1. Faculty of Physics, St. Petersburg State University, Universitetskaya Embankment 13B, St. Petersburg 199034, Russia
2. Laboratory of Semiconductor Materials, Institute of Materials, Faculty of Engineering, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland
3. Institute of Physics, Faculty of Basic Sciences, Ecole Polytechnique Fédérale de Lausanne, Lausanne 1015, Switzerland