1. Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravska cesta 9, 841 04 Bratislava, Slovakia
2. Institute for Solid State Electronics, TU Wien, Floragasse 7, A-1040 Vienna, Austria
3. Microelectronics Research Group (MRG), IESL, FORTH, P.O. Box 1385, 71110 Heraklion, Greece
4. Department of Physics, University of Crete, 71203 Heraklion, Greece