1. Institute of Electron Physics, Ukr. Nat. Acad. Sci., 21 Universytetska Str., 88017 Uzhhorod, Ukraine
2. Uzhhorod National University, 46 Pidhirna Str, 88000 Uzhhorod, Ukraine
3. Semiconductor Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany
4. Tomsk Polytechnic University, 30 Lenina Ave., 634034 Tomsk, Russia
5. V. Lashkaryov Institute of Semiconductor Physics, Ukr. Nat. Acad. Sci., 45 Prospect Nauky, 03028 Kyiv, Ukraine
6. Solid Surface Analysis, Chemnitz University of Technology, D-09107 Chemnitz, Germany