1. Institute of Quantum Matter / Semiconductor Physics Group, University of Ulm, Albert-Einstein-Allee 45, 89069 Ulm, Germany
2. Institute of Optoelectronics, University of Ulm, Albert-Einstein-Allee 45, 89069 Ulm, Germany
3. Institut für Experimentelle Physik, Otto-von-Guericke-Universität Magdeburg, Universitätsplatz 2, 39106 Magdeburg, Germany