Atomic-Scale Scanning\nof Domain Network in the Ferroelectric\nHfO<sub>2</sub> Thin Film

Ferroelectric HfO<sub>2</sub>-based thin films have attracted\nmuch\ninterest in the utilization of ferroelectricity at the nanoscale for\nnext-generation electronic devices. However, the structural origin\nand stabilization mechanism of the ferroelectric phase are not understood\nbecause the film is typically nanocrystalline with active yet stochastic\nferroelectric domains. Here, electron microscopy is used to map the\nin-plane domain network structures of epitaxially grown ferroelectric\nY:HfO<sub>2</sub> films in atomic resolution. The ferroelectricity\nis confirmed in free-standing Y:HfO<sub>2</sub> films, allowing for\ninvestigating the structural origin for their ferroelectricity by\n4D-STEM, high-resolution STEM, and iDPC-STEM. At the grain boundaries\nof <111>-oriented Pca2<sub>1</sub> orthorhombic grains, a high-symmetry\nmixed-(<i>R</i>3<i>m</i>, Pnm2<sub>1</sub>) phase\nis induced, exhibiting enhanced polarization due to in-plane compressive\nstrain. Nanoscale Pca2<sub>1</sub> orthorhombic grains and their grain\nboundaries with mixed-(<i>R</i>3<i>m</i>, Pnm2<sub>1</sub>) phases of higher symmetry cooperatively determine the ferroelectricity\nof the Y:HfO<sub>2</sub> film. It is also found that such ferroelectric\ndomain networks emerge when the film thickness is beyond a finite\nvalue. Furthermore, in-plane mapping of oxygen positions overlaid\non ferroelectric domains discloses that polarization is suppressed\nat vertical domain walls, while it is active when domains are aligned\nhorizontally with subangstrom domain walls. In addition, randomly\ndistributed 180° charged domain walls are confined by spacer\nlayers.