Abstract
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.
Highlights
Introduction of Si nanostructures into Si-based thermoelectric devices has been widely studied in order to enhance their performance by reducing their thermal conductivity through the suppression of phonon transport.1–4 Several studies reported the reduction of thermal conductivity in a low dimensional structure owing to the promotion of the boundary scattering in phonon transport.5–8 Martin et al reported that the surface roughness strongly affects the thermal conductivity in thin Si nanowires, which originates from the phonon-surface scattering.9 The phononscattering at the boundaries primarily depends on the surface roughness and the incident phonon wavelength
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
The blue dotted line shows the linear fitted and the red broken line indicates the calculated Se of bulk Si with a carrier concentration of 3.6 × 1017 cm−3.20,23 The arrow at the right axis corresponds to the reported value of the bulk Si with a carrier concentration of ∼1015 cm−3.17 From Fig. 1, the Seebeck coefficient is clearly observed to be higher than the calculated Se, which indicates the contribution of the phonon-drag effect
Summary
Introduction of Si nanostructures into Si-based thermoelectric devices has been widely studied in order to enhance their performance by reducing their thermal conductivity through the suppression of phonon transport. Several studies reported the reduction of thermal conductivity in a low dimensional structure owing to the promotion of the boundary scattering in phonon transport. Martin et al reported that the surface roughness strongly affects the thermal conductivity in thin Si nanowires, which originates from the phonon-surface scattering. The phononscattering at the boundaries primarily depends on the surface roughness and the incident phonon wavelength. Diffusive scattering occurs when the phonons are reflected by a boundary that has surface roughness comparable to or higher than the phonon wavelength. This dependency is characterized by the specularity parameter, p, expressed as p = exp(−4η2k2 cos θ),. The impact of the surface roughness on the phonon-boundary scattering has been studied in the scope of thermal transport.. The Seebeck coefficient of a semiconductor comprises a phonon-drag factor Sph as well as an electronic factor Se originating from carrier transport.. We investigate the Seebeck coefficient of Si wires with micro/nanometer dimensions and discuss the influence of the sample dimensions on its phonon-drag component from the viewpoint of the specularity
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