1. NCSR “Democritos,” Institute of Microelectronics, 15310 Aghia, Paraskevi, Greece
2. Laboratoire de Physique des Composants a Semiconducteurs, URA 840 CNRS, ENSERG, BP 257, 38016 Grenoble Cedex, France
3. Department of Physics, University of Thessaloniki, 54006 Thessaloniki, Greece
4. LETI, Centre d'Etudes Nucleaires de Grenoble, 38041 Grenoble Cedex, France