High crystallinity Si films containing silicide nanodots (NDs) were epitaxially grown on Si substrates at high temperature (∼750 °C), where the silicide phase of NDs (metallic α-FeSi2 or semiconductor β-FeSi2) was selectable by tuning the Fe deposition amount. The high crystallinity high-temperature-grown Si films with NDs exhibited lower thermal conductivity (5.4 W m−1 K−1) due to the phonon scattering at the ultrasmall ND interfaces than bulk Si-silicide nanocomposites that have ever been reported. In this ND system with extremely low thermal conductivity, due to the less point defects and high quality ND interface, the thermoelectric power factor (∼28 μW cm−1 K−2) was observed to be the same as the high value of Si films without NDs at room temperature, which is the highest value among Si-silicide bulk nanocomposites ever reported. The simultaneous achievement of a high power factor and low thermal conductivity in the high quality ND system will provide the key for design of high thermoelectric performance of Si-based nanostructured films.

Thin film thermoelectric materials have drawn much attention for realizing one-chip stand-alone power sources of Internet of Things devices. Here, we fabricate two types of the nanostructured Si films with high crystallinity: Si films containing β-FeSi 2 nanodots with a wider nanodot size distribution of ∼5-120 nm and Si films containing α-FeSi 2 nanodots with a narrow size distribution of ∼5-20 nm. The thermal conductivity of these films is lower than those of Si-silicide nanocomposite bulks. Interestingly, Si films containing β-FeSi 2 nanodots show about two times lower thermal conductivity than Si films containing α-FeSi 2 nanodots. This is because the widely-size-distributed β-FeSi 2 nanodots can effectively work as phonon scattering centers due to hierarchical architectures. These films also exhibited a high power factor due to the small amount of point defects and single crystalline epitaxial interfaces, regardless of the iron silicide phase of nanodots. These detailed investigations will open a road for realizing high-performance thin film thermoelectric materials.