In-plane thermoelectric properties of silicon/germanium superlattices

In recent years, investigation of thermoelectric enhancement through low dimensional material property engineering is a popular topic in research filed. Several low dimensional material systems, including Si/Ge superlattice, have been studied for that purpose. In this thesis, we investigated the in-plane thermoelectric transport properties in Si/Ge superlattices. The basic principle of low dimensional thermoelectric enhancement is the quantum size effect. The proof-of-principle ZT enhancement has been observed experimentally on Si/Ge superlattice. However, it was found that electron transport properties are not only affected by the quantum size effect at low dimension. Based on that, we developed a hybrid model that incorporate both quantum and classical size effects for the electron in-plane transport in the superlattice system. The model, with strain induced band regulation in consideration, was successfully applied to characterize the measured thermoelectric transport properties obtained from the strained Si/Ge superlattice samples. It demonstrated that the classical size, which is ignored in previous theoretical works, is the cause of the power factor degradation. We developed the 2-wire 3o measurement technique and measured the anisotropic thermal conductivity of Si/Ge superlattices. The measured data demonstrated great thermal conductivity reduction effect along both in-plane and cross-plane directions. The phonon scattering mechanisms at low dimension along both in-plane and cross-plane directions are analyzed with different models. Theoretical analysis demonstrated that the phonon partial diffuse and partial specular scattering at the interface is the major mechanism that account for the great thermal conductivity reduction at both in-plane and cross-plane directions. The in-plane temperature dependent ZT values of Si/Ge superlattice are obtained for the first time from experiment. The results indicated that both quantum size effect and classical size effect are important at nanometer dimension. Further increase of ZT rely on the improvement of material growth technology and new approaches to engineering the material properties.