1. Department of Electrical Engineering, Stanford University, Stanford, California 94305, USA
2. Geballe Laboratory for Advanced Materials, Department of Applied Physics, Stanford University, 476 Lomita Mall, Stanford, California 94305, USA
3. Department of Inorganic Materials Science, Faculty of Science and Technology and MESA+ Institute for Nanotechnology, University of Twente, P.O. Box 217, 7500 AE Enschede, The Netherlands
4. Stanford Institute for Materials and Energy Sciences, SLAC National Accelerator Laboratory, 2575 Sand Hill Road, Menlo Park, California 94025, USA