1. Physics and Technology Department, Vasyl Stefanyk Preсarpathian National University, Ivano-Frankivsk, Ukraine
2. V.Lashkaryov Institute of Semiconductors Physics, National Academy of Sciences of Ukraine, Kyiv, Ukraine
3. Physico-Mathematical Modelling of Low-Dimensional Systems Department, Pidstryhach Institute for Applied Problems of Mechanics and Mathematics, Lviv, Ukraine