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