1. V. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, Pr. Nauky 41, 03680 Kiev, Ukraine
2. Institute for Nanoscience and Engineering, University of Arkansas, West Dickson 731, Fayetteville, Arkansas 72701, USA
3. State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, 610054 Chengdu, China