Atomic-Scale Compensation Phenomena at Polar Interfaces

The interfacial screening charge that arises to compensate electric fields of dielectric or ferroelectric thin films is now recognized as the most important factor in determining the capacitance or polarization of ultrathin ferroelectrics. Here we investigate using aberration-corrected electron microscopy and density-functional theory to show how interfaces cope with the need to terminate ferroelectric polarization. In one case, we show evidence for ionic screening, which has been predicted by theory but never observed. For a ferroelectric film on an insulating substrate, we found that compensation can be mediated by an interfacial charge generated, for example, by oxygen vacancies.

Figure 1

Measured displacements in the PZT/SRO/STO interfacial region. (a) Phase-contrast image of the $⟨100⟩$ projection of the PZT/SRO/STO film. Contrast has been inverted to ease feature location. A schematic of the $⟨100⟩$ projection of the PZT lattice, included to the left of the image, shows the displacements of the $\mathrm{Zr}/\mathrm{Ti}$ (red) and O (blue) columns relative to the Pb (yellow) reference frame. (b) Measured displacements of the $\mathrm{Zr}/\mathrm{Ti}$, Ru, and apical O1 columns in the reference frame of the $\mathrm{Pb}/\mathrm{Sr}$ columns. The O1 oxygen columns are located in the vertical $\mathrm{Zr}/\mathrm{Ti}$ (red-filled circle) O (blue-filled, smaller circles) plane seen in (a). Distance from the interface is measured in $c$-axis unit cells. These dimensions vary from 0.3905 nm in STO to 0.3828 nm in SRO to 0.427 nm in PZT.

Figure 2

Measured displacements at the PZT/STO interface. (a) Phase-contrast image of the interfacial region between ferroelectric PZT and dielectric STO, seen along the $⟨110⟩$ direction. The contrast has been reversed to ease feature location. A magnified view of the PZT matrix and a schematic of ferroelectric PZT are included to the left of the image. (b) Measured displacements of the $\mathrm{Ti}/\mathrm{Zr}$ and equatorial O2 columns. The O2 oxygen columns are located in the horizontal $\mathrm{Zr}/\mathrm{Ti}$ (red-filled circles) O (blue-filled, smaller circles) plane seen in (a). Distance from the interface is measured in $c$-axis unit cells. These dimensions vary from 0.3905 nm in STO to 0.427 nm in PZT.

Figure 3

Calculated ferroelectric displacements at the PTO/STO and PTO/SRO interfaces. The shaded regions show an interfacial arrangement not seen experimentally. (a) PTO/STO interface without vacancies, (b) PTO/STO interfaces with O and Sr vacancies, and (c) PTO/SRO interface without vacancies.