1. Institute for Physics of Microstructures of Russian Academy of Sciences, 603950 Nizhny Novgorod, Russia
2. Lobachevsky State University of Nizhny Novgorod, 603950 Nizhny Novgorod, Russia
3. A.V. Rzhanov Institute of Semiconductor Physics, Siberian Branch of Russian Academy of Sciences, 630090 Novosibirsk, Russia