1. Institute of Physics, Wrocław University of Technology, Wybrzeże Wyspiańskiego 27, 50-370 Wrocław, Poland
2. Institute of Experimental Physics, University of Warsaw, Hoża 69, 00-681 Warsaw, Poland
3. Institute of Solid State Physics, University of Bremen, Postfach 330 440, D-28334 Bremen, Germany
4. Institute of Electron Technology, al. Lotników 32/46, 02-668 Warsaw, Poland