1. Institute of Semiconductors, South China Normal University, Guangzhou 510631, P. R. China
2. College of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China
3. Department of Physics, Xiamen University, Xiamen 361005, P. R. China
4. Guangdong Key Lab of Chip and Integration Technology, Guangzhou 510631, P. R. China