1. V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 41 Nauky Avenue, 03028 Kyiv, Ukraine
2. State University of Information and Communication Technologies, 7 Solomenska Street, 03110 Kyiv, Ukraine
3. Physics Department, Taras Shevchenko National University of Kyiv, 4 Hlushkova Avenue, 03022 Kyiv, Ukraine