Ferroelectric ZrO2 phases from infrared spectroscopy

We investigate ferroelectric thin films of ZrO2 experimentally and theoretically using infrared (IR) absorption spectroscopy coupled with density functional theory (DFT) calculations. The IR absorbance is measured using IR synchrotron radiation, while the theoretical investigations are performed using CRYSTAL suite of quantum chemical programs. Theoretically, we consider the two polar experimentally observed orthorhombic Pbc21, and rhombohedral R3m phases, as well as two non-polar phases, tetragonal P42/nmc, and ground state monoclinic P21/c. Experimentally, we follow two approaches to enable IR measurements: (i) direct growth of ZrO2 films on high-resistive silicon substrates with either a La0.67Sr0.33MnO3 (LSMO) or ZnO buffer layer to induce tensile and compressive strain, respectively, and (ii) transfer of ZrO2 films, grown initially on LSMO-buffered SrTiO3 or DyScO3 substrates, onto high-resistive silicon substrate as membranes. The far-IR structural signature of the films is assigned to a tetragonal phase, mainly when the film is under tensile strain; however, the ZrO2 film shows a ferroelectric signature. Under compressive strain, an early stage of phase transition from the non-polar P42/nmc phase to the polar R3m phase is observed. Our findings open new paths for investigating the origin of ferroelectricity in ZrO2-based ferroelectric thin films.