1. School of Semiconductor Science and Technology, South China Normal University, Foshan ,Guangdong 528225, P. R. China
2. School of Electronic Engineering, Chaohu University, Hefei, Anhui 238000, P. R. China
3. College of Optical Science and Engineering, Zhejiang University, Hangzhou ,Zhejiang 310027, P. R. China
4. Provincial Key Laboratory of Chip and Integration Technology, Guangzhou ,Guangdong 510631, P. R. China