Stabilization of the epitaxial rhombohedral ferroelectric phase in ZrO 2 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 new 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.