Composition dependence of ferroelectric properties in (111)-oriented epitaxial HfO 2 -CeO 2 solid solution films

The composition dependence of ferroelectric properties was investigated for (111)-oriented epitaxial HfO 2 -CeO 2 solid solution films. Twenty nanometer thick films with different compositions were prepared on (111)ITO//(111)YSZ substrates at room temperature by pulsed laser deposition and subsequent heat treatment at 1000 °C under atmospheric N 2 or O 2 gas flow. All the films had fluorite structures, and their crystal symmetries changed from monoclinic through orthorhombic to tetragonal/cubic phases as x increased for the (Hf 1− x Ce x )O 2 ( x = 0.12–0.25) films. The orthorhombic phase was confirmed by X-ray diffraction analysis for films with x = 0.15 and 0.17. On the other hand, ferroelectric properties were observed in films with x = 0.15–0.20, suggesting that a field-induced phase transition takes place for films with x = 0.20. The film composition showing ferroelectricity was the widest range of doping concentration for reported epitaxial HfO 2 -based films. Their remanent polarization ( P r ) and coercive field ( E c ) were almost identical, at 17–19 μ C cm −2 and 2.0–3.0 MV cm −1 . This wide ferroelectric composition range with relatively similar ferroelectricity is due to the solid solution of the same fluorite structure of HfO 2 and CeO 2 with monoclinic and cubic symmetries, that are respectively lower and higher crystal symmetries of the ferroelectric orthorhombic phase.