Polar metals as electrodes to suppress the critical-thickness limit in ferroelectric nanocapacitors

Abstract: Enhancing the performance of nanoscale ferroelectric (FE) field-effect transistors and FE capacitors for memory devices and logic relies on miniaturizing the metal electrode/ferroelectric area and reducing the thickness of the insulator. Although size reductions improve data retention, deliver lower voltage threshold switching, and increase areal density, they also degrade the functional electric polarization. There is a critical, nanometer length $t_\mathrm{FE}^*$ below which the polarization disappears owing to depolarizing field effects. Here we show how to overcome the critical thickness limit imposed on ferroelectricity by utilizing electrodes formed from a novel class of materials known as polar metals. Electronic structure calculations on symmetric polar-metal electrode/FE capacitor structures demonstrate that electric polarizations can persist to the sub-nanometer scale with $t_\mathrm{FE}^*\rightarrow0$ when a component of the polar axis in the electrode is perpendicular to the electrode/insulator interface. Our results reveal the importance of interfacial dipolar coherency in sustaining the polarization, which provides a platform for atomic scale structure-based design of functions that deteriorate in reduced dimensions.