Thermoelectric power factor enhancement based on carrier transport physics in ultimately phonon-controlled Si nanostructures

Abstract

We thoroughly investigated carrier and phonon transports related to thermoelectric properties using absolutely-controlled Si nanostructures, namely Si films containing epitaxial nanodots (NDs). In the Si nanostructure, various ND materials were introduced selectively such as Ge and β-FeSi2. This brings the ND/Si hetero-interface control; interface energy band offset and lattice mismatch strain in Si films. Ultrahigh density stacking faults can also be intentionally introduced using ultrathin SiO2 films. We reduced thermal conductivity (<2 Wm−1K−1) close to amorphous Si due to phonon transport control in absolutely-controlled Si nanostructures: scattering both by NDs and intentionally-doped atomic-scale impurities, namely wide-scale length phonon scattering. The electron transport related to Seebeck coefficient and electrical conductivity was manipulated by intentionally-introduced nanostructures: ND/Si hetero-interfaces and stacking faults. In the epitaxial Si thin films containing β-FeSi2 NDs, energetic carrier transports through stacking faults with several tens of meV barrier height enhanced the power factor. In the strained epitaxial Si thin films containing Ge NDs, it was found that strain-induced band splitting leading to high electron mobility enhanced the power factor. These results demonstrated that thermoelectric performance determined by electron and phonon transports were strongly dependent on the crystal characters (strain, crystal defects and the interfaces) relating to intentionally-introduced nanostructures. This suggests that it is important to design nanostructured materials on nanoscale and atomic-scale for enhancement of thermoelectric performance: introduction of stacking faults with proper energy barriers, NDs and dopants scattering phonons, and the less point defects. This also demonstrates that its nanostructuring methodology opens a load to realization of thermoelectric Si thin films.