1. Institute
of Physics, Center
for Interdisciplinary Nanostructure Science and Technology (CINSaT), University of Kassel, Heinrich-Plett-Strasse 40, D-34132
Kassel, Germany
2. Laser Zentrum Hannover e.V., Semiconductor and Photovoltaics-Group, Hollerithallee
8, D-30419 Hannover, Germany