[1] Linkoping Univ, Dept Phys Chem & Biol, SE-58183 Linkoping, Sweden
[2] Norstel AB, SE-60238 Norrkoping, Sweden
[3] Budapest Univ Technol & Econ, Dept Atom Phys, H-1111 Budapest, Hungary
[4] Hungarian Acad Sci, Res Inst Solid State Phys & Opt, H-1525 Budapest, Hungary