Insights into the nucleation, grain growth and phase transformation behaviours of sputtered metastable β-W films

Metastable phase in tungsten film is of great interests in recent years due to its giant spin Hall effects, however, little information has been known on its nucleation, growth and phase transformation. In this paper, a 900 nm-thick tungsten film with double-layer structure (α-W underlayer and β-W above it) was produced on SiO2/Si substrate by high vacuum magnetron sputtering at room temperature. The structural properties of β-W were systemically investigated by X-ray diffraction, transmission electron microscopy, thermodynamic calculation, first-principle and phase-field simulations. It is found that the β-W nucleation is energetically favoured on the SiO2 surface compared to the α-W one. As the film thickening proceeds, β-W[211] turns to be preferred direction of growth owing to the elastic strain energy minimization, which is verified by phase-field simulations. Moreover, the β → α phase transformation takes place near the film-substrate interface while the rest of the film keeps the β-W phase, leading to a double-layer structure. This localized phase transition is induced by lower Gibbs free energy of α-W phase at larger grain sizes, which can be confirmed by thermodynamic calculation. Further in-situ heating TEM analysis of the as-deposited film reveals that the β→α phase transformation is fulfilled by α/β interface propagation rather than local atomic rearrangements. Our findings offer valuable insights into the intrinsic properties of metastable phase in tungsten.