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 $\mathrm{Si}{\mathrm{O}}_2$ layers by means of electron energy-loss spectroscopy (EELS). Core-diameter dependence of the EELS spectra was systematically studied. It was clarified that both $E_1$ and $E_2$ direct interband transitions of Si core are explicitly enhanced, owing to monopolar surface plasmons at a $\mathrm{Si}∕\mathrm{Si}{\mathrm{O}}_2$ interface whenever core diameter is small. We also discuss the effects of thickness of the oxide layers on the direct interband transitions.

Figure 1

HRTEM images of (a) a thick-shell SiNW and (b) a thin-shell SiNW.

Figure 2

The dependence of EELS spectra of thick-shell SiNWs on core diameters $d$ (a) before and (b) after the zero-loss tails are subtracted. [(c) and (d)] HRTEM images of SiNWs utilized for the EELS measurements.

Figure 3

EELS spectra from thin-shell SiNWs (a) before and (b) after the zero-loss tails are subtracted.

Figure 4

$I_{\mathit{IT}}∕I_{p5}$ and $I_{\mathit{IT}}∕I_{p3}$ as a function of core diameter.

Figure 5

Plots of surface-response functions of a model SiNW surrounded by an infinite $\mathrm{Si}{\mathrm{O}}_2$ medium for (a) monopolar mode $(m=0)$ and (b) one of multipolar mode $(m=1)$.

Figure 6

Plots of (a) surface-response function of a model SiNW embedded in a vacuum for monopolar mode $(m=0)$ (Ref. 8) and (b) $\zeta =\mathrm{Im}\left[{\gamma }_0^{\mathrm{vac}}\right]-\mathrm{Im}\left[{\gamma }_0\right]$.