1. Department of Electrical and Computer Engineering, National University of Singapore, Singapore 117583, Singapore
2. NUS Graduate School, National University of Singapore, Singapore 119077, Singapore
3. Engineering Product Development, Singapore University of Technology and Design (SUTD), Singapore 487372, Singapore
4. Key Laboratory of Microelectronic Devices and Circuits (MOE), School of Integrated Circuits, Peking University, Beijing 100871, China
5. Institute of Materials Research and Engineering, Agency for Science, Technology and Research (A*STAR), Singapore 138634, Singapore
6. School of Physics, University of Electronic Science and Technology of China, Chengdu 611731, China
7. Department of Materials Science and Engineering, University of California, Berkeley, and Lawrence Berkeley National Laboratory, California 94720, United States