Low thermal conductivity of complex thermoelectric barium silicide film epitaxially grown on Si

We achieved substantially low thermal conductivity by introducing some crystal disorder into complex material BaSi₂ films on Si substrates for realization of a high-performance thermoelectric material. The BaSi₂ films/Si exhibited a low thermal conductivity of 0.96 W m⁻¹ K⁻¹ without nanostructuring, which is less than about two-thirds value of bulk BaSi₂ and is the lowest among ecofriendly silicide materials. This substantially low thermal conductivity was brought by both the use of complex material with intrinsically low phonon group velocity and the introduction of point defects as the crystal disorder. The first-principles calculations revealed that the point defect modulates the phonon dispersion relation lowering longitudinal acoustic phonon group velocity. However, the transverse acoustic phonon group velocity was increased at the same time, resulting in a negligible change in average acoustic phonon group velocity. This indicated that the crystal disorder effect related to point defects in this system is enhancement of phonon scattering, not lowering phonon group velocity. The BaSi₂ films/Si with point defects exhibited a higher thermoelectric power factor (2.9 μW cm⁻¹ K⁻²) than bulk BaSi₂. These results highlight that complex material BaSi₂ film/Si with point defects, having substantially low thermal conductivity, is a candidate as a thermoelectric power generator material in the sensor network.