[1] Russian Acad Sci, Inst Analyt Instrumentat, St Petersburg 198063, Russia
[2] Russian Acad Sci, AF Ioffe Physicotech Inst, St Petersburg 194021, Russia
[3] Max Planck Inst Mikrostrukturphys, D-06120 Halle Saale, Germany
[4] Russian Acad Sci, Inst Semicond Phys, Siberian Div, Novosibirsk 630090, Russia