1. Institute of Materials Research and Engineering; Agency for Science, Technology and Research (A*STAR); 2 Fusionopolis Way, Innovis Singapore 138634 Singapore
2. Institute of Microelectronics; Agency for Science, Technology and Research (A*STAR); 2 Fusionopolis Way, Innovis Singapore 138634 Singapore
3. Division of Physics and Applied Physics; School of Physical & Mathematical Sciences; Nanyang Technological University; Singapore 637371 Singapore
4. Department of Electrical and Computer Engineering; National University of Singapore; Singapore 117567 Singapore
5. State Key Laboratory on Integrated Opto-Electronics; College of Electronic Science and Engineering; Jilin University; Changchun 130012 P. R. China