On the basis of detailed first-principles calculations the anisotropic thermoelectric transport properties of biaxially strained silicon were studied with the focus on a possible enhancement of the power factor. Electron as well as hole doping was examined in a broad doping and temperature range. In the low temperature and low doping regime an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case, and for compressive strain in the hole-doped case. In the thermoelectrically more important high temperature and high doping regime a slight enhancement of the power factor was only found for the hole-doped case under small biaxial tensile strain. The results are discussed in terms of band structure effects. An analytical model is presented to understand the fact that the thermopower decreases if degenerate bands are energetically lifted due to a strain-induced redistribution of states.

The anisotropic thermoelectric transport properties of bulk silicon strained in the [111]-direction were studied by detailed first-principles calculations focusing on a possible enhancement of the power factor. Electron and hole doping were examined in a broad doping and temperature range. At low temperature and low doping an enhancement of the power factor was obtained for compressive and tensile strain in the electron-doped case and for compressive strain in the hole-doped case. For the thermoelectrically more important high-temperature and high-doping regime a slight enhancement of the power factor was only found under small compressive strain with the power factor overall being robust against applied strain. To extend our findings the anisotropic thermoelectric transport of a [111]-oriented Si/Ge superlattice was investigated. Here, the cross-plane power factor under hole doping was drastically suppressed due to quantum-well effects, while under electron doping an enhanced power factor was found. For this, we state figures of merit of ZT = 0.2 and 1.4 at T = 300 and 900 K for the electron-doped [111]-oriented Si/Ge superlattice. All results are discussed in terms of band structure features.

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...