1. School of Materials Science and Engineering, University of Science and Technology Beijing, Beijing, China
2. Beijing National Laboratory for Condensed Matter Physics, Institute of Physics, Chinese Academy of Sciences, Beijing, China
3. Department of Materials Science and Engineering, City University of Hong Kong, Hong Kong, China
4. Semiconductor Technology Innovation Center (Beijing) Corporation, Beijing, China
5. Department of Physics, City University of Hong Kong, Hong Kong, China
6. Key Laboratory of Vacuum Physics, School of Physical Sciences, University of Chinese Academy of Sciences, Beijing, China