Size-Dependent Localized Phonon Population in Semiconducting Si Nanowires

The optical phonons in semiconductor nanostructures play an indispensable role in fundamental phenomenon and device applications based on these nanostructures. We study the Raman spectroscopy of optical phonons in Si nanowires (NWs) whose sizes are beyond the phonon confinement regime. The peak shift and unusual asymmetric broadening by one phonon mode in Si NWs is observed during far-field Raman studies. Using an appropriate thermal anchoring and localized Raman measurements on single NWs by near-field tip-enhanced Raman spectroscopy (TERS), we demonstrate the decoupling of multiple origins responsible for the peak shift accompanied by asymmetric broadening of the one-phonon mode and the appearance of multiple phonon peaks from a single measurement area. A model based on the localized phonon population induced by NW size-dependent charge depletion is proposed to explain the observed dependence of phonon characteristics on NW size. The scanning Kelvin probe force microscopy measurements confirm the size-dependent intrinsic semiconductor surface and interface states-induced charge depletions in single Si NWs. The study clearly suggests the size-dependent phonon characteristics of Si NWs which are crucial for several NW-based photovoltaic and thermoelectric devices.