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At low temperatures, in reasonably pure conductors subjected to a thermal gradient, charge carriers (electrons and holes) are swept (dragged) by out of equilibrium phonons, giving rise to a large contribution to the Seebeck coefficient called phonon drag. We demonstrate a spectacular influence of substrate phonons on charge carriers in thin films of Bi2Te3. We show that one can control and tune the position and magnitude of the phonon-drag peak over a wide range of temperatures by depositing thin films on substrates with vastly different Debye temperatures. Our experiments also provide a way to study the nature of the phonon spectrum in thin films, which is rarely probed but clearly important for a complete understanding of thin film properties and the interplay of the substrate and films.

a b s t r a c tWe report on the growth of high-quality topological insulator BiSbTe 3 films (thickness of 30 nm) on sapphire (0001) substrates through the molecular beam epitaxy (MBE) technique and discuss the possibility to improve the film quality and surface flatness through annealing. Instead of using elemental Sb, the choice of Sb 2 Te 3 as well as the use of solid sources of Bi and Te assures excellent stability during thermal evaporation enabling layer-by-layer epitaxial growth of high-quality films of BiSbTe 3 . The crystallinity and the terrace size of the BiSbTe 3 films are found to be improved through increasing the deposition temperature and/or after annealing at 540 K-620 K for 1-4 hours. The films grown at 485 K, 500 K, and 515 K exhibit root-mean-square (RMS) roughness of 2.9 nm, 2.3 nm, and 2.3 nm, respectively, whereas the RMS roughness is reduced to 0.6 nm or less when the films are annealed at 580 K for 2-4 hours. Annealing the film at too high temperature, such as 620 K, introduces a rougher surface due to the loss of material during annealing. A relatively low electron density of$ 2.2 Â 10 18 cm -3 (at 2 K) is achieved for the as-grown films deposited at 485 K and 500 K. Significantly enhanced electron density is found in the case of either increasing growth temperature or increasing annealing temperature. An exception is the film annealed at 620 K, which became a p-type conductor. In addition, weak antilocalization effects are evident for the n-type films, but they nearly vanish for p-type conducting films. The significant influence of temperature on the crystallinity, the surface roughness, and the electronic transport in BiSbTe 3 films will be instructive for further investigations of the transport behavior of surface states in BiSbTe 3 films.

In this research, we report the enhanced thermoelectric power factor in topologically insulating thin films of Bi0.64Sb1.36Te3 with a thickness of 6–200 nm. Measurements of scanning tunneling spectroscopy and electronic transport show that the Fermi level lies close to the valence band edge, and that the topological surface state (TSS) is electron dominated. We find that the Seebeck coefficient of the 6 nm and 15 nm thick films is dominated by the valence band, while the TSS chiefly contributes to the electrical conductivity. In contrast, the electronic transport of the reference 200 nm thick film behaves similar to bulk thermoelectric materials with low carrier concentration, implying the effect of the TSS on the electronic transport is merely prominent in the thin region. The conductivity of the 6 nm and 15 nm thick film is obviously higher than that in the 200 nm thick film owing to the highly mobile TSS conduction channel. As a consequence of the enhanced electrical conductivity and the suppressed bipolar effect in transport properties for the 6 nm thick film, an impressive power factor of about 2.0 mW m−1 K−2 is achieved at room temperature for this film. Further investigations of the electronic transport properties of TSS and interactions between TSS and the bulk band might result in a further improved thermoelectric power factor in topologically insulating Bi0.64Sb1.36Te3 thin films.