1. Hunan Key Laboratory of Super-microstructure and Ultrafast Process, School of Physics and Electronics, Central South University, Changsha 410083, Hunan, China
2. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences & Center of Materials Science and Optoelectronics Engineering, University of Chinese Academy of Sciences, Beijing 100083, China
3. State Key Laboratory of Precision Measuring Technology and Instruments, Tianjin University, Tianjin 300072, China
4. Department of Applied Physics, School of Physics and Electronics, Hunan University, Changsha 410082, Hunan, China
5. School of Physics and Optoelectronic Engineering, Shandong University of Technology, Zibo 255049, Shandong, China
6. Institute of Semiconductors, South China Normal University, Guangzhou 510631, China