基金Project supported by the National Natural Science Foundation of China (Grant No 60376001), the National Basic Research Program of China (Grant No 2002CB311904) and the National Defense Basic Research Program of China (Grant No 51327020202). The authors express their gratitude to Ma Nongnong of Tianjin Electronic Materials Research Institute for performing SIMS measurements and to Li Ying of Xi'an University of Technology for performing XPS measurements.