1. Faculty of Fundamental Problems of Technology, Department of Experimental Physics, Wrocław University of Science and Technology, Wyb. Wyspiańskiego 27, 50-370 Wrocław, Poland
2. Faculty of Fundamental Problems of Technology, Department of Semiconductor Materials Engineering, Wrocław University of Science and Technology, Wyb. Wyspiańskiego 27, 50-370 Wrocław, Poland
3. Department of Physics, The University of Warwick, Gibbet Hill Road, CV4 7AL Coventry, U.K.