1. Institute
of Solid State Physics, Bulgarian Academy of Sciences, 72 Tzarigradsko
Chaussee, 1784 Sofia, Bulgaria
2. Fraunhofer Institute for Integrated Systems and Device Technology, Schottkystrasse 10, 91058 Erlangen, Germany
3. Chair
of Electron Devices, Friedrich-Alexander University Erlangen-Nuremberg, Cauerstrasse 6, 91058 Erlangen, Germany