Crucial role of oxygen on the bulk and surface electronic properties of stable β phase of tungsten

The A15 β phase of tungsten has recently attracted great interest for spintronic applications due to the finding of giant spin-Hall effect. As β phase is stabilized by oxygen, we have studied the electronic structure of O-doped β-W from first principles calculations. It is found that 20 at.% O-doping makes β phase lower in energy than α-W. These results are in good agreement with energy dispersive X-ray spectroscopy which also shows ~ 16.84 at.% O in 60 nm thick W films. The latter has predominantly β phase as confirmed by grazing incidence X-ray diffraction (XRD). The simulated XRD of bulk β having 15.79 at.% O also agrees with XRD results. Oxygen binds strongly on the surface and affects the Dirac fermion behavior in pure β-W. There is structural disorder, O-inhomogeneity, and higher density-of-states in O-doped β-W at EF compared with pure α. These results are promising to understand the properties of β-W.

www.nature.com/scientificreports/ with ~ 5.33 μΩ-cm for α-W at room temperature need a proper understanding of the crucial role of O in its stability as well as atomic and electronic structure. Moreover, Derunova et al. 27 have shown a giant spin-Hall effect in a group of A15 materials and emphasized the location of E F in the energy bands to be very important.
In this direction the doping of Ta has been shown to even further increase SHA [27][28][29] . Here we study the effects of oxygen on the electronic structure of β-W.

Results and discussion
Atomic structure and oxygen composition from experiments and calculations. The GIXRD patterns of 35 nm (film A) and 60 nm (film B) thick W samples in the 2 θ range of 20º to 80º are exhibited in Fig. 1 along with the calculated powder diffraction of a 2 × 2 × 2 supercell of β-W doped with 15.79 at.% O. Here θ is the Bragg angle. The atomic structure of O doped β-W was simulated using ab initio molecular dynamics (MD) at a high temperature of 3500 K in order to overcome barriers for O diffusion. The peak centred around 40º for film B belongs to the reflection from either (210) β-W or (110) α-W or both the phases (JCPDS #03-065-6453 for β-W, #04-0806 for α-W). The other two characteristic reflections from the (200) and (211) planes of the β phase are also identified in film B along with small peaks around 70º. Importantly, the same diffraction pattern was obtained almost a year ago suggesting the stability of the β-W phase. The diffraction pattern of the simulated O-doped β-W resembles the one obtained from GIXRD experiments on film B. The optimized O-doped β-W supercell (inset, Fig. 1) shows a slight deviation from a cube. There is a small increase in the lattice parameters to 10.32 Å, 10.36 Å, and 10.41 Å compared with the calculated value of 5.059 Å for the unit cell of pure β-W. There is some disorder in the structure and clustering of O atoms. In our earlier work 18  Our calculated diffraction peaks are slightly shifted to lower 2 θ values compared with the experimental results. This is due to the slight overestimation of the calculated lattice parameters of pure α (3.171 Å) and β (5.059 Å) phases compared with the experimental values of 3.16 Å and 5.04 Å, respectively. Another possible reason could be the variation in the O concentration in the sample. In general, an increase in O concentration leads to an increase in the lattice parameters. Our EDX experiments suggest that oxygen concentration of about 15 at.% on an average is good to achieve a stabilized polycrystalline β-W film as discussed below. With this O concentration, all Figure 1. X-ray diffraction and atomic structure. X-ray diffraction pattern of film B (60 nm thick) with characteristic peaks of the β phase and possible presence of α phase due to the overlapping (110) peak. The calculated powder X-ray diffraction of 15.79 at. % O doped β-W obtained from ab-initio MD simulations reproduces the features quite well. The corresponding atomic structure is shown in the inset. The green and red balls are W and O atoms, respectively. The measured diffraction pattern of film A (35 nm thick) is shown in the inset. There is a broad peak signifying the presence of disorder in the structure. The peak marked with a grey box is the reflection from the Si substrate. www.nature.com/scientificreports/ the W atoms in a sample may not be in the β phase due to O clustering as discussed above. Therefore, our results suggest the possibility of the co-existence of α phase with low O concentration together with β-W with much higher O concentration. A coexistence of α and β phases was also reported by Petroff et al. 9 using transmission electron microscopy (TEM). We also performed simulated annealing calculations with ab initio MD on a 2 × 2 × 2 supercell of β-W and 4 × 4 × 4 supercell of α-W with the same O concentration. It is found that the doping of 20 at.% O makes the optimized β-W phase lower in energy compared with α-W. Therefore, a transition to β phase must occur by this O concentration. This is in good agreement with our experimental results. We further performed EDX measurements in cross-sectional TEM (XTEM) mode parallel to the film surface to understand the distribution of the constituent elements.  Table S1 in Supplementary Information.

Electronic density of states of bulk phases with O doping.
The calculated DOS of β-W with 15.79 at.% O is shown in Fig. 3. There is a broad peak at ~ 2.65 eV with a shoulder at ~ 1.5 eV below E F . There is also a weak peak at ~ 5 eV below E F . The angular momentum as well as site decomposed partial DOS (PDOS) show that   Fig. 3c]. This is interesting for the understanding of higher T c in β-W films within the BCS formalism but other factors such as the electron-phonon interaction, the microstructure and defects need also to be considered. Figure 3a shows the DOS for β-W with ~ 15.79 at.% O while Fig. 3d shows it for a simulated bulk α-W with ~ 12.3 at.% O. Interestingly, both show broadly similar features, although the atomic structures are different. The inset of Fig. 3d also shows the structure. It is found that even in the simulated and optimized α phase with lower O concentration, there is a clustering of oxygen due to attractive interaction as it could be expected. Both the O doped phases show a bonding peak due to oxygen at ~ 7 eV below E F . Furthermore, our calculations for O-doped bulk β-W also give a peak (Fig. 3) in the region of 8.6-9.4 eV below E F , while for the O-doped bulk α-phase the peak extends to slightly higher binding energies. These peaks are also identified to arise from the hybridization of O and W valence orbitals. The inclusion of spin-orbit coupling (SOC) does not change the DOS significantly except for a small decrease at E F .  www.nature.com/scientificreports/ of surface W atoms. The DOS of the slab shows a broad peak in the W 5d band region. Also, there is a peak at around 7 eV below E F in the DOS of both α and β slabs due to O adsorption as shown in Fig. 4a and b, respectively. It is attributed to O bonding interaction with the slab as it was also found in the case of the O doped bulk β-W [ Fig. 3a] and α-W [ Fig. 3d]. However, in contrast to the O-doped bulk phases, the DOS is small at around 9 eV below E F . These results are not affected much by including SOC. Therefore, the peak at ~ 7 eV below E F can be considered to have the joint contribution of W 5d and O 2p states from bulk and surface of α and β phases, but the peak around 9 eV below E F will have contributions mainly from bulk α and β phases. These results may vary to some extent depending upon the presence of defects, different surfaces, and/or interfaces such as between α and β phases in actual samples which are not considered in our calculations. The adsorption energy of an O atom, defined as the gain in energy when a free O atom adsorbs on the surface is 7.50 eV and 7.95 eV, respectively, for a fourfold site of α-W and a threefold site of β-W on a (001) surface. When 9 (16) O atoms are adsorbed on the (001) surface of α-W (β-W) in the supercell, the adsorption energy becomes 6.76 eV (6.74 eV) per O atom. These values are comparable to 6.55 eV and 6.79 eV reported 32 for (1 × 1) and (2 × 1) structures of O on W(110) surface. It is found that there is a decrease in the adsorption energy of an O atom with increasing coverage. However, the values of the adsorption energy are much higher than the energy gain of 4.031 eV (4.478 eV) per O atom for 12.3 at.% (15.79 at.%) O in a supercell of bulk α-W (β-W). Therefore, O would be there on the surface of both α and β phases, as evidenced from our experiments (see Fig. 2). Further calculations for an O atom in the sub-surface region give only a small (0.223 eV) decrease for α-W but a significantly lower value for β-W. Accordingly, O is likely to be present in sub-surface sites also for α-W, but may go to the bulk region in β-W.  www.nature.com/scientificreports/ DOS are also shown in Fig. S4. These results show O induced peaks in the energy region of − 7.5 eV to − 10 eV and no peak at around − 7 eV. Therefore the latter peak in α-and β-W with higher concentration of O can be associated to arise from clustering of O atoms. There is increase in the DOS at E F with O-doping as we also discussed earlier.

Effects of O doping
The band structures for the undoped β-W without and with SOC are shown in Fig. 5a and b, respectively. One can see multiple Direc points and nodal lines near E F and this agrees well with the published results 13 . Inclusion of SOC lifts the degeneracy of the bands and opens up small band gaps [ Fig. 5b] at some places where the bands cross. Doping of O clearly leads to significant changes in the band structure as it can be seen in Fig. 5c and d for the doping of 1 O and 2 O in the unit cell of β-W, respectively. It has been further found that inclusion of SOC leads to splitting of the bands as shown in Fig. S5 in the Supplementary Information. In order to further compare the results, we have chosen nearly equivalent directions in the Brillouin zones of pure and doped W (see Fig. S6 in Supplementary Information) and the corresponding bands are shown in Fig. 6. Here, the energy bands for the pure β case in Γ-X-M directions [ Fig. 6a] having multiple Dirac points near E F , get affected when one O is doped [ Fig. 6b], but the Dirac point-like feature can still be seen along the Y 2 -C 2 direction. The bands in the Γ-Y 2 direction change significantly compared with the Γ-X 1 direction for the pure case. Further, with the doping of 2 O atoms, the bands have changed more significantly as shown in Fig. 6c. When SOC is included, there is opening of a band gap at the band crossings as shown in Fig. S7. These will contribute to SHA even in the case of O-doped β-W.

Bonding nature of O and Bader charge analysis.
We further explored the nature of bonding by performing Bader charge analysis. It is found that there is ~ 1.6 e excess charge on O atoms predominantly due to charge transfer from the nearest neighbour W atoms. A large fraction of W atoms has charge in the range of 5.5-6.0 e and only a few have charge in the range of 5.0-5.5 e. This suggests that the charge transfer from W to O is much smaller than in WO 3 . The hybridization of partially filled 5d orbitals of W with the 2p orbitals of O gives rise to an increase in DOS near E F and below ~ − 6 eV (Fig. 3), while a decrease in the remaining d-band region of W. The disorder in the structure due to the doping of O also leads to broadening of peaks in the DOS. The isosurfaces of the charge density and electron localization function (ELF) for β-W doped with 15.79 at.% O are shown in Fig. S8. One can see localization of charge around O ions. There is a characteristic broad peak at around 2.6 eV in the W 5d band region of β-W in contrast to peaks at ~ 2 and ~3 eV in pure α-W. There is an increase in DOS at E F in the O-doped β phase with respect to pure α-W. This result is interesting to understand the higher T c in β-W within the BCS formalism. Oxygen is found to interact strongly on W surfaces and therefore it supports the observed increase in O concentration in the surface region. These results would be interesting for further experimental and theoretical studies of the surfaces and interfaces of β-W films and their frontier applications. The presence of O induced disorder in the structure as well as its inhomegenous distribution will contribute to the higher resistivity of β-W as observed compared with a pure BCC phase due to increased scattering of electrons. But real samples are polycrystalline with small grain size and a proper understanding will need such consideration including the role of defects and interfaces. We believe that our combined experimental and theoretical study will help in further understanding of the technologically interesting β phase of W.

Methods
Sample preparation and measurements. A 500 µm thick Si(100) wafer was diced into pieces (area 1 × 1 cm 2 ) and cleaned by dipping in trichloroethylene, acetone, isopropanol, and deionized water followed by heating for 2 min in each step. After cleaning of the Si surface, W films of about 35 nm (film A) and 60 nm (film B) thicknesses were grown at room temperature (RT) by electron beam deposition technique. The deposition rate was 0.01 nm/s, while the chamber base pressure and the working pressures were ~ 6.65 × 10 -7 mbar and 1.33 × 10 -6 mbar, respectively. We used 99.95% pure W powder (Alfa Aesar) for depositing films whose thickness was estimated by a surface profilometer (DektakXT, Bruker). The β-W phase was confirmed by GIXRD measurements (Bruker, D8-Discover) using the Cu-K α radiation (λ = 0.154 nm). The elemental analysis has been performed by EDX with a 2 nm spot size in the cross-sectional geometry of TEM. The mapping has been taken parallel to the film surface. We used a C s corrected TEM system from Tecnai-FEI operated with an acceleration voltage of 200 kV. A Pt layer was deposited on the W film to avoid any damage during the XTEM sample preparation by focused ion beam. The full range of the EDX hypermap has 172 (69) values in a column for each element in every raster scan for sample B (A).
Calculations on bulk and surfaces. The calculations have been performed within the framework of DFT as implemented in the Vienna Ab initio Simulation Package (VASP) 33 . We used Perdew-Burke-Ernzerhof (PBE) form of the generalized gradient approximation (GGA) 34 for the exchange-correlation functional and projected www.nature.com/scientificreports/ augmented wave (PAW) pseudopotentials 35 for electron-ion interaction. The kinetic energy cut-off for the plane wave expansion of the wave function was set to 500 eV. The atomic structures in all cases were completely relaxed until the absolute value of each component of the Hellmann-Feynman force on each ion became less than 0.005 eV/Å. We also performed the volume relaxation. A grid of 15 × 15 × 15 (9 × 9 × 9) Monkhorst-Pack k-points was used for the unit cell calculations of the α (β) phase, whereas 5 × 5 × 5 k-points were used for the 4 × 4 × 4 (2 × 2 × 2) supercell calculations for structural relaxation and the convergence of the charge density. The lattice parameters of the optimized unit cells of 1) α-W (bcc, Im-3 m) with 2 atoms per unit cell and 2) β-W (A15, Pm-3n) containing 8 atoms per unit cell, are found to be 3.171 Å and 5.056 Å, respectively. These are in good agreement with the previously reported values of 3.17 Å and 5.05 Å as well as our experimental results 1,2,4,18 .
For the 2 × 2 × 2 supercell of β-W phase (64 atoms) the optimized lattice parameter was found to be 10.110 Å. Oxygen with 15.79 at.% concentration was initially distributed in β-W supercell keeping the O-O separation of about 2.04 Å as obtained for the case of two O atoms. This oxygen concentration lies in the range of 13-19% obtained from the EDX measurement 18 on sample B. Subsequently we performed ab initio MD simulations to explore energetically the most favorable configuration using the simulated annealing method. For this, the system was heated to 3500 K and equilibrated for 3 ps. Then the system was cooled from 3500K to room temperature continuously by simulating for 3.5 ps. In these finite temperature calculations, we used only the Γ point to represent the Brillouin zone. After cooling to room temperature, we optimized the structure by relaxing the ions as well as the cell parameters using again 5 × 5 × 5 k-points mesh with high precision. Similar calculations were performed for other oxygen concentrations as well as for α-W. The (001) surface of β-W was modelled by a slab of 3 unit-cell thickness (108 W atoms) with about 15 Å vacuum space. We considered 2 × 2 (3 × 3) supercell in the plane of the slab for β-W (α-W with 99 atoms) and a 5 × 5 × 1 k-points mesh to perform Brillouin zone integrations. The ions were relaxed keeping the cell dimensions fixed. Further we studied adsorption of O atoms at fourfold (threefold) site on the (001) surface of α (β) as well as on a sub-surface site in order to compare the behavior with bulk. Bader charge analysis has been conducted to check the valence electrons on W and O atoms.