1. State Key Laboratory of Electronic Thin Films and Integrated Devices University of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. Institute of Microelectronics of Chinese Academy of Sciences Beijing 100029 P. R. China
3. University of Chinese Academy of Sciences Beijing 100049 P. R. China
4. Hunan Provincial Key Laboratory of Thin Film Materials and Devices School of Materials Science and Engineering Xiangtan University Xiangtan 411105 P. R. China