1. Department of Microsystems Engineering (IMTEK), University of Freiburg, Freiburg, Germany
2. Institute of Solid State Physics, Technische Universität Berlin, Berlin, Germany
3. Ferdinand-Braun-Institut, Leibniz-Institut für Höchstfrequenztechnik, Berlin, Germany
4. Materials Department, University of California, Santa Barbara, California 93106, USA
5. Electrical and Computer Engineering Department, University of California, Santa Barbara, California 93106, USA
6. Institute of Physics, Technische Universität Chemnitz, Chemnitz, Germany