1. School of Material Science and Engineering State Key Laboratory of Reliability and Intelligence of Electrical Equipment Hebei University of Technology Dingzigu Road 1 Tianjin 300130 P. R. China
2. Institute for Solid State Physics Leibniz University Hannover Appelstrasse 2 30167 Hannover Germany
3. State Key Laboratory of Integrated Optoelectronics College of Electronic Science and Engineering Jilin University 2699 Qianjin Street Changchun 130012 P. R. China
4. Key Laboratory of Optoelectronic Technology & Systems (Ministry of Education) College of Optoelectronic Engineering Chongqing University Chongqing 400044 P. R. China