BiFeO3 Films Research Articles

The phase composition and structure of the BiFeO-=SUB=-3-=/SUB=- film grown on MgO(001) substrate by high frequency cathode deposition in oxygen atmosphere

The crystal structure and Mossbauer spectroscopy studies results for BiFeO3 film growth on the MgO(001) single crystal substrate are present in the paper. It been shown that film have high crystal perfection and low defectiveness which results in appearing of narrow lines during the theta-2theta and φ scanning and the small (lower than 0.7o) disorientation of film and substrate crystal axes. It is been revealed that unit cell of BiFeO3/MgO(001) heterostructure possess monoclinic symmetry and deformation of unit cell is negligible. The Mossbauer study shows that magnetic subsystem of film has spatial spin-modulated structure with zero value of anharmonicity parameter (m). This indicate that at room temperature the magnetic anisotropy changes from the "easy axis" type to "easy plane" type. Keywords: thin films, bismuth ferrite, Mossbauer effect.

Фазовый состав и структура пленки BiFeO-=SUB=-3-=/SUB=-, выращенной на подложке MgO(001) методом ВЧ-катодного распыления в атмосфере O-=SUB=-2-=/SUB=-

The crystal structure and Mossbauer spectroscopy studies results for BiFeO3 film growth on the MgO(001) single crystal substrate are present in the paper. It been shown that film have high crystal perfection and low defectiveness which results in appearing of narrow lines during the θ-2θ and φ scanning and the small (lower than 0.7°) disorientation of film and substrate crystal axes. It is been revealed that unit cell of BiFeO3/MgO(001) heterostructure possess monoclinic symmetry and deformation of unit cell is negligible. The Mossbauer study shows that magnetic subsystem of film has spatial spin-modulated structure with zero value of anharmonicity parameter (m). This indicate that at room temperature the magnetic anisotropy changes from the "easy axis" type to "easy plane" type.

Coupled Current Jumps and Domain Wall Creeps in a Defect‐Engineered Ferroelectric Resistive Memory

AbstractFerroelectric (FE) resistive switching has attracted considerable interest as a promising candidate for applications in non‐volatile memory technology. In this work, via judiciously controlling the defect states of oxygen vacancy through Sm‐doping, the authors obtain multiple current jumps/discrete resistance states in the resistive switching memories based on a model FE BiFeO3 (BFO). These hitherto unreported current jumps are attributed to the space‐charge‐limited current correlated with electron trapping by oxygen vacancies in the BFO film. Concurrently, oxygen vacancies serve as the pinning centers for the FE domains, leading to the domain wall creep behavior. These results illustrate the strong interplay between the defect, resistive switching, and domain wall creep behavior in FE diodes, providing a new insight into the mechanism of FE resistive switching. Overall, the large on/off ratio of ≈5 × 105, multiple resistance states, and fast switching speed of ≈30 ns, promise their potential applications in multi‐level data storage memories.

Microstructure and Optical Properties Study of Nd-doped BiFeO3 (Ba1-xNdxFeO3) Films on Quartz Substrate

Bismuth ferrite oxide (BFO), due to its remarkable properties, has become one of the most attractive multiferroic materials to be extensively studied. BFO doped with various materials, including Neodymium (Nd), could improve its properties that apply to numerous electronic devices. However, the studies related to the properties of Nd-doped BFO (Ba1-xNdxFeO3) thin films on a quartz substrate, especially the optical properties, are relatively scarce. This study aimed to investigate the microstructure and optical properties of the Nd-doped BFO (BNFO) as the variation of the Nd concentrations. The BNFO thin films with Nd concentrations of 0.05 (BNFO5); 0.1 (BNFO10); and 0.2 (BNFO20) have been deposited on the quartz substrates via the sol-gel method and using spin coating. The films were annealed at 600 °C for 1.5 h. The XRD result of the BNFO films revealed a single phase of BFO with a cubic structure. The lattice constants and volume cells of the films declined with more Nd. Meanwhile, the crystallite size and lattice strain changed due to the change in the Nd number. Additionally, the morphology images showed the pores on the films’ surface and the different film thicknesses of each BNFO film. From the optical characterization, the transmittance spectra of the BNFO films tended to rise as the more Nd amount doped, in which the BNFO20 had the highest transmittance. The BNFO10 had the highest refractive index, followed by the BNFO5 and BNFO20. Contrarily, the BNFO20 had the highest extinction coefficient and spectra, followed by the BNFO5 and BNFO10. Further, the bandgap values of the BNFO5, BNFO10, and BNFO20 were 2.75, 2.85, and 2.64 eV, respectively. Accordingly, due to the highest Nd amount that most impacted its microstructure, the BNFO20 exhibited the lowest bandgap value compared to the other films that are good for photovoltaic applications.

Deterministic Dual Control of Phase Competition in Strained BiFeO3: A Multiparametric Structural Lithography Approach

The realization of a mixed-phase microstructure in strained BiFeO3 (BFO) thin films has led to numerous novel effects derived from the coexistence of the tetragonal-like monoclinic phase (T phase) and rhombohedral-like monoclinic phase (R phase). Strong strain and polarization differences between the phases should result in a high level of transformation plasticity, which enables the continuous alteration of the relative proportion of R and T states in response to external forces. Although the potential for utilizing such plasticity to control mixed-phase populations under external stimuli is evident, direct experimental evidence backed by equilibrium predictions has not yet been fully demonstrated. Here we demonstrate deterministic control of mixed-phase populations in an epitaxially strained BFO thin film through the application of localized stresses and electric fields in a reversible manner. The results illustrate and rationalize deterministic control of mixed phases in strained BFO films, which could be crucial in tuning their functional properties. The findings also highlight a new multiparametric technique in the scanning probe lithography toolbox based on tip-assisted electric and strain field manipulation of functional properties that might find application beyond the ferroelectric domain and structural phase lithography.

Photoresponse of high quality epitaxial BiFeO3 films grown through hydrothermal method and rapid microwave assisted method

High quality BiFeO3 film with enhanced remanent polarization and magnetization attracts much interest for its great application potential in information storage and photoelectric devices. Here we grew single-crystal epitaxial BiFeO3 film on SrTiO3 substrate through hydrothermal method at low temperature, the film has a quality higher than any reported BiFeO3 films grown through hydrothermal method. Furthermore, BiFeO3 film was grown through a 25 min rapid microwave-assisted hydrothermal process. The two BiFeO3 films exhibited obvious light response to periodically switching on and off of a LED light, and the film grown through microwave-assisted hydrothermal method produced a 4.7V open-circuit voltage exceeding the band gap of BiFeO3 originating from the ferroelectric photovoltaic effect. These results demonstrate that hydrothermal and microwave-assisted hydrothermal method are simple, inexpensive and rapid to grow high quality epitaxial BiFeO3 film with potential application in photoelectric detection and photovoltaic.

Effect of (Zn, Mn) co-doping on the structure and ferroelectric properties of BiFeO3 thin films

BiFe1-2xZnxMnxO3 (BFZMO, with x = 0–0.05) thin films were synthesized via sol–gel method. Effects of (Zn, Mn) co-doping on the structure, ferroelectric, dielectric, and optical properties of BiFeO3 (BFO) films were investigated. BFZMO thin films exhibit rhombohedral structure. Scanning electron microscopy (SEM) images indicate that co-doping leads to a decrease in grain size and number of defects. Leakage current density (4.60 × 10−6 A/cm2) of BFZMO film with x = 0.02 was found to be two orders of magnitude lower than that of pristine BFO film. Owing to decreased leakage current density, saturated P–E curves were obtained. Maximum double remnant polarization of 413.2 μC/cm2 was observed for BFZMO thin film with x = 0.02, while that for the BFO film was found to be 199.68 μC/cm2. The reason for improved ferroelectric properties is partial substitution of Fe ions with Zn and Mn ions, which resulted in a reduction in the effect of oxygen vacancy defects. In addition, co-doping was found to decrease optical bandgap of BFO film, opening several possible routes for novel applications of these (Zn, Mn) co-doped BFO thin films.

Atomic-Scale Tunable Flexoelectric Couplings in Oxide Multiferroics.

Flexoelectricity is an effective tool in modulating the crystallographic structures and properties of oxides for multifunctional applications. However, engineering the nonuniform strain to obtain tunable flexoelectric behaviors at the atomic scale remains an ongoing challenge in conventional substrate-imposed ferroelectric films. Here, the regulatable flexoelectric behaviors are demonstrated at atomic scale in [110]-oriented BiFeO3 thin films, which are triggered by the strain-field coupling of high-density interfacial dislocations. Using aberration-corrected scanning transmission electron microscopy, the asymmetric polarization rotation around the single dislocation is revealed, which is induced by the gradient strain fields of the single dislocation. These strain fields are highly correlated to generate huge strain gradients between neighboring dislocations, and thereby, serial flexoelectric responses are engineered as a function of dislocation spacings in thicker BiFeO3 films. This work opens a pathway for the modulation of flexoelectric responses in ferroelectrics, which could be extended to other functional materials to create exotic phenomena.

The effect of thickness-dependent strain relaxation on magnetoelectric behaviors for highly c-axis oriented BiFeO3 films on Si substrate

The thickness-dependent strain-relaxation behavior and the associated impacts upon the magnetic, electric properties and magnetoelectric coupling effect for the highly c-axis oriented BiFeO3 (BFO) film films grown on (001) oriented Si substrate were studied. The result shown that the value of remnant polarization and coercive field were suppressed as the strain relaxes with increasing thickness. The BFO films also presented typical weak ferromagnetic hysteresis loops with a low coercive field and small remnant magnetization, and the saturated magnetization and remnant magnetization decreasing with increasing film thickness. The room temperature dynamic magnetoelectric coupling was approved by the transverse magnetoelectric voltage coefficients, which also decreased with strain relaxation due to the increase of thickness, and the maximum was around 265 mV/cm.Oe when the film thickness is 50 nm.

High-Quality Sputtered BiFeO3 for Ultrathin Epitaxial Films

BiFeO3 films were grown by RF magnetron sputtering with various O2 gas flow ratios and substrate temperatures. The optimal sputtering conditions for a slightly excess Bi content produced high-quality parameters: an atomically flat surface (Ra < 0.4 nm), low leakage current (Jc < 10–6 A/cm2), high ferroelectric polarization (72 μC/cm2//[001]pc), and large exchange bias (∼140 Oe). In addition to these typical characterizations, the following two advanced analyses were performed: (i) The lattice constant was identified by Bragg’s diffraction specific to a space group of R3c using X-ray diffraction; it was precisely determined as an expanded a-axis (abulk = 0.568 → aepi = 0.572 nm) and a shrunk c-axis (cbulk = 1.398 → cepi = 1.373 nm). (ii) The ferroelectricity was analyzed by first-order reversal curve (FORC) diagrams, which revealed that ferroelectric switching was packed in a narrow electric field area; an internal electric field in the film body was not observed despite the fact that the BiFeO3 films were as-grown samples. A 3 nm thick BiFeO3 film with a continuous and flat surface/interface was confirmed over a wide area. The crystal symmetry might be identified as a space group of R3c in the case of the 3 nm thick film by comparing the nanobeam selected area electron diffraction patterns with the patterns based on structural calculations. The ferroelectricity might be confirmed by the piezoresponse force microscopy of a 2 nm thick BiFeO3 epitaxial film, owing to the optimal condition of low Jc and uniform ferroelectric switching properties. Furthermore, a 0.4 nm thick ultrathin BiFeO3 film was confirmed to be a continuous one-unit cell perovskite (∼0.4 nm) layer, owing to the optimal condition of low Ra. This study provides a method for investigating the crystal symmetry that affects the multiferroic properties of ultrathin films, which can be used as barrier layers in multiferroic tunnel junctions for highly functional sensors.

Magnetic-Electric Behaviors and Physical Properties of The Thin Films on ITO-Glass Substrate

Polycrystalline BiFeO3 thin films on ITO glass substrates were prepared by radio frequency magnetron sputtering using a Bi1.1FeO3 target. The samples which were annealed with different annealing conditions are pure without impurities. We measured the magnetic properties and ferroelectricity of the BiFeO3 films. The measurement results show that the magnetic and electrical properties of the BiFeO3 films are significantly different under different annealing conditions.

Design of oxygen vacancy in BiFeO3-based films for higher photovoltaic performance

Photovoltaic technology has been a research hotspot in the field of renewable and clean solar energy. More recently, the ferroelectric photovoltaic effect (FEPV) of BiFeO3 has attracted much attention, since it could obtain a giant open-circuit voltage (VOC) far beyond its bandgap and possibly break through the energy conversion limit of traditional solar cells. However, there has been controversy about the origin and the mechanism of FEPV in BiFeO3. In this work, (Pr, Ni) co-doped BiFeO3 film photovoltaic devices were well constructed and the intrinsic mechanism of the photovoltaic effect was comprehensively investigated. The results and analysis suggested that oxygen vacancies play significant roles on the photovoltaic effect of BiFeO3. Oxygen vacancies not only introduce impurity band as an intermediate band among the band gap of ferroelectric semiconductor to enhance the photocurrent, but also restructure the energy band of Schottky barrier at the interfaces through their electromigration that acts as a driving force to achieve the switchable photovoltaic response.

Training the Polarization in Integrated La0.15 Bi0.85 FeO3 -Based Devices.

The functionalities of BiFeO3 -based magnetoelectric multiferroic heterostructures rely on the controlled manipulation of their ferroelectric domains and of the corresponding net in-plane polarization, as this aspect guides the voltage-controlled magnetic switching. Chemical substitution has emerged as a key to push the energy dissipation of the BiFeO3 into the attojoule range but appears to result in a disordered domain configuration. Using non-invasive optical second-harmonic generation on heavily La-substituted BiFeO3 films, it is shown that a weak net in-plane polarization remains imprinted in the pristine films despite the apparent domain disorder. It is found that this ingrained net in-plane polarization can be trained with out-of-plane electric fields compatible with applications. Operando studies on capacitor heterostructures treated in this way show the full restoration of the domain configuration of pristine BiFeO3 along with a giant net in-plane polarization enhancement. Thus, the experiments reveal a surprising robustness of the net in-plane polarization of BiFeO3 against chemical modification, an important criterion in ongoing attempts to integrate magnetoelectric materials into energy-efficient devices.

Integrating Band Engineering and the Flexoelectric Effect Induced by a Composition Gradient for High Photocurrent Density in Bismuth Ferrite Films.

Photovoltaic energy as one of the important alternatives to traditional fossil fuels has always been a research hot spot in the field of renewable and clean solar energy. Very recently, the anomalous ferroelectric photovoltaic effect in multiferroic bismuth ferrite (BiFeO3) has attracted much attention due to the above-bandgap photovoltage and switchable photocurrent. However, its photocurrent density mostly in the magnitudes of μA/cm2 resulted in a poor power conversion efficiency, which severely hampered its practical application as a photovoltaic device. In this case, a novel approach was designed to improve the photocurrent density of BiFeO3 through the cooperative effect of the gradient distribution of oxygen vacancies and consequently induced the flexoelectric effect realized in the (La, Co) gradient-doped BiFeO3 multilayers. Subsequent results and analysis indicated that the photocurrent density of the gradient-doped multilayer BiFeO3 sample was nearly 3 times as much as that of the conventional doped single-layer sample. Furthermore, a possible mechanism was proposed herein to demonstrate roles of band engineering and the flexoelectric effect on the photovoltaic performance of the gradient-doped BiFeO3 film.

Enhancing the bulk photovoltaic effect by tuning domain walls in epitaxial BiFeO3 films

In this letter, the role of domain wall (DW) on bulk photovoltaic effect (BPV) effect in BiFeO3 (BFO) films was studied by x-ray reciprocal space mapping and conductive atomic force microscope. It was found that the domain structure and DW can be tuned by controlling the epitaxial orientation of BFO film. Remarkably, under 1 sun AM 1.5 G illumination, the 109° DW enhances the transport of photogenerated carriers and simultaneously improves the conductivity and power conversion efficiency (PCE). The short-circuit current density and PCE can reach 171.15 μA cm−2 and 0.1127%, respectively. Therefore, our study reveals the correlation between the DW and the BPV effect in BFO film and provides a new pathway to improve the PCE of BFO-based photovoltaic device.

Thin film processing of multiferroic BiFeO3: From sophistication to simplicity. A review

The obtaining of BiFeO3 in the form of a thin film represents a critical issue for its application in electronic devices since this is the required geometry for the integration of this material in microelectronic circuits. There are several techniques for this purpose, from those that use a gas or plasma phase to transport the precursors to the substrate, which would be included within the PVD or CVD techniques (for its acronym Physical Vapor Deposition and Chemical Vapor Deposition) to those that use a phase liquid for this transport, which would be included under the CSD (Chemical Vapor Deposition) techniques. However, among the large number of published papers, there is much controversy about which of them would be most suitable. It has been clearly demonstrated that all of them have the sufficient capacity to produce uniform and homogeneous thin films in thickness throughout the entire substrate, and although in many articles pure BiFeO3 films have been obtained with an exploitable functional response, others have reported the typical drawbacks that usually entails the obtaining this material: appearance of secondary phases, high leakages or a poor functional response. Nevertheless, there is a common aspect in the specialized literature that seems to be in agreement: the first group of techniques require very sophisticated equipment that involves high energy consumption in terms of temperature and pressure (vaccum), while the techniques that are based on solutions are characterized by their higher simplicity. In this context, the purpose of the present review is to summarize the main aspects of each technique in the obtaining of BiFeO3 thin films, from those that are more sophisticated to the simplest and environmentally benevolent ones, in order to provide an easier understanding of them.

Enhanced magnetoelectric coupling response in hot pressed BiFeO3 and polymer composite films: Effect of magnetic field on grain boundary and grain resistance

We report the effect of grain boundary resistance (GBR) and grain resistance (GR) on the magnetoelectric coupling in the multiferroic composite thick films of BiFeO3 (BFO) nanocrystalline powder and poly vinylidene-fluoride (PVDF) polymer. These composite thick films of thickness ∼ 0.2 mm were prepared via hot pressing with the weight ratio of BFO powder and PVDF polymer as 70:30 (abbreviated as BFO70), 60:40 (abbreviated as BFO60), 50:50 (abbreviated as BFO50) and 40:60 (abbreviated as BFO40), respectively. Impendence investigations (Cole-Cole plots) of these thick films demonstrate that the GBR is significantly changed on the application of magnetic field (8 kOe) in comparison to the GR. Our results indicate that the GBR plays an important role in strengthening of magnetoelectric (ME) effect in BFO-PVDF system. In addition, the ME coupling study indicates that the BFO50 sample shows better results (α33 ∼ 2.7 mV/cm-Oe) and may be suitable for device and sensor applications.

Low temperature deposition of BiFeO3 films on Ti foils for piezoelectric applications

Predominantly (100)-oriented BiFeO3 films with excellent ferroelectric and piezoelectric properties were successfully fabricated on Ti foils via RF-magnetron sputtering at 450 °C. Well-saturated P-E loops with a high remnant polarization (Pr ~72 μC/cm2) and a large built-in field (~145 kV/cm) were observed in the BiFeO3 films. A large transverse piezoelectric coefficient (|e31,f| ~ 2.2 ± 0.2 C/m2) and a decent dielectric property (εr~260–270, tanδ~1.6–2.0%), together with a remarkable fatigue-resistance (~1.1% loss of |e31,f| after 1.2 × 106 piezoelectric actuation cycles) were demonstrated in patterned BiFeO3-Ti cantilevers. These results indicate a great potential of BiFeO3 films as piezo-active components in metal-based micro-electro-mechanical systems (MEMS).

Tunable multiferroic and forming-free bipolar resistive switching properties in multifunctional BiFeO3 film by doping engineering

The advent of multiferroic-based materials has opened the plethora for high tunable multifunctional materials and ultra-fast operation for future non-volatile memory technology. Multifunctional rhombohedral Bi0.94Y0.06Fe0.95Mn0.05O3 (BYFMO) film is grown on fluorine-doped tin oxide to investigate the electromechanical and resistive switching properties in Ag/BYFMO/FTO RRAM configuration. UV–visible absorbance spectra reveal the semiconducting behavior of BYFMO, and the band-gap is found to be 2.37 eV. The magnetic hysteresis curve manifests the soft ferromagnetic nature by suppressing the spiral spin modulated structure, supported by MFM imaging. The Y-Mn co-doped BFO possesses highly tunable piezoelectric and ferroelectric features with maximum domains preferred along 710 and 1090. Lateral domain growth is observed with the increase in tip bias voltage. The Ag/BYFMO/FTO RRAM shows distinct bipolar resistive switching behavior at the SET (ON), and RESET (OFF) processes are obtained at voltage VSET = +1.7V and VRESET = −2.8V, respectively. The memory window (ON/OFF) between high resistance state and low resistance state is about ~ 100, which can be sustained up to 100 testing cycles and 103s without any degradation, indicating that the BYFMO based device exhibits better endurance and retention properties. Moreover, the resistive switching mechanism of the device can be well explained by space charge limited current conduction, which is well supported by conducting a filamentary model. With excellent piezoelectric and resistive switching performance, the multifunctional BYFMO has enough potential for future non-volatile memory technology.

Prominent ferroelectric properties in Mn-doped BiFeO3 spin-coated thin films

BiFe1-xMnxO3 (BFMO, x = 0–0.05) thin films were successfully deposited onto fluorine-doped tin oxide (FTO)/glass substrates via sol–gel technique. According to results obtained from XRD, Raman spectroscopy, SEM, and energy-dispersive EDS, it was found that films have rhomboid perovskite structure and Mn ions were successfully and uniformly incorporated into BiFeO3 (BFO) films. Doping process was accompanied by the reduction in grain size. From XPS pattern, it was found that both Fe2+ concentration and oxygen vacancies are reduced as a result of Mn doping. Compared with pure BFO samples, ferroelectric, dielectric, and optical properties of BFMO films are noticeably enhanced. BiFe0.96Mn0.04O3 film exhibited excellent remnant polarization (2Pr of 312 μC/cm2) accompanied by a low leakage current. Underlying mechanism can be explained in terms of Mn doping-induced structural transition, reduction in grain size, and oxygen vacancies. Additionally, increased bandgap (Eg = 2.68 eV) was obtained in BiFe0.98Mn0.02O3 film compared with BFO film (Eg = 2.58 eV). This work demonstrates remarkable effect of Mn doping on BFO, indicating that BFMO is promising candidate for ferroelectric photovoltaic device applications.