Experimental Evidence of Superdiffusive Thermal Transport in Si_0.4Ge_0.6 Thin Films

Superdiffusive thermal transport represents a unique phenomenon in heat conduction, which is characterized by a size (L) dependence of thermal conductivity (κ) in the form of κ ∝ L^β with a constant β between 0 and 1. Although superdiffusive thermal transport has been theoretically predicted for SiGe alloys, direct experimental evidence is still lacking. Here, we report on a systematic experimental study of the thickness-dependent thermal conductivity of Si_0.4Ge_0.6 thin films grown by molecular beam epitaxy. The cross-plane thermal conductivity of Si_0.4Ge_0.6 thin films spanning a thickness range from 20 to 1120 nm was measured in the temperature range 120-320 K via a differential three-omega method. Results show that the thermal conductivity follows a consistent κ ∝ t^0.26 power law with the film thickness (t) at different temperatures, providing direct experimental evidence that alloy-scattering dominated thermal transport in SiGe is superdiffusive.