The research on thermoelectrics experienced a renaissance due to the theoretical predictions made by Hicks and Dresselhaus that thermoelectric efficiency could be greatly enhanced through nanostructural engineering. The nanoalloying method has proved to be a promising approach to synthesize binary and nanostructured thin films. The films were analyzed in a combined approach utilizing the FhIPM expertise in thermoelectrical measurement techniques with that of the groups from the University of Kiel for structural analysis and nanoscale characterization of structure and composition via advanced electron microscopy techniques. It is believed that the high degree of thermal stability is caused by the epitaxial growth of the materials for which interdiffusion of the binary constituents is suppressed by a strong degree of c-orientation and the absence of grain boundaries. The feasibility of highly stable, low periodic Bi2Te3/Bi2(Se,Te)3 SLs by molecular beam epitaxy (MBE) has already been demonstrated.

Roughly a decade ago an outstanding thermoelectric figure of merit ZT of 2.4 was reported for nanostructured Bi2Te3/Sb2Te3-based thin film superlattice (SL) structures. The published results strongly fueled and renewed the interest in the development of efficient novel nanostructured thermoelectric materials. This review article shall give an overview over the most recent theoretical and experimental advances on Bi2Te3/Sb2Te3 SLs and related superlattice systems. The presented theoretical models are subdivided into electronic and phononic aspects. The experimental results are summarized with regard to the method used. A more detailed elaboration on structural and transport properties is given in the subsequent sections.

Bi2Te3, (Bi1−xSbx )2Te3 and layered Bi2Te3/(Bi1−xSbx)2Te3 superlattices fabricated by nanoalloying. Our approach is based on the sequential sputtering of nanoscale layers of the elements and subsequent annealing in order to induce a solid state reaction. While conventionally Bi2(SexTe1−x )3 compounds are used as n-type V2VI3 material system, the deposition of Se proves to be problematic especially for sputtering deposition and is therefore replaced by (Bi1−xSbx )2Te3. A superlattice consisting of 25 nm Bi2Te3/25 nm (Bi0:9Sb0:1)2Te3 – ML (periodicity of 50 nm) was synthesized and annealed at temperatures of 150, 200, 225, and 250°C. The layers are slightly rough and polycrystalline, and the grain sizes increase with increasing annealing temperature. The XRD analysis shows a pronounced (00l) texture of the sputtered layers. SIMS depth profiles reveal that the chemical separation into layers is present, yet smeared out to some degree after annealing at 200°C. High Seebeck coefficients of up to ~−190 μV/K were achieved. A high maximum power factor of 22 μW/cmK2 can be attained after annealing at 250 °C for 12 h. The superlattice system Bi2Te3 / (Bi1−xSbx )2Te3 can compete with Bi2Te3 / Bi2(SexTe1−x )3 in terms of electrical properties while representing a good practical alternative for the sputter deposition due to the substitution of problematic Se with Sb. Cross-plane thermal conductivities are in the range of 0.55 to 0.6 W/mK. The thermal conductivity is generally reduced due to the nanocrystallinity of the material, however, there seems to be no measurable reduction of the thermal conductivity by the superlattice-type 2D nanostructuring.

Sb2Te3 and Bi2Te3 thin films were grown on SiO2 and BaF2 substrates at room temperature using molecular beam epitaxy. Metallic layers with thicknesses of 0.2 nm were alternately deposited at room temperature, and the films were subsequently annealed at 250°C for 2 h. x-Ray diffraction and energy-filtered transmission electron microscopy (TEM) combined with high-accuracy energy-dispersive x-ray spectrometry revealed stoichiometric films, grain sizes of less than 500 nm, and a texture. High-quality in-plane thermoelectric properties were obtained for Sb2Te3 films at room temperature, i.e., low charge carrier density (2.6 × 1019 cm−3), large thermopower (130 μV K−1), large charge carrier mobility (402 cm2 V−1 s−1), and resulting large power factor (29 μW cm−1 K−2). Bi2Te3 films also showed low charge carrier density (2.7 × 1019 cm−3), moderate thermopower (−153 μV K−1), but very low charge carrier mobility (80 cm2 V−1 s−1), yielding low power factor (8 μW cm−1 K−2). The low mobilities were attributed to Bi-rich grain boundary phases identified by analytical energy-filtered TEM.

The effect of dimensionality and nanostructure on thermoelectric properties in Bi2Te3-based nanomaterials is summarized. Stoichiometric, single-crystalline Bi2Te3 nanowires were prepared by potential-pulsed electrochemical deposition in a nanostructured Al2O3 matrix, yielding transport in the basal plane. Polycrystalline, textured Sb2Te3 and Bi2Te3 thin films were grown at room temperature using molecular beam epitaxy and subsequently annealed at 250°C. Sb2Te3 films revealed low charge carrier density of 2.6 × 1019 cm−3, large thermopower of 130 μV K−1, and large charge carrier mobility of 402 cm2 V−1 s−1. Bi2(Te0.91Se0.09)3 and (Bi0.26Sb0.74)2Te3 nanostructured bulk samples were prepared from as-cast materials by ball milling and subsequent spark plasma sintering, yielding grain sizes of 50 nm and thermal diffusivities reduced by 60%. Structure, chemical composition, as well as electronic and phononic excitations were investigated by x-ray and electron diffraction, nuclear resonance scattering, and analytical energy-filtered transmission electron microscopy. Ab initio calculations yielded point defect energies, excitation spectra, and band structure. Mechanisms limiting the thermoelectric figure of merit ZT for Bi2Te3 nanomaterials are discussed.

Energy-filtered transmission electron microscopy (EFTEM) yields new possibilities for the investigation of Bi2Te3 based nanomaterials. Combined low-loss electron energy-loss spectroscopy (EELS) and energy-dispersive x-ray microanalysis (EDS) and energy-filtered TEM were applied on a Zeiss 912Ω TEM to investigate nanowires, thin films, and bulk materials. Multilayered Bi-Sb-Te nanowires with a diameter of 65 nm and a period of 200 nm and stoichiometric Bi2Te3 nanowires were grown by potential-pulsed electrochemical deposition. Tellurium elemental maps of the multilayered nanowires were obtained by two-window edge-jump ratio images (EJI). EDS chemical analysis showed that small Te fluctuations of 3 at.% yielded significant contrast in EJI. Energy-filtered TEM applied on nano-alloyed Bi2Te3 thin films grown by molecular beam epitaxy (MBE) revealed 10-20 nm thick Bi-rich blocking layers at grain boundaries. Plasmon spectroscopy by EELS was applied on Bi2(Te0.91Se0.09)3 bulk and yielded a plasmon energy of 16.9 eV. Finally, plasmon dispersion was measured for Bi2(Te0.91Se0.09)3 bulk by angle-resolved EELS, which yields a fingerprint of the anisotropy and the dimensionality of the electronic structure of the materials.

In this work, Bi2Te3-Sb2Te3 superlattices were prepared by the nanoalloying approach. Very thin layers of Bi, Sb, and Te were deposited on cold substrates, rebuilding the crystal structure of V2VI3 compounds. Nanoalloyed super- lattices consisting of alternating Bi2Te3 and Sb2Te3 layers were grown with a thickness of 9 nm for the individual layers. The as-grown layers were annealed under different conditions to optimize the thermoelectric parameters. The obtained layers were investigated in their as-grown and annealed states using x-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive x-ray (EDX) spectroscopy, transmission electron microscopy (TEM), and electrical measurements. A lower limit of the elemental layer thickness was found to have c-orientation. Pure nanoalloyed Sb2Te3 layers were p-type as expected; however, it was impossible to synthesize p-type Bi2Te3 layers. Hence the Bi2Te3-Sb2Te3 superlattices consisting of alternating n- and p-type layers showed poor thermoelectric properties.

Nano-alloyed p -type Sb 2 Te 3 and n -type Bi 2 Te 3 thin films were grown on SiO 2 /Si and BaF 2 substrates by molecular beam epitaxy (MBE) in two steps: (i) Repeated deposition of five-layer stacks with sequence Te-X-Te-X-Te (X = Sb or Bi) with elemental layer thicknesses of 0.2 nm on substrates at room temperature, (ii) annealing at 250 °C for two hours at which phase formation of Sb 2 Te 3 or Bi 2 Te 3 occurred. The room temperature MBE deposition method reduces surface roughness, allows the use of non lattice-matched substrates, and yields a more accurate and easier control of the Te content compared to Bi 2 Te 3 thin films, which were epitaxially grown on BaF 2 substrates at 290 °C. X-ray diffraction revealed that the thin films were single phase, poly-crystalline, and textured. The films showed grain sizes of 500 nm for Sb 2 Te 3 and 250 nm for Bi 2 Te 3 , analyzed by transmission electron microscopy (TEM). The in-plane transport properties (thermopower S , electrical conductivity σ, charge carrier density n , charge carrier mobility μ, power factor S 2 σ) were measured at room temperature. The nano-alloyed Sb 2 Te 3 thin film revealed a remarkably high power factor of 29 μW cm -1 K -2 similar to epitaxially grown Bi 2 Te 3 thin films and Sb 2 Te 3 single crystalline bulk materials. This large power factor can be attributed to a high charge carrier mobility of 402 cm 2 V −1 s -1 similar to high- ZT Bi 2 Te 3 /Sb 2 Te 3 superlattices. However, for the nano-alloyed Bi 2 Te 3 thin film a low power factor of 8 μW cm −1 K -2 and a low charge carrier mobility of 80 cm 2 V −1 s −1 were found. Detailed microstructure and phase analyses were carried out by energy-filtered TEM in cross-sections. Quantitative chemical analysis by energy-dispersive x−ray spectroscopy (EDS) was also applied. In Bi 2 Te 3 thin films, few nanometer thick Bi-rich blocking layers at grain boundaries and Te fluctuations by 1.3 at.% within the grains were observed. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing.

Nano-alloyed p-type Sb2Te3 and n-type Bi2Te3 thin films were grown on SiO2/Si and BaF2 substrates by molecular beam epitaxy (MBE) in two steps: (i) Repeated deposition of five-layer stacks with sequence Te-X-Te-X-Te (X = Sb or Bi) with elemental layer thicknesses of 0.2 nm on substrates at room temperature, (ii) annealing at 250 °C for two hours at which phase formation of Sb2Te3 or Bi2Te3 occurred. The room temperature MBE deposition method reduces surface roughness, allows the use of non lattice-matched substrates, and yields a more accurate and easier control of the Te content compared to Bi2Te3 thin films, which were epitaxially grown on BaF2 substrates at 290 °C. X-ray diffraction revealed that the thin films were single phase, poly-crystalline, and textured. The films showed grain sizes of 500 nm for Sb2Te3 and 250 nm for Bi2Te3, analyzed by transmission electron microscopy (TEM). The in-plane transport properties (thermopower S, electrical conductivity σ, charge carrier density n, charge carrier mobility μ, power factor S2σ) were measured at room temperature. The nano-alloyed Sb2Te3 thin film revealed a remarkably high power factor of 29 μW cm-1 K-2 similar to epitaxially grown Bi2Te3 thin films and Sb2Te3 single crystalline bulk materials. This large power factor can be attributed to a high charge carrier mobility of 402 cm2 V−1 s-1 similar to high-ZT Bi2Te3/Sb2Te3 superlattices. However, for the nano-alloyed Bi2Te3 thin film a low power factor of 8 μW cm−1 K-2 and a low charge carrier mobility of 80 cm2 V−1 s−1 were found. Detailed microstructure and phase analyses were carried out by energy-filtered TEM in cross-sections. Quantitative chemical analysis by energy-dispersive x−ray spectroscopy (EDS) was also applied. In Bi2Te3 thin films, few nanometer thick Bi-rich blocking layers at grain boundaries and Te fluctuations by 1.3 at.% within the grains were observed. The small charge carrier densities are explained by a reduced antisite defect density due to the low temperatures to which the thin films were exposed during annealing.

In this work, p-type nanoscale “soft superlattices” consisting of multilayer stacks of 25 nm Sb2Te3 on 25 nm (Bi0.2Sb0.8)2Te3 were fabricated by nanoalloying. With this technique, nanoscale layers of the elements Bi, Sb, and Te are deposited by sputtering onto a Si/SiO2 substrate and subsequently annealed to induce interdiffusion and a solid-state reaction to form the final superlattices. Different combinations of annealing temperatures were used in the annealing process. The in-plane electronic properties (Seebeck coefficient, electrical conductivity, charge carrier concentration, and carrier mobility) of these soft superlattices were examined. The cross-plane thermal conductivity was determined using time-domain thermal reflectance (TDTR). Secondary-ion mass spectrometry (SIMS) depth profiles reveal that the nanostructured thin films exhibit high stability against thermal interdiffusion during the annealing process. X-ray patterns of the samples display very strong texture with preferred c-orientation of the crystallites after the heat treatment. Scanning electron microscopy (SEM) cross-section images of the films show distinctly polycrystalline structure with increasing grain size for higher annealing temperatures, as confirmed by x-ray diffraction (XRD) analysis. Very high power factors exceeding 40 μW/cm K2, similar to values for bulk single crystals with comparable compositions, are observed for the soft superlattices. The nanostructure appears to be stable up to 300°C. For a sample annealed at 150°C, a thermal conductivity as low as 0.45 W/mK was determined. Based on different assumptions concerning the degree of anisotropy of the transport properties, a cross-plane figure of merit ZT of 0.6 to 1.9 can be estimated for the thin films annealed at 300°C.

A tool has been developed at Fraunhofer-IPM to calculate the transport properties of thermoelectric material by using its band structure described in terms of effective masses and the location of the ellipsoids in reciprocal space. The calculated transport properties are compared with experimental data measured on bismuth telluride, antimony telluride, and bismuth antimony telluride. Polycrystalline specimens have been prepared by spark plasma sintering (Fraunhofer-IFAM). Electron backscattering diffraction analysis of sample cross-sections yields the frequency distribution of grain orientations. This texture information permits the generation of appropriate finite-element models of the polycrystalline microstructure (TU Dresden). By means of the commercial code COMSOL, which allows anisotropic thermoelectric properties to be taken into account, the effective electrical and thermal conductivities as well as the Seebeck coefficient both parallel and perpendicular to the pressing direction have been calculated.

The 3 omega method is the best established method for measuring the thermal properties of thin films (>100 nm) and nanowires. Theoretically, the method could be applied to many more types of samples, leading to new knowledge, but to date little effort has been made to extend its applicability. An enabling set of technologies has been tested at the Fraunhofer-IPM. The technologies developed encompass a new design of microheater, the measurement of bulk samples with a prefabricated microheater on adhesive tapes and polymer sheets, the measurement of tiny bulk samples glued to the underside of polymer sheets, a fully automated experimental setup, and a new numerical tool adapted to the new type of heater. The new design of microheater and software were validated using float glass as a reference material. A microheater on adhesive tape was used to measure accurately the thermal properties of sintered thermoelectric materials. The thermal conductivity of a very small melt-spun nanocomposite sample glued to the underside of a Kapton™ sheet was measured. The potential of the new design of microheater to measure very thin (~nm) films is discussed.

In this paper the evolution of thermoelectric material parameters of V-VI and V-VI/IV_VI compounds like thermal and electrical conductivity, Seebeck coefficient and the dimensionless figure of merit (ZT) is discussed in dependence on the fabrication and doping of the base material as well as on the treatment during and after preparation using melt spinning and sparc plasma sintering (SPS). This material is used as the basis for the fabrication of thermoelectric modules to be integrated into power supplies for the operation of wireless sensors. They employ particularly tailored dc-dc-converter topologies which are configured to cope with extremely low input voltage sources.

Thermoelectric compounds based on doped bismuth telluride and its alloys have recently attracted increasing interest. Due to their structural features they show increased values of the thermoelectric figure of merit (ZT). A promising approach to improve the thermoelectric properties is to manufacture nanocomposite materials exhibiting lower thermal conductivities and higher ZT. The ZT value of compounds can be shifted reasonably to higher values (>1) by alloying with IV-Te materials and adequate preparation methods to form stable nanocomposites. The influence of PbTe and Sn on the thermoelectric properties is studied as a function of concentration and preparation methods. Melt spinning and spark plasma sintering were applied to form nanocomposite materials that were mechanically and thermodynamically stable for applications in thermoelectric devices. The structural properties are discussed based on analysis by transmission electron microscopy and x-ray diffraction.

A survey of state of the art of the development of high temperature materials is presented and will be discussed in comparison to the situation in the 1990th. An attempt will be made to assess the state of the art of the materials thermoelectric properties, their technical level, and possible potential for standardized device technology. Also a first assessment based on current commodity prices for some important thermoelectric compounds will be made. As a roundup advantages and drawbacks for some classical and upcoming compounds will be given. The main challenges, which will have to be overcome to finally enable thermoelectric power generation as a recycling technology of “nomadic” energy, will be summarized. As a result, thermoelectrics should play an important role in the field of green energies.

Nanometerscale textured layers were achieved by annealing corresponding elemental layers, deposited in nanometerscale thickness which fits to the intended stoichiometry of the resulting compounds.The compound formation as well as the accompanying thermoelectric properties are reported.The compound formation corresponds in particular to the evolution of the Seebeck coefficient.

Bismuth telluride samples are compared with respect to the evolution of their thermoelectric material parameters like thermal and electrical conductivity. The Seebeck coefficient is discussed in dependence on the melt spinning fabrication technique. The melt spinner used is only able to produce small thin ribbon shaped specimens, some as thin as 10 mu m. This limits melt spinning to mainly production of research specimens for alloys with high critical cooling rate, which are difficult to fabricate with other techniques. Additional parameters are alloying or doping of the base material by comparing the properties as prepared to different annealing conditions. The intrinsic p- and n-doped material was alloyed with up to 0.5% lead telluride by rapidly cooling the bulk material to improve the thermoelectric properties analysed from RT up to about 600 K. A Seebeck coefficient of well above 200 mu V/K could be obtained for p- and n-type materials. (C) 2007 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim.

For room temperature applications, normally V-VI-compounds with ZT values around 1 are used in the field of bulk thermoelectrics. In the case of thin film materials, several challenges arise from the complex crystal structure of these compounds due to strongly anisotropic thermoelectric transport parameters. This limits, or at least complicates the use of V-VI based thin films. Therefore, materials with a comparably simple crystal structure and isotropic transport properties are desirable. These needs are fulfilled in the case of IV-VI-compounds. Unfortunately, those materials suffer from their relatively low ZT of /spl sim/0.2 around room temperature. Here, we report on structural and in particular thermoelectric properties of molecular beam epitaxy grown PbSe and PbTe thin films alloyed with tin. It was found that increasing the tin concentration without changing the other growth parameters results in increased charge carrier concentrations and thermopower values. Special care was taken to evaluate the in-plane thermal conductivity of insulated free standing Pb/sub 1-x/Sn/sub x/Te and Pb/sub 1-x/Sn/sub x/Se thin films. As expected, also the lattice thermal conductivity decreases due to alloy scattering. Increasing the tin concentration is known to result in decreasing bandgaps. By this, the optimum operating temperature is shifted towards ambient temperature. All those effects strongly enhance the thermoelectric properties of Pb/sub 1-x/Sn/sub x/Te and Pb/sub 1-x/Sn/sub x/Se in the room temperature region. For tin contents of x/spl sim/0.08, ZT>0.6 were determined in both cases. Even higher ZT values should be achievable using e.g. adequate doping levels and/or quaternary Pb/sub 1-x/Sn/sub x/Te/sub 1-y/Se/sub y/ compounds.