1. Semiconductor Physics Institute, A. Goštauto 11, LT- 01108 Vilnius, Lithuania
2. Department of Physics, Vilnius Gediminas Technical University, Saulėtekio Av. 11, LT-10223 Vilnius, Lithuania
3. Department of Radiophysics, Vilnius University, Saulėtekio Av. 9, LT-10222 Vilnius, Lithuania