The Growth of Polarization Domains in Ultrathin Ferroelectric Films Seeded by the Tip of an Atomic Force Microscope

Abstract

Piezoresponse force microscopy is used to study the velocity of the polarization domain wall in ultrathin ferroelectric barium titanate (BTO) films grown on strontium titanate (STO) substrates by molecular beam epitaxy. The electric field due to the cone of the atomic force microscope tip is demonstrated as the dominant electric field for domain expansion in thin films at lateral distances greater than about one tip diameter away from the tip. The velocity of the domain wall under the applied electric field by the tip in BTO for thin films (less than 40 nm) followed an expanding process given by Merz’s law. The material constants in a fit of the data to Merz’s law for very thin films are reported as about 4.2 KV/cm for the activation field, $$E_{\mathrm{a}}$$ E a , and 0.05 nm/s for the limiting velocity, $$v_{\infty }$$ v ∞ . These material constants showed a dependence on the level of strain in the films, but no fundamental dependence on thickness.

Ferroelectric Size Effects on Statics and Dynamics of Domain Wall

“…[ 40,55 ] In particular, the compressive strain has been found to have a significant impact on DW motion in epitaxial ferroelectric films by increasing the overall activation energy. [ 48,56,57 ] Therefore, the precise extraction of lattice parameters, epitaxial strain, and tetragonality is crucial to understand the nature of thickness size effects. To determine the lattice parameters, high‐resolution X‐ray reciprocal space mapping (X‐RSM) around the asymmetric ($$\bar{1}03$$) reflection was performed (Figure S2A, Supporting Information), revealing pseudocubic (00 l )‐oriented fully strained growth of LSMO, while the tetragonal strain state of BTO changes with thickness.…”

“…[48,56] Interestingly, the observed 𝜇 or d eff step-saturation-type transition in the 10-25 nm thickness range is close enough to the critical thickness range (15-40 nm) for the strain relaxation (shaded regions in Figure 5). Here, BTO films up to 15 nm in thickness are fully strained, constraining DW velocities due to the large compressive strain or relatively higher barrier energy, [42,57] resulting in lower creep exponents 𝜇 ≈ 0.23 and dimensionality d eff ≈ 1.6. Likewise, larger creep exponents 𝜇 ≈ 0.54 and d eff ≈ 2.3 may correspond to the greater DW velocities associated with lower barrier energy or relatively low strain in partially relaxed films.…”

Writing‐Speed Dependent Thresholds of Ferroelectric Domain Switching in Monolayer α‐In2Se3

“…[34] These results were similar to the speeddependent domain wall widths [21] and the simulated electric field distribution under the tip bias. [35][36][37] Thus, successful domain writing with increased speed required a higher PFM tip voltage.…”