1. Department of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, P. R. China
2. Research Center for Wide Gap Semiconductor, Peking University, Beijing 100871, China
3. CAS Key Laboratory of Standardization and Measurement for Nanotechnology, CAS Center of Excellence for Nanoscience, National Center for Nanoscience and Technology, Beijing 100190, P. R. China
4. Department of Physics, Tsinghua University, Beijing 100084, P. R. China
5. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371