1. Energy and Semiconductor Research Laboratory
2. Department of Physics
3. Carl von Ossietzky University of Oldenburg
4. 26111 Oldenburg
5. Germany
6. Semiconductor Physics
7. Technische Universität Chemnitz
8. 09107 Chemnitz
9. V. E. Lashkaryov Institute of Semiconductors Physics
10. Dipartimento di Fisica
11. Politecnico di Milano
12. 20133 Milano
13. Italy