Domain patterns and domain wall dynamics in small-scale ferroelectrics
This thesis attempts to address two major themes: firstly, the intrinsic dynamics of switching in meso/nanoscale ferroelectrics incorporated into device geometries, and secondly, the development of control over the precise mechanics of switching by altering the ferroelectric morphology therein. In addressing the first theme, thin single-crystal slices of BaTiO^ (or ‘lamellae’), have been incorporated into co-planar electroded test structures. Piezoresponse Force Microscopy (PFM) imaging revealed that switching was carried out by redistribution of highly-ordered packets of 90° ferroelastic domains, or ‘superdomains’, which exhibit a net in-plane polarisation aligned (or anti-aligned) with the switching field. Ferroelectric switching was mediated mainly by the motion of boundaries separating these superdomains whilst the ferroelastic substructure remained largely inactive. In-situ PFM (i.e. imaging during planar switching) revealed that superdomains exhibit the classic nucleation, forwards growth, and sidewards growth modes observed in simple 180° domain switching experiments. Real-time imaging of field-driven superdomain boundary motion showed that the boundary slowed down dramatically after it was set into motion. This bore a striking similarity to the ballistic dynamics of a particle. Hence Newtonian equation of motion fits were attempted to describe the dynamics. However, such an inertial description of the boundary dynamics was found to be unphysical. Instead, interplay between applied and residual depolarising fields in the system was suspected to be at the heart of the apparent ballistic-like dynamics. In studying the long-term relaxation of switched states a large fluxclosing quadrant domain configuration was seen to form, spanning micrometre spatial scales. Its development was rationalised to be driven by residual inplane depolarising fields in the system due to incomplete bound charge compensation at the electrodes. The unusually large size of the quadrant configuration was facilitated by the observed fine subdomain structure within each quadrant which offset the otherwise prohibitively large stresses associated with this geometry. Detailed study of the relaxation dynamics showed that both single and double closure patterns could form depending to some extent on the initial post-switched domain configuration. In addressing the second theme in this thesis the influence of antinotches on switching dynamics in FIBed ferroelectric nanowires has been investigated. This provided a complementary study to an ongoing body of work at QUB which previously characterised switching in simple and notched BaTiO^ wires. By combining capacitance-voltage hysteresis measurements and Rayleigh analysis (of capacitance as a function of a.c. bias) it was determined that antinotches serve to inhibit switching compared to plain wires without antinotches. This behaviour could be described by a finite-element modelled depression in electric field strength in the vicinity of the antinotch feature. Furthermore, this is completely consistent with the conclusions of the notched wire study [McMillen et al. Appl. Phys Lett. 96, 042904 (2010).] where an enhanced switchability was correlated with a local field amplification in the region of the notch. In general, it seems that manipulation of wire morphology in ferroelectric wires affects switching indirectly, through the manner in which the spatial electric field distribution is altered. This idea of ‘field-engineering’ culminated in a final investigation attempting to control the number and position of domain walls created during switching of single-crystal KTiP04 lamellae (chosen for displaying only simple 180° domain patterns). Finite-element field models were used to develop patterned lamella designs, based on simple triangular and circular hole features, with consideration as to how these factors would affect field amplitude distribution under an externally applied field. In real KTiP04 lamellae, with FIBed circular holes, domain walls imaged using PFM were repeatedly observed to rest along regions of high field amplitude. By this means a predictable and repeatable domain configuration could be obtained. Furthermore, in-situ PFM imaging strongly implied that domain nucleation was initiated at these regions of high field amplitude adjacent to the patterned holes, rather than at the electrodes. This deterministic nucleation behaviour contrasted strongly with the unpredictable stochastic nucleations observed by PFM in unpatterned KTP lamellae. Finally, the effect of field-engineering on domain wall mobility was investigated by in-situ PFM imaging of field-driven domain walls in regions of differing local field amplitude. It was seen that a domain wall moving through a region of higher field amplitude travelled with a higher velocity. This provides direct confirmatory evidence for the previous interpretation of how domain wall mobility is affected by field-engineering in notched and antinotched wires.
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