[1] Microelectronics Laboratory, Department of Electrical Engineering, National University of Singapore, 4 Engineering Drive 3, Singapore 117576, Singapore
[2] Microelectronics Laboratory, Santa Clara University, 500 El Camino Real, Santa Clara, CA 95053, United States
[3] IMRE, NUS, Blk S7, Singapore 119260, Singapore
[4] Surface Science Laboratory, Department of Physics, National University of Singapore, 10 Kent Ridge Crescent, Singapore 119260, Singapore