1. Department of Physics; Faculty of Science; Anadolu University; Yunus Emre Campus Eskisehir 26470 Turkey
2. Faculty of Science; Department of Physics; Gazi University; Teknikokullar Ankara 06500 Turkey
3. Institute of Semiconductor Physics of Siberian Branch of Russian Academy of Sciences; Acad. Lavrentiev ave 13 Novosibirsk 630090 Russia
4. Novosibirsk State University; 2, Pirogov Street Novosibirsk 630090 Russia