1. School of Physics Henan Normal University Xinxiang Henan 453007 P. R. China
2. School of Physics and Microelectronics Key Laboratory of Materials Physics of Ministry of Education of China Zhengzhou University Zhengzhou 450052 P. R. China
3. Henan Key Laboratory of Infrared Materials and Spectrum Measures and Applications Henan Normal University Xinxiang 453007 P. R. China
4. Institute of Semiconductors South China Normal University Guangzhou 510631 P. R. China