Fabrication of 5-20 nm thick <i>β</i>-W films

Narasimham, Avyaya J.; Medikonda, Manasa; Matsubayashi, Akitomo; Khare, Prasanna; Chong, Hyuncher; Matyi, R. J.; Diebold, Alain C.; LaBella, V. P.

doi:10.1063/1.4903165

Cited by 22 publications

(9 citation statements)

References 20 publications

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“…Our materials' characterization shows that the incorporation of oxygen stabilizes the β−W. This is consistent with previous studies, which have shown that the formation of β−W is extremely sensitive to the oxygen incorporation during the growth process 37 38 39 40 41 , and an oxygen concentration of only 10% is sufficient to stabilize β−W. In our experiments, at n =12.1%, we obtain a SHA of , which is higher than any previous measurements in conventional metal-based systems.…”

Section: Discussionsupporting

confidence: 92%

Enhanced spin–orbit torques by oxygen incorporation in tungsten films

et al. 2016

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The origin of spin–orbit torques, which are generated by the conversion of charge-to-spin currents in non-magnetic materials, is of considerable debate. One of the most interesting materials is tungsten, for which large spin–orbit torques have been found in thin films that are stabilized in the A15 (β-phase) structure. Here we report large spin Hall angles of up to approximately –0.5 by incorporating oxygen into tungsten. While the incorporation of oxygen into the tungsten films leads to significant changes in their microstructure and electrical resistivity, the large spin Hall angles measured are found to be remarkably insensitive to the oxygen-doping level (12–44%). The invariance of the spin Hall angle for higher oxygen concentrations with the bulk properties of the films suggests that the spin–orbit torques in this system may originate dominantly from the interface rather than from the interior of the films.

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Section: Discussionsupporting

confidence: 92%

Enhanced spin–orbit torques by oxygen incorporation in tungsten films

et al. 2016

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“…2(a) (purple curve), strongly limiting the use of seed and adhesion materials. However, there are reports showing that the presence of oxygen or nitrogen adsorbed onto the surface provides nucleation sites for the growth of metastable β-W [37][38][39].…”

Section: Extension Of Tungsten β Phase Windowmentioning

confidence: 99%

“…Such thin layers impose severe constraints on the choice of thickness and on the etching conditions, possibly leading to a strong distribution of the SOT resistance and efficiency from device to device. We report in this paper the possibility of optimizing the growth of β-W using dopants such as nitrogen and oxygen [37][38][39]. We develop a process in which growing a bilayer of oxygen-and nitrogendoped tungsten, W(O, N), results in process control of the tungsten β phase to more than 10 nm, while concurrently improving the PMA as well as improving the SOT efficiency to −44.4%.…”

Section: Introductionmentioning

confidence: 99%

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

et al. 2021

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Switching induced by spin-orbit torque (SOT) is being vigorously explored, as it allows the control of magnetization using an in-plane current, which enables a three-terminal magnetic-tunnel-junction geometry with isolated read and write paths. This significantly improves the device endurance and the read stability, and allows reliable subnanosecond switching. Tungsten in the β phase, β-W, has the largest reported antidamping SOT charge-to-spin conversion ratio (θ AD ≈ −60%) for heavy metals. However, β-W has a limitation when one is aiming for reliable technology integration: the β phase is limited to a thickness of a few nanometers and enters the α phase above 4 nm in our samples when industryrelevant deposition tools are used. Here, we report our approach to extending the range of β-W, while simultaneously improving the SOT efficiency by introducing N and O doping of W. Resistivity and XRD measurements confirm the extension of the β phase from 4 nm to more than 10 nm, and transport characterization shows an effective SOT efficiency larger than −44.4% (reaching approximately −60% for the bulk contribution). In addition, we demonstrate the possibility of controlling and enhancing the perpendicular magnetic anisotropy of a storage layer (Co-Fe-B). Further, we integrate the optimized W(O, N) into SOT magnetic random-access memory (SOT-MRAM) devices and project that, for the same thickness of SOT material, the switching current decreases by 25% in optimized W(O, N) compared with our standard W. Our results open the path to using and further optimizing W for integration of SOT-MRAM technology.

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“…In many of the prior studies of W deposition, the formation of -W phase was achieved by deliberately introducing oxygen into the chamber (see, for example, Ref. 35) when was the desired phase, or was the unwanted consequence of the presence of residual oxygen impurities in the chamber when ↵ was the desired phase (see, for example, Ref. 6).…”

Section: ( C/min)mentioning

confidence: 99%

Transformation of topologically close-packed β-W to body-centered cubic α-W: Comparison of experiments and computations

Barmak

Liu

Harlan

et al. 2017

The Journal of Chemical Physics

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The enthalpy and activation energy for the transformation of the metastable form of tungsten, β-W, which has the topologically close-packed A15 structure (space group Pm3¯n), to equilibrium α-W, which is body-centered cubic (A2, space group Im3¯m), was measured using differential scanning calorimetry. The β-W films were 1 μm-thick and were prepared by sputter deposition in argon with a small amount of nitrogen. The transformation enthalpy was measured as -8.3 ± 0.4 kJ/mol (-86 ± 4 meV/atom) and the transformation activation energy as 2.2 ± 0.1 eV. The measured enthalpy was found to agree well with the difference in energies of α and β tungsten computed using density functional theory, which gave a value of -82 meV/atom for the transformation enthalpy. A calculated concerted transformation mechanism with a barrier of 0.4 eV/atom, in which all the atoms in an A15 unit cell transform into A2, was found to be inconsistent with the experimentally measured activation energy for any critical nucleus larger than two A2 unit cells. Larger calculations of eight A15 unit cells spontaneously relax to a mechanism in which part of the supercell first transforms from A15 to A2, creating a phase boundary, before the remaining A15 transforms into the A2 phase. Both calculations indicate that a nucleation and growth mechanism is favored over a concerted transformation. More consistent with the experimental activation energy was that of a calculated local transformation mechanism at the A15-A2 phase boundary, computed as 1.7 eV using molecular dynamics simulations. This calculated phase transformation mechanism involves collective rearrangements of W atoms in the disordered interface separating the A15 and A2 phases.

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Fabrication of 5-20 nm thick β-W films

Cited by 22 publications

References 20 publications

Enhanced spin–orbit torques by oxygen incorporation in tungsten films

Enhanced spin–orbit torques by oxygen incorporation in tungsten films

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

Transformation of topologically close-packed β-W to body-centered cubic α-W: Comparison of experiments and computations

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