Thermal and thermoelectric transport in superlattice and quantum wells

Low dimensional structures such as superlattice and quantum wells may offer a new approach to achieve high thermoelectric figure-of-merit for solid-state energy conversion applications. This work addresses the effects of low dimensionality on the fundamental thermophysical properties and experimentally studies the thermoelectric transport in superlattice and quantum wells.; The mechanisms of thermal transport in superlattice are under hot debate. Current models on thermal transport fall into two groups: particle models and wave models. The effects of phonon confinement are investigated in both in-plane and cross-plane directions of superlattice based on a lattice dynamics model, which treats phonons as coherent wave. It is found that the in-plane thermal conductivity drop, caused by the suppressed group velocity, is very small, and cannot explain the experimentally observed values. Even in the cross-plane direction, the calculated thermal conductivity is many times higher than the experimental data. Similarly, only very small reduction in thermal conductivity in quantum wells is predicted based on the lattice dynamics model. The discrepancy between the lattice dynamics models and the experiment is due to the absence of the diffuse interface scattering. Two different approaches, namely the unified wave-particle model and partially coherent phonon heat conduction model, have been developed to combine the effects of phonon confinement and diffuse interface scattering on thermal conductivity in superlattice. The experimental data, including period thickness dependence and temperature dependence in both in-plane and cross-plane directions of superlattice, can be well explained by these two models.; It is extremely challenging to measure the thermoelectric properties in superlattice. A novel method is developed to simultaneously measure the Seebeck coefficient and thermal conductivity across thin films. Moreover, the thermoelectric properties in both in-plane and cross-plane directions of an n-type Si/Ge superlattice have been measured, and their anisotropy and temperature dependence have been studied. A 2-wire 3o method is employed to measure the in-plane and cross-plane thermal conductivities. The cross-plane electrical conductivity is determined through a modified transmission-line method. The thermal conductivity and electrical conductivity show similar anisotropy in the Si/Ge superlattice, but the difference of the in-plane and cross-plane Seebeck coefficients is much smaller.