ABSTRACT
The A15 ß phase of tungsten has recently attracted great interest for spintronic applications due to the finding of giant spin-Hall effect. As ß phase is stabilized by oxygen, we have studied the electronic structure of O-doped ß-W from first principles calculations. It is found that 20 at.% O-doping makes ß 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 ß phase as confirmed by grazing incidence X-ray diffraction (XRD). The simulated XRD of bulk ß having 15.79 at.% O also agrees with XRD results. Oxygen binds strongly on the surface and affects the Dirac fermion behavior in pure ß-W. There is structural disorder, O-inhomogeneity, and higher density-of-states in O-doped ß-W at EF compared with pure α. These results are promising to understand the properties of ß-W.
ABSTRACT
Thin films of ß-W are the most interesting for manipulating magnetic moments using spin-orbit torques, and a clear understanding of α to ß 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 ß-W. It is found that the W films on SiO2/Si have 13-22 at.% oxygen in A15 ß structure. Ab initio calculations show higher solution energy of oxygen in ß-W, and a tendency to transform locally from α to ß phase with increasing oxygen concentration. X-ray absorption spectroscopy also revealed local geometry of oxygen in ß-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 ß-W phase for device applications.
