1. Chalmers University of Technology, Department of Microtechnology and Nanoscience, SE-412 96 Gothenburg, Sweden
2. Nizhny Novgorod State Technical University n.a. R.E. Alekseev, Nizhny Novgorod 603951, Russia
3. Institute for Physics of Microstructures, Russian Academy of Sciences, Nizhny Novgorod 603950, Russia