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 2000.28em0excm^-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 ~5.80.28em0exW0.16em0excm^-10.16em0exK^-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.