Misfit dislocations in nanoscale ferroelectric heterostructures

Abstract

We present a quantitative study of the thickness dependence of the polarization and piezoelectric properties in epitaxial (001) ${\mathrm{PbZr}}_{0.52}{\mathrm{Ti}}_{0.48}{\mathrm{O}}_3$ films grown on (001) ${\mathrm{SrRuO}}_3$ -buffered (001) ${\mathrm{SrTiO}}_3$ substrates. High-resolution transmission electron microscopy reveals that even the thinnest films $(\sim 8\mathrm{nm})$ are fully relaxed with a dislocation density close to ${10}^{12}{\mathrm{cm}}^{-2}$ and a spacing of approximately 12 nm. Quantitative piezoelectric and ferroelectric measurements show a drastic degradation in the out-of-plane piezoelectric constant $\left(d_{33}\right)$ and the switched polarization $\left(\Delta P\right)$ as a function of decreasing thickness. In contrast, lattice-matched ultrathin ${\mathrm{PbZr}}_{0.2}{\mathrm{Ti}}_{0.8}{\mathrm{O}}_3$ films that have a very low dislocation density show superior ferroelectric properties. Supporting theoretical calculations show that the variations in the strain field around the core of the dislocation leads to highly localized polarization gradients and hence strong depolarizing fields, which result in suppression of ferroelectricity in the vicinity of a dislocation.