1. Institute of Physics, Polish Academy of Sciences, al. Lotników 32/46, 02-668 Warsaw, Poland
2. College of Science, Cardinal S. Wyszyński University, Warsaw, Poland
3. Dept. of Physics and Meas. Techn., Linköping University, 581 83 Linköping, Sweden
4. Institute of Semiconductor Physics, SB RAN, Novosibirsk 630090, Russia
5. Institute of Solid State Physics, Bremen University, 28334 Bremen, Germany