1. Institute of Physics, Polish Academy of Sciences, aleja Lotników 32/46, Warsaw 02-668, Poland
2. Lviv Polytechnic National University, S. Bandera Str. 12, Lviv 79013, Ukraine
3. Taras Shevchenko National University of Kyiv, Volodymyrska Str. 60, Kyiv 01033, Ukraine
4. Institute of Physics, University of Tartu, W. Ostwald Str. 1, Tartu 50411, Estonia
5. Institute of Solid State Physics, University of Latvia, Kengaraga Str. 8, Riga 1063, Latvia
6. College of Physics, Jilin University, 2699 Qianjin Str., Changchun 130012, China
7. V.E. Lashkaryov Institute of Semiconductor Physics, National Academy of Sciences of Ukraine, 41 prospekt Nauki, Kyiv 03028, Ukraine