1. Solid Surface Analysis, Institute of Physics, Chemnitz University of Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany
2. Semiconductor Physics, Institute of Physics, Chemnitz University of Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany
3. Institute for Solid State Physics, Leibniz University of Hannover, Appelstr. 2, 30167 Hannover, Germany
4. Department of Applied Physics, Hebrew University of Jerusalem, Jerusalem, Israel
5. Center for Nanoscience and Nanotechnology, Hebrew University of Jerusalem, Jerusalem, Israel
6. Semiconductor Physics group, Institute of Physics, Chemnitz University of Technology, Reichenhainer Str. 70, 09126 Chemnitz, Germany