1. Semiconductor Physics Institute, A. Goštauto 11, 01108 Vilnius, Lithuania
2. Vilnius Gediminas Technical University, Saulėtekio al. 11, 10223 Vilnius, Lithuania
3. Department of Physics, UMIST, University of Manchester, Manchester M60 1QD, United Kingdom
4. IMP, School of Electronic and Electrical Engineering, University of Leeds, Leeds LS2 9JT, United Kingdom
5. Department of Electronic and Electrical Engineering, University of Sheffield, Sheffield S1 3JJD, Sheffield, United Kingdom