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 Si O2 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/Si O2 interface whenever core diameter is small. We also discuss the effects of thickness of the oxide layers on the direct interband transitions.