1. School of Physical Electronics, University of Electronic Science and Technology of China, Chengdu 610054, People's Republic of China
2. Department of Electrical and Computer Engineering, Iowa State University, Ames, Iowa 50011, USA
3. Ames Laboratory, U.S. Department of Energy, Ames, Iowa 50011, USA
4. North Electronic Device Research Institute, Beijing 100141, People's Republic of China