Long-range ferroelectric interactions in KTaO3/KNbO3 superlattice structures

Abstract

Symmetric superlattice structures consisting of alternating atomic-scale layers of ${\mathrm{KTaO}}_{\mathrm{3}}$ and ${\mathrm{KNbO}}_{\mathrm{3}}$ with variable periodicity were grown on ${\mathrm{KTaO}}_{\mathrm{3}}$ substrates by pulsed laser deposition. The in-plane structure of ${\mathrm{KNbO}}_{\mathrm{3}}$ closely matches that of the ${\mathrm{KTaO}}_{\mathrm{3}}$ substrate, resulting in ${\mathrm{KTaO}}_{\mathrm{3}}{\mathrm{/KNbO}}_{\mathrm{3}}$ heterostructures that are uniformly strained in-plane without misfit dislocations. This strain imposes an in-plane ${\mathrm{KNbO}}_{\mathrm{3}}$ lattice spacing identical to that of the ${\mathrm{KTaO}}_{\mathrm{3}}$ substrate for the temperature range $\text{30\hspace{0.17em}°}\mathrm{C}\text{<T<700\hspace{0.17em}°}\mathrm{C},$ and a tetragonal-to-tetragonal transition is observed whose phase transition temperature $T_c$ depends on the ${\mathrm{KNbO}}_{\mathrm{3}}$ layer thickness. The in-plane strain results in a significant increase in this ferroelectric-paraelectric $T_c$ for superlattices with relatively thick ${\mathrm{KNbO}}_{\mathrm{3}}$ layers ($T_c\text{=535\hspace{0.17em}°}\mathrm{C}$ for a 17 nm thick layer, as compared to 435 °C for bulk ${\mathrm{KNbO}}_{\mathrm{3}}$ ) and for ${\mathrm{K(Nb}}_{\mathrm{0.5}}{\mathrm{Ta}}_{\mathrm{0.5}}{\mathrm{)O}}_{\mathrm{3}}$ random-alloy thin films. As the superlattice period decreases, a reduction of $T_c$ is observed. For superlattices with periodicities of 50 Å or less, the Curie temperature is identical to that of the ${\mathrm{K(Ta}}_{\mathrm{0.5}}{\mathrm{Nb}}_{\mathrm{0.5}}{\mathrm{)O}}_{\mathrm{3}}$ random-alloy film, indicating significant long-range ferroelectric coupling across the ${\mathrm{KTaO}}_{\mathrm{3}}$ layers.