the decrease of negative fixed charges can be explained by the generation of positively charged [Si-2 = OH](+) centers in the oxide layer. The positively charged centers can account for a compensation of the negative fixed oxide charge. Rapid thermal annealing at temperatures up to 800 degrees C in nitrogen did not influence the fixed oxide charges in the HfxSi1- xOy films. (c) 2006 The Electrochemical Society.