Enhanced direct interband transitions in silicon nanowires studied by electron energy-loss spectroscopy

We have experimentally observed abnormally enhanced direct interband transitions in silicon nanowires covered with SiO₂ layers by means of electron energy-loss spectroscopy (EELS). Core-diameter dependence of the EELS spectra was systematically studied. It was clarified that both E₁ and E₂ direct interband transitions of Si core are explicitly enhanced, owing to monopolar surface plasmons at a Si∕SiO₂ interface whenever core diameter is small. We also discuss the effects of thickness of the oxide layers on the direct interband transitions.