1. School of Materials Science and Engineering, College of Engineering, Peking University, Beijing 100871, China
2. Research Center for Wide Gap Semiconductor, Peking University, Beijing 100871, China
3. Division of Physics and Applied Physics, School of Physical and Mathematical Sciences, Nanyang Technological University, Singapore 637371
4. State Key Laboratory of Low-Dimensional Quantum Physics, Department of Physics, Tsinghua University, Beijing 100084, China