Thermal transport through SiGe superlattices

Understanding thermal transport in nanoscale is important for developing nanostructured thermolelectric materials and for heat management in nanoelectronic devices. This dissertation is devoted to understand thermal transport through SiGe based superlattices. First, we systematically studied the cross-plane thermal conductivity of SiGe superlattices by varying the thickness of Si(Ge) spacers thickness. The observed additive character of thermal resistance of the SiGe nanodot/planar layers allows us to engineer the thermal conductivity by varying the interface distance down to ~1.5 nm. Si-Ge intermixing driven by Ge surface segregation is crucial for achieving highly diffusive phonon scattering at the interfaces. By comparing the thermal conductivity of nanodot Ge/Si superlattices with variable nanodot density and superlattices with only wetting layers, we find that the effect of nanodots is comparable with that produced by planar wetting layers. This is attributed to the shallow morphology and further flattening of SiGe nanodots during overgrowth with Si. Finally, the experiments show that the interface effect on phonon transport can be weakened and even eliminated by reducing the interface distance or by enhancing Si-Ge intermixing around the interfaces by post-growth annealing. The results presented in this dissertation are expected to be relevant to applications requiring optimization of thermal transport for heat management and for the development of thermoelectric materials and devices based on superlattice structures.