The influence of crystal defects on domain wall motion in thin film Pb(Zr,Ti)O3

This work describes the interactions of domain walls in ferroelectric Pb(Zr,Ti)O3 with grain boundaries and PbO non-stoichiometry. This was studied for a variety of Zr:Ti ratios by analyzing the local piezoelectric response using band excitation piezoresponse force microscopy. Measurements were conducted on a variety of tilt and twist bicrystals with angles ranging from 10° to 30°. For the >15° tilt and ≥10° twist grain boundaries, a local minimum in the nonlinear response was observed at the grain boundary. The 10° tilt grain boundaries exhibited a maximum nonlinear response at the grain boundary.;Variations in the nonlinear response at a 24° tilt grain boundary was measured for three different Zr:Ti ratios. Films with a ratio of 20:80, far into the tetragonal regime, exhibited a complex distribution of nonlinear response alternating between low and high with distance from the grain boundary. Films with a ratio of 45:55 and 52:48, tetragonal and near morphotropic phase boundary rhombohedral, respectively, exhibited a minimum in nonlinear response at the grain boundary neighbored by a maximum in nonlinear response.;The nonlinear response was correlated to the domain structure. The domain structure was characterized before and after poling using piezoresponse force microscopy and transmission electron microscopy. It was found that the domain structure was controlled by the local strain and electric field for the largest angle grain boundaries. Domain walls were pinned at the grain boundary by the local strain and electric field. At 360-650 nm from the grain boundary, domain wall -- domain wall interactions dominated the nonlinear response. A smaller width of reduced nonlinear response for the film with Zr:Ti ratio of 52:48 was attributed to enhanced relaxation of the local strain and electric field due to the small ∼6 nm domain size.;Phase field models were used to determine the primary factors involved in forming domains at large angle tilt grain boundaries. The models suggest that domains form to minimize the local change in strain across the grain boundary, explaining correlated domain organization in neighboring grains previously observed by piezoresponse force microscopy and transmission electron microscopy. However, strain compatibility could not account for the observed formation of head to head domain structures observed at the grain boundary for Pb(Zr0.2Ti0.8)O3; it is believed that these are stabilized by built-in charge or additional stress compensation.;It was demonstrated that the pinning for 24° tilt angle grain boundaries influences domain wall motion over a longer lateral distance (0.45 -- 0.80 microm) than 30° twist angle grain boundaries (∼0.35 microm). Additionally, the pinning energy reduced with the grain boundary angle. The maximum in nonlinear response observed for small angle tilt grain boundaries (≤ 10°) was attributed to an increased concentration of low energy pinning sites reducing the reversible response and increasing the irreversible response. Similarly, minimal variation in the nonlinear response was observed at the grain boundary for intermediate grain boundary angles (15° tilt) due to the grain boundary energy being similar to other defects present in the film.;A furnace providing a controlled PbO atmosphere was developed so that the effect of PbO defects on the functional properties of Pb(Zr,Ti)O 3 could be determined. Minimal variation in the permittivity, Rayleigh parameters, and aging rates was observed for films with a range of PbO contents. A decreasing remanent polarization was observed with increasing PbO content in minor polarization -- electric field hysteresis loops. An increase in the area of low nonlinear response regions with decreasing PbO content was measured by band excitation piezoresponse force microscopy. This suggests that PbO deficiencies act to reduce domain wall motion where it is already low. It was determined that VPb″ -- VO″ defect dipoles, if they exist, have only a modest influence on domain wall motion compared to defects already present in the film.;This work helps to determine the mechanisms responsible for emergent properties in ferroelectric materials and, as such, provides a basis for superior models representing the functional properties of ferroelectric materials. The measurements of the effect of grain boundaries and PbO concentration on nonlinear response support the framework for the representations of mobile interfaces interacting with defects in materials. By correlating measurements by various characterization and modeling methods, a deeper understanding of ferroelectric materials is provided.