Ferroelectric Switching Characteristics Research Articles

Hydrogen Bonding-Assisted Complete Ferroelectric β-Phase Conversion in Poly(vinylidene fluoride) Thin Films: Exhibiting an Excellent Memory Window and Long Retention.

Organic nonvolatile memory with low power consumption is a critical research demand for next-generation memory applications. Ferroelectric switching characteristics of poly(vinylidene fluoride) (PVDF) thin films modified with a trace amount of hydrated Cu salt (CuCl2·2H2O) are explored in the present study. Herein, a Cu salt-mediated PVDF (Cu/PVDF) thin film with preferential edge-on β-crystallites is fabricated through the orientation-controlled spin coating (OCSC) technique. This work proposes a convenient and effective approach to produce edge-on-oriented electroactive PVDF thin films with a high degree of polar β-phase, so as to realize the favorable switching under low operating voltages. Herein, chemically modified PVDF is anticipated to form a complex intermediate, which attains its stability by undergoing favorable hydrogen bonding that reorients the C-C structure of PVDF to obtain the β-conformation. Such information is verified by X-ray photoelectron spectroscopy (XPS). Grazing incidence Fourier transform infrared (GI-FTIR) spectroscopy revealed that the Cu salt incorporated into the PVDF matrix favored the formation of the electroactive β-phase with edge-on crystallite lamellae. Consequently, the Cu/PVDF thin film demonstrates a good contrast between electric field-assisted written and erased data bits in the piezoresponse force microscopy (PFM) phase image. Furthermore, to obtain the ferroelectric memory window, a metal-ferroelectric-insulator-semiconductor (MFIS) diode with Cu/PVDF as a ferroelectric layer has been fabricated. The capacitance-voltage (C-V) characteristic of the MFIS diode exhibits a memory window of 12 V with a long-term retention behavior (∼longer than 7 days). In a nutshell, we tried to represent a clear understanding of the interfacial interactions of the Cu salt with PVDF, which favor the edge-on formation that results in the promising low-voltage ferroelectric switching and excellent retention response, where any additional electrical poling and/or external stretching is completely possible to be ruled out, thus offering a new prospect for the evolution of devices with long-lasting nonvolatile memories.

In-Grain Ferroelectric Switching in Sub-5 nm Thin Al0.74 Sc0.26 NFilms at 1 V.

Analog switching in ferroelectric devices promises neuromorphic computing with the highest energy efficiency if limited device scalability can be overcome. To contribute to a solution, one reports on the ferroelectric switching characteristics of sub-5 nm thin Al0.74 Sc0.26 Nfilms grown on Pt/Ti/SiO2 /Siand epitaxial Pt/GaN/sapphiretemplates by sputter-deposition. In this context, the study focuses on the following major achievements compared to previously available wurtzite-type ferroelectrics: 1) Record low switching voltages down to 1V are achieved, which is in a range that can be supplied by standard on-chip voltage sources. 2) Compared to the previously investigated deposition of ultrathin Al1-x Scx Nfilms on epitaxial templates, a significantly larger coercive field (Ec ) to breakdown field ratio is observed for Al0.74 Sc0.26 Nfilms grown on silicon substrates, the technologically most relevant substrate-type. 3) The formation of true ferroelectric domains in wurtzite-type materials is for the first time demonstrated on the atomic scale by scanning transmission electron microscopy (STEM) investigations of a sub-5 nm thin partially switched film. The direct observation of inversion domain boundaries (IDB) within single nm-sized grains supports the theory of a gradual domain-wall driven switching process in wurtzite-type ferroelectrics. Ultimately, this should enable the analog switching necessary for mimicking neuromorphic concepts also in highly scaleddevices.

Intersecting multiaxial domain walls in plastic ferroelectric crystal films

Plastic ferroelectrics comprise a class of solution-processable molecular ferroelectrics, in which spherical polar molecules can rotate and/or freeze within crystal lattices. Some compounds exhibit superior multiaxial ferroelectricity and very sharp ferroelectric switching characteristics, although the domain wall (DW) characteristics have not yet been explored. Here, we report distinctive DW characteristics in drop-casted monocrystalline thin films of a typical plastic/ferroelectric $[\mathrm{AH}][\mathrm{Re}{\mathrm{O}}_{4}]$. We demonstrate that the films form ${71}^{\ensuremath{\circ}}, {109}^{\ensuremath{\circ}}$, and ${180}^{\ensuremath{\circ}}$ DWs, unique to the pseudocubic rhombohedral lattice, that emerge and compete with each other, depending on the crystallographic orientation of the films. Particularly, it is observed that all three types of DWs intersect at several single points, whose unique nature should be ascribed to the small elastic deformation. The observed domains and ${180}^{\ensuremath{\circ}}$ DWs also comprise much larger periodicity and scaling than those of conventional inorganic ferroelectric films, which should be associated with the unique polarization reversal mechanism in the compound films. These observed features reveal the exotic multiaxial ferroelectricity of soft plastic/ferroelectrics, paving the way for developing printable ferroelectronics.

Ferroelectric switching and current characteristics dependence on ferroelectric polarization direction of microwave synthesized BiFeO3 nanocubes

Herein, we studied the ferroelectric switching and current characteristics of BiFeO3 (BFO) nanocubes dispersed on the surface of a Nb-doped SrTiO3 (Nb:STO) substrate based on the ferroelectric polarization orientation. The microwave synthesis method afforded BFO nanocubes with an average size of ∼50 nm, which were dispersed on the Nb:STO substrate surface and the substrate was subsequently subjected to heat treatment at 500 °C for 1 h. The piezoelectric d33 hysteresis loop, ferroelectric domain structure, and ferroelectric polarization switching characteristics of the 50-nm-sized BFO nanocubes were examined using piezoresponse force microscopy. Finally, atomic force microscopy confirmed the dependency of current characteristics on the ferroelectric polarization orientation of the BFO nanocubes, verifying the applicability of BFO nanocubes as storage media for ferroelectric polarization information.

Imprint phenomenon of ferroelectric switching characteristics in BaTiO3/PbTiO3 multilayer thin films

Materialsin which ferroelectric polarization can be locally controlled have potential for high-density information storage applications. Herein, we investigated the ferroelectric polarization switching characteristics of epitaxial BaTiO3/PbTiO3 (BTO/PTO) multilayer thin films by using piezoresponse force microscopy. Pulsed laser deposition was used to form an epitaxial BTO layer on an epitaxial ferroelectric PTO single thin film. This epitaxial BTO/PTO multilayer thin film exhibited an imprint phenomenon, in which the internal electric field was asymmetrically induced and the ferroelectric hysteresis loop was shifted. Owing to this imprint phenomenon, a large difference occurred between the two coercive electric fields of the ferroelectric hysteresis loop. Because the coercive electric field was reduced in one direction, a relatively small switching voltage was required to form ferroelectric polarization nanobits in a local region and the size of the switched nanobits was minimized. This behavior can be utilized as the basis for developing high-density ferroelectric-based storage media.

Size and shape effects on polarization reversal of nanoscale BaTiO3

We report piezo force microscopy (PFM) investigations on the ferroelectric switching characteristics of spherical and rod shaped non-interacting nanocrystalline BaTiO3 (BTO). Polarization-voltage hysteresis loop (P-E hysteresis loop) measurement shows that the strong polarization reversal observed for the nanorod (NR) shaped BTO than the spherical nanoparticle (NP). Asymmetric P-E loop is observed for all kind of BTO NRs. From the PFM spectroscopy and ferroelectric switching studies reveal that the defect dipoles present in the BTO NR at below 60 nm have no significant effect on the asymmetric P-E hysteresis loop. It is strongly depends on the strain between the substrate and BTO NRs.

Comparison of the evolution of internal bias field of doped hafnia ferroelectric capacitors for the field-cycling reliability

The discovery of ferroelectricity in HfO2 thin films extends the range of research on next-generation electronic devices. However, for commercial applications, reliable ferroelectric switching characteristics under electric field cycling of HfO2 thin films must be secured. Despite recent reports of improved reliability under electric cycling of HfO2 with various dopants, a deep understanding of the experimental results is still needed. Our research has confirmed that finer domains and grains formed in thin films doped with Si than in those doped with Zr. This difference leads to a difference in the size of the domain and the number of domain walls, and a large number of domain walls function to suppress the diffusion of oxygen vacancies, which is known to significantly affect the stability of hafnia. This dependence was verified by the observation of relatively limited changes in the internal electric field of the Si-doped HfO2 films during the progression of cycling, using the first-order reversal curve method and various electrical measurements.

Birefringent Field-Modulation Imaging of Transparent Ferroelectrics

Observations of ferroelectric domains and domain walls are essential for understanding ferroelectric switching characteristics. Here, we demonstrate that the use of optical birefringence, coupled with a first-order electro-optic effect (Pockels effect), allows the visualization of ferroelectric domains in transparent ferroelectric materials. We utilize a field-modulation imaging technique, with the use of a complementary metal-oxide-semiconductor (CMOS) area image sensor to detect slight differences in parallel- and crossed-Nicols polarized optical microscopy images between forward- and reverse-electric-field applications. The ferroelectric domains with antiparallel polarizations are clearly discriminated from each other by the sign of the ellipticity modulation of outgoing elliptically polarized light. The observed images are consistent with piezoresponse force microscopy images of transparent single-crystal films of 2-methylbenzimidazole (MBI). The modulation signal at both polarization configurations exhibit clear sign changes in the wavelength dispersion, owing to the anisotropic nature of the MBI films. The unique optical-probe nature also reveals the existence of two types of domain walls with different three-dimensional orientations.

Switching Dynamics Analysis by Various Models of Hf0.5Zr0.5O2 Ferroelectric Thin Films

Recent discoveries of ferroelectric properties in ultrathin doped hafnium oxide (HfO2) have led to the expectation that HfO2 could overcome the shortcomings of perovskite materials and be applied to electron devices such as Fe-Random access memory (RAM), ferroelectric tunnel junction (FTJ) and negative capacitance field effect transistor (NC-FET) device. As research on hafnium oxide ferroelectrics accelerates, several models to analyze the polarization switching characteristics of hafnium oxide ferroelectrics have been proposed from the domain or energy point of view. However, there is still a lack of in-depth consideration of models that can fully express the polarization switching properties of ferroelectrics. In this paper, a Zr-doped HfO2 thin film based metal-ferroelectric-metal (MFM) capacitor was implemented and the polarization switching dynamics, along with the ferroelectric characteristics, of the device were analyzed. In addition, a study was conducted to propose an applicable model of HfO2-based MFM capacitors by applying various ferroelectric switching characteristics models.

3D hierarchical Ho-doped ZnO micro-flowers assembled with nanosheets: A high temperature ferroelectric material

A novel high temperature ferroelectric (Tc ∼205 °C, Ec ∼4.72 kV/cm & Pr ∼0.05 μC/cm2) marigold-like holmium (Ho)-doped ZnO nanocrystals were developed using wet chemical co-precipitation method. Herein, we have studied the impact of Ho-doping on structural, morphological, dielectric and ferroelectric properties of ZnO nanocrystals. Interestingly as a result of Ho-doping, the 1D nanopencils (NPCs) of pure ZnO were transformed to 3D hierarchical micro-flowers made up of nanosheets (NSs). Crystallite size (L), deformation stress (σ), energy density (u) and lattice strain of both nanopencils and nanosheets were evaluated using the Scherrer equation and Williamson-Hall methods. The temperature dependent plot of dielectric constant (ε′) displayed a (dielectric anomaly) ferro-to para-electric transition peak at 205 °C. The flower-like Ho-doped ZnO nanosheets exhibit a weak ferroelectric polarization hysteresis characteristic with a coercive field (Ec) of 4.72 kV/cm and a remnant polarization (Pr) of 0.05 μC/cm2 at room temperature. Electric conduction properties were studied for both pure ZnO NPCs and Ho-doped ZnO NSs. Our study opens up the door of inducing high temperature ferroelectric switching characteristics in bio compatible pristine ZnO nanostructures by doping it with rare earth holmium (Ho3+) and to use the ferroelectric material in designing components for memory applications.

Ferroelectric Liquid Crystals: Synthesis and Thermal Behavior of Optically Active, Three-Ring Schiff Bases and Salicylaldimines.

The chiral ferroelectric smectic C (SmC*) phase, characterized by a helical superstructure, has been well exploited in developing high-resolution microdisplays that have been effectively employed in the fabrication of a wide varieties of portable devices. Although, an overwhelming number of optically active (chiral) liquid crystals (LCs) exhibiting a SmC* phase have been designed and synthesized, the search for new systems continues so as to realize mesogens capable of meeting technical necessities and specifications for their end-use. In continuation of our research work in this direction, herein we report the design, synthesis, and thermal behavior of twenty new optically active, three-ring calamitic LCs belonging to four series. The first two series comprise five pairs of enantiomeric Schiff bases whereas the other two series are composed of five pairs of enantiomeric salicylaldimines. In each pair of optical isomers, the configuration of a chiral center in one stereoisomer is opposite to that of the analogous center in the other isomer as they are derived from (3 S)-3,7-dimethyloctyloxy and (3 R)-3,7-dimethyloctyloxy tails. To probe the structure-property correlations in each series, the length of the n-alkoxy tail situated at the other end of the mesogens has been varied from n-octyloxy to n-dodecyloxy. The measurement of optical activity of these chiral mesogens was carried out by recording their specific rotations. As expected, enantiomers rotate plane polarized light in the opposite direction but by the same magnitude. The thermal behavior of the compounds was established by using a combination of optical polarizing microscopy, differential scanning calorimetry, and powder X-ray diffraction. These complementary techniques demonstrate the existence of the expected, thermodynamically stable, chiral smectic C (SmC*) LC phase besides blue phase I/II (BPI or BPII) and chiral nematic (N*) phase. However, as noted in our previous analogous study, the vast majority of the Schiff bases show an additional metastable, unfamiliar smectic (SmX) phase just below the SmC* phase. Notably, the SmC* phase persists over the temperature range ≈80-115 °C. Two mesogens chosen each from Schiff bases and salicylaldimines were investigated for their electrical switching behavior. The study reveals the ferroelectric switching characteristics of the SmC* phase featuring the spontaneous polarization (PS ) in the range 69-96 nC cm-2 . The helical twist sense of the SmC* phase as well as the N* phase formed by a pair of enantiomeric Schiff bases and salicylaldimines has been established with the help of circular dichroism (CD) spectroscopic technique. As expected, the SmC* and the N* phase of a pair of enantiomers showed mirror image CD signals. Most importantly, the reversal of helical handedness from left to right and vice versa has been evidenced during the N* to SmC* phase transition, implying that the screw sense of the helical array of the N* phase and the SmC* phase of an enantiomer is opposite.

Local ferroelectric polarization switching properties of BiFeO3 nanodots and thin films depending on the ferroelectric polarization nanobit diameter

We investigated the local ferroelectric polarization switching characteristics of an epitaxial (001) BiFeO3 (BFO) thin film and BFO nanodots having 100–300-nm diameters. The 30-nm-thick BFO thin films were deposited on (100) Nb-doped SrTiO3 (Nb:STO) substrates via pulsed laser deposition. This large switching voltage of the BFO thin film originates from the neighboring domains that stabilize the ferroelectric polarization nanobit against the switching electric field induced by the external bias. The coercive voltages of the piezoelectric d33 hysteresis loops, which depend on the diameters of the BFO nanodots, were saturated to that of the BFO thin film, indicating that the influence of the neighboring domains is limited to within a diameter of approximately 300 nm.

Asymmetric ferroelectric switching characteristics of ferroelectric poly(vinylidene fluoride-ran-trifluoroethylene) thin films grown on highly-oriented pyrolytic graphite substrates

Abstract Ferroelectric poly(vinylidene fluoride-ran-trifluoroethylene)(PVDF-TrFE) thin films were deposited on highly-oriented pyrolytic graphite (HOPG) substrates by spin coating. The PVDF-TrFE thin films on HOPG substrates exhibited good ferroelectric properties, with a remnant polarization of 10.2 μC/cm 2 (2Pr ∼20.4 μC/cm 2 ) and a coercive electric field of approximately 1.0 MV/cm. A Pt/PVDF-TrFE/HOPG capacitor had an asymmetric ferroelectric hysteresis loop and asymmetric switching speeds. The fast switching speed of a Pt/PVDF-TrFE/Pt capacitor from downward polarization to upward polarization supported the conclusion that HOPG or graphene electrodes provided fast polarization switching when used as one electrode of a ferroelectric PVDF-TrFE capacitor.

Size effect on polarization switching kinetics of P(VDF-TrFE) copolymer nanodots

We demonstrate polarization switching kinetics of poly(vinylidene fluoride-ran-trifluoroethylene) P(VDF-TrFE) nanodots depending on their sizes. Ferroelectric copolymer P(VDF-TrFE) nanodots, ranging from 20 to 250 nm in lateral size, were deposited on a Nb-doped SrTiO3 substrate by a position-controlled dip-pen nanolithography technique. Ferroelectric switching characteristics of P(VDF-TrFE) nanodots was investigated by piezoresponse force microscopy. Asymmetric switching behavior was observed during switching process, indicating that the threshold electric fields for polarization reversal were measured differently under the same conditions according to the switching polarity. Significantly, when the size of P(VDF-TrFE) nanodot approached around 250 nm, electric fields for polarization switching were compatible with those of the P(VDF-TrFE) thin film. We suggest that the electric fields required for polarization reversal of the P(VDF-TrFE) nanodots present asymmetric switching behaviors, and asymptotic behavior be observed as the nanodots increase in lateral size.

Nanoscale magnetoelectric coupling study in (111)-oriented PZT-Co ferrite multiferroic nanobilayer thin film using piezoresponse force microscopy: Effect of Co ferrite composition

The perfect (111)-oriented PZT/Co0.8Fe2.2O4 and PZT/Co0.6Mn0.2Fe2.2O4 bilayer multiferroic thin films were grown on Pt(111)/Si substrate using pulsed laser deposition technique. The PZT/Co0.6Mn0.2Fe2.2O4 bilayer showed higher magnetization and lower coercivity than the PZT/Co0.8Fe2.2O4 bilayer film, which is due to the Mn substitution in the cobalt ferrite layer. The effect of composition of cobalt ferrite underlayer on local magnetoelectric coupling of samples was investigated using piezoresponse force microscopy under external magnetic field, showing magnetically induced evolution of piezoresponse and ferroelectric switching characteristics as a result of interfacially strain transferred from magnetostrictive cobalt ferrite underlayer. The change in piezoresponse amplitude of PZT/Co0.6Mn0.2Fe2.2O4 by applying the magnetic field was higher than that of PZT/Co0.8Fe2.2O4 film and both were significantly higher than that for PZT/CoFe2O4, which has been discussed based on their magnetostriction properties. These results revealed that in comparison to PZT/Co0.8Fe2.2O4, the Mn-substituted bilayer film can be used in lower magnetic fields to reach the same magnetoelectric response making it feasible to the work at lower level of magnetic field in the sensor and actuator devices.

Ferroelectric polarization and resistive switching characteristics of ion beam assisted sputter deposited BaTiO3 thin films

In this work, 150nm thick polycrystalline BaTiO3 (BTO) films were deposited on Pt/TiO2/SiO2/Si substrate by ion beam assisted sputter deposition technique. The bias voltage dependent resistive switching (RS) and ferroelectric polarization characteristics of Au/BTO/Pt devices are investigated. The devices display the stable bipolar RS characteristics without an initial electroforming process. Fittings to current–voltage (I–V) curves suggest that low and high resistance states are governed, respectively, by filamentary model and trap controlled space charge limited conduction mechanism, where the oxygen vacancies act as traps. Presence of oxygen vacancies is evidenced from the photoluminescence spectrum. The devices also display P–V loops with remnant polarization (Pr) of 5.7μC/cm2 and a coercive electric field (Ec) of 173.0kV/cm. The coupling between the ferroelectric polarization and RS effect in BTO films is demonstrated.

Improved ferroelectric property and domain structure of highly a-oriented polycrystalline CaBi2Nb2O9 thin film

A Lead-free ferroelectric CaBi2Nb2O9 (CBNO) thin film was deposited on Si substrate by pulsed laser deposition. TiO2 buffer layer was employed and Pt electrode was used for nano-scale capacitor. The x-ray diffraction reveals that the CBNO thin film has highly a-oriented polycrystalline structure. The highly a-oriented polycrystalline CBNO thin film significantly exhibit the enhanced ferroelectric property with a remnant polarization of 10μC/cm2 compared to other values reported previously. In particular, the highly a-oriented polycrystalline CBNO thin film show faster ferroelectric switching characteristics than the epitaxially c-oriented CBNO thin film.

Enhanced switching characteristics and piezoelectric response in epitaxial BiFeO3–TbMnO3 thin films

High-quality (001) oriented epitaxial 0.9BiFeO3–0.1TbMnO3 thin films were grown on La2/3Sr1/3MnO3 and SrRuO3 buffered SrTiO3 substrate using pulsed laser deposition. X-ray diffraction showed that the films are single-phase perovskite without secondary impurity phases. Domain structures and upward ferroelectric self-poling phenomenon were distinctly observed in both films with compressive epitaxial strains. Furthermore, the upward self-poling disappears in polycrystalline 0.9BiFeO3–0.1TbMnO3 thin film on Pt/TiO2/SiO2/Si substrates. Through local switching spectroscopy measurements, the evidence of enhanced ferroelectric switching and piezoelectric response characteristics have been provided.

Ferroelectric domain structures and polarization switching characteristics of polycrystalline BiFeO3 thin films on glass substrates

We investigated ferroelectric characteristics of BiFeO3 (BFO) thin films on SrRuO3 (SRO)/yttria-stabilized zirconia (YSZ)/glass substrates grown by pulsed laser deposition. YSZ buffer layers were employed to grow highly crystallized BFO thin films as well as SRO bottom electrodes on glass substrates. The BFO thin films exhibited good ferroelectric properties with a remanent polarization of 2Pr = 59.6 μC/cm2 and fast switching behavior within about 125 ns. Piezoelectric force microscopy (PFM) study revealed that the BFO thin films have much smaller mosaic ferroelectric domain patterns than epitaxial BFO thin films on Nb:SrTiO3 substrates. Presumably these small domain widths which originated from smaller domain energy give rise to the faster electrical switching behavior in comparison with the epitaxial BFO thin films on Nb:SrTiO3 substrates.

Ferroelectric Switching of Vinylidene and Trifluoroethylene Copolymer Thin Films on Au Electrodes Modified with Self-Assembled Monolayers.

The ferroelectric switching characteristics of a vinylidene fluoride and trifluoroethylene copolymer were significantly changed via the chemical modification of a gold electrode with an alkanethiol self-assembled monolayer (SAM). The alkanethiol SAM-Au electrode successfully suppressed the leakage current (dark current) from the electrode to the bulk ferroelectric. Smaller leakage currents led to the formation of an effective electric field in the bulk ferroelectric. At switching cycles ranging from 10 to 100 kHz, significant changes in the ferroelectric properties were observed. At 100 kHz, a remanent polarization (Pr) of 68 mC·m−2 was measured, whereas a very small Pr value of 2.4 mC·m−2 was measured for the sample without a SAM. The switching speed of the SAM-Au electrode is as twice as fast as that of the unmodified electrode. A large potential barrier was formed via the insertion of a SAM between the Au electrode and the ferroelectric, effectively changing the ferroelectric switching characteristics.