1. Faculty of Physics, Taras Shevchenko National University of Kyiv, 64 Volodymyrs’ka St., 01601 Kyiv, Ukraine
2. V. E. Lashkaryov Institute of Semiconductors Physics, NAS of Ukraine, 41 pr. Nauki, 03028 Kyiv, Ukraine
3. O.O. Chuiko Institute of Surface Chemistry, 17 Generala Naumova St., 03164 Kyiv, Ukraine
4. Semiconductor Physics, Chemnitz University of Technology, D-09107 Chemnitz, Germany