Aberration Corrected Transmission Electron Microscopy Investigation Of Domain Structures In High-Index Ferroelectric Films

Ferroelectric materials have a spontaneous polarization below the Curie temperature,and this polarization can be coupled with various external fields,which makes these materials have great potential in functional devices such as non-volatile memories,brakes,and microelectronic mechanical systems.With the demands of miniaturization and energy conservation of such functional devices,great advantages have appeared in the form of ferroelectric films,whose performances are depended on their microstructure and domain structure especially their evolution under external electric field.Here,we designed and grew a variety of ferroelectric films based on classical tetragonal ferroelectric PbTi03 by using pulsed laser deposition.A combination of the aberration transmission electron microscopy,piezoelectric force microscopy and high-resolution X-ray diffraction was used to analyze the microstructures and domain structures of the prepared ferroelectric films at multiple scales.Finally,we analyzed the evolution process of the domain structure under external field and its influence on related properties.Related research results have shown that the performance characteristics of[111]-oriented ferroelectric thin films are distinct from the[001]-oriented ones,such as lower coercive field,higher dielectric properties and negligible substrate clamping effect on piezoelectric response.However,thorough investigations of the domain structures and their modulation are lacking for high-index ferroelectric thin films for their complexity.Here,we deposited a serial of[111]-oriented PbTiO3 thin films with different thicknesses on orthogonal GdSc03(101)o substrates and investigated the domain evolutions and the structural details.Under the asymmetric strain applied by the substrates,the original three ferroelastic variants(d1,d2 and d3)in bulk PbTi03 are reduced to two(d2 and d3),forming two kinds of domain configurations:d2+/d3+ and d2-/d3+.With the increase of PbTi03 film thickness,the domain structures of the PbTi03 films evolve from a mixed state containing d2+/d3+ and d2-/d3+ domains to a single state only containing d2+/d3+ domain.The period of the d2+/d3+ domain structure is proportional to the square root of the film thickness,which meets the KMF law.Further aberration-corrected scanning transmission electronic microscopy and piezoelectric force microscopy demonstrate that the lattice and polarization characteristics of the two kinds domain structures are consistent with 90°ferroelastic domains,and the distribution of lattice strain is correlated with c-axis projection component.This work enriches our understanding of domain configuration and its modulation,and provides guidance to fabricating ferroelectric domain based nano-devices.The performance of ferroelectric is strongly correlated with the domain structure,especially its evolution under external electric field.For example,electric field induced domain reorientation was thought to be the origin of excellent piezoelectric responses near the morphotropic phase boundaries of ferroelectric oxides.However,the insight mechanism that causes the outstanding properties has been a long-standing debate,and the other two possible contributions were attributed to intrinsic lattice response of monoclinic phase and electric-field induced phase transformation.Here,we fabricated[001]and[111]-oriented PbTi03 films,and the crystal of[001]-oriented ones are proven to be tetragonal forming single c domain,while the[111]-oriented ones show MPB-like monoclinic crystal structure forming complicated domain configurations with {201} domain walls.Subsequent PFM switching studies manifest only domain reorientation occurred in[001]-oriented PbTi03 films,while the[111]-oriented ones undergo domain reorientation and electric-field induced reversible monoclinic-tetragonal phase transformation successively.Finally,by analyzing the piezoelectric response process,we give direct evidence that the field-induced structural transformation contributes mostly to piezoelectricity near the morphotropic phase boundary compared with intrinsic lattice response and domain orientation.This study not only reveals the domain structure of monoclinic phase,but also clarifies the origin of piezoelectric response,which may facilitate the development of high-performance piezoelectric materials for electronic device applications.Variety of studies have revealed the properties are not only correlated with the domain structure,but also dependent on the microstructure,especially the inhomogeneous strain distribution caused by local defect,which could affect retention properties through flexoelectric effect,but still lack of direct experimental evidence.Here,we designed two kinds PbTiO3 films:normal PbTiO3 films and strain-gradient PbTi03 films.Using a combination of piezoelectric force microscopy,atomic-scale scanning transmission electron microscopy,and reciprocal space mapping,we find that the polarization retention failure can be caused by a strain gradient induced flexoelectric field in PbTi03 films.Atomic imaging reveals that the strain gradient is introduced by tensile strains,which is resultant from the vertically distributed Pb-rich anti-phase domains.This strain gradient couples with polarization and results in flexoelectric fields in the films,leading to retention failure in the films.Our study directly underlines the atomic mechanisms behind the strain gradient induced macroscopic retention failure behavior and clarifies the effect of local strain state on domain relaxation processes,thus can shed light on further understanding and improving the retention properties of oxide ferroelectrics.