1. Institute for Physics and Chemistry of Solid State, Uzhhorod National University, Uzhhorod, Ukraine
2. Physical Faculty, Uzhhorod National University, Uzhhorod, Ukraine
3. Institute of Electron Physics, Ukr. Nat. Acad. Sci., Uzhhorod, Ukraine
4. Faculty of Science and Technology, Jan Dlugosz University in Czestochowa, Czestochowa, Poland