Lawrence Berkeley National Laboratory

Ferroelectric HfO₂-based thin films have attracted much interest in the utilization of ferroelectricity at the nanoscale for next-generation electronic devices. However, the structural origin and stabilization mechanism of the ferroelectric phase are not understood because the film is typically nanocrystalline with active yet stochastic ferroelectric domains. Here, electron microscopy is used to map the in-plane domain network structures of epitaxially grown ferroelectric Y:HfO₂ films in atomic resolution. The ferroelectricity is confirmed in free-standing Y:HfO₂ films, allowing for investigating the structural origin for their ferroelectricity by 4D-STEM, high-resolution STEM, and iDPC-STEM. At the grain boundaries of <111>-oriented Pca2₁ orthorhombic grains, a high-symmetry mixed-(R3m, Pnm2₁) phase is induced, exhibiting enhanced polarization due to in-plane compressive strain. Nanoscale Pca2₁ orthorhombic grains and their grain boundaries with mixed-(R3m, Pnm2₁) phases of higher symmetry cooperatively determine the ferroelectricity of the Y:HfO₂ film. It is also found that such ferroelectric domain networks emerge when the film thickness is beyond a finite value. Furthermore, in-plane mapping of oxygen positions overlaid on ferroelectric domains discloses that polarization is suppressed at vertical domain walls, while it is active when domains are aligned horizontally with subangstrom domain walls. In addition, randomly distributed 180° charged domain walls are confined by spacer layers.