Epitaxy‐Driven Ferroelectric/Non‐Ferroelectric Polymorph Selection in an All‐Fluorite System

Films of ferroelectric hafnia have hitherto been deposited on electrodes with a non-fluorite crystal structure. As a result, they are polycrystalline, contain fractions of non-ferroelectric polymorphs or have poor crystal quality. Here, a strategy that circumvents all these limitations is shown. Seven nanometers-thick epitaxial Hf0.5Zr0.5O2 (HZO) films are deposited directly on yttria-stabilized zirconia (YSZ) single-crystals. The fluorite structure of the whole system enables coherent epitaxy, while the substrate orientation induces polymorph-selective growth, being the HZO films orthorhombic on YSZ(111) and monoclinic on YSZ(001). Besides, the YSZ substrate can play the role of a buried floating electrode under the appropriate measuring conditions (temperature and frequency) thanks to its thermally-activated oxygen conductivity. Indeed, out-of-plane ferroelectric switching is confirmed in the orthorhombic HZO samples at 185 degrees C and 0.01 Hz frequency. This original approach avoids the need to deposit conducting bottom layers, allowing high-quality orthorhombic hafnia to be obtained directly on the substrate and its ferroelectric nature to be studied. Moreover, it constitutes a case of ion-driven ferroelectric switching, and thus gives support to the recently proposed relationship between ionic conductivity, and ferroelectricity in fluorite systems. The structure studied here consisting of a Hf0.5Zr0.5O2 (HZO) epitaxial film and an yttria-stabilized zirconia (YSZ) substrate demonstrates polymorph-selective and fully-coherent growth of hafnia films with high crystal quality. Besides, the thermally-activated oxygen conductivity of the substrate enables the ferroelectric switching in the orthorhombic film. This constitutes an ion-driven electronic effect, and supports the relationship between ferroelectricity and oxygen mobility. image