Raman Scattering Spectroscopy of Metal-assisted Chemically Etched Rough Si Nanowires

We have shown that SiNWs formed on (100) Si substrates by metal-assisted chemical etching can exhibit different Raman scattering processes that are dependent on the orientation of examined NWs to the incident excitation light. The SiNWs retain the single crystal orientation and doping from the original bulk substrate and form as rough and mesoporous NWs with a SiOx shell surrounding the NW. The Raman scattering spectra of vertical and horizontally lying SiNWs showed quantum confined phonon scattering processes from narrow and roughened NWs, whose spectral resolution was increased by orienting NW horizontal to the beam to maximize probe cross-section. SiOx contributions were not evident and specific substrate Raman modes were suppressed for horizontal NWs. Localized beam induced heating to 425 K showed pronounced and specific red-shifting and asymmetry of the TO, 2TO and 2TA phonon modes consistent with phonon quantum confinement effects not observable when the NW were oriented parallel to the incident excitation light.