1. Key Lab of Semiconductor Materials Science, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China
2. CETC Key Laboratory of Carbon-based Electronics, Nanjing Electronic Devices Institute, Nanjing 210016, P. R. China
3. Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100049, P. R. China