Boosting Seebeck coefficient through electron-phonon interaction by phonon frequency control

In Dirac and Weyl semimetals with a flat band, the Seebeck coefficient ($S$) can be enhanced by the energy filtering effect in wave number space through electron-phonon interaction (EPI). Especially, topological B20-type semimetal thin films are promising materials because their Dirac or Weyl fermions are robust against defect scattering, making EPI dominant carrier scattering in the thin films. Here, we propose a strategy of raising this EPI-induced $S$ enhancement effect (ESE) by enhancing EPI at room temperature (RT) via phonon frequency control. By using two atoms with a similar mass in topological B20 materials, the projected phonon density of states (DOS) of two atoms are mainly tuned within the frequency range below $200\phantom{\rule{0.28em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}$ (RT thermal energy), leading to the increased total phonon DOS at RT related to EPI enhancement. In this study, focusing on topological B20-CoGe with Dirac-like and flat bands, where Co and Ge have similar masses, we demonstrate raising ESE in an epitaxial B20-CoGe thin film experimentally and theoretically. Although B20-CoGe is unstable under atmospheric pressure, the epitaxial growth of B20-CoGe thin films is achieved on Si substrates by the seed-assisted epitaxy method. The good agreement between experimental results and calculated $S$ values with EPI is observed. The epitaxial B20-CoGe thin film shows the thermoelectric power factor of $\ensuremath{\sim}5.8\phantom{\rule{0.28em}{0ex}}\textmu{}\mathrm{W}\phantom{\rule{0.16em}{0ex}}\mathrm{c}{\mathrm{m}}^{\ensuremath{-}1}\phantom{\rule{0.16em}{0ex}}{\mathrm{K}}^{\ensuremath{-}2}$, which is comparable to group IV element-based typical thermoelectric semiconductor thin films although B20-CoGe is a semimetal. Furthermore, the use of a heavier Ge atom resulted in lower thermal conductivity. The proposed strategy opens a new approach to realize a high thermoelectric performance (at RT) thin film on a Si platform.