For highly efficient thermoelectric devices with Si nanostructures, we have fabricated and characterized micro/nanometer-scaled Si wires preserving the phonon-drag effect in order to observe the impact of phonon-boundary scattering on the phonon-drag factor in its Seebeck coefficient. The observed phonon-drag factor in the Seebeck coefficient decreases with a decrease in the wire width, which is considered due to an increase in the boundary scattering of phonons. Since the boundary scattering is characterized by the specularity parameter, we measured the surface roughness of the wire and evaluated the specularity. It was found that the top surface of the Si wire has higher specularity compared with the sidewall of the wire in the range of phonon wavelength contributing to the phonon drag. This result qualitatively explains the fact that the phonon drag in the Seebeck coefficient is hardly affected by the wire thickness with a nanometer order, whereas the wire width influences it significantly even on a micrometer scale. Moreover, it is demonstrated that the phonon-drag effect in the Seebeck coefficient of Si nanostructures can be preserved while their thermal conductivity is lowered.
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