1. Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695
2. Department of Material Science and Engineering, North Carolina State University, Raleigh, North Carolina 27695
3. Department of Electrical and Computer Engineering, North Carolina State University, Raleigh, North Carolina 27695, and Microelectronics Center of North Carolina, Research Triangle Park, North Carolina 27709
4. Department of Physics and Astronomy, University of North Carolina, Chapel Hill, North Carolina 27599