1. Wuhan National Laboratory for Optoelectronics; Huazhong University of Science and Technology; Wuhan 430074 China
2. Department of Physics, Pen-Tung Sah Institute of Micro-Nano Science and Technology; Xiamen University; Xiamen 361005 China
3. Key Laboratory of Semiconductor Materials Science and Beijing Key, Laboratory of Low-dimensional Semiconductor Materials and Devices, Institute of Semiconductors; Chinese Academy of Sciences; Beijing 100083 China