1. Institute of Semiconductor Science and Technology, South China Normal University, Foshan 528225, P.R. China
2. Songshan Lake Material Laboratory, Dong Guan 523808, P.R. China
3. Institute of Physics, Chinese Academy of Sciences, Beijing 100190, P.R. China
4. Department of Physics, The University of Hong Kong, Hong Kong 999077, P.R. China
5. Zhongshan Institute of Changchun University of Science and Technology, Zhongshan 528437, P.R. China
6. Guangdong Engineering Research Center of Optoelectronic Functional Materials and Devices, Guangzhou 510631, P.R. China