Unravelling oxygen driven α to β phase transformation in tungsten

Abstract

Thin films of β-W are the most interesting for manipulating magnetic moments using spin–orbit torques, and a clear understanding of α to β phase transition in W by doping impurity, especially oxygen, is needed. Here we present a combined experimental and theoretical study using grazing incidence X-ray diffraction, photoelectron spectroscopy, electron microscopy, and ab initio calculations to explore atomic structure, bonding, and oxygen content for understanding the formation of β-W. It is found that the W films on SiO2/Si have 13–22 at.% oxygen in A15 β structure. Ab initio calculations show higher solution energy of oxygen in β-W, and a tendency to transform locally from α to β phase with increasing oxygen concentration. X-ray absorption spectroscopy also revealed local geometry of oxygen in β-W, in agreement with the simulated one. These results offer an opportunity for a fundamental understanding of the structural transition in α-W and further development of β-W phase for device applications.

Optimization of Tungsten β -Phase Window for Spin-Orbit-Torque Magnetic Random-Access Memory

“…Thin films of W can exist in two crystalline phases [32,34,36,37,44,45]: an α phase (α-W), which has a low resistivity (5-20 μ cm) and low ξ AD J (note that α-W is widely used in industry for contact metallization), and a β phase (β-W), with a larger resistivity (100-300 μ cm) and, importantly, a very large ξ AD J . Therefore, to check for the presence of the β phase in different samples, we measure the sheet resistance by a four-point-probe technique as a function of the thickness t SOT of the W, W(N), W(O), and W(O, N) samples.…”

Oblique angle co-deposition of nanocolumnar tungsten thin films with two W sources: Effect of pressure and target current

“…The lattices parameters are equal to a = 0.316 nm and a = 0.504 nm for α and β phases, respectively. The β-W phase is stabilized by the presence of oxygen acting as impurities, but without forming a tungsten oxide compound [42][43][46][47]. Even if -W is often described as an ordered A-15 unit cell with a W3O stoichiometry, the tungsten thin films designated as -W do not present an ordered structure and the oxygen concentration in the films is often below 15 at.% but does not reach the 25 at.%, corresponding to a W3O stoichiometry [42][43]47].…”

“…The β-W phase is stabilized by the presence of oxygen acting as impurities, but without forming a tungsten oxide compound [42][43][46][47]. Even if -W is often described as an ordered A-15 unit cell with a W3O stoichiometry, the tungsten thin films designated as -W do not present an ordered structure and the oxygen concentration in the films is often below 15 at.% but does not reach the 25 at.%, corresponding to a W3O stoichiometry [42][43]47]. The stabilization of β-W has also been attributed to interstitial incorporation of argon or nitrogen impurities as well as to the stacking fault W3W in the A15 structure [48][49].…”

Elucidating the role of disorder introduced by nitrogen in the superconducting properties of tungsten thin films

“…Tungsten characterizes to present two different phases: alpha (α)-W (body-centered cubic structure) and beta (β)-W (A15 structure) 1 . The latest is metastable 2 , and it is usually stabilized in sputtered films by modifying the deposition parameters 3 or adding nitrogen 4,5 or oxygen impurities in a reactive atmosphere 6,7,8,9 . The electrical resistivity of β-W is much higher than the α-W 1 .…”