1. Semiconductor Physics Institute, Center for Physical Sciences and Technology, A. Goštauto 11, LT-01108, Vilnius, Lithuania
2. Physikalisches Institut, Johann Wolfgang Goethe Universität, Max-von-Laue-Str. 1, Frankfurt am Main, Germany
3. Department of Physics, Lougborough University, Lougborough, LE113TU, United Kingdom
4. Fraunhofer-Institut für Angewandte Festkörperphysik, Tullastr. 72, D-79108 Freiburg, Germany