Phonon transport in the nano-system of Si and SiGe films with Ge nanodots and approach to ultralow thermal conductivity

Phonon transport in the nano-system has been studied using well-designed nanostructured materials to observe and control the interesting phonon behaviors like ballistic phonon transport. Recently, we observed drastic thermal conductivity reduction in the films containing well-controlled nanodots. Here, we investigate whether this comes from the interference effect in ballistic phonon transport by comparing the thermal properties of the Si or Si0.75Ge0.25 films containing Ge nanodots. The experimentally-obtained thermal resistance of the nanodot layer shows peculiar nanodot size dependence in the Si films and a constant value in the SiGe films. From the phonon simulation results, interestingly, it is clearly found that in the nanostructured Si film, phonons travel in a non-diffusive way (ballistic phonon transport). On the other hand, in the nanostructured SiGe film, although simple diffusive phonon transport occurs, extremely-low thermal conductivity (similar to 0.81 W m(-1) K-1) close to that of amorphous Si0.7Ge0.3 (similar to 0.7 W m(-1) K-1) is achieved due to the combination of the alloy phonon scattering and Ge nanodot scattering.

Automated summary

Phonon transport in nano-systems has been studied using nanostructured materials, revealing interesting behaviors like ballistic phonon transport. A drastic reduction in thermal conductivity was observed in films containing nanodots, with different materials showing peculiar effects on thermal resistance. Additionally, phonon simulation results show non-diffusive transport in nanostructured Si films and low thermal conductivity in SiGe films due to alloy and nanodot scattering.