New modalities of strain-based engineering of ferroelectrics: domain structure and properties of PbZr1-xTixO3 thin films

Epitaxial strain has been widely used to modify the crystal and domain structure, and ultimately the dielectric, ferroelectric, and pyroelectric responses of ferroelectric thin-films for a wide variety of applications including memories, transducers, energy harvesters, sensors, and many more. Traditionally, the ability to engineer materials using epitaxial strain has been confined to a limited range of materials systems which are closely lattice matched to commercially available substrates. In turn, considering the PbZr1-xTixO3 system, a model ferroelectric, study of strain effects has been primarily limited to the Ti-rich variants where a wealth of closely lattice matched substrates (~±1%) exists, enabling nearly-coherently-strained films to be obtained. While these studies have generated a wealth of knowledge on the basic effects of epitaxial strain and have demonstrated the ability to enhance ferroelectric susceptibilities, these improvements have only been incremental. In the present work, we seek to expand the bounds of epitaxial strain engineering through the use of chemistry, controlled epitaxial strain (and relaxation), and compositional and strain gradients with the directive of generating phase competition and high-energy ferroelastic domain structures to enhance ferroelectric susceptibilities. We, for the first time, grow epitaxial thin films of PbZr1-xTixO3 across the compositional phase diagram, and show that the rate of strain relaxation is significantly enhanced at the morphotropic phase boundary (PbZr0.52Ti0.48O3), as the result of the high adaptability of the crystal and domain structure. Despite the appearance of nearly “relaxed” crystal structures, PbZr0.52Ti0.48O3 thin films were shown to exhibit significantly different dielectric responses when grown on various substrates. Highlighting the more nuanced effects of partial strain relaxation in tuning ferroelectric susceptibilities. We then proceed to extend the bounds of epitaxial strain by growing compositionally-graded heterostructures which facilitates the retention of strains in excess of 3.5%, large strain gradients (~4.35x10-5 m-1), and has the ability to stabilize the tetragonal crystal and domain structure, in PbZr1-xTixO3 solid solutions that are rhombohedral in the bulk. Furthermore, we show that by varying the compositional-gradient form (in terms of the nature of the compositional gradient and heterostructure thickness) that it is possible to generate large built-in potentials, which significantly suppresses the dielectric response, but has minimal detrimental impact on the ferroelectric and pyroelectric susceptibilities, giving rise to dramatically enhanced pyroelectric figures of merit. Additionally, we show that the magnitude of the built-in potential does not follow predictions based on the magnitude of the strain gradient alone, and instead, is enhanced by local strain gradients which occur when traversing chemistries associated with structural phase boundaries (where there are abrupt changes in the lattice parameter) and at/near ferroelastic domain boundaries. Finally, we explore the nanoscale response of these ferroelastic domains in compositionally-graded heterostructures using a combination of transmission electron microscopy-based nanobeam diffraction strain mapping and multi-dimensional band-excitation switching spectroscopy. We observe that the presence of compositional and strain gradients can preferentially stabilize highly-energetic, needle-like domains, which under electrical excitation are highly labile in the out-of-plane direction (while remaining spatially fixed in the plane). These labile domain walls act like domain wall springs (enhancing the magnitude of the built-in potential), transferring elastic energy between the a and c domains (depending on the direction of the applied bias), and giving rise to locally enhanced piezoresponse at the a domains, as the result of a switching mechanism where the a domains expand towards the free surface or are nearly excluded from the film (once again, depending on the direction of applied bias). These studies demonstrate the efficacy of compositional and strain gradients as a new modality of strain engineering capable of significantly altering the crystal and domain structure, and ferroelectric susceptibilities of ferroelectric thin films inconceivable in the absence of compositional/strain gradients