1. Institute of Electrical Engineering, Slovak Academy of Sciences, Dúbravska cesta 9, 84104 Bratislava, Slovakia
2. Microelectronics Research Group (MRG), IESL, FORTH, P.O. Box 1385, 71110 Heraklion, Greece
3. Department of Physics, University of Crete, 71203 Heraklion, Greece