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. College of Optoelectronic Engineering Chengdu University of Information Technology Chengdu 610225 P. R. China
3. College of Science and Institute of Oxygen Supply Center of Tibetan Studies (Everest Research Institute) Tibet University Lhasa 850000 P. R. China