1. Institute of Semiconductors, Guangdong Academy of Sciences, Guangzhou 510650, China
2. School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, China
3. Research center of advanced information materials, School of Physics & Materials Science, Guangzhou University, Guangzhou 510006, China
4. State Key Laboratory of Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China