1. Department of Electrical and Computer Engineering, University of Washington, Seattle, WA 98195, USA.
2. Department of Materials Science and Engineering, University of Maryland, College Park, MD 20742, USA.
3. Institute for Research in Electronics and Applied Physics, University of Maryland, College Park, MD 20742, USA.
4. School of Microelectronics, Tianjin University, Tianjin 300072, China.
5. Department of Physics, University of Washington, Seattle, WA 98195, USA.