Atomic-scale mechanism of internal structural relaxation screening at polar interfaces

The effective screening of the polarization bound charge is a prerequisite to stabilize the ferroelectricity in ferroelectric thin films. Here, by combining annular bright field imaging and electron energy-loss spectroscopy (EELS) in an aberration-corrected scanning transmission electron microscope with phase-field simulations, we investigate the screening mechanism by quantitatively measuring the structural relaxation at $\mathrm{Pb}(\mathrm{Z}{\mathrm{r}}_{0.2}\mathrm{T}{\mathrm{i}}_{0.8}){\mathrm{O}}_{3}/\mathrm{SrTi}{\mathrm{O}}_{3}$ interfaces. We find that the thickness of the interfacial layer is $\ensuremath{\sim}3.5$ unit cells ($\ensuremath{\sim}1.4$ nm) in a domain with upward polarization and $\ensuremath{\sim}5.5$ unit cells ($\ensuremath{\sim}2.2$ nm) in a domain with downward polarization. Phase-field simulations, an EELS analysis, and a lattice parameter analysis verify the existence of interfacial oxygen vacancies accounting for the narrower interfacial layer in the domain with upward polarization. Our study indicates the internal structural relaxation at the interface is the dominant mechanism for the polarization charge screening for ferroelectric films grown on insulating substrates and has implications for our understanding of domain switching in ferroelectric devices.