1. Department of Physics and Astronomy/iNANO, Aarhus University, Gustav Wieds Vej 14, DK-8000 Aarhus C, Denmark
2. Belarusian State University, prosp. Nezavisimosti 4, 220030 Minsk, Belarus
3. Department of Semiconductor Materials, Helmholtz-Zentrum Dresden-Rossendorf, Bautzner Landstraße 400, 01328 Dresden, Germany