Enhanced Thermoelectric Performance in Sputtered Epitaxial Fe₂VAl Thin Films via Crystal Orientation Selection

Heusler intermetallic compounds present a wide variety of compositions that fulfill the requirement of avoiding toxic or scarce elements. The Fe₂VAl family of Heusler alloys shows a large potential and versatility for thermoelectric thin films with tunable properties via doping with additional elements such as Ti, Ta, Si, W, and others [1-3]. The doping allows the fabrication of p- and n-type alloys in the same family, which simplifies future thermoelectric device fabrication. While the undoped alloy Fe₂VAl is a p-type material, the introduction of W, Si, or Ta results in a n-type one. P-type alloys are obtained with Ti and Zr, both with an increase in Seebeck values compared to the undoped case. The addition of these materials can also be performed in an off-stoichiometric manner [4,5].<br/>Sputter deposition is a well-established, inexpensive, and very versatile technique for the fabrication of a wide variety of material systems. The chemical composition can be highly controlled by the simultaneous use of several magnetrons with different elements (codeposition) to achieve the desired film content or doping concentration, which is needed to optimize the thermoelectric properties [6].<br/>From a fundamental point of view, the possibility to explore the influence of crystal orientation on thermoelectric performance is very appealing, offering a perfect scope for experiment versus theory comparison. In this sense, physical vapor deposition techniques are excellent tools for obtaining thin films with desired orientations through the use of adequate substrates in terms of crystal symmetry and lattice match. In this contribution, we present results on Fe₂VAl thin films fabricated by magnetron dc sputtering. Two different sample series have been prepared with varying deposition temperatures (RT-950°C) and with two different substrates and, consequently, two crystalline orientations owing to the different epitaxial relations. Films grown on MgO (100) are (100)-oriented, while Al₂O₃ (11-20) substrates induce a (110) orientation. In both cases, the (111) diffraction peak Fe₂VAl corresponding to the L21 fully ordered phase can be observed in asymmetric configuration diffraction measurements.<br/>The results of the characterization of the thin films with X-Ray Diffraction and SEM imaging are shown, proving epitaxial growth for both series. The electric conductivity, Seebeck parameter, and power factor are reported. We observe a large difference in the thermoelectric performance depending on the growth orientation of the films.<br/>With these results, we prove the large potential of sputtering to produce high-quality Fe₂VAl films ready for further desired compositional variation and tuned doping via codeposition.<br/><br/><i>References</i><br/><br/>[1] Nishino, Y. IOP Conference Series: Materials Science and Engineering, IOP Publishing, 142001 (2011)<br/>[2] K. Renard, et al., Journal of Applied Physics, <b>115</b> (3), 033707 (2014).<br/>[3] B. Hinterleitner, <i>et al.</i>, Nature, <b>576</b> (7785), 85-90 (2019).<br/>[4] B. Hinterleitner, <i>et al</i>., Physical Review B <b>102</b> (7), 075117 (2020).<br/>[5] M. Mikami, et al., Journal of Applied Physics <b>111</b> (9), 093710 (2012).<br/>[6] A. Conca, <i>et al</i>., ACS Applied Electronic Materials (2023), DOI: https://doi.org/10.1021/acsaelm.2c01772<br/><br/><i>Acknowledgments</i><br/>Financial support by the Ministerio de Ciencia e Innovación with the ThermHeus (TED2021-131746B-I00) project and the ERC Advanced Grant POWERbyU (ERC-2021-ADG-101052603) is acknowledged. We acknowledge the service from the MiNa Laboratory at IMN, and funding from CM (project S2018/NMT-4291 TEC2SPACE), MINECO (project CSIC13-4E-1794) and EU (FEDER, FSE).