Pulsed Hybrid Reactive Magnetron Sputtering for High zT Cu₂Se Thermoelectric Films

Abstract

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

High Thermoelectric zT in n‐Type Silver Selenide films at Room Temperature

“…Ag 2+ x Se films have been prepared by a reactive sputtering method that gives rise to highly crystalline films of controlled stoichiometry. We have previously reported the development of the PHRMS system . This method includes a fine control on the amount of selenium present in the alloy, a fast growth rate, accurate control over the stoichiometry, it takes place at low temperatures and it is easily scalable for industrial production.…”

“…In particular, this is due to the low value of its thermal conductivity, 0.34 W m −1 K −1 , which derives from the scattering of phonons by atomic dislocations, nanocrystalline grain boundaries, and nanopores . Recently, we have reported values of 0.4 at room temperature for thin films of Cu 2 Se grown through a novel technique developed called pulsed hybrid reactive magnetron sputtering (PHRMS) …”

“…Moreover, some of these fabrication methods present further problems as far as the homogeneity of the samples is concerned, making it difficult to establish a correlation between carrier concentration and the Ag/Se ratio. In this work, we present a study on the thermoelectric performance of Ag 2+ x Se thin films that have been fabricated by a novel system, PHRMS, which provides an accurate control over the stoichiometry of the films. It is, also, a low temperaturature process, so the problems with the silver mobility at high temperatures are avoided.…”

Flexible Thermoelectric Materials and Generators: Challenges and Innovations

“…As shown in Figure b, zT as high as 0.21 was achieved at 300 K with an optimal n . Based on the reactive‐sputtering method, pulsed hybrid reactive magnetron sputtering (PHRMS) has also been developed to further accurately control the stoichiometry of films. Basically, PHRMS combines direct reactive sputtering with periodic pulse‐controlled sputtering.…”

Advances in Scanning Thermal Microscopy Measurements for Thin Films

“…This experimental setup was successfully implemented to obtain thermal images of different thermoelectric films and to determine their thermal conductivity. For instance, in the work of Perez-Taborda et al, a novel fabrication method to obtain thin films Cu 2 Se with high control over the stoichiometry was achieved [40]. In this case, the thermal conductivity at room temperature under ambient conditions obtained via SThM measurements, arouse a value as low as 0.8 AE 0.1 W/mÁK, which results in a TE figure of merit of 0.4.…”