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

Abstract

Bestowing complementary metal-oxide semiconductor-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 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 experimental evidence for anti-distortive polaronic transport recently predicted in WO₃.