Stabilization of the epitaxial rhombohedral ferroelectric phase in ZrO2 by surface energy

Abstract

Doped ${\mathrm{HfO}}_{2}$ and ${\mathrm{HfO}}_{2}\text{\ensuremath{-}}{\mathrm{ZrO}}_{2}$ compounds are gaining significant interest thanks to their ferroelectric properties in ultrathin films. Here, we show that ${\mathrm{ZrO}}_{2}$ could be a playground for doping and strain engineering to increase the thickness in epitaxial thin films. Based on surface-energy considerations supported by ab initio calculations, we find that pure ${\mathrm{ZrO}}_{2}$ exhibits a ferroelectric rhombohedral phase ($r$ phase, with $R3m$ space group) more stable than for the HZO and pure ${\mathrm{HfO}}_{2}$ cases. In particular, for a thickness up to 37 nm we experimentally evidence a single (111)-oriented $r$ phase in ${\mathrm{ZrO}}_{2}$ films deposited on ${\mathrm{La}}_{2/3}{\mathrm{Sr}}_{1/3}{\mathrm{MnO}}_{3}$-buffered ${\mathrm{DyScO}}_{3}(110)$ substrate. The formation of this $r$ phase is discussed and compared between ${\mathrm{HfO}}_{2}$, ${\mathrm{ZrO}}_{2}$ and HZO, highlighting the role of surface energy.