1. Guangdong Provincial Key Laboratory of Chip and Integration Technology, School of Semiconductor Science and Technology, Faculty of Engineering, South China Normal University, Foshan 528225, P. R. China
2. School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China
3. College of Optical Science and Engineering, Zhejiang University, Hangzhou 310027, P. R. China