1. Institute of Semiconductor Science and Technology, South China Normal University, Guangzhou 510631, P. R. China
2. Guangdong Province Key Lab of Chip and Integration Technology, Guangzhou 510631, P. R. China
3. Key Laboratory of Micro-systems and Micro-structures Manufacturing of Ministry of Education, Harbin Institute of Technology, Harbin 150080, P. R. China
4. School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
5. SCNU Qingyuan Institute of Science and Technology Innovation Co., Ltd., Qingyuan 511517, P. R. China