1. Department of Microelectronic Science and Engineering, Faculty of Science, Ningbo University, Ningbo 315211, P. R. China
2. State Key Laboratory of Functional Materials for Informatics, Shanghai Institute of microsystem and Information Technology, Chinese Academy of Sciences, Shanghai 200050, P. R. China
3. University of Chinese Academy of Sciences, Beijing 100049, P. R. China
4. Department of Materials Science, Fudan University, Shanghai 200433, P. R. China
5. State Key Laboratory of Integrated Optoelectronics, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P. R. China