1. Institute of Low Temperature and Structure Research, Polish Academy of Sciences, ul. Okolna 2, Wrocław 50-422, Poland
2. Departamento de Fisica, Universidade Federal do Ceara, P.O. Box 6030, Fortaleza 60455-970, Brazil
3. Department of Semiconductor Materials Engineering, Faculty of Fundamental Problems of Technology, Wrocław University of Science and Technology, Wybrzeże Wyspiańskiego 27, Wrocław 50-370, Poland