Combinatorial Sputtering of 2D Layered ScTaN2 Thin Films

Ternary nitrides, composed of two metal cations and a nitrogen anion, exhibit unique electronic and thermal properties that make them suitable for a wide range of applications, including ceramic coatings, semiconductors, permanent magnets, spintronics, topological materials, and superconductors. Among these, ABN₂ nitrides, where A and B are metals, form layered structures analogous to layered oxides and are promising candidates for thermoelectric materials due to their two-dimensional electronic characteristics induced by the inherent nanolaminate character [1]. Despite their potential, the thin film synthesis of ternary layered nitrides, such as ScTaN₂, remains challenging due to the tendency of physical vapor deposition techniques to produce cation-disordered metastable phases [2].<br/>In this study, we investigate the Sc-Ta-N system, specifically focusing on ScTaN₂, which has already been synthesized in bulk but never as a thin film. We employ a high-throughput combinatorial sputtering approach to create composition-graded libraries of Sc_xTa_1-xN, followed by rapid thermal annealing (RTA) to induce the transition from cation-disordered to ordered layered phases. Our results demonstrate that an annealing temperature of 1200°C is necessary to initiate the nucleation of the desired layered structure (s.g. <i>P63/mmc</i>). We further explore the stability of these layered motifs under off-stoichiometric conditions, highlighting unexpected stabilization of the layered phase for Ta-rich compounds, and revealing the formation of the layered nitride Ta₅N₆(s.g. <i>P63/mcm</i>), structurally analogous to ScTaN₂. The tie line between the two compounds and the ability of Ta to accommodate multiple coordination environment shows that it is possible to gradually substitute Sc by Ta in the octahedral layers, inducing site-specific disorder, and keeping the inherent layered structure symmetry.<br/>X-ray diffraction, including synchrotron grazing-incidence wide angle scattering (GIWAXS), is utilized to characterize the crystal structures and identify various phases. Complementary oxidation state characterizations and crystal field analyses provide insights into the stabilization mechanisms of Sc-Ta-N layered compounds. Our findings contribute to the understanding of kinetic pathways for synthesizing new layered ternary nitrides by controlling cation ordering and therefore tuning properties, advancing their potential for energy harvesting applications.<br/> <br/> <br/>[1] R. Pilemalm, L. Pourovskii, I. Mosyagin, S. Simak, and P. Eklund, <i>Thermodynamic Stability, Thermoelectric, Elastic and Electronic Structure Properties of ScMN2-Type (M = V, Nb, Ta) Phases Studied by Ab Initio Calculations</i>, Condens. Matter <b>4</b>, 36 (2019).<br/>[2] A. Zakutayev, S. R. Bauers, and S. Lany, <i>Experimental Synthesis of Theoretically Predicted Multivalent Ternary Nitride Materials</i>, Chem. Mater. <b>34</b>, 1418 (2022).