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.
Epitaxial layers of ferroelectric orthorhombic HfO2 are frequently investigated as model systems for industrially more relevant polycrystalline films. The recent success in stabilizing the orthorhombic phase in the solid-solution cerium oxide – hafnium oxide system allows detailed investigations of external influences during fabrication. This report analyzes the ferroelectric properties of two thin film capacitors, which were post-deposition annealed in N2 and O2 atmospheres to achieve the orthorhombic phase after room temperature deposition. The samples, which exhibit very similar constituent phase, appear identical in conventional polarization-field hysteresis measurements. However, a significant switching speed difference is observed in pristine devices. Continued field cycling reduces the difference. Deeper analysis of switching transients based on the Nucleation Limited Switching model suggests that the O2 heat treatment atmosphere results in an altered oxygen vacancy profile, which is reverted during ferroelectric cycling.
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