Crystal growth and flat-band effects on thermoelectric properties of Fe2TiAl-based full-Heusler thin films

In this study, various Fe-based thermoelectric full-Heusler thin films were fabricated on MgO substrates by a post-annealing process. It is clarified that crystal growth through the post-annealing process is prevented by both an initial crystallization and a lattice mismatch between the thin films and the substrate. One of the thermoelectric materials, namely, Fe2TiAl, was almost epitaxially grown on the substrate from an initial amorphous state owing to a small mismatch of less than 3%. The thermoelectric properties of Fe2TiAl-based thin films were modulated by changing the material composition. We found that they strongly depend on not only the valence electron concentration and the Fe amount as was observed in typical thermoelectric full-Heusler, Fe2VAl, but also the electronic band structures. The flat band in the conduction band strongly affects not only the n-type thermoelectric performance but also the p-type ones; the large density of states in the conduction band prevents the p-type Seebeck coefficient from increasing. The Seebeck coefficient of a V-added Fe2TiAl thin film with a composition of Fe2.01Ti0.56V0.67Al0.76 was increased to 99 µV/K by controlling the flat band in the conduction band away from the valence band to reduce the effects of the flat band, resulting in a dimensionless figure of merit of 0.12 at room temperature.