1. Arkansas Institute for Nanoscale Materials Science and Engineering, University of Arkansas, Fayetteville, Arkansas 72701, USA
2. Departamento de Fisica, Universidade Federal de São Carlos, 13565-905 São Carlos, São Paulo, Brazil
3. State Key Laboratory of Electronic Thin Film and Integrated Devices, University of Electronic Science and Technology of China, Chengdu, People's Republic of China
4. Institute of Semiconductor Physics, National Academy of Sciences, pr. Nauki 45, Kiev 03028, Ukraine
5. Instituto de Fisica de São Carlos, Universidade de São Paulo, 13.566-590 São Carlos, São Paulo, Brazil