Growth-dependent structural ordering and stability in β-tungsten films for spintronic applications

Abstract

The β phase of tungsten has attracted great interest for spintronic applications due to its higher spin Hall angle compared to other elemental solids and large spin–orbit torque, but the stability of this phase is yet to be well understood as many different results are there in the literature mainly based on the film thickness, temperature, and overall growth conditions. The growth of films by sputter deposition has emerged as a promising technique to achieve β-W owing to its compatibility with current spintronic technology. We demonstrate here the efficient ability of dc magnetron sputtering to grow stable β-W films up to a thickness of ∼180 nm at room temperature by varying a set of deposition parameters like pressure, power, and deposition time and discuss the various underlying mechanisms. From these results, the optimized set of deposition parameters for growing β-W films is given. A clear understanding of the influence of oxygen in the atomic structure of β-W is obtained by varying the thickness of the films. This is confirmed from the ab initio molecular dynamics (MD) simulations, where the atomic structure is influenced by the oxygen doping concentration. A stable polycrystalline β phase can be achieved by controlled doping of oxygen. Additionally, a phase transformation from α to β with the doping of oxygen is also evident by MD simulations.