1. Applied Physics and Semiconductor Spectroscopy, Brandenburg University of Technology Cottbus-Senftenberg, 03046Cottbus, Germany
2. Institute of Solid State Physics, University of Bremen, 28359Bremen, Germany
3. MAPEX Center for Materials and Processes, University of Bremen, 28359Bremen, Germany
4. iNANO - Interdisciplinary Nanoscience Center, Aarhus University, 8000Aarhus, Denmark