1. b R&D Department , Chartered Semiconductor Manufacturer Pte Ltd. , 2 Science Park Drive, Singapore Science Park , 0511 , Singapore
2. a Division of Applied Sciences , Harvard University , Cambridge , MA , 02138 , USA
3. c Department of Electrical Engineering , Beijing Polytechnic University , Beijing , 100022 , P.R. China
4. d Department of Physics , Boston College , Chestnut Hill , MA , 02167 , USA