1. School of Microelectronics, Xidian University, Xi'an 710071, People's Republic of China
2. Engineering Research Center of Thin Film Optoelectronics Technology, Ministry of Education, Nankai University, Tianjin 300350, People's Republic of China
3. State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, People's Republic of China
4. Hangzhou Institute of Technology, Xidian University, Hangzhou 311231, People's Republic of China