1. School of Physics and Optoelectronic Engineering, Guangdong University of Technology, Guangzhou 510006, P. R. China
2. School of Materials and Energy, Guangdong University of Technology, Guangzhou 510006, P. R. China
3. Guangdong Provincial Key Laboratory of Chip and Integration Technology, School of Electronic Science and Engineering, South China Normal University, Foshan 528225, P. R. China