1. State Key Laboratory for Artificial Microstructure & Mesoscopic Physics
2. School of Physics
3. Peking University
4. Beijing 100871
5. China
6. Department of Electronics Engineering
7. National Changhua University of Education
8. Changhua 50007
9. Taiwan
10. State Key Laboratory of Superlattices and Microstructures
11. Institute of Semiconductors
12. Chinese Academy of Sciences
13. Beijing 100083
14. National Institute for Materials Science
15. Tsukuba 305-0044
16. Japan