1. a Electrical and Computer Engineering, McGill University , Montreal , Quebec , H3A 2T8 , Canada
2. b Tsinghua National Laboratory for Information Science and Technology (TNList) , Institute of Microelectronics, Tsinghua University , Beijing , 100084 , P. R. China
3. c State Key Laboratory of Electronic Thin Films and Integrated Devices , University of Electronic Science and Technology of China , Chengdu , 610054 , P. R. China