1. Key Laboratory of Microelectronic Devices & Integrated Technology Institute of Microelectronics Chinese Academy of Sciences Beijing 100029 P. R. China
2. University of the Chinese Academy of Sciences Beijing 100049 P. R. China
3. Department of Electrical Engineering and Computer Sciences University of California Berkeley CA 94720 USA
4. School of Microelectronics University of Science and Technology of China Hefei Anhui 230026 P. R. China
5. Key Laboratory of Advanced Materials (MOE) School of Materials Science and Engineering Tsinghua University Beijing 100084 P. R. China
6. Institute of Physics Chinese Academy of Sciences Beijing 100190 China
7. Center of Materials Science and Optoelectronics Engineering University of Chinese Academy of Sciences Beijing 100049 China