Structural defects and local chemistry across ferroelectric–electrode interfaces in epitaxial heterostructures

We present a detailed investigation of the chemistry at the growth interface between the bottom electrode and ferroelectric film in (001)-oriented epitaxial ferroelectric thin-film heterostructures. Three different ferroelectric systems, namely PbZr_0.2Ti_0.8O₃, PbZr_0.52Ti_0.48O₃, and BaTiO₃ deposited on SrRuO₃/SrTiO₃, were investigated to compare and contrast the role of lattice mismatch versus the volatility of the deposited cation species. A combination of transmission electron microscopy-based imaging and spectroscopy reveals distinct correlations among the ferroelectric thin-film composition, the deposition process, and chemical gradients observed across the ferroelectric–electrode interface. Sr diffusion from the electrode into the ferroelectric film was found to be dominant in PbZr_0.2Ti_0.8O₃/SrRuO₃/SrTiO₃ thin films. Conversely, Pb diffusion was found to be prevalent in PbZr_0.52Ti_0.48O₃/SrRuO₃/SrTiO₃ thin films. The BaTiO₃/SrRuO₃/SrTiO₃ heterostructure was found to have atomically sharp interfaces with no signature of any interdiffusion. We show that controlling the volatility of the cation species is as crucial as lattice mismatch in the fabrication of defect-free ferroelectric thin-film devices.     

Epitaxial PbZrxTi1−xO3 Ferroelectric Bilayers with Giant Electromechanical Properties

Giant electromechanical response viaferroelastic domain switching is achieved in epitaxial (001) ferroelectric tetragonal (T) PbZr_0.3Ti_0.7O₃/rhombohedral (R) PbZr_0.55Ti_0.45O₃ bilayers, grown on La_0.67Sr_0.33MnO₃ buffered SrTiO₃ substrates. X‐ray diffraction and transmission electron microscopy show that the domain structure of the T films is tuned as a function of its thickness, from a fully a₁/a₂‐domains (30 nm thick T layer) to a three domain stress‐free c/a₁/c/a₂ polytwin state (100 nm thick T layer). A large switchable polarization is found up to 65 μC cm⁻². Quantitative piezoelectric force microscopy reveals enhanced piezoelectric coefficients, with d₃₃ coefficients ranging from 250 to 350 pm V⁻¹, which is up to seven times higher than the nominal PbZr_xTi_1−xO₃ thin film values. These are attributed to the motion of nanoscale ferroelastic domains. Fatigue testing proves that these domains are reversible and repeatable up to 10⁷ cycles. In‐situ X‐ray synchrotron measurements reveal that the ferroelastic domain switching is promoted by a pulsating strain effect imposed by the R layer. The study reports a fundamental understanding of the origin of giant piezoelectric coefficients in epitaxial ferroelectric bilayers.     

Tuning Susceptibility via Misfit Strain in Relaxed Morphotropic Phase Boundary PbZr1‐xTixO3 Epitaxial Thin Films

Epitaxial strain is a powerful tool to manipulate the properties of ferroelectric materials. But despite extensive work in this regard, few studies have explored the effect of epitaxial strain on PbZr_0.52Ti_0.48O₃. Here we explore how epitaxial strain impacts the structure and properties of 75 nm thick films of the morphotropic phase boundary composition. Single‐phase, fully epitaxial films are found to possess “relaxed” or nearly “relaxed” structures despite growth on a range of substrates. Subsequent studies of the dielectric and ferroelectric properties reveal films with low leakage currents facilitating the measurement of low‐loss hysteresis loops down to measurement frequencies of 30 mHz and dielectric response at background dc bias fields as large as 850 kV/cm. Despite a seeming insensitivity of the crystal structure to the epitaxial strain, the polarization and switching characteristics are found to vary with substrate. The elastic constraint from the substrate produces residual strains that dramatically alter the electric‐field response including quenching domain wall contributions to the dielectric permittivity and suppressing field‐induced structural reorientation. These results demonstrate that substrate mediated epitaxial strain of PbZr_0.52Ti_0.48O₃ is more complex than in conventional ferroelectrics with discretely defined phases, yet can have a marked effect on the material and its responses.     

Understanding the Role of Ferroelastic Domains on the Pyroelectric and Electrocaloric Effects in Ferroelectric Thin Films

Temperature‐ and electric‐field‐induced structural transitions in a polydomain ferroelectric can have profound effects on its electrothermal susceptibilities. Here, the role of such ferroelastic domains on the pyroelectric and electrocaloric response is experimentally investigated in thin films of the tetragonal ferroelectric PbZr_0.2Ti_0.8O₃. By utilizing epitaxial strain, a rich set of ferroelastic polydomain states spanning a broad thermodynamic phase space are stabilized. Using temperature‐dependent scanning‐probe microscopy, X‐ray diffraction, and high‐frequency phase‐sensitive pyroelectric measurements, the propensity of domains to reconfigure under a temperature perturbation is quantitatively studied. In turn, the “extrinsic” contributions to pyroelectricity exclusively due to changes between the ferroelastic domain population is elucidated as a function of epitaxial strain. Further, using highly sensitive thin‐film resistive thermometry, direct electrocaloric temperature changes are measured on these polydomain thin films for the first time. The results demonstrate that temperature‐ and electric‐field‐driven domain interconversion under compressive strain diminish both the pyroelectric and the electrocaloric effects, while both these susceptibilities are enhanced due to the exact‐opposite effect from the extrinsic contributions under tensile strain.     

Impact of Zr/Ti ratio in the PZT on the photoreduction of silver nanoparticles

Silver nanoparticle deposition from an aqueous solution of silver nitrate onto the surface of PZT thin films of stoichiometric compositions PbZr_0.3Ti_0.7O₃ and PbZr_0.52Ti_0.48O₃ has been investigated. The impact of Zr/Ti ratio on the photochemical properties of PZT is shown by the preferential growth of silver nanoparticles onto the surface. Photoreduction of silver occurs on both c⁺ and c⁻ domains on PbZr_0.52Ti_0.48O₃ whereas it occurs only on c⁺ domains on a PbZr_0.3Ti_0.7O₃ surface. The difference in deposition pattern is attributed to difference in magnitude of spontaneous polarization, effective hole concentration and band gap of the two samples which impacts shape and width of space charge layer in the two samples resulting in a change in band bending at the surface.     

Fabrication of a plane-parallel interface in Ni/PbZr<Subscript>0.2</Subscript>Ti<Subscript>0.8</Subscript>O<Subscript>3</Subscript> heterostructures

A process has been proposed for producing a plane-parallel ferromagnetic/ferroelectric interface, which ensures a reproducible magnetoelectric performance of Ni/PbZr_0.2Ti_0.8O₃ (PZT) heterostructures. Its principle is to smooth the initial surface profile of the PZT ceramic substrate to a submicron level by sequential deposition/sputtering of three layers 0.2, 0.1, and 0.05 μm in thickness through ion beam sputtering of a target having the same composition as the substrate, followed by the growth of a nickel film on the smoothed surface. This allows one to preclude film bulging and spalling and ensures high quality of the interface.     

Better,Faster, and Less Biased Machine Learning: Electromechanical Switching in Ferroelectric Thin Films

Machine-learning techniques are more and more often applied to the analysis of complex behaviors in materials research. Frequently used to identify fundamental behaviors within large and multidimensional datasets, these techniques are strictly based on mathematical models. Thus, without inherent physical or chemical meaning or constraints, they are prone to biased interpretation. The interpretability of machine-learning results in materials science, specifically materials’ functionalities, can be vastly improved through physical insights and careful data handling. The use of techniques such as dimensional stacking can provide the much needed physical and chemical constraints, while proper understanding of the assumptions imposed by model parameters can help avoid overinterpretation. These concepts are illustrated by application to recently reported ferroelectric switching experiments in PbZr_0.2Ti_0.8O₃ thin films. Through systematic analysis and introduction of physical constraints, it is argued that the behaviors present are not necessarily due to exotic mechanisms previously suggested, but rather well described by classical ferroelectric switching superimposed by non-ferroelectric phenomena, such as electrochemical deformation, electrostatic interactions, and/or charge injection.     

Heterolayered PZT thin films of different thicknesses and stacking sequence

The effects of stacking sequence and thickness toward the texture and electrical properties of heterolayered PbZr _x Ti_1−x O₃ (PZT) films, consisting of alternating PbZr_0.7Ti_0.3O₃ and PbZr_0.3Ti_0.7O₃ layers, have been studied. Thickness dependence is observed in the ferroelectric and dielectric behavior of the heterolayered PZT films whereby the remanent polarization (P _r) and relative permittivity (ε) increase with thickness, while coercive field (E _c) decreases. When baked at 500 °C and thermally annealed at 650 °C, the heterolayered PZT films regardless of their stacking sequence exhibit perovskite phase with (001)/(100) preferred orientation. Interestingly, the stacking sequence of the heterolayered PZT films dictates the morphology of the films which eventually affects the ferroelectric and dielectric performance. The heterolayered PZT film with PbZr_0.7Ti_0.3O₃ as the first layer (heterolayered PZ₇₀T₃₀ film) exhibits a large grain size in the range of 1–3 μm and shows superior properties as compared to the heterolayered PZT films with PbZr_0.3Ti_0.7O₃ as the first layer (heterolayered PZ₃₀T₇₀ film), which exhibits a much smaller grain size. From the sub-switching field measurement according to the Rayleigh law, there appears a lower concentration or mobility of domain walls in the small-grained heterolayered PZ₃₀T₇₀ films.     

Comparative investigating the properties of Pb-based multilayer ferroelectric thin films for FeRAM

The PbZr_0.3Ti_0.7O₃(PZT) thin film and multilayer PbZr_0.3Ti_0.7O₃/PbTiO₃(PZT/PT), PbTiO₃/PbZr_0.3Ti_0.7O₃/ PbTiO₃(PT/PZT/PT) thin films were prepared by a Sol-Gel method on the Pt(111)/Ti/SiO₂/Si(100) substrate for FeRAM application. The microstructure, ferroelectric, fatigue, dielectric, and leakage current properties of these thin films were investigated. The results indicate that the multilayer PT/PZT/PT thin film have a better ferroelectric, fatigue, dielectric and leakage current density properties. Its remanent polarization P_r reaches a maximum value of 21.2 μC/cm² and the coercive field E_c gets to a minimum value of 64.2kV/cm. After 10¹⁰ cycles, it still has more than 80% remnant polarization. The PT/PZT/PT thin film exhibits lower dielectric constant and lower dielectric loss, which is beneficial for FeRAM application. It also has the lowest leakage current density. The leakage current mechanism of the PZT, PZT/PT and PT/PZT/PT thin films is correlated to the microstructure and can be modeled in terms of GBLC and SCLC theory. The microstructure and electric properties of these films are correlated. The double-sided PT seed layers enhance not only the microstructure but also the electric properties. It is evident that the PT/PZT/PT multilayer thin film is a promising candidate for FeRAM application.     

Ferroelectric field effect in SrCuO2 and SrRuO3 films

We report on ferroelectric field effect experiments on Pb(Zr_0.52Ti_0.48)O₃/SrCuO₂ and Pb(Zr_0.52Ti_0.48)O₃/SrRuO₃ epitaxial heterostructures with an emphasis on the material characterization. Upon reversing the polarization of the Pb(Zr_0.52Ti_0.48)O₃ ferroelectric layer, we measured a nonvolatile change in the resistivity of ultrathin layers of SrCuO₂ and SrRuO₃. In thin SrRuO₃ films (30 Å) up to a 9% resistivity change has been observed at room temperature.     

Microstructure, domain structures and ferroelectric properties in (Pb0.8,La0.1,Ca0.1)TiO3/Pb(Zr0.2,Ti0.8)O3 bilayered thin film

Ferroelectric (Pb_0.8,La_0.1,Ca_0.1)TiO₃/Pb(Zr_0.2,Ti_0.8)O₃ (PLCT/PZT) bilayered thin film was prepared on Pt(111)/Ti/SiO₂/Si(100) substrate by RF magnetron sputtering technique. Pure perovskite crystalline phase, determined by X-ray diffraction, was formed in the PLCT/PZT bilayer. The bilayered film exhibited a very dense and smooth surface morphology with a uniform grain size distribution. The ferroelectric domain structures were investigated by a combination of vertical and lateral piezoresponse force microscopy (VPFM and LPFM, respectively). It is demonstrated by both VPFM and LPFM observations that out-of-plane and in-plane lamellar ferroelectric domains coexist in the bilayered thin film. The PLCT/PZT bilayered film possesses good ferroelectric properties with relatively high spontaneous polarization (2P_s = 82 µC/cm²) and remnant polarization (2P_r = 26.2 µC/cm²).     

Nanoscale domain switching in Bi3.15Eu0.85Ti3O12 thin films annealed at different temperature by scanning probe microscopy

The nanoscale domain switching behavior of Bi_3.15Eu_0.85Ti₃O₁₂ (BET) thin films annealed at temperatures of 600, 650 and 700 °C is investigated by scanning probe microscopy (SPM) via the direct observation on their domain structure. It is shown that most ferroelectric domains are clearly detectable and confined in the grains. Some domains opposite to the polarizing electric field were observed by SPM, and their formations are attributed to the grain boundaries playing an active roles in pinning a preferential polarization state. The remnant polarization (2P_r) values of BET thin films increase with the annealing temperature due to the increase of nanoscale domains switched into the direction of polarizing electric field.     

Microstructure and Nanoscale Piezoelectric/Ferroelectric Properties in La2Ti2O7 Thin Films Grown on (110)‐Oriented Doped Nb:SrTiO3 Substrates

(00l)‐Oriented La₂Ti₂O₇ (LTO) thin films with monoclinic perovskite‐layer structure [a = 7.806(2) Å, b = 5.552(3) Å, c = 13.015(5) Å, β = 98.62(2)°] have been grown by a sol–gel route on conducting (110)‐oriented doped Nb:SrTiO₃ (STO) substrates. The narrow rocking curves (0.24° width for 004_LTO peak) demonstrate the sharp mosaïcity of the films. Using high‐resolution X‐ray diffraction (HR‐XRD), epitaxial relationships between the LTO, and the STO substrate are given. In addition, HR‐XRD evidences the existence of (212)‐oriented crystallites 1.5° disoriented with respect to the plane of the substrate. We confirm, by DFT calculations, that the polarization vector lies in the b‐axis of the LTO cell and consequently, the existence of these (212)‐oriented crystallites enables to explain the origin of the various contrasts observed both on the in‐plane and out‐of‐plane images when collected by piezoresponse force microscopy. Finally, both successful poling experiments performed via the tip of atomic force microscope and the existence of local piezoloops within the domains, unambiguously confirm the ferroelectric state of the films at the nanoscale level. Once again, this study demonstrates that a clear understanding of nanoscale piezoelectric/ferroelectric phenomena in oriented thin films passes through a carefully structural analysis as performed by HR‐XRD.     

Magnetic properties and magnetoelectric effect in Ni0.8Co0.1Cu0.1Fe2O4 + PbZr0.2Ti0.8O3 composites

Composites with compositions x (Ni_0.8Co_0.1Cu_0.1Fe₂O₄) + (1−x) PbZr_0.2Ti_0.8O₃ (x-mole fraction varies as 0.15, 0.25, 0.35 and 0.45) were prepared by standard ceramic method. The presence of constituent phases, namely ferrite and the ferroelectric were confirmed by X-ray diffraction. The structural analyses were carried out using the obtained powder pattern X-ray data. The porosity of the samples was calculated and the values obtained lie between 10% to 19%. To date, the variations in the magnetic properties with variation in ferrite phase in composites and thereby its influence on magnetoelectric effect is not yet reported. The saturation magnetization (Ms) and magnetic moment (η_B) in Bohr magneton were calculated for all the composites. The static value of magnetoelectric voltage coefficient (dE/dH) was measured as a function of intensity of magnetic field. The maximum value of ME coefficient was observed for a composite with 35% ferrite + 65% ferroelectric phase.     

Mechanical and electrical switching of local ferroelectric domains of K<Subscript>0.5</Subscript>Bi<Subscript>4.5</Subscript>Ti<Subscript>4</Subscript>O<Subscript>15</Subscript> film

Ferroelectric/Piezoelectric K_0.5Bi_4.5Ti₄O₁₅ (KBT) film was fabricated by pulsed laser deposition method and confirmed by ferroelectric, dielectric measurements and local butterfly-type piezoresponse hysteresis loops. Importantly, ferroelectric domain switching by both electrical field and mechanical force in KBT film was demonstrated. The dark and bright contrast represents the PFM response of the up and down polarized domains, which can be written by a dc bias of ±12 V or a mechanical force of 40–50 nN. The successful demonstration of mechanical force switching of ferroelectric domain in KBT film other than electric field provides a novel mean for information storage and sensors.     

Giant Ferroelectric Polarization in Ultrathin Ferroelectrics via Boundary‐Condition Engineering

Tailoring and enhancing the functional properties of materials at reduced dimension is critical for continuous advancement of modern electronic devices. Here, the discovery of local surface induced giant spontaneous polarization in ultrathin BiFeO₃ ferroelectric films is reported. Using aberration‐corrected scanning transmission electron microscopy, it is found that the spontaneous polarization in a 2 nm‐thick ultrathin BiFeO₃ film is abnormally increased up to ≈90–100 µC cm^?2 in the out‐of‐plane direction and a peculiar rumpled nanodomain structure with very large variation in c /a ratios, which is analogous to morphotropic phase boundaries (MPBs), is formed. By a combination of density functional theory and phase‐field calculations, it is shown that it is the unique single atomic Bi₂O_3?x layer at the surface that leads to the enhanced polarization and appearance of the MPB‐like nanodomain structure. This finding clearly demonstrates a novel route to the enhanced functional properties in the material system with reduced dimension via engineering the surface boundary conditions.     

The study of the electric and magnetic properties of PbZr0.2Ti0.8O3-BiFeO3 multilayers

The results of the electric and magnetic measurements performed on PbZr_0.2Ti_0.8O₃-BiFeO₃ symmetric structures, deposited on Pt/Si wafers, were compared for different number of layers in order to analyse the effect of interfaces over the macroscopic properties. It was found that the shape and magnitude of the capacitance-voltage characteristic, as well as the shape and parameters of the ferroelectric and magnetic hysteresis, depend on the number of interfaces in the intended multilayer structure. A temperature induced gradual transition from a magnetically disordered spin glass like phase of low temperature to an uncompensated antiferromagnetic phase at room temperature takes place in the BiFeO₃ films, under low applied magnetic fields. A partial ferromagnetic like order can be obtained at low temperatures by increasing the field. The observed changes in the electric and magnetic behaviour of the systems were related to an increased degree of disorder for electric dipoles and magnetic moments, due to the increased number of layers and crystallization treatments.     

Effect of seed layer on the ferroelectric properties and leakage current characteristics of sol–gel derived vanadium doped PbZr0.53Ti0.47O3 films

In this paper, we report the ferroelectric properties and leakage current characteristics of vanadium-doped PbZr_0.53Ti_0.47O₃ (PZTV) films grown on various seed layers prepared by a sol–gel process. The PZTV multilayered film of ~250-nm-thick showed excellent ferroelectric properties, with a large remnant polarization (P _r) of ~30 μC/cm² (E _c ~ 90 kV/cm), a high saturation polarization (P _s) of ~85 μC/cm² for an applied field of 1,000 kV/cm, fatigue-free characteristics of up to ≥ 10¹⁰ switching cycles, and a low leakage current density of 7 × 10⁻⁸ A/cm² at 100 kV/cm. X-ray diffraction (XRD), atomic force microscopy (AFM), and scanning electron microscopy (SEM) investigations indicated that PZTV films grown on PbZr_0.53Ti_0.47O₃/PbLa_0.05TiO₃ (PZT/PLT) seed layers exhibited a dense, well-crystallized microstructure with random orientations and a rather smooth surface morphology.     

Quantitative Nanometer‐Scale Mapping of Dielectric Tunability

Two scanning probe microscopy techniques—near‐field scanning microwave microscopy (SMM) and piezoresponse force microscopy (PFM)—are used to characterize and image tunability in a thin (Ba,Sr)TiO₃ film with nanometer scale spatial resolution. While sMIM allows direct probing of tunability by measurement of the change in the dielectric constant, in PFM, tunability can be extracted via electrostrictive response. The near‐field microwave imaging and PFM provide similar information about dielectric tunability with PFM capable to deliver quantitative information on tunability with a higher spatial resolution close to 15 nm. This is the first time, information about the dielectric tunability is available on such length scales.     

Ferroelectric and magnetic studies on unpoled Poly (vinylidine Fluoride)/Fe3O4 magnetoelectric nanocomposite structures

The fabrication of flexible Poly (vinylidine Fluoride)/Fe₃O₄ magnetoelectric nanocomposite films with different weight fractions of Fe₃O₄ nanoparticles is explained in this paper. The effects of nano Fe₃O₄ content on the structural, chemical, thermal, and magnetoelectric properties of PVDF matrix are discussed. XRD and FTIR results reveal the interaction between the filler and the matrix and also they suggest that the β phase contribution can be significantly controlled by the inclusion of nano Fe₃O₄. Thermal stability and melting point behavior of the composite films are investigated through TG/DTA. The existence of ferrimagnetism and ferroelectric properties in the films are proved through the magnetization and polarization studies. The hysteresis loops show the largest polarization and magnetization values at 0.14 wt% of Fe₃O₄. Also, the dependence of ferroelectric polarization on temperature is investigated and reported.