Formation of polar phase in Fe-doped ZrO ₂ epitaxial thin films

The existence of a polar phase in epitaxially grown Fe-doped ZrO ₂ thin films has been demonstrated. The films were prepared on yttria-stabilized zirconia substrates by ion-beam sputtering and subsequent annealing, where the composition was controlled by changing the FeO _1.5 -ZrO ₂ ratio of the sputtering target. The chemical states of elements in the 30 nm-thick xFeO _1.5 -(1-x)ZrO ₂ thin films (x = 0, 0.06, 0.10, and 0.14 nominal composition) have been measured by X-ray photoelectron spectroscopy, and the crystal structure has been examined by using X-ray diffraction and scanning transmission electron microscopy. These observations revealed that the undoped ZrO ₂ thin film (x = 0) is dominated by the monoclinic phase (space group: P2 ₁ /c), while the films containing Fe exhibit multi-domain structures consisting of monoclinic and orthorhombic (Pbc2 ₁ ) phases, the latter being a polar phase, showing that the orthorhombic phase is stabilized by the addition of Fe. Concurrently, the polarization-electric field loops of the Fe-doped films exhibited hysteresis for the film with x = 0.06.