1. School of Materials Science and Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
2. School of Electrical and Electronic Engineering; Nanyang Technological University; 50 Nanyang Avenue Singapore 639798 Singapore
3. State Key Laboratory of Silicon Materials; School of Materials Science & Engineering; Zhejiang University; Hangzhou Zhejiang 310027 P. R. China
4. Department of Physics; National University of Singapore; Singapore 117542 Singapore
5. School of Physical and Mathematical Sciences; Nanyang Technological University; Singapore 639798 Singapore