1. Institute of Microsystems Engineering, University of Freiburg, Georges-Koehler-Allee 103, 79110 Freiburg, Germany
2. Max Planck Institute of Microstructure Physics, Weinberg 2, 06120 Halle, Germany
3. Fraunhofer Institute for Applied Solid State Physics, Tullastrasse 72, 79108 Freiburg, Germany