1. School of Electronic Science and Engineering and State Key Laboratory of Electronic Thin Films and Integrated Devices University of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. School of Physics University of Electronic Science and Technology of China Chengdu 611731 P. R. China
3. Materials Science and Engineering Program & Texas Materials Institute the University of Texas at Austin Austin TX 78712 USA
4. College of Materials Science and Engineering Sichuan University Chengdu 610065 P. R. China
5. School of Science and Institute of Oxygen Supply Tibet University Lhasa 850000 P. R. China