P.L. Kurshev - Post-graduate Student, Department of Physics of Semiconductors and Microelectronics, Voronezh State University E-mail: mepavel@mail.ru E.N. Bormontov - Dr. Sc. (Phys.-Math.), Professor, Head of Department of Physics of Semiconductors and Microelectronics, Voronezh State University E-mail: me144@phys.vsu.ru