Experimental and theoretical exploration of oxygen impurities in sputter grown β-Ta films

Abstract

The β phase of tantalum (β-Ta) has gained significant interest in spintronic applications due to its large spin Hall angle and spin-orbit coupling. However, the sputter-grown β-Ta films in these applications are used without having a comprehensive theoretical and experimental understanding of this phase formation and its stabilization. Here, we report the efficacy of the DC magnetron sputtering technique to prepare stable β-Ta films at room temperature by controlling various deposition parameters such as pressure, power, and time. Detailed x-ray diffraction and microstructural analyses confirm the formation of a stable tetragonal structure, while chemical and depth-dependent elemental analyses show the importance of oxygen impurity under the high vacuum condition of ∼ 2 × 10⁻⁷ Torr in achieving the β-Ta phase. This has further been evidenced by ab initio molecular dynamics simulations which suggest the β phase to become favorable with ∼13 at.% O in Ta films. In addition, we discuss the formation of the β-Ta phase instead of the A15 structure found in neighboring tungsten theoretically by performing calculations with controlled oxygen doping.