1. Institute of Optoelectronics, Ulm University; Ulm Germany
2. Institute of Experimental Physics, Otto-von-Guericke-University; 39106 Magdeburg Germany
3. Institute of Solid State Physics, Technical University of Berlin; Berlin Germany
4. Institute of Electron Devices and Circuits, Ulm University; Ulm Germany
5. Institute of Quantum Matter / Semiconductor Physics Group, Ulm University; Ulm Germany