Epitaxial ferroelectric hafnia stabilized by symmetry constraints

Ferroelectric memories experienced a revival in the last decade due to the discovery of ferroelectricity in HfO2-based nanometer-thick thin films. These films exhibit exceptional silicon compatibility, overcoming the scaling and integration obstacles that impeded perovskite ferroelectrics' use in high-density integrated circuits. The exact phase responsible for ferroelectricity in hafnia films remains debated with no single factor identified that could stabilize the ferroelectric phase thermodynamically. Here, supported by density functional theory (DFT) high-throughput (HT) calculations that screen a broad range of epitaxial conditions, we demonstrate conclusively that specific epitaxial conditions achievable with common substrates such as yttria-stabilized zirconia (YSZ) and SrTiO3 can favor the polar Pca21 phase thermodynamically over other polar phases such as R3m and Pmn21 and nonpolar P21/c phase. The substrate's symmetry constraint-induced shear strain is crucial for the preference of Pca21. The strain-stability phase diagrams resolve experiment-theory discrepancies and can guide the improvement of ferroelectric properties of epitaxial hafnia thin films.

Automated summary

Ferroelectric memories have seen a resurgence due to the discovery of ferroelectricity in HfO2-based nanometer-thick films. These films have excellent silicon compatibility and overcome the challenges faced by perovskite ferroelectrics in high-density integrated circuits. The exact phase responsible for ferroelectricity in hafnia films is still debated, but researchers have used computational methods to identify specific epitaxial conditions that thermodynamically favor the polar Pca21 phase. The strain-stability phase diagrams help resolve discrepancies between experiments and theory and can guide the improvement of ferroelectric properties in epitaxial hafnia thin films.