Stacking Design And Electrical Properties Of Freestanding Perovskite Ferroelectric Thin Films

The ferroelectric oxide material Pb(Zr_1-xTi_x)O₃with perovskite structure has been widely used in various microelectronic devices due to its excellent dielectric,piezoelectric,ferroelectric,and pyroelectric properties.In recent years,with the rapid development of flexible electronics,researchers have employed various physical or chemical techniques to convert high-performance perovskite ferroelectric oxides from rigid to flexible states to meet the current demands of electronic devices.One important approach involves the preparation of ferroelectric thin films on rigid substrates,followed by their separation from the substrate through chemical etching,mechanical force peeling,laser delamination,or other means,and then transferring them onto flexible substrates to achieve the freestanding ferroelectric thin films.Among these methods,the use of chemical etching and peeling techniques can yield high-quality single-crystal freestanding ferroelectric thin films,which have the potential to achieve better electrical performance and facilitate the study of the polarization evolution mechanism in freestanding ferroelectric films under new flexible states.However,most research efforts have focused only on the preparation and study of single-layer freestanding PZT thin films,with little in-depth investigation into superlattices and freestanding multilayer films.There is a lack of understanding of the performance and principles behind multilayer stacked films.Therefore,in this thesis,we selected Pb(Zr_0.52Ti_0.48)O₃（PZT）material and prepared epitaxial PZT thin films and superlattices.We systematically investigated the piezoelectric,ferroelectric,leakage,and other electrical characteristics of the superlattices and freestanding PZT thin films,and discussed their underlying principles.This research aims to achieve unrestricted manipulation,integration,and utilization of the thin films.The specific research content is as follows:1.High-quality PZT_m/STO₃（m=2,3,6,...,u.c.）ferroelectric superlattices were prepared on（001）-oriented La_0.67Sr_0.33Mn O₃/Sr Ti O₃（LSMO/STO）substrates using pulsed laser deposition（PLD）technique.The microstructure,energy storage performance,ferroelectric properties,and dielectric properties of the PZT/STO superlattices were systematically studied.It was found that the energy storage density and efficiency(W_rec≈22.22J/cm³,η≈55%)of the PZT/STO superlattices were significantly higher than those of the PZT films.Compared to pure PZT thin films,the dielectric constant of the PZT₆/STO₃ superlattice increased by approximately 160%.We observed a process in which the in-plane domains gradually transformed from microdomains to nanodomains as the thickness of the PZT period decreased,which explained the enhanced response to electrostatic and ferroelectric relaxation under electric field and the improvement in energy storage performance.The results demonstrate that the modulation of the electrostatic boundary conditions and the control of interface multi-domain structures can be achieved by varying the periodic thickness of the ferroelectric/dielectric layers in the superlattice,thereby improving the dielectric,piezoelectric,and energy storage properties of the films.Importantly,we obtained self-supported PZT films by sacrificial layer technique using LSMO as the sacrificial layer,which lays the material foundation for the preparation of high-performance flexible ferroelectric microelectronic devices.2.A series of high-performance PZT thin films with different thicknesses were prepared on LSMO/STO substrates using PLD technology.The sacrificial LSMO layer was etched using a concentrated acidic KI solution,Obtaining a series of freestanding PZT thin films with high polarization properties and different thicknesses.Controlled stacking design of freestanding thin films was achieved,including single-layer（each layer 200 nm）,double-layer（each layer100 nm）,and four-layer（each layer 50 nm）,all with a total thickness of 200 nm,creating novel hetero-interfaces.Subsequently,the influence of the novel hetero-interfaces generated by the stacking of flexible PZT thin films on their electrical properties was systematically studied.It was found that the breakdown voltage of the four-layer 50 nm PZT thin film was 2 MV/cm,which was 33%higher than that of the single-layer 200 nm PZT thin film.The leakage current of the four-layer film was nearly 5 orders of magnitude lower than that of the single-layer film,and the longitudinal piezoelectric coefficient(referred to as d₃₃,approximately 80 pm/V)of the four-layer film was more than double that of the single-layer film.The study showed that the interfaces generated by thin film stacking significantly affected the piezoelectric,ferroelectric,and leakage properties of the thin films,laying the foundation for the fabrication of flexible micro-nano devices and the design of specific functional devices in the future.