1. Research Center for Wide Gap Semiconductors
2. State Key Laboratory for Artificial Microstructure and Microscopic Physics
3. School of Physics
4. Peking University
5. 100871 Beijing
6. Institute of Condensed Matter Physics
7. PR China
8. School of Electronics and Information Engineering
9. Tianjin Polytechnic University
10. Tianjin 300387
11. China