1. Department of Physics, Humboldt University Berlin, Newton Str. 15, 12489 Berlin, Germany
2. Institute Kurz GmbH, Stöckheimer Weg 1, 50829 Cologne, Germany
3. V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Pr. Nauki 45, 03028 Kyiv, Ukraine
4. Paul Drude Institute for Solid State Electronics, Hausvogteiplatz 5-7, 10117 Berlin, Germany