1. Faculty of Physics, University of Vienna, Strudlhofgasse 4, A-1090 Vienna, Austria
2. Physics and Materials Science Research Unit, University of Luxembourg, Campus Limpertsberg, L-1511 Luxembourg, Luxembourg
3. Institute for Electronics, Microelectronics, and Nanotechnology (IEMN), CNRS-UMR 8520, Dept. ISEN, 59652 Villeneuve d’Ascq, France