In this work, a zT value as high as 1.2 at room temperature for n‐type Ag2Se films is reported grown by pulsed hybrid reactive magnetron sputtering (PHRMS). PHRMS is a novel technique developed in the lab that allows to grow film of selenides with different compositions in a few minutes with great quality. The improved zT value reported for room temperature results from the combination of the high power factors, similar to the best values reported for bulk Ag2Se (2440 ± 192 µW m−1 K−2), along with a reduced thermoelectric conductivity as low as 0.64 ± 0.1 W m−1 K−1. The maximum power factor for these films is of 4655 ± 407 µW m−1 K−2 at 103 °C. This material shows promise to work for room temperature applications. Obtaining high zT or, in other words, high power factor and low thermal conductivity values close to room temperature for thin films is of high importance to develop a new generation of wearable devices based on thermoelectric heat recovery.

In this work, we measure the thermal and thermoelectric properties of large-area Si0.8Ge0.2 nano-meshed films fabricated by DC sputtering of Si0.8Ge0.2 on highly ordered porous alumina matrices. The Si0.8Ge0.2 film replicated the porous alumina structure resulting in nano-meshed films. Very good control of the nanomesh geometrical features (pore diameter, pitch, neck) was achieved through the alumina template, with pore diameters ranging from 294 ± 5nm down to 31 ± 4 nm. The method we developed is able to provide large areas of nano-meshes in a simple and reproducible way, being easily scalable for industrial applications. Most importantly, the thermal conductivity of the films was reduced as the diameter of the porous became smaller to values that varied from κ = 1.54 ± 0.27 W K−1m−1, down to the ultra-low κ = 0.55 ± 0.10 W K−1m−1 value. The latter is well below the amorphous limit, while the Seebeck coefficient and electrical conductivity of the material were retained. These properties, together with our large area fabrication approach, can provide an important route towards achieving high conversion efficiency, large area, and high scalable thermoelectric materials.

Scandium nitride (ScN) is an emerging indirect bandgap rocksalt semiconductor that has attracted significant attention in recent years for its potential applications in thermoelectric energy conversion devices, as a semiconducting component in epitaxial metal/semiconductor superlattices and as a substrate material for high quality GaN growth. Due to the presence of oxygen impurities and native defects such as nitrogen vacancies, sputter-deposited ScN thin-films are highly degenerate n-type semiconductors with carrier concentrations in the (1–6) × 1020 cm−3 range. In this letter, we show that magnesium nitride (MgxNy) acts as an efficient hole dopant in ScN and reduces the n-type carrier concentration, turning ScN into a p-type semiconductor at high doping levels. Employing a combination of high-resolution X-ray diffraction, transmission electron microscopy, and room temperature optical and temperature dependent electrical measurements, we demonstrate that p-type Sc1-xMgxN thin-film alloys (a) are substitutional solid solutions without MgxNy precipitation, phase segregation, or secondary phase formation within the studied compositional region, (b) exhibit a maximum hole-concentration of 2.2 × 1020 cm−3 and a hole mobility of 21 cm2/Vs, (c) do not show any defect states inside the direct gap of ScN, thus retaining their basic electronic structure, and (d) exhibit alloy scattering dominating hole conduction at high temperatures. These results demonstrate MgxNy doped p-type ScN and compare well with our previous reports on p-type ScN with manganese nitride (MnxNy) doping.

Si x Ge1-x alloys are well-known thermoelectric materials with a high figure of merit at high temperatures. In this work, metal-induced crystallization (MIC) has been used to grow Si0.8Ge0.2 films that present improved thermoelectric performance (zT = 5.6 × 10(-4) at room temperature)--according to previously reported values on films--with a relatively large power factor (σ · S (2) = 16 μW · m(-1) · K(-2)). More importantly, a reduction in the thermal conductivity at room temperature (κ = 1.13 ± 0.12 W · m(-1) · K(-1)) compared to other Si-Ge films (∼3 W · m(-1) · K(-1)) has been found. Whereas the usual crystallization of amorphous SiGe (a-SiGe) is achieved at high temperatures and for long times, which triggers dopant loss, MIC reduces the crystallization temperature and the heating time. The associated dopant loss is thus avoided, resulting in a nanostructuration of the film. Using this method, we obtained Si0.8Ge0.2 films (grown by DC plasma sputtering) with appropriate compositional and structural properties. Different thermal treatments were tested in situ (by heating the sample inside the deposition chamber) and ex situ (annealed in an external furnace with controlled conditions). From the studies of the films by: x-ray diffraction (XRD), synchrotron radiation grazing incidence x-ray diffraction (SR-GIXRD), micro Raman, scanning electron microscopy (SEM), x-ray photoemission spectroscopy (XPS), Hall effect, Seebeck coefficient, electrical and thermal conductivity measurements, we observed that the in situ films at 500 °C presented the best zT values with no gold contamination.

to its figure of merit, zT, defined as zT = (S 2 •σ)•κ −1 •T, an ideal thermoelectric material must exhibit high electrical conductivity (σ), high Seebeck coefficient (S), and low thermal conductivity (κ), simultaneously, to maximize its efficiency for the desired temperature range. In this context, there has been a significant increase of reports in the literature on Cu 2−x Se as a p-type material with high power factor (PF) [3] (being the PF = S 2 • σ). [1,3,4] Therefore, copper selenides have become a hot topic in the TE field, with reported figures of merit as high as zT ≈ 1.6 @ 727 °C. [5] Moreover, Cu 2−x Se has a crystallographic phase transition at T ≈ 130 °C, and it has been shown that around this transition temperature zT can reach values as high as 2.3. [6] Thermoelectric thin films occupy an industrial niche for microfabricated multielement planar devices on flexible substrates as low-current voltage generators for room temperature (RT) applications. In this range of temperatures, the highest zT reported value for bulk crystalline material is 0.28 (Liu et al. [5]). Cu 2−x Se films are typically p-type, highly conducting, semitransparent, and with a bandgap varying between 1.1 and 1.4 eV. Numerous methods have been reported for the deposition of Cu 2−x Se films at low substrate temperatures, such as a chemical bath deposition, [7-9] galvanic synthesis, [10] solution growth, [11] hydrothermal method, [12] or electrochemical deposition. [10,13] Other methods, such as adsorption/diffusion (selenization), [14-16] SILAR method, [17] and pulsed laser deposition [18,19] require high-temperature post growth treatments to improve and stabilize the thermoelectric properties. In any case, those different manufacturing film methods have not been able to surpass the thermoelectric efficiencies at room temperature of the Cu 2−x Se bulk samples prepared by solid-state reaction. [20,21] In the case of bulk samples other methods, such as spark plasma sintering, [4,5,22-28] ball milling followed by hot pressing, [24,29] and quenched bulk [30] have also been reported. In all these cases, high temperatures and long manufacturing times (even weeks) are necessary. In this work, we have developed a fabrication approach namely pulsed hybrid reactive magnetron sputtering (PHRMS) based on reactive sputtering, a vacuum technique that is widely used in industry as particularly suitable for thin film devices

Scandium Nitride (ScN) is an emerging rocksalt semiconductor with octahedral coordination, and an indirect bandgap. ScN has attracted significant attention in recent years for its potential thermoelectric applications, as a component material in epitaxial metal/semiconductor superlattices, and as a substrate for defect-free GaN growth. Sputter-deposited ScN thin films are highly degenerate n-type semiconductors, and exhibit a large thermoelectric power factor of ~3.5 10-3 W/m-K 2 at 600K-800K. Since practical thermoelectric devices require both n-and p-type materials with high thermoelectric figures-of-merit, development and demonstration of highly efficient p-type ScN is extremely important. Recently, the authors have demonstrated p-type Sc 1x Mg x N thin film alloys with low Mg x N y mole-fractions within the ScN matrix. In this article, we demonstrate temperature dependent thermal and thermoelectric transport properties, including large thermoelectric power factors in both n-and p-type Sc 1-x Mg x N thin film alloys at high temperatures (up to 850K). Employing a combination of temperature dependent Seebeck coefficient, electrical conductivity and thermal conductivity measurements, as well as detailed Boltzmann transport based modeling analyses of the transport properties, we demonstrate that ptype Sc 1-x Mg x N thin film alloys exhibit a maximum thermoelectric power factor of ~0.8 10-3 W/m-K 2 at 850K. The thermoelectric properties are tunable by adjusting the Mg x N y molefraction inside the ScN matrix, thereby shifting the Fermi energy in the alloy films from inside the conduction band in case of undoped n-type ScN to inside the valence band in highly holedoped p-type Sc 1-x Mg x N thin film alloys. The thermal conductivities of both the n-and p-type films were found to be undesirably large for thermoelectric applications. Thus, future work should address strategies to reduce the thermal conductivity of Sc 1-x Mg x N thin-film alloys, without affecting the power factor for improved thermoelectric performance.

Silicon and germanium present distinct and interesting transport properties. However, composites made of silicon-germanium (SiGe) have resulted in a breakthrough in terms of their transport properties. Currently, these alloys are used in different applications, such as microelectronic devices and integrated circuits, photovoltaic cells, and thermoelectric applications. With respect to thermoelectricity, in the last decades, Si 0.8 Ge 0.2 has attracted significant attention as an energy harvesting material, for powering space applications and other industrial applications. This chapter focuses on the recent advances and new approaches in silicon-germanium (Si 1−x Ge x) nanostructures for thermoelectric devices with high thermoelectric efficiency obtained through magnetron sputtering.

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