1 School of Electrical and Electronic Engineering, Nanyang Technological University, Singapore 639798, Singapore
2 State Key Laboratory of Superlattices and Microstructures, Institute of Semiconductors, Chinese Academy of Sciences, Beijing 100083, China
3 Applied Materials, Inc., Sunnyvale, California 95054, USA
4 Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117576, Singapore
5 e-mail: shaoteng.wu@ntu.edu.sg
6 e-mail: rongqiao.wan@ntu.edu.sg
7 e-mail: chenqm@ntu.edu.sg