1. Department of Physics, Taras Shevchenko National University of Kyiv, 64 Volodymyrs’ka St., Kyiv 01601, Ukraine
2. Institute for Nano Science and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States
3. V. E. Lashkaryov Institute of Semiconductors Physics, NAS of Ukraine, 41 pr. Nauki, Kyiv 03028, Ukraine
4. Departamento de Física, Universidade Federal de São Carlos, São Carlos, São Paulo 13565-905, Brazil
5. Department of Electrical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States