1. Institute of Optoelectronics, Ulm University, Albert-Einstein-Allee 45; 89081 Ulm Germany
2. Home address: Institute of Physics, Azerbaijan National Academy of Sciences, H. Javid pr., 33; Baku 1143 Azerbaijan
3. Institute of Quantum Matter/Semiconductor Physics Group, Ulm University, Albert-Einstein-Allee 45; 89081 Ulm Germany