1. Microelectronics Research Group (MRG), IESL, FORTH, P.O. Box 1385, GR 711 10 Heraklion Crete, Greece and Physics Department, University of Crete, GR 710 03 Heraklion Crete, Greece
2. Physics Department, Aristotle University of Thessaloniki, GR 541 24, Thessaloniki, Greece