1. School of Materials Science and Engineering, Peking University, Beijing 100871, China
2. The Institute of Technological Sciences, Wuhan University, Wuhan 430072, China
3. State Key Laboratory for Artificial Microstructure and Mesoscopic Physics, School of Physics, Peking University, Beijing 100871, China
4. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China