Composition dependence of ferroelectric properties in (111)-oriented epitaxial HfO₂-CeO₂ solid solution films

Abstract The composition dependence of ferroelectric properties was investigated for (111)-oriented epitaxial HfO2-CeO2 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 oC under atmospheric N2 and O2 flows. All of the films had fluorite structures, and their crystal symmetries changed from monoclinic to tetragonal/cubic through orthorhombic phases as the (Hf1-xCex)O2 (x =0.12-0.25) increased in the film. The orthorhombic phase was confirmed 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 for films with x = 0.20. The film composition showing ferroelectricity was the widest for the reported epitaxial HfO2-based films. Their remanent polarization (P r) and coercive field (E c) were almost the same, at 17-19 μC/cm2 and 2.0-3.0 MV/cm, respectively. This wide ferroelectric composition range with relatively similar ferroelectricity is due to the solid solution of the same fluorite structure of HfO2 and CeO2 with monoclinic and cubic symmetries, that is respectively lower and higher crystal symmetries of the ferroelectric orthorhombic phase.