1. State Key Laboratory of Electronic Thin Films and Integrated Devices, School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China (UESTC), Chengdu 610054, P. R. China
2. College of Science, Sichuan Agriculture University, Yaan 625014, P. R. China
3. College of Optoelectronic Engineering, Chengdu University of Information Technology, Chengdu 610225, P. R. China