1. School of Microelectronics, University of Science and Technology of China, Hefei 230029, P. R. China
2. Department of Materials Science and Engineering, Southern University of Science and Technology, Shenzhen 518055, P. R. China
3. State Key Laboratory of Optoelectronic Materials and Technologies, School of Materials, Sun Yat-sen University, Guangzhou 510275, P. R. China