1. Center for Integrated Sensor Systems, Danube University Krems, 2700 Wiener Neustadt, Austria
2. Institute of Physics, University of Augsburg, 86159 Augsburg, Germany
3. Laboratory for Mesoscopic Systems, Department of Materials, ETH Zurich, 8093 Zurich, Switzerland
4. Laboratory for Micro- and Nanotechnology, Paul Scherrer Institute, 5232 Villigen PSI, Switzerland