1. Solid State Physics Laboratory, ETH Zurich, 8093 Zurich, Switzerland, Micro and Nanosystems, ETH Zurich, 8092 Zurich, Switzerland, and Institute for Electronics, Microelectronics, and Nanotechnology (IEMN), CNRS-UMR 8520, B.P. 60069, 59652 Villeneuve d'Ascq Cedex, France