1. Key Laboratory of Optoelectronic Technology & Systems, Education Ministry of China and College of Optoelectronic Engineering, Chongqing University, 400044 Chongqing, People’s Republic of China.
2. Chongqing Pingchuang Institute of Semiconductors, Chongqing 400044, People’s Republic of China.
3. School of Materials Science and Engineering, Georgia Institute of Technology, Atlanta, GA 30332, USA.