1. Institute for Nano Science and Engineering, University of Arkansas 1 , Fayetteville, Arkansas 72701, USA
2. VE Lashkaryov Institute of Semiconductors Physics, NAS of Ukraine 2 , Kyiv 03028, Ukraine
3. Department of Physics, Taras Shevchenko National University of Kyiv 3 , Kyiv 01601, Ukraine
4. Department of Electrical Engineering, University of Arkansas 4 , Fayetteville, Arkansas 72701, USA