Scaling of structure and electrical properties in ultrathin epitaxial ferroelectric heterostructures

Scaling of the structural order parameter, polarization, and electrical properties was investigated in model ultrathin epitaxial SrRuO ₃/PbZr _0.2Ti _0.8O ₃/SrRuO ₃/SrTiO ₃ heterostructures. High-resolution transmission electron microscopy images revealed the interfaces to be sharp and fully coherent. Synchrotron x-ray studies show that a high tetragonality (c/a∼1.058) is maintained down to 50 Å thick films, suggesting indirectly that ferroelectricity is fully preserved at such small thicknesses. However, measurement of the switchable polarization (Δ3) using a pulsed probe setup and the out-of-plane piezoelectric response (d ₃₃) revealed a systematic drop from ∼140 μC/cm ² and 60 pm/V for a 150 Å thick film to 11 μC/cm ² and 7 pm/V for a 50 Å thick film. This apparent contradiction between the structural measurements and the measured switchable polarization is explained by an increasing presence of a strong depolarization field, which creates a pinned 180° polydomain state for the thinnest films. Existence of a polydomain state is demonstrated by piezoresponse force microscopy images of the ultrathin films. These results suggest that the limit for a ferroelectric memory device may be much larger than the fundamental limit for ferroelectricity.