1. School of Electrical Science and Engineering, University of Electronic Science and Technology of China, Chengdu, Sichuan 611731, P. R. China
2. Department of Electrical and Computer Engineering, University of Wisconsin-Madison, Madison, Wisconsin 53705, United States
3. Science and Technology on Monolithic Integrated Circuits and Modules Laboratory, Nanjing Electronic Device Institute, Nanjing, Jiangsu 210016, P. R. China
4. Department of Physics, School of Science, Tibet University, Lhasa 850000, P. R. China