Correlation between Thermal Transport Mechanisms and Microstructure of Epitaxially Grown (Sb1-xBix)2Te3 Thin Films

Abstract

This thesis experimentally studies the mechanisms associated with the very low lattice thermal conductivities of the ternary van der Waals layered compound (Sb1-xBix)2Te3. This compound presently constitutes one of the most promising thermoelectric systems for room temperature applications. Epitaxial thin films of high crystalline quality have been grown by a thermal evaporation technique by our project partner, the group of Prof. S. Schultz, University of Duisburg-Essen. The author has conducted unique low temperature thermal conductivity measurements along the c-axis using the so called 3 technique. In contrast to most other works on that field, the thermal boundary resistances have been determined precisely and were taken into account to access the intrinsic thermal conductivity of the films. Very regular growth, low planar defect concentrations and the presence of solid solutions have been confirmed by High Resolution Transmission Electron Microscopy. The results demonstrate how the low thermal conductivity of the undoped Sb2Te3 can be further reduced by the substitution of Sb by Bi atoms. For a high doping level, the presence of a phonon glass state is shown, discernible with an almost temperature independent lattice thermal conductivity despite the distinct crystalline nature of the sample. Numerical analysis reveal an extraordinary increase of phonon Umklapp scattering rates with increasing Bi contents, along with strong Rayleigh scattering. The present work proves that the low lattice thermal conductivity of (Sb1-xBix)2Te3 can be explained as an intrinsic material property and does not require nanostructuring. It is presumably the result of a combined effect, where Umklapp scattering due to anharmonicity of the van der Waals and resonant bonding might be further enhanced by Rayleigh scattering.