1. School of Electronic Science and Engineering University of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. State Key Laboratory of Electronic Thin Films and Integrated Devices University of Electronic Science and Technology of China Chengdu 610054 P. R. China
3. Electronic Materials Research Laboratory Key Laboratory of the Ministry of Education and International Center for Dielectric Research School of Electronic Science and Engineering Xi'an Jiaotong University Xi'an 710049 China