1. School of Physics and Technology
2. Inner Mongolia University
3. Hohhot 010021
4. P. R. China
5. State Key Laboratory of Superlattices and Microstructures
6. Institute of Semiconductors
7. Chinese Academy of Sciences
8. Beijing 100083
9. School of Information Science and Engineering
10. Shandong University
11. Jinan
12. School of Physics
13. Jining Normal University
14. Jining 010021