1. Institute of Molecular and Atomic Physics, National Academy of Sciences, F. Skaryna Ave 70, Minsk 220072 Belarus, Department of Microelectronics, Belarusian State University of Informatics and Electronics, Minsk, P. Brovka Str., 6, 220027 Belarus, and Material Science Department, Technical University of Valencia, Paseo de Viaducto 1, 03801 Spain