1. School of Optoelectronic Science and EngineeringUniversity of Electronic Science and Technology of China Chengdu 610054 P. R. China
2. National Laboratory of Solid State MicrostructuresSchool of Electronic Science and Engineering Nanjing 210093 P. R. China
3. Collaborative Innovation Center of Advanced MicrostructuresNanjing University Nanjing 210093 P. R. China
4. State Key Laboratory of Electronic Thin Films and Integrated DevicesUniversity of Electronic Science and Technology of China Chengdu 610054 P. R. China