Study Of Ferroelastic Superdomain And Electrical Properties In Epitaxial PZT Ferroelectric Thin Film

In ferroelectric devices,the ferroelectric domain switching is the fundamental physics of their functional properties.The non-volatile ferroelastic strain driven by electric fields can provide an alternative way for the ferroelectric domain switching.This plays a crucial role in the realization of low-energy-consumption devices in the future,such as sensing,driving,information storage,and magnetoelectric coupling.Because of the clamping of the substrate to the ferroelectric thin film,the ferroelastic strain generated in the ferroelastic domain switching process is suppressed,which makes the electric field driven non-volatile ferroelastic strains are difficult to achieve in the integrated devices.If the a/c domains of the PZT thin film are arranged regularly in a large scale,an effective net polarization can be used macroscopically,so called ferroelastic "superdomain".These superdomains in the film will effectively reduce the mismatch stress.Because of mechanical and electrostatic incompatibility,the superdomain boundaries will likely exhibit new physical properties.Compared to uncharged domain walls and bulk materials,partial charged domain wall will produce greater conductivity.In addition,the superdomains may be transformed into larger-scale structures,such as a flux-closure structure with in-plane polarization.In this thesis,high quality epitaxial Pb(Zr,Ti)O3(PZT)ferroelectric thin films were prepared on SrTiO3 single crystal substrates by pulsed laser deposition(PLD).Different epitaxial orientations of the PZT thin films were controlled by the orientation of the single crystal SrTiO3 substrate.The ferroelastic superdomains in the film were comprehensively investigated,including the formation of the domains,microstructures,domain switching,and electrical properties.In addition,a new kind of epitaxial PZT thin films were fabricated based on van der Waals epitaxial growth,in which the substrate clamping effect is released.In the frame of the ferroelastic superdomain switching,the relationship between the formation of the ferroelastic superdomain and the macro electrical properties is discussed.The main results and conclusions are as following:1.In the PLD system,a well-designed laser focusing system was used to fine tune the laser energy density and fabricate high-quality epitaxial PbZr0.20Ti0.80O3(PZT)films.These films have different densities of a/c ferroelastic domain on(100)SrTiO3 single crystal substrates.Transmission electron microscopy(TEM)was used to characterize the microstructure and the distribution of the ferroelastic domains.The correlation between the ferroelastic domain distribution and the surface morphology of thin films has been established by means of piezoresponse force microscopy(PFM).When the composition and structure of the PZT film are fixed,the morphology of the film is fine-tuned via the energy density of the laser,and growth mode of the film gradually changes from a two-dimensional layer-by-layer to a three-dimensional island.In the middle of this prossessing,the density of a/c ferroelastic domain in the films decreases significantly and disappears immediately.The stripe-like ferroelastic a-domain,penetrating in the whole films,transforms into needle-like a-domain terminating inside the films,accompanied by high-conductivity 180° domain walls.Due to the reduction of substrate clamping and domain interactions,these needle-like ferroelastic domains exhibit an in-plane electrostrictive effects under the control of an out-of-plane electric field.2.High-quality(101)-oriented PZT epitaxial films were prepared on a(110)SrTiO3 single crystal substrate by PLD.Since the c/a ratio of the tetragonal PZT single crystal film is greater than 1,the PZT film orientation changes from(001)to(101),and the stress in the film changes from impressive stress to tensile stress.The control of the distribution and density of the ferroelastic domains is achieved by controlling the stress state between the film and the substrate.PFM and TEM results indicate that the(101)-PZT films have higher density of a/c ferroelastic domains,and present an ordered band distribution in the local area,i.e.ferroelastic superdomain.When an out-of-plane electric field was applied,these ferroelastic superdomains can be merged into super-sized superdomains.We found that the formation of a superdomain structure significantly improves the ferroelectric switching characteristics of the films.Not only that,we applied a(101)-PZT thin film with a superdomain structure to a photoelectrochemical cell and found that the electric-field-modulated ferroelectric switching based on the superdomain structure can change the direction of the photocurrent.It shows great potential of application in non-volatile energy storage devices.3.A method for transforming a(111)-PZT thin film from a disordered domain structure to a ferroelastic superdomain structure by loading a cyclic external electric field is designed.The formation of a superdomain structure can significantly enhance the macroscopic ferroelectric,dielectric,and especially fatigue properties of PZT films.In addition,we also epitaxially grown PZT ferroelectric thin films on(111)SrTiO3 single crystal substrate,PFM results show that the original(111)-PZT ferroelectric film exhibits a disordered nano-domain structure.A stable ferroelastic superdomain is formed after repeated loading for two to five cycles above the coercive voltage.We have found that an ordered superdomain structure enhances the ferroelectric properties of PZT films,including a 40%reduction in coercive voltage and a 50%reduction in leakage current.Not only that,there is less fatigue(<15%)after 109 pulse cycles.4.A preparation method for the ferroelastic superdomains based on crystallographic engineering was designed to realize the formation of a regular parallel-band distributed engineered ferroelastic superdomain in the(111)-PZT films.Rapid cooling produces an asymmetrical mechanical boundary condition in the films and forms large anisotropic misfit strain.Under the aforesaid cyclic electric field,the a/c ferroelastic domain preferentially forms a regular and stable engineered superdomain along the crystal direction of the maximum mismatch strain.During the PLD epitaxial growth of(111)-PZT ferroelectric films,we used an ultra-fast(>50? min-1)cooling rate.Next,we applied an electric field on the surface of the sample using a PFM conductive probe.The PFM results show that the samples exhibits a regular parallel band-shaped engineered ferroelastic superdomain along the(11-2)direction.The cross-sectional TEM results suggest that the band-like a/c ferroelastic domain wall are an angle of 33° to the plane of film surface.Instead of a large number of smaller-sized ferroelectric domains,these engineered superdomains are switched in a whole,so that the pinning effect of the domain wall is minimized,which will significantly improve the macroscopic electrical properties of the film.The results show that the(111)-PZT ferroelectric thin film exhibits higher ferroelectric polarization and dielectric constant than the aforementioned samples in(4),and presents fatigue-free characteristics after a 109-pulse cycles.5.Using van der Waals epitaxial technology,PZT epitaxial thin films were fabricated without substrate clamping,so that the ferroelectric domain reversal in the film is not clamped by the substrate.First,an ultra-thin(<5 nm)CoFe2O4 seeding layer was grown on the layered mica substrate,and then the bottom electrode SrRuO3 and PZT thin film were sequentially deposited.XRD and TEM results show that PZT films present(111)-orientation epitaxial growth.By applying a cyclic electric field to the film through a PFM conductive tip,a ferroelastic superdomain structure can still be produced.Compared to the macroscopic electrical properties of the(111)-PZT film deposited on the rigid-SrTiO3 substrates in(3),the remnant polarization and fatigue resistance of this ferroelectric film are significantly improved,which can be attributed to the ferroelectric domain switching without clamping from the substrates.At the same time,the prototype transistor devices based on van der Waals epitaxial film exhibits excellent bending fatigue characteristics and high temperature resistance characteristics.