1. State
Key Laboratory for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China.
2. Wuhan
National Laboratory for Optoelectronics (WNLO) and College of Optical and Electronic Information, Huazhong University of Science and Technology (HUST), Wuhan 430074, P. R. China.