1. Institute for Nanoscience and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States
2. V. E. Lashkaryov Institute of Semiconductors Physics, NAS of Ukraine, Kyiv 03028, Ukraine
3. The National Institute for Research and Development in Microtechnologies (IMT), Bucharest 077190, Romania
4. Physics Department, Federal University of São Carlos, 13565-905 São Carlos, São Paulo, Brazil
5. Department of Electrical Engineering, University of Arkansas, Fayetteville, Arkansas 72701, United States