1. School of Electrical and Electronic Engineering, Nanyang Technological University, 50 Nanyang Avenue, 639798 Singapore
2. State Key Lab for Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, No.A35 QingHua East Road, Beijing 100083, China
3. State Key Lab of Luminescence and Applications, Changchun Institute of Optics, Fine Mechanics and Physics, Chinese Academy of Sciences, No.3888 Dong Nanhu Road, Changchun 130033, China
4. Temasek Laboratories, Nanyang Technological University, 50 Nanyang Drive, 637553 Singapore