1. Physics Department, Aristotle University of Thessaloniki, 54624 Thessaloniki, Greece
2. Department of Materials Science and Technology, University of Crete, P.O. Box 2208, 70013 Heraklion, Greece
3. Microelectronics Research Group, IESL-FORTH, P.O. Box 1385, 71110 Heraklion, Greece
4. Department of Physics, University of Crete, P.O. Box 2208, 70013 Heraklion, Greece