Determination of Seebeck coefficient originating from phonon-drag effect using Si single crystals at different carrier densities

Abstract

The phonon-drag effect is useful for improving the thermoelectric performance, especially the Seebeck coefficient. Therefore, the phonon and electron transport properties of Si single crystals at different carrier densities were investigated, and the relationship between these properties and the phonon-drag effect was clarified. Phonon transport properties were determined using nanoindentation and spot-periodic heating radiation thermometry. The electron transport properties were determined based on the electrical conductivity of Si. The diffusive Seebeck coefficient derived from the electron transport properties was in good agreement with previous reports. However, the value of the phonon-drag Seebeck coefficient derived from the phonon transport properties is very low. This phenomenon suggests that phonons with a normal mean free path (MFP) do not contribute to the increase in the Seebeck coefficient; however, phonons with a long MFP and low frequency increase the Seebeck coefficient via the phonon-drag effect. Moreover, the phonon-drag effect was sufficiently pronounced even at 300 K and in the heavily doped region. These features are key in designing thermoelectric materials with enhanced performance derived from the phonon-drag effect.

Ultralow thermal conductivity of amorphous silicon–germanium thin films for alloy and disorder scattering determined by 3ω method and nanoindentation

“…41) In our previous studies, we developed a method to estimate thermal conductivity, group velocity, and phonon MFP by combining thermal conductivity measurements and nanoindentation. [42][43][44][45][46] Consequently, we showed that the primary cause of the ultralow thermal conductivity of amorphous materials is a notable reduction in phonon MFP. 47) One approach to further decreasing the lattice thermal conductivity of amorphous materials is to alloy them.…”

Origin of ultralow thermal conductivity in amorphous Si thin films investigated using nanoindentation, 3ω method, and phonon transport analysis

“…[18][19][20][21] In addition, the phonon frequency in singlecrystal Si was estimated using the combined method and modeling. 22) Nanoindentation is frequently used to measure the elastic modulus and hardness of structural materials, 23,24) and group velocity can be estimated from the elastic modulus. Therefore, the combined method has the potential to measure the phonon transport properties of amorphous materials, which can be used to elucidate the origin of their ultralow thermal conductivity.…”

“…To elucidate the origin of the ultralow thermal conductivity in amorphous Si thin films, we compared the phonon transport properties of amorphous Si with those of single-crystal Si reported in our previous studies and extracted the distinctive properties of amorphous Si. 20,22) This method can be applied not only to thermoelectric materials but also to materials with disordered structures as a new analysis method. Furthermore, it can be used to control the thermal properties of amorphous materials.…”

“…10,17,[36][37][38] In contrast, the thermal conductivity of single-crystal Si obtained in our previous study was 132 W/(m•K). 22) Therefore, the amorphous structure reduces the thermal conductivity to approximately 1/40 that of single-crystal Si. These results demonstrate that the disordered structure has little effect on the group velocity but significantly affects the thermal conductivity.…”