Modelling & Simulation
Nanostructured semiconductors for thermoelectrics
Peter Kratzer, University of Duisburg-Essen, Duisburg, Germany
Nanostructuring offers a promising route to modified properties of semiconductor materials. In my presentation, I will demonstrate how the capabilities of nanostructuring may be exploited to enhance the thermoelectric performance of semiconductors, following the strategy of reducing their thermal conductivity while retaining large thermopower and electrical conductivity. Atomistic calculations are presented for the high-temperature thermoelectric material Si/Ge, as well as for recently developed ternary semiconductors in the C1_b “half-Heusler” structure.
From a theoretical point of view, the simplest nanostructured material is a stack of layers. The interface between materials of different acoustic impedance hinders the transport of heat perpendicular to the layers. I will show that the harmonic phonon spectra, which are computationally accessible via Density Functional Perturbation Theory, form the basis for a simple estimate of the interface thermal resistance. From the class of half-Heusler compounds, I will consider the electronic properties of the closely lattice-matched heterostructure ZrNiSn/ZrCoBi and show that the high thermoelectric power factor of these materials persists, while the cross-plane thermal conductivity may be reduced by a factor of three compared to the bulk materials.
In materials systems with a larger lattice mismatch, such as Si/Ge, the internal strain may be further exploited for band structure engineering. Epitaxial growth techniques allow for fabrication of a regular array of nanoscale Ge inclusions in a Si host crystal. Due to the strain in these type-II quantum dot crystals, conducting channels for electrons open up right below the conduction band edge in the Si host. These channels allow for a large thermoelectric power factor of the compound material already at room temperature, while the thermal conductivity drops even below the limit of a random SiGe alloy.