Synergistic phonon scattering in epitaxial silicon multilayers with germanium nanodot inclusions

Temperature-dependent thermal conductivity of epitaxial silicon (Si)/ultrathin silica multilayers film with epitaxial germanium (Ge) nanodot inclusions is measured over the range of temperature from 50 K to room temperature using time-domain thermoreflectance. The measured thermal conductivity with 5-nm Ge nanodots is much smaller than the reported values for Si/Ge superlattices, bulk SiGe, and nanostructured SiGe in the entire temperature range. The thermal conductivity of the film is analyzed with a kinetic model incorporating multiple phonon scattering processes, where intrinsic three-phonon scattering inside the Si layers is calculated by first principles, boundary scattering at the ultrathin silica layer is calculated by the atomistic Green's function, and scattering by the Ge nanodots is approximated with nanovoids. The analysis reveals that summing the multiple scattering rates by Matthiessen's rule cannot explain the extremely low thermal conductivity. The Monte Carlo ray tracing calculation that incorporates the multiple scattering effect reveals that the synergistic effect of ultrathin silica interfaces and Ge nanodots enhances phonon scattering. This suggests the merit in synergistically designing multiple nanostructures to reduce thermal conductivity, which is beneficial for developing thermoelectric materials.