Epitaxial Superlattices as a Platform for Maximizing the Spontaneous Polarization and Cyclability of Ferroelectric Hafnia

The discovery of ferroelectricity in a metastable orthorhombic (Pca2₁) phase of hafnia (HfO₂) in 2011 came as a pleasant surprise to a ferroelectrics community that had long sought after CMOS compatible ferroelectric materials capable of presenting a spontaneous polarization at the nanoscale. Since then the field of hafnia ferroelectrics has exploded, both in search of the mechanisms behind this peculiar ferroelectric material and in the pursuit of maximal ferroelectric performance (that is, high spontaneous polarization, P_s, lasting over long device lifetimes). In epitaxial hafnia thin films, an additional metastable ferroelectric phase has since been found in the form of a rhombohedral phase that appears in hafnia films under high compressive strains.

In the present work, ferroelectric hafnia is grown in superlattice stacks with other binary oxides (ZrO₂, CeO₂) by pulsed laser deposition (PLD) on SrTiO₃ (001) with a La_0.67Sr_0.33MnO₃ buffer layer. Bicolor (HfO₂-XO₂)_n stacks serve to highlight the impact oxygen diffusion has on device performance and how the additional interfaces between superlattice sublayers can be exploited to maximize device lifetimes. Furthermore, it is observed that maintaining the strain state within each sublayer can enable larger P_s values compared to their monolayer (solid solution) equivalents. Both the endurance and polarization can be further improved through adjustment of each sublayer composition, where the inclusion of Zr in (Hf_1-xZr_xO₂- ZrO₂)_n stacks allows for an increased polarization, reaching a record maximum 2P_s=84 µC/cm² with excellent endurance of 10⁹ cycles.

Tricolor (HfO₂-BO₂-CO₂)_n stacks provide an additional breaking of symmetry associated with a built-in field around the polar hafnia sublayers that helps enhance the ferroelectric phase stability. Here we show our results on (ZrO₂-HfO₂-CeO₂)_n, which similarly show improved P_s values when compared to both a HfO₂ monolayer and their solid solution equivalent. For such superlattices, adjustment of the sublayer composition is necessary to achieve superlattice stacks containing the three binary oxides with homogeneous phase and orientation throughout the stack. Overall, this work demonstrates that the ferroelectric behaviour of hafnia is highly tuneable in superlattice constructions, which enable engineering of the spontaneous polarization, coercive field and device endurance.