1. Institute of Materials Science and Department of Electrical and Information Engineering, University of Wuppertal, Gauss Strasse 20, 42097 Wuppertal, Germany
2. Department of Physics, University of Durham, South Road, Durham, DH1 3LE, UK
3. Groupe d'Etude des Semiconducteurs CC074, Université Montpellier II, Place Bataillon, 34095 Montpellier, Cedex 05, France
4. LASMEA Université Blaise Pascal, Clermont‐Ferrand II, 24 av. des Landais, 63177 Aubiere, Cedex, France
5. Department of Electronics, University of York, Heslington, York YO10 5DD, UK
6. Institute of Physical Chemistry, University of Hamburg, Bundesstr. 45, 20146 Hamburg, Germany
7. Department of Physics, University of Munich, Amalienstr. 54, 80799 Munich, Germany