1. Key Laboratory of Photovoltaic Materials Henan University Kaifeng 475004 P. R. China
2. Henan Key Laboratory of Infrared Materials & Spectrum Measures and Applications Henan Normal University Xinxiang 453007 P. R. China
3. Key Laboratory of Semiconductor Materials Science Beijing Key Laboratory of Low Dimensional Semiconductor Materials and Devices Institute of Semiconductors Chinese Academy of Sciences Beijing 100083 P. R. China
4. Fachgebiet Angewandte Nanophysik Institut für Physik & IMN MacroNano Technische Universität Ilmenau 98693 Ilmenau Germany