Impact of the Topological Surface State on the Thermoelectric Transport in Sb₂Te₃ Thin Films

Abstract

Ab initio electronic structure calculations based on density functional theory and tight-binding methods for the thermoelectric properties of ptype Sb 2 Te 3 films are presented. The thicknessdependent electrical conductivity and the thermopower are computed in the diffusive limit of transport based on the Boltzmann equation. Contributions of the bulk and the surface to the transport coefficients are separated which enables to identify a clear impact of the topological surface state on the thermoelectric properties. By tuning the charge carrier concentration, a crossover between a surface-state-dominant and a FuchsSondheimer transport regime is achieved. The calculations are corroborated by thermoelectric transport measurements on Sb 2 Te 3 films grown by atomic layer deposition.Almost all proposed three-dimensional (3D) Z 2 topological insulators (TIs) 1,2 are efficient thermoelectric materials. That is not by coincidence, since the link between an efficient thermoelectric * To whom correspondence should be addressed † Institute of Physics, Martin Luther University HalleWittenberg, D-06099 Halle, Germany ‡ Institute of Nanostructure and Solid State Physics, Universität Hamburg, Jungiusstrasse 11, D-20355 Hamburg, Germany ¶ Max Planck Institute of Microstructure Physics, Weinberg 2, D-06120 Halle, Germany material and the topological character is the inverted band gap. 3,4 The last is due to spin-orbit coupling which switches parity of the bands and leads, if strong enough, to narrow band gaps which are favourable for efficient room-temperature thermoelectrics. Usually strong spin-orbit coupling is mediated by heavy elements which in turn also tend to reduce the material's lattice thermal conductivity, another requirement for desirable thermoelectrics.In the early 90's, Hicks and Dresselhaus 5,6 proposed the concept of low-dimensionality to increase further the thermoelectric efficiency; primarily in thin films, the thermopower S should be enlarged. However, in contrast to previous theoretical model calculations, 7,8 decreased values of S were found experimentally 9-11 for Bi 2 Te 3 and Sb 2 Te 3 thin films and were recently corroborated by both model 12,13 and ab initio calculations 4 of our groups.Thus, to resolve this discrepancy, the potential impact of the surface state (SS) of TIs on thermoelectricity needs to be investigated in more detail. In this Letter, we present ab initio calculations and transport measurements of the thermoelectric properties of Sb 2 Te 3 films at varying thickness, temperature and charge carrier concentration. (1/ cm)(g) Theoretical resultsIn the following, we discuss the doping-and temperature-dependent electrical conductivity and thermopower, as shown in Fig. 1, exemplary for a Sb 2 Te 3 film thickness of 18 quintuple-layer (QL), i.e. about 18 nm. A discussion of films with other thicknesses is given in the supplemental material. 14 The converged electronic structure results serve as input to obtain the thermoelectric transport properties at temperature T and fixed extri...

Robustness of a Topologically Protected Surface State in a Sb2Te2Se Single Crystal

“…To consider a two-band model of a TI, the total thermopower and electrical conductivity come from the surface state and the bulk carriers. The total electrical conductivity is expressible as σ total  =  σ bulk  +  σ surface ; the total thermopower is expressed as S total  = ( σ bulk S bulk  +  σ surface S surface )/ σ total 26. In a thin-film system, the contribution of electrical conductivity from bulk bands can be decreased on controlling the thickness of the TI.…”

“…In early work, Hicks and Dresselhaus proposed that low-dimensional systems could enhance the thermoelectric efficiency through an enlarged thermopower22, but a decreased thermopower value was observed in Sb 2 Te 3 thin films; the discrepancy was resolved in recent work on Sb 2 Te 3 thin films232425. The separate contributions to thermopower from the bulk and the surface could cause a decreased total thermopower26. A surface state existing in the energy gap with a small concentration of bulk carriers could benefit an enhanced thermopower.…”

Tuning thermoelectricity in a Bi2Se3topological insulator via varied film thickness

“…Instead, it exhibits a peculiar sign anomaly with the Hall effect induced by the drastically different transport properties of the bulk and surface states [12]. In a more recent study of p-type Sb 2 Te 3 TI thin films [13], it was found that tuning the E F position and film thickness has a significant impact on the total thermopower, also demonstrating the effect of topological surface states on the thermoelectric properties of TI.…”

Thermoelectric properties of 3D topological insulator: Direct observation of topological surface and its gap opened states

“…The observed TE properties of pure m-TSDS allows us to discuss the accuracy of theoretical predictions. TE properties of 3D-TIs are mainly discussed in the framework of a semi-classical Boltzmann transport equation (SBTE) [19][20][21][22][23][24][25] . For the conventional materials, the physical parameters required for comparing the experimental data to the theoretical calculations are the chemical potential μ and relaxation time τ.…”