[1] Ukr Nat Acad Sci, Inst Electron Phys, Univ Etska Str 21, UA-88017 Uzhgorod, Ukraine
[2] Uzhgorod Natl Univ, Appl Phys, Pidhirna Str 46, UA-88000 Uzhgorod, Ukraine
[3] Ukr Nat Acad Sci, L Pysarzhevsky Inst Phys Chem, Prospekt Nauky 29, UA-03028 Kiev, Ukraine
[4] Tech Univ Chemnitz, Semicond Phys, D-09107 Chemnitz, Germany
[5] Ukr Nat Acad Sci, V Lashkaryov Inst Semicond Phys, 45 Prospekt Nauky, UA-03028 Kiev, Ukraine