1. Department of Electronic & Computer Engineering The Hong Kong University of Science and Technology Clear Water Bay Kowloon Hong Kong SAR China
2. State Key Laboratory for Superlattices and Microstructures Institute of Semiconductors Chinese Academy of Sciences & Center of Materials Science and Optoelectronic Engineering University of Chinese Academy of Sciences Beijing 100083 China
3. MIIT Key Laboratory of Advanced Display Materials and Devices Institute of Optoelectronics & Nanomaterials School of Materials Science and Engineering Nanjing University of Science and Technology Nanjing 210094 China