1. Institute of Microelectronics, National Research Center for Physical Sciences “Democritos,” Aghia Paraskevi 15310, Athens, Greece
2. Laboratoire de Physique des Composants à Semiconducteurs, UA‐CNRS 840, ENSERG, 38016 Grenoble Cedex, France
3. Physics Department, Aristotle University of Thessaloniki, Thessaloniki 54006, Greece