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Controlled pore formation on mesoporous single crystalline silicon nanowires: threshold and mechanisms

Silicon nanowires are prepared by the method of the two-step metal-assisted wet chemical etching. We have analyzed the structure of solid, rough and porous nanowire surfaces of boron-doped silicon substrates with resistivities of ρ> 1000 Ωcm, ρ= 14-23 Ωcm, ρ< 0.01 Ωcm by scanning electron microscopy and nitrogen gas adsorption. Silicon nanowires prepared from highly-doped silicon reveal mesopores on their surface. However, we found a limit for pore formation. Pores were only formed by etching below a critical H$_2$O$_2$ concentration (c$_{\mbox{H}_2\mbox{O}_2}$ < 0.3 M). Furthermore, we have determined the pore size distribution in dependence on the etching parameters and characterized the morphology of the pores on the nanowire surface. The pores are in the regime of small mesopores with a mean diameter of 9-13 nm. Crystal and surface structure of individual mesoporous nanowires have been investigated by transmission electron microscopy. The vibrational properties of nanowire ensembles have been investigated by Raman spectroscopy. Heavily boron-doped silicon nanowires are highly porous and the remaining single crystalline silicon nanoscale mesh leads to a redshift and a strong asymmetric line broadening for Raman scattering by optical phonons at 520 cm$^{-1}$. This redshift, λ$_{\mbox{Si bulk}}$ = 520 cm$^{-1}$ $\rightarrow$ λ$_{\mbox{Si nanowire}}$ = 512 cm$^{-1}$, hints to a phonon confinement in mesoporous single crystalline silicon nanowire.