Polarity and anti-distortive polarons in WO3 through epitaxial shear strain

TL;DR

The paper tackles the challenge of realizing CMOS-compatible oxide functionalities by stabilizing a polar phase in crystalline WO3 through epitaxial shear strain on (001)-oriented YAlO3 substrates.The authors combine epitaxial thin-film growth with extensive PFM, STEM, XRD, SEM, and electrical measurements to reveal a low-symmetry triclinic P1 phase that yields four domain variants and a stripe-domain polarization pattern.A notable finding is the enhanced conductivity at neutral stripe-domain walls, accompanied by a suppression of the triclinic distortive mode, which they interpret as experimental evidence for anti-distortive polarons predicted in WO3.Overall, the work demonstrates a route to domain-wall-enabled nanoelectronics in WO3 and broadens the understanding of strain-stabilized polar phases in transition-metal oxides.

Abstract

Bestowing CMOS-compatible binary oxides with additional functionalities is a powerful strategy toward the realization of oxide electronics. Ideal candidates are thin films which display a strong sensitivity to strain, chemical doping or nanoscale confinement. Among these, crystalline tungsten trioxide WO3 exhibits exceptional structural flexibility, enabling a wide range of functionalities. Here, we reveal the emergence of a previously unreported polar phase in epitaxial WO3 thin films. We accomplish this by imposing epitaxial shear strain, which stabilizes a low-symmetry triclinic structure that persists up to large film thicknesses and elevated temperatures. At the atomic scale, a change in the oxygen octahedral tilt pattern facilitates this symmetry lowering into a polar phase, which manifests as a periodic in-plane polarized stripe domain configuration with needle-like bifurcations at the microscale. The stripe domain walls further exhibit a strongly enhanced electrical conductivity in conjunction with a pronounced reduction of a distortive structural mode, providing the first experimental evidence for the formation of anti-distortive polarons recently predicted in WO3.

Figures (19)

• Figure 1: Angle-resolved lateral piezoresponse force microscopy (PFM) of a WO3 film on a (001)-oriented YAlO3 substrate at sample rotations of (a,b) 0 and (d,e) 90 with respect to the reference axes displayed on the top, showing lateral PFM (a,d) amplitude and (b,d) phase. The inset in (d) shows a vertical PFM amplitude image with a stripe-like buckling contrast corresponding to the domain configuration seen in (a), obtained via a 90° rotation of the fast scanning axis. c) Sample topography. f) Schematic illustration of the polarization vector. The polarization is predominantly oriented along the stripe domains seen in (a,b) with a small component in the orthogonal direction (d,e).
• Figure 2: Structural characterization of WO3 films on (001)-oriented YAO. a,b) Projections of reciprocal lattice planes of increasing order of a 157WO3 film along the a) [010]YAO and the [100]YAO directions. Each image represents the a) (2+n 1+n 4) or b) (1+n 2-n 4) WO3 reflections, using pseudo-orthorhombic indices. The vertical and horizontal directions in each image are a) [001]YAO and [100]YAO; and b) [001]YAO and [010]YAO, respectively. c) Symmetric -2 XRD line scans of the (002) reflection of WO3 films with varying thicknesses. d) Diagram showing the relationship between two structural WO3 domains and the YAO substrate. The other two domains are related to the ones shown here by a 180 rotation about the [001] axis perpendicular to the page. e) Reciprocal space map of the pseudo-cubic 103 reflections of the WO3 film at 450 showing the four triclinic domains. The spots labeled '1', '2', and '3' were used to define the order parameters in (f). f) Evolution of the order parameters of the phase transition from the low-temperature triclinic phase into the high-temperature pseudo-orthorhombic phase.
• Figure 3: Atomic resolution structural characterization of the WO3 film on (001)-oriented YAO. a) iDPC-STEM images of the film near a domain wall. b) Blow-ups of the image in (a) showing the change of structure in the surface and domain wall region and in the interior of a domain. Arrows in the left panel show the direction of the zigzag modulation of W atomic column positions. A half-unit-cell phase shift of this modulation occurs at the dashed line. c) iDPC-STEM image along the in-plane zone axis perpendicular to that in (a), showing no modulation of the oxygen column ellipticity (shown at the bottom of the image). The colors of the bars are correlated with the magnitude of the ellipticity d) Map of the out-of-plane (i.e. vertical in the image) strain in the image in (a). Strain is defined as the local lattice parameter divided by the average lattice parameter in the image. e) Vertically-averaged profiles of the out-of-plane and in-plane strain in the image in (a) showing out-of-plane contraction and in-plane expansion of the unit cell. The symbol colors are correlated to the strain magnitude f) Vertically-averaged ellipticity of atomic columns of oxygen in the V AO layers showing a modulation of the ellipticity in the interior of the domains, which disappears in the domain wall region.
• Figure 4: Domain width dependence on film thickness. (a-c) Lateral PFM amplitude images for a a) 40, b) 79 and c) 157 thick film. The arrows represent the predominant in-plane polarization axis. d) rocking curves around the (004) peak of the WO3 films, showing pronounced thickness-dependent domain satellite peaks. qx corresponds to the (110) axis of the WO3 film. The dashed line represents the fit to the data. e) Log-log plot of the domain periodicity as a function of film thickness, extracted from the domain satellite peak positions in d). The dashed line is a fit to the data.
• Figure 5: Micro- and macroscopic transport characteristics of the triclinic stripe domain walls in WO3. a) cAFM image showing domain-wall contrast with an enhanced domain-wall conductivity. b) SEM image showing a reduced secondary electron yield at the location of the stripe domain walls. c) Temperature-dependent conductivity parallel and perpendicular to the stripe domain walls measured using Hall bars. The solid lines represent the fit according to a small polaron transport model.