1. College of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China
2. Department of Physics, Xiamen University, Xiamen 361005, P. R. China
3. Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China
4. College of Materials Science and Opto-Electronic Technology, University of Chinese Academy of Sciences, Beijing 100049, China