1. College of Optoelectronic Engineering Chongqing University of Posts and Telecommunications Chongqing 400065 P. R. China
2. Key Laboratory of Optoelectronic Technology and Systems (Ministry of Education) College of Optoelectronic Engineering Chongqing University Chongqing 400044 P. R. China
3. School of Material Science and Engineering Zhengzhou University Zhengzhou 450001 P. R. China
4. College of Materials Science and Engineering Sichuan University Chengdu 610065 P. R. China
5. State Key Discipline Laboratory of Wide Band Gap Semiconductor Technology School of Microelectronics Xidian University Xi'an 710071 P. R. China