1. Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland
2. Semiconductor Physics Institute, A. Gostauto 11, 2600 Vilnius, Lithuania
3. Hamburger Synchrotronstrahlungslabor HASYLAB am Deutschen Elektronen-Synchrotron DESY, Notkestr. 85, D-22603 Hamburg, Germany