1. Institute for Solid State Electronics, Vienna University of Technology, Floragasse 7, A-1040 Vienna, Austria
2. Institute for Solid State Physics, Vienna University of Technology, Wiedner Hauptstr. 8/052, A-1040 Vienna, Austria
3. Institute for Microelectronics, Vienna University of Technology, Gusshausstrasse 27-29, A-1040 Vienna, Austria