1. CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P.R. China
2. University of Chinese Academy of Sciences, Beijing 100049, P.R. China
3. CAS Key Laboratory of Nanosystem and Hierarchical Fabrication, CAS Center for Excellence in Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P.R. China
4. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P.R. China
5. Research Center for Wide Band Semiconductor, Peking University, Beijing 100871, P.R. China
6. Key Laboratory of Semiconductor Materials Science, Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, P.R. China