1. Institute
of Semiconductor and Solid State Physics, Johannes Kepler University, Altenbergerstrasse 69, A-4040 Linz, Austria
2. Institute
of Solid State Physics, Graz University of Technology, Petersgasse
16, A-8010 Graz, Austria
3. Faculty
of Aerospace Engineering, Delft University of Technology, 2629 HS Delft, The Netherlands