Macroscopic Ferroelectric Properties Research Articles

Structural, dielectric and ferroelectric properties of BiFeO3–BiInO3 solid solution thin films obtained by sol-gel method

The construction of phase boundaries is a well-known strategy to enhance the performances of ferroelectrics. In the present work, BiFeO3 and BiInO3 with attractive high Curie temperature of 830 °C and 600 °C were considered to reveal an unexplored phase boundary. (100-x)BiFeO3-xBiInO3 (x = 10, 20, 30, 40, 41, 50, 60, and 80) films, with thicknesses of approximately 289 nm, were obtained through industrial-scalable sol-gel method on Pt/Ti/SiO2/Si substrates. Complete solid solution can form between BiFeO3 and BiInO3, changing from rhombohedral to orthorhombic structure with the increase of BiInO3 content. The macroscopic dielectric, ferroelectric and piezoelectric properties of the thin films were systematically studied to understand the effect of the solid solution, with its phase diagram being proposed. A phase boundary near 60BiFeO3–40BiInO3 is formed with much superior polarization switch and piezoelectric response, of which the Curie temperature is as high as 470 °C. The results not only deepen the understanding of BiFeO3-based solid solutions, but also push forward potential applications of BiFeO3-based films in devices particularly for potential application at elevated temperature.

Flexible ferroelectric Pb(Zr0.53Ti0.47)O3 epitaxial films with large radiation and high-temperature tolerance

Radiation and temperature hardness are important for electronic devices operating in harsh environments such as modern medical equipment, outer space, and nuclear energy industries. In this work, we report radiation and temperature-hardened flexible ferroelectric Pb(Zr0.53Ti0.47)O3 epitaxial films that were grown on flexible inorganic mica substrates via van der Waals heteroepitaxy and can be further used for building high-performance electronic devices. We systematically investigated the effect of 60Co-gamma radiation and temperature on the ferroelectric properties of Pb(Zr0.53Ti0.47)O3 films and their flexibility as well. Pb(Zr0.53Ti0.47)O3 films exhibit a radiation and temperature tolerance of up to 20 Mrad and 175 °C, respectively. In particular, the 20 Mrad-irradiated Pb(Zr0.53Ti0.47)O3 film has an excellent retention even after 105 s with extrapolate retention time longer than 10 years and polarization fatigue up to 1010 cycles as well as moderate flexibility and bending stability after bending 105 cycles. Furthermore, we identified that the degradation of macroscopic ferroelectric properties of irradiated Pb(Zr0.53Ti0.47)O3 films originates from the ferroelectric domain pinning effect induced by the ionization effect. Our work demonstrates that flexible ferroelectric Pb(Zr0.53Ti0.47)O3 films are promising for developing novel flexible electronic devices for applications in harsh environments.

Intrinsic 90° charged domain wall and its effects on ferroelectric properties

The domain is an important characteristic of ferroelectric materials, and the type of domain can have significant effects on the performance of ferroelectric materials. In particular, the unique wake-up and fatigue effects of Hf0.5Zr0.5O2 (HZO) films severely restrict improvements in terms of film performance. Recently, using integrated differential phase contrast (iDPC) scanning transmission electron microscopy (STEM) imaging, we observed the existence of intrinsic tail-to-tail 90° domain structures, charged domain walls (CDWs), and polarization relaxation on the TiN/HZO/TiN interfaces of HZO films. Based on this experimental investigation, we establish a phase-field model of the tail-to-tail 90°CDWs based on the time-dependent Ginzburg–Landau equation. In our model, the tail-to-tail 90°CDW affects the mesoscopic domain switching through a built-in electric field, which, in turn, affects the macroscopic ferroelectric properties of the thin film. The following conclusions are drawn: (a) the CDWs that are distributed horizontally or vertically along the thickness of the film cause a A-wake-up effect (antiferroelectric-like wake-up) with double hysteresis loops or a S-wake-up effect (spindle-like wake-up), with thick portions in the middle and both ends points of the hysteresis loop; (b) the S-wake-up caused by the CDWs distributed along the thickness of the film performs badly in fatigue tests and is easier to break down. The model proposed herein reveals the mechanism of the wake-up effect in hafnium oxide-based ferroelectric films. It provides a theoretical basis for the development of a new type of hafnium oxide-based ferroelectric memory system.

Structural evolution and ferroelectric properties in lead-free (1−x)(Bi0.5Na0.4K0.1)TiO3-xSrTiO3 solid solutions

Bi0.5Na0.5TiO3 (BNT)-based dielectric ceramics have received a lot of attention due to the increased demand for pulse ceramic capacitors. However, comprehensive study on the relationship between their internal phase structure, dielectric characteristics, and ferroelectric properties is still lacking. The phase evolution and its impact on dielectric and ferroelectric properties of an important BNT-based solid solution, Bi0.5Na0.4K0.1TiO3-xSrTiO3 (x = 0, 0.1, 0.2, 0.3 and 0.4), were investigated systematically in this work using structural, dielectric, and ferroelectric characterization techniques. X-ray diffraction indicated the coexistence of rhombohedral and tetragonal phases. The frequency- and temperature-dielectric characterization was then used to derive the characteristic temperatures TB, Tm, Ts, and Td, and a phase diagram was developed. Furthermore, the temperature-dependent current against electric field curves and polarization versus electric field loops were used to derive the characteristic temperatures connected to high electric field features. This study not only explains the phase evolution of the Bi0.5Na0.4K0.1TiO3-xSrTiO3 solid solution, but it also correlates microscopic domains and polar nanoregions to macroscopic dielectric and ferroelectric properties.

Enhanced domain wall conductivity in photosensitive ferroelectrics Sn2P2S6 with full-visible-spectrum absorption

Recent optical stimulation suggests a vital non-contact pathway to manipulate both macroscopic and microscopic ferroelectric properties and paves the foundation for optoelectronics devices. However, up to date, most optical-related manipulation of ferroelectric properties is restricted due to their intrinsic bandgap and limited visible light spectrum absorption. Here, we reveal non-oxide Sn2P2S6 single crystal possesses full-visible-spectrum absorption (from 300 to 800 nm) with a unique disproportionation mechanism of photoexcited Sn ions and Urbach tail, which is not contradicting to the intrinsic band gap. Interestingly, we observed the existence of conductive domain walls (c-DW) and the light illumination induced significant enhancement of the domain wall conductivity caused by such disproportionation reaction. In addition, the domains separated by c-DW also exhibited noticeable electrical conductivity difference in the presence of optical illumination owing to the interfacial polarization charge with opposite signs. The result provides a novel opportunity for understanding the electrical conductivity behavior of the domains and domain walls in ferroelectrics with full-visible-spectrum absorption and achieving greatly enhanced performances for optoelectronics.

Effect of strain and stoichiometry on the ferroelectric and pyroelectric properties of the epitaxial Pb(Zr0.2Ti0.8)O3 films deposited on Si wafers

Properties of epitaxial PbZr0.2Ti0.8O3 (PZT) films deposited on Si substrates were investigated for integration in the present CMOS technology. Polarization is downward oriented, in association with the presence of an internal electric field, and has a lower value compared to the PZT films deposited on single crystal perovskite SrTiO3 (STO) substrates (40 µC/cm2 versus 80 µC/cm2), while the dielectric constant is larger (180 versus 120). Large value for the pyroelectric coefficient was also found, 1.22 × 10−3C/m2K, as for PZT grown on single crystal STO. The macroscopic ferroelectric and pyroelectric properties appear to be affected by the structural quality and stoichiometry of the PZT film. The changes in the electric properties are an effect of the strain gradients induced by the large difference between the thermal expansion coefficients of PZT and Si substrate, leading in turn to Pb oxidation and antisite defect formation compared to PZT films deposited on STO substrates.

Grain size effects in donor doped lead zirconate titanate ceramics

The ferroelectric, ferroelastic, and dielectric properties as well as the crystal structure were investigated for polycrystalline donor doped lead zirconate titanate (PZT) with grain sizes ranging from 0.25 to 5 μm, which were prepared using a novel zirconium titanium hydrate precursor (ZTH) with a specific surface area of 310 m2/g. Piezoforce microscopy was used to investigate the change in the domain structure, revealing a change in the domain configuration from a complex 3D structure to a simple lamellar domain formation at a 1 μm grain size that corresponded to a rapidly increasing internal mechanical stress observed with in situ synchrotron x-ray experiments. The correlation between the change in domain configuration, increasing internal stresses, effects of poling on the crystal structure, and the macroscopic ferroelectric and ferroelastic properties are discussed in detail, allowing a deeper understanding of size effects in polycrystalline donor doped PZT ceramics.

PVDF–ferrite composites with dual magneto-piezoelectric response for flexible electronics applications: synthesis and functional properties

In the present work, a magnetodielectric flexible thick film composite with a quaternary Ba12Fe28Ti15O84 ferrite filler (≤ 9 vol%) embedded into PVDF polymer matrix was investigated. The role of filler volume on the macroscopic dielectric, magnetic and ferroelectric properties as well as on the nanoscale magnetic and piezoelectric response was analyzed. The formation of small amounts of polar phases besides the majority α-phase of PVDF was shown by XRD and FTIR combined analysis. The electrical properties are dominated by the polymer response, while magnetic order is derived as sum property from the ferrite ones, having a predominant soft magnetic character with small coercivity of Hc ~ 60 Oe and high saturation magnetization of Ms = 2.6 emu/g for the highest concentration of 9 vol%. Permittivity and losses slightly increase with ferrite filler addition, and the composite maintains a dielectric character for all the compositions. Local PFM and MFM investigations have shown a combined ferro/piezoelectric character and magnetic order, with magnetoelectric coupling demonstrated by the reorientation of filler particles and modifications of local piezoresponse when applying a static magnetic field.

Tunable Ferroelectricity in Ruddlesden-Popper Halide Perovskites.

Ruddlesden-Popper (RP) halide perovskites are the new kids on the block for high-performance perovskite photovoltaics with excellent ambient stability. The layered nature of these perovskites offers an exciting possibility of harnessing their ferroelectric property for photovoltaics. Adjacent polar domains in a ferroelectric material allow the spatial separation of electrons and holes. Presently, the structure-function properties governing the ferroelectric behavior of RP perovskites are an open question. Herein, we realize tunable ferroelectricity in 2-phenylethylammonium (PEA) and methylammonium (MA) RP perovskite (PEA)2(MA) n̅-1Pb n̅I3 n̅+1. Second harmonic generation (SHG) confirms the noncentrosymmetric nature of these polycrystalline thin films, whereas piezoresponse force microscopy and polarization-electric field measurements validate the microscopic and macroscopic ferroelectric properties. Temperature-dependent SHG and dielectric constant measurements uncover a phase transition temperature at around 170 °C in these films. Extensive molecular dynamics simulations support the experimental results and identified the correlated reorientation of MA molecules and ion translations as the source of ferroelectricity. Current-voltage characteristics in the dark reveal the persistence of hysteresis in these devices, which has profound implications for light-harvesting and light-emitting applications. Importantly, our findings disclose a viable approach for engineering the ferroelectric properties of RP perovskites that may unlock new functionalities for perovskite optoelectronics.

Temperature-dependent anhysteretic behavior of co-doped PZT

The anhysteretic behavior of a soft Pb(Zr,Ti)O3 was measured from 25 °C to 175 °C. The experimental determination of the anhysteretic polarization curve, combined with classical P-E and S-E loop measurements, allows for an experimental separation of the reversible and dissipative contributions to the ferroelectric behavior. This approach offers insight into the different mechanisms originating at the microscopic scale and the contribution to the macroscopic ferroelectric properties. It was found that the reversible anhysteretic susceptibility χa of the unpoled material increases by 30% from room temperature to 150 °C. On the other hand, the effect on the total susceptibility for a null polarization χc increases only by 17% over the same temperature range. Since the difference between χa and χc reflects the dissipative contribution to the macroscopic ferroelectric behavior, this reveals that dissipation reduces the improvement of susceptibility under increasing temperature. This work illustrates the benefits of separating experimentally the reversible and dissipative contributions to describe the ferroelectric behavior, which can serve as a basis for advanced modeling approaches.

Multiscale simulations of defect dipole–enhanced electromechanical coupling at dilute defect concentrations

The role of defects in solids of mixed ionic-covalent bonds such as ferroelectric oxides is complex. Current understanding of defects on ferroelectric properties at the single-defect level remains mostly at the empirical level, and the detailed atomistic mechanisms for many defect-mediated polarization-switching processes have not been convincingly revealed quantum mechanically. We simulate the polarization–electric field (P–E) and strain–electric field (ε–E) hysteresis loops for BaTiO3 in the presence of generic defect dipoles with large-scale molecular dynamics and provide a detailed atomistic picture of the defect dipole–enhanced electromechanical coupling. We develop a general first-principles-based atomistic model, enabling a quantitative understanding of the relationship between macroscopic ferroelectric properties and dipolar impurities of different orientations, concentrations, and dipole moments. We find that the collective orientation of dipolar defects relative to the external field is the key microscopic structure feature that strongly affects materials hardening/softening and electromechanical coupling. We show that a small concentration (≈0.1 at. %) of defect dipoles dramatically improves electromechanical responses. This offers the opportunity to improve the performance of inexpensive polycrystalline ferroelectric ceramics through defect dipole engineering for a range of applications including piezoelectric sensors, actuators, and transducers.

Polarization dependent ferroelectric photovoltaic effects in BFTO/CuO thin films

Bi5FeTi3O15 (BFTO) and BFTO/CuO films were deposited by a sol-gel technique, which exhibited macroscopic ferroelectric properties. It was found that the BFTO/CuO films showed a short circuit photocurrent density (Jsc) enhanced by nearly 10 times and power conversion efficiency increased by 13-fold compared to those of the BFTO film. The significant increase in the photovoltaic (PV) response may be attributed to the p-n junction internal electric field acting as the driving force of photogenerated carriers. Furthermore, both BFTO and BFTO/CuO films indicated a switchable PV response in both polarities. The open circuit voltage (Voc) and Jsc for BFTO and BFTO/CuO were observed to be −0.59 V and +43.88 μA/cm2 and −0.23 V and +123.16 μA/cm2, respectively, after upward poling, whereas after downward poling, +0.11 V and −6.26 μA/cm2 and +0.17 V and −83.21 μA/cm2 for BFTO and BFTO/CuO were observed, respectively. The switchable PV responses were explained by the ferroelectric depolarization field, whose direction could be altered with the variation in the applied poling field. This work provides an efficient approach to developing ferroelectric film based PV devices with low cost.

Ferroelectric domain continuity over grain boundaries

Formation and mobility of domain walls in ferroelectric materials is responsible for many of their electrical and mechanical properties. Domain wall continuity across grain boundaries has been observed since the 1950's and is speculated to affect the grain boundary-domain interactions, thereby impacting macroscopic ferroelectric properties in polycrystalline systems. However detailed studies of such correlated domain structures across grain boundaries are limited. In this work, we have developed the mathematical requirements for domain wall plane matching at grain boundaries of any given orientation. We have also incorporated the effect of grain boundary ferroelectric polarization charge created when any two domains meet at the grain boundary plane. The probability of domain wall continuity for three specific grain misorientations is studied. Use of this knowledge to optimize processing techniques in manipulating the micro-structure and domain structure to result in desired interactions between neighbouring grains could prove to be beneficial for future polycrystalline ferroelectric materials.

Correlation between the macroscopic ferroelectric material properties of Si:HfO2 and the statistics of 28 nm FeFET memory arrays

ABSTRACTWith the discovery of ferroelectric hafnium oxide (FE-HfO2), the ferroelectric field effect transistor (FeFET), a long-term contender for non-volatile data storage, has finally managed to scale to the 2× nm technology node. Here for the first time, we correlate the thickness dependent ferroelectric properties of Si:HfO2 with the memory characteristics of small (56 bit) FeFET arrays. First, an electrical and structural analysis of metal-ferroelectric-metal capacitors is given. Even though possessing room-temperature deposited top electrodes, TiN / Si:HfO2 (20 nm) / TiN capacitors are showing deteriorated polarization characteristics as compared to their 10 nm Si:HfO2 counterparts. This could be attributed to an increased monoclinic phase fraction, as indicated by small-signal capacitance voltage and grazing incidence X-ray diffraction measurements. Identical Si:HfO2 thin films with thicknesses of 10 nm and 20 nm respectively, were utilized in a 28 nm high-k metal-gate CMOS flow to form small FeFET memory arrays of AND architecture. After extracting the most suitable operating conditions from erase matrix, single cell evaluation was performed by standard VP/3 program and a novel VP/3 positive-source drain erase scheme. Array cells incorporating 10 nm Si:HfO2 films showed a maximum memory window of 1.03 V whereas cells incorporating 20 nm Si:HfO2 films could reach up to 1.57 V. Moreover, in accordance to the basic material properties, the previously observed increased monoclinic phase fraction in 20 nm Si:HfO2 thin films correlate well with a reduced number of functional FeFET cells.

Periodicity Dependence of the Built-in Electric Field in (Ba0.7Ca0.3)TiO3/Ba(Zr0.2Ti0.8)O3 Ferroelectric Superlattices.

Symmetric ferroelectric superlattices consisting of (Ba0.7Ca0.3)TiO3 (BCT) and Ba(Zr0.2Ti0.8)O3 (BZT) layers were successfully grown on La0.7Sr0.3MnO3 electroded (001)-oriented SrTiO3 substrates by laser molecular beam epitaxy. With the monitor of reflective high-energy electron diffraction, the growth mode and rate were precisely controlled to realize the desired superlattice periodicity as confirmed by both X-ray diffraction and transmission electron microscopy results. The microscopic piezoelectric response and macroscopic ferroelectric properties were investigated as a function of periodicity of the BCTm/BZTm (m = 3, 5, 10, and 15 unit cells) superlattices. The existence of a built-in electric field was confirmed in all the superlattices and its strength was highly dependent on the periodicity. The excellent tunability of built-in electric field opens a path for designing microelectronic devices with various functionalities based on BCTm/BZTm superlattices.

Ferroelectric hysteresis and improved fatigue of PZT (53/47) films fabricated by a simplified sol–gel acetic-acid route

Double-layer Pb(Zr0.53Ti0.47)O3 films were fabricated by spin coating of a sol–gel acetic-acid-based precursor solution deposited onto commercial Pt–Si substrates. The structural properties of the samples were studied by several diffraction, spectroscopy and microscopy techniques. The annealed ferroelectric films were crystallized to a pure PZT perovskite phase. A significant monoclinic phase content was found together with a relatively large tetragonal c/a ratio, according to the diffraction pattern refinement results. No traces of organic material were observed. Good film densification with relatively large grain sizes and low surface roughness was achieved. Ferroelectric domain distribution and local piezoresponse hysteresis loops were investigated by piezoresponse force microscopy. The films showed good local ferroelectric properties and a relatively large d33 piezoelectric coefficient was derived. A degree of self-polarization of the film was also found from the domain distribution-map analysis. Good macroscopic ferroelectric properties were also achieved, specially for the film with less rhombohedral content. An improved ferroelectric fatigue behavior was observed as the films proved to sustain down to 108 fatigue cycles with only a 10 % decrease of the initial remnant polarization.

Crystal structure and local piezoelectric properties of strain-controlled (001) BiFeO3 epitaxial thin films

In this study, we investigated how controlling lattice-mismatch strain affected the crystal structure as well as the macroscopic and microscopic ferroelectric properties of (001) BiFeO3 epitaxial thin films. Lattice-mismatch strain was controlled by introducing a BaTiO3 buffer layer. Structural analysis indicated that the film with a lattice mismatch of −1.3% has a crystal structure of the rhombohedral-like monoclinic phase (MA). In contrast, the crystal structure of the film with a lattice mismatch of 0.5% is MA with a (001) plane tilted from the normal of the substrate surface. Scanning probe microscopy with a conducting probe revealed that the density of leakage spots of the film with the lattice mismatch of 0.5% was lower than that of the film with a lattice mismatch of −1.3%. In addition, the inverse piezoelectric response of the film with the lattice mismatch of 0.5% is about two times larger than that of the film with the lattice mismatch of −1.3%. We suggest that the domain engineering more strongly affects the tilted MA phase.

Effect of Substrate on Structure and Multiferrocity of (La,Mn) CoSubstituted BiFeO3 Thin Films

In this article, we report the substrate effect on ferroelectric and magnetic properties of epitaxial BiFeO3‐based thin films at room temperature. (La, Mn) cosubstituted BiFeO3 (BFOLM) thin films were deposited on differently lattice mismatched single‐crystal substrates to manipulate the strain states in the as‐deposited films. All the films with 30‐nm thick CaRuO3 bottom electrodes exhibited highly epitaxial growth behavior with a slightly monoclinic distorted lattice structure while their strain states are drastically different as confirmed by X‐ray reciprocal space mapping. These films possessed significantly different macroscopic ferroelectric properties with giant remanent polarization of 101 ± 2, 65 ± 2, and 48 ± 2 μC/cm2 for the films grown on SrTiO3, (La, Sr)(Al, Ta)O3, and LaAlO3, respectively. It is found that the room‐temperature magnetic properties are also in accordance with their strain state, having a reciprocal relationship with polarization. For example, the enhanced magnetization is associated with the suppressed polarization and vice versa. The stain tunability of multiferroic properties in BFOLM thin films are presumably ascribed to the polarization rotation and oxygen octahedral tilts.

Electrical properties of (110) epitaxial lead-free ferroelectric Na0.5Bi0.5TiO3 thin films grown by pulsed laser deposition: Macroscopic and nanoscale data

We report the electrical properties, measured both at the macroscopic and nanometric scales, of epitaxial (110)-Na0.5Bi0.5TiO3 (NBT) thin films grown on (110)Pt/(110)SrTiO3 by pulsed laser deposition (PLD). The influence of the A-site composition (Na and/or Bi excess) on both the structural/microstructural characteristics and the electrical properties is discussed. Whatever the composition of the NBT target, the final layers are systematically epitaxially grown, with NBT crystallites mainly (110)-oriented, and as well (100)-oriented for some minor proportion. Atomic force microscopy (AFM) images reveal the coexistence of two kinds of grains presenting different shapes: namely flat and elongated grains, corresponding to (100)- and (110)-oriented NBT crystallites, respectively. The macroscopic ferroelectric properties were measured at room temperature. A rather well-defined shape of the hysteresis loops was obtained: the incorporation of a Bi excess in the target clearly improves the saturation of the loops. The ferroelectric performances are a remanent polarization (Pr) value, ranging from 7 to 14 μC/cm2, associated with a coercive field (Ec) in the range 68–85 kV/cm. In addition, at 105 Hz, the relative permittivity was about ɛr ∼ 255–410 and the dielectric losses (tan δ) were ∼6%–7%. Finally, the electrical properties at the local scale were investigated by coupling piezoresponse force microscopy (PFM) and tunneling AFM (TUNA) measurements. The collected data reveal that the two types of grains behave differently. The PFM amplitude signal of (110)-oriented grains is very contrasted and such grains are often divided in ferroelectric bi-domains of nanometric sizes, whereas the response of (100)-oriented grains is less contrasted and more homogeneous. The interpretation of the PFM signal is provided. The piezoloop recorded on a (110)NBT grain is strongly distorted and shifted along the vertical axis, in agreement with the vertical drift observed for macroscopic ferroelectric data. Finally, TUNA data clearly indicate that flat grains are leakier than elongated grains, highlighting the anisotropy of the electrical properties at the local scale.

Strain effect on the surface potential and nanoscale switching characteristics of multiferroic BiFeO3 thin films

The BiFeO3 films were deposited on the SrTiO3 (001) substrates via tuning the thickness of the SrRuO3 (SRO) bottom electrode by pulsed laser deposition. The macroscopic ferroelectric and dielectric properties were dramatically impacted by the various nanoscale domain structures for both films due to the tunable SRO thickness. The nanoscale domain switching behaviors for both films were investigated via piezoresponse force microscopy, and results suggest that the domain structure could be changed by tuning the strain state. The surface potential investigation indicates that strain helps increase data storage density and stability.