Intrinsically large effective mass and multi-valley band characteristics of n-type Bi2Bi2Te3 superlattice-like films

Thermoelectric superlattices are expected to decouple the strong correlation between various thermoelectric parameters, and are an important strategy for excellent thermoelectric performances. The superlattices of (Bi2)m(Bi2Te3)n homologous series are well-known for low lattice thermal conductivity and intriguing topological surface states. However, the impacts of electronic structure on the thermoelectric performance were still not well-understood in (Bi2)m(Bi2Te3)n. To cope with this issue, Bi2Bi2Te3 superlattice-like films with adjustable Bi2/(Bi2+Bi2Te3) molar ratio (R) were successfully fabricated by the molecular beam epitaxy technique. Angle-resolved photoemission spectroscopy measurements combined with theoretical calculations revealed the conduction band evolution from single-valley to multi-valley as R ≥ 0.30, leading to intrinsically high carrier effective mass and improved thermoelectric power factor. Also, the superlattice film (R = 0.46) with the structure close to Bi4Te3 possesses the topological surface state feature around the high symmetry point. As a result of the high effective mass of 3.9 m0 and very high electron density of 2.31 × 1021 cm−3, the film with R = 0.46 acquired the highest power factor of 1.49 mW·m−1·K−2 at 420 K, outperforming that of other (Bi2)m(Bi2Te3)n superlattices. This work lays an essential foundation on understanding the electronic structure and further improving thermoelectric performances of (Bi2)m(Bi2Te3)n homologous series.