Deposition pressure-controlled phase tailoring and stability of β-W for spintronic applications

Abstract

Understanding the nucleation and growth of tungsten (W) is technologically important in spin-to-charge interconversion for realizing energy-efficient spintronic devices. Here, we have systematically investigated the effect of Ar deposition pressure (PAr) on the nucleation and growth of W. The observed surface topography as a function of PAr reveals a microstructural transition from zone T to zone 1 in the structure zone model. The physical origin for the increasing roughness as a function of PAr correlates with the surface diffusion of adatoms and growth kinetics in the Volmer–Weber growth mechanism. Grazing incidence x-ray diffraction (GIXRD) results show that W exhibits a structural phase transition from a mixed phase of (α+β)-W to a single phase of β-W as a function of PAr. The analysis of the electron diffraction patterns obtained from the films grown on amorphous-SiNx windows also supports these observations. The observed transition is fundamentally correlated with the growth kinetics in zone T and zone I. Thickness-dependent GIXRD results qualitatively prove that the film grown in zone T exhibits compressive strain, whereas that grown in zone I exhibits only tensile strain. The critical thickness for the phase transition is strongly attributed to the strain during nucleation and growth. The increasing resistivity as a function of PAr corroborates the change in structural phases. Thickness-dependent resistivity measurements correlate with the degree of crystallinity via relative intensity observed from the GIXRD results. Our results strongly suggest that W structural phases can be deterministically controlled via PAr for developing low-power spintronic devices.