1. Key Laboratory of Information Materials of Sichuan Province, Southwest Minzu University, Chengdu 610041, P.R. China
2. School of Optoelectronic Science and Engineering, University of Electronic Science and Technology of China, Chengdu 611731, P.R. China
3. State Key Laboratory of Electronic Thin Films and Integrated Devices, University of Electronic Science and Technology of China, Chengdu 611731, P.R. China