1. Institute of Semiconductor Science and Technology, South China Normal University, Guangzhou 510631, P. R. China
2. Guangdong Province Key Lab of Chip and Integration Technology, Guangzhou 510631, P. R. China
3. Key Laboratory of Micro-systems and Micro-structures Manufacturing of Ministry of Education, Harbin Institute of Technology, Harbin 150080, P. R. China
4. Department of Physics, Harbin Institude of Technology, Harbin 150080, P. R. China
5. SCNU Qingyuan Institute of Science and Technology Innovation Co., Ltd, Qingyuan 511517, P. R. China
6. Department of Materials Science, Fudan University, Shanghai 200433, P. R. China