Cross-plane thermal transport in SrRuO₃ thin films investigated by time-domain thermoreflectance technique

A SrRuO₃ thin film has been widely used as a metal electrode in electronic devices based on transition metal oxides, and hence it is important to understand thermal transport properties through the SrRuO₃ thin film to minimize a thermal degradation problem during the device operation. Using the time-domain thermoreflectance measurement technique, we investigate the cross-plane thermal conductivity of the SrRuO₃ thin films with a thickness variation from 1 um to 8 nm. We find that the thermal conductivity becomes reduced from 5.0 W/m-K for the 1 um thick film to 0.94 W/m-K for the 8 nm thick film, and attribute such a large reduction of the thermal conductivity to the boundary scattering of thermal carriers, i.e., electron and phonon. In addition, we find a signature of the ballistic phonon transport at low temperature particularly when the film thickness becomes much smaller than the phonon mean free path.