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

Doped HfO2 and HfO2-ZrO2 compounds are gaining significant interest thanks to their ferroelectric properties in ultrathin films. Here, we show that ZrO2 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 ZrO2 exhibits a ferroelectric rhombohedral phase (r phase, with R3m space group) more stable than for the HZO and pure HfO2 cases. In particular, for a thickness up to 37 nm we experimentally evidence a single (111)-oriented r phase in ZrO2 films deposited on La2/3Sr1/3MnO3-buffered DyScO3(110) substrate. The formation of this r phase is discussed and compared between HfO2, ZrO2 and HZO, highlighting the role of surface energy.