BiFeO3 Thin Films Research Articles

A Comparative Analysis on the Crystalline Behavior, Topographical, Photoluminescence, and Leakage Current Characteristics of BiFeO3 Nanopowders and Thin Films

AbstractThin films and nanocrystalline powders of BiFeO3 (BFO) are prepared from pulsed laser deposition (PLD) and sol–gel process, respectively. Their grain sizes, microstrain, morphological features, leakage current characteristics, and photoluminescence (PL) emission behavior are investigated and compared with each other. X‐ray diffraction (XRD) technique has been utilized to compute the grain sizes and crystalline strain values for the thin films and nanopowders of BFO through Williamson–Hall (W–H) plots. XRD and Raman peaks of the deposited thin film and synthesized nanopowders of BFO confirm the formation of rhombohedral unit cell character and its vibrational structure. Topographical analysis of nanopowders and thin films of BFO from scanning electron microscopy (SEM) and atomic force microscopy (AFM) measurements reveals the spherical morphology of grown particles. Suppression of emission peaks, as noticed from PL spectra for BFO nanopowders and thin film, indicates the possible electron defect level transitions due to oxygen vacancies. Leakage current density studies suggest the detectable flow of current in both study of specimen.

Improved polarization retention in epitaxial BiFeO3 thin films induced by strain relaxation

This study investigated the impact of lattice mismatch strain (LMS) on the polarization retention of epitaxial ferroelectric BiFeO3 (BFO) thin films. The application of varying degrees of LMS on the BFO films was achieved by tuning the film thickness, which was verified through X-ray diffraction and reciprocal space mapping. As the strain was gradually eliminated, the film demonstrated a more saturated hysteresis loop with a reduced negative coercive electric field. Additionally, a significant increase in polarization retention was observed with increasing film thickness, strongly indicating the crucial role of LMS in driving the formation of preferred polarization, which is intimately connected with the built-in field induced by oxygen vacancies-relevant space charge alignment. Moreover, the anisotropic LMS state of the BFO films was utilized to horizontally reverse local polarization in a sub-micrometer scale, revealing that strain accelerates the back-switching of polarization to eliminate the artificially created ferroelastic domain walls. To counteract the strain effect, charge injection of electrons was proposed to enhance the retention of switched polarization by fully suppressing the built-in field and pinning the artificially created domain walls. The findings provide valuable insights into the impact of strain on polarization retention for the development of ferroelectric memories.

Defect-Induced Control on Current Conduction of Cobalt-Doped BiFeO3 Thin-Film Devices

Defect-Induced Control on Current Conduction of Cobalt-Doped BiFeO3 Thin-Film Devices

Polarization-independent enhancement in UV photoconductivity of BiFeO3/Sn:In2O3 heterostructure

Currently, polarization-dependent transport in ferroelectric materials under optical illumination is gaining a lot of attention in optoelectronics. This idea is implemented on BiFeO3 to illustrate its UV–Visible light photodetection property, however, naturally occurring bismuth or oxygen vacancies serve a major disadvantage as they interfere with its polarization ability. It is very difficult to overcome this defect issue in bismuth-based perovskites, therefore, based on this herein an enhancement in the photoconductive property of polycrystalline BiFeO3 thin film deposited via spray pyrolysis technique on Sn:In2O3 coated glass substrate is demonstrated without polarizing it. An extraordinary photodetection behavior with higher photoresponsivity (1.01 A/W) and external quantum efficiency (364) as well as faster response speed (6 ms) even under low UV illumination (340 nm) and lower applied bias of 2 V is observed. The roles of the BiFeO3–Sn:In2O3 interface, as well as the adsorption/desorption of oxygen molecules on the surface of BiFeO3 layers, were highlighted for the ferroelectric material's UV photodetector application.

Temperature Dependence of the Hyperfine Magnetic Field at Fe Sites in Ba-Doped BiFeO3 Thin Films Studied by Emission Mössbauer Spectroscopy

Emission 57Fe Mössbauer spectroscopy (eMS), following the implantation of radioactive 57Mn+ ions, has been used to study the temperature dependence of the hyperfine magnetic field at Fe sites in Ba-doped BiFeO3 (BFO) thin films. 57Mn β decays (t1/2 = 90 s) to the 14.4 keV Mössbauer state of 57Fe, thus allowing online eMS measurements at a selection of sample temperatures during Mn implantation. The eMS measurements were performed on two thin film BFO samples, 88 nm and 300 nm thick, and doped to 15% with Ba ions. The samples were prepared by pulsed laser deposition on SrTiO3 substrates. X-ray diffraction analyses of the samples showed that the films grew in a tetragonal distorted structure. The Mössbauer spectra of the two films, measured at absorber temperatures in the range 301 K–700 K, comprised a central pair of paramagnetic doublets and a magnetic sextet feature in the wings. The magnetic component was resolved into (i) a component attributed to hyperfine interactions at Fe3+ ions located in octahedral sites (Bhf); and (ii) to Fe3+ ions in implantation induced lattice defects, which were characterized by a distribution of the magnetic field BDistr. The hyperfine magnetic field at the Fe probes in the octahedral site has a room temperature value of Bhf = 44.5(9) T. At higher sample temperatures, the Bhf becomes much weaker, with the Fe3+ hyperfine magnetic contribution disappearing above 700 K. Simultaneous analysis of the Ba–BFO eMS spectra shows that the variation of the hyperfine field with temperature follows the Brillouin curve for S = 5/2.

Mechanically Driven Reversible Polarization Switching in Imprinted BiFeO3 Thin Films

AbstractMechanically driven polarization switching via scanning probe microscopy provides a valuable voltage‐free strategy for designing ferroelectric nanodomain structures. However, it is still challenging to realize reversible polarization switching with mechanical forces. Here, the mechanically driven reversible polarization switching observed in imprinted ferroelectric BiFeO3 thin films is reported, i.e., up‐to‐down switching by a sharp scanning tip and down‐to‐up switching by a blunt tip. Free energy calculations, phase‐field simulations, and piezoresponse force microscopy reveal that reversible mechanical switching arises from the interplay among the flexoelectric effect, the piezoelectric effect, and the internal upward built‐in field in BiFeO3 films. This study gains a deeper insight into the mechanism and control of mechanically driven polarization switching, and provides guidance for exploring potential ferroelectric‐based electro‐mechanical microelectronics.

Size-dependent polarization retention in ferroelectric BiFeO3 domain wall memories

Reliable polarization retention in a single crystal-like ferroelectric thin film is crucial for the effective implementation of ferroelectric domain wall (DW) memories. This ensures that the saved information can be non-destructively read out by identifying the resistive state of the memory with a voltage signal below the coercive voltage (Vc) of the ferroelectric film. However, stabilizing a small volume of switched polarization for ultra-high-density information storage applications is still challenging due to its back-switching behavior at zero electric field. In this study, we investigated the polarization retention of ferroelectric BiFeO3 (BFO) thin film devices with size-variable Pt electrodes. With the scaling down of the device size, the switched polarizations became more susceptible to depolarization field and ferroelastic stress arising at the artificially created domain boundary. We found that BFO devices featuring a switched polarization area larger than 7 × 106 nm2 demonstrated both long-term polarization retention and a current rectification ratio of 4.5:1 induced by DW manipulation. As the device size decreases, the retention time of the switched polarization reduces gradually, which results in the disappearance of the device current due to the DW being eliminated by the back-switching of the polarization. A plot was created to illustrate the relationship between the current retention time and the switched polarization area on the surface of the BFO thin films. Furthermore, the unsustainable polarization retention facilitates a higher current rectification ratio of 48:1, which is suitable for non-destructive information readout with a voltage pulse larger than Vc in high-density ferroelectric DW memories.

Preparation of epitaxial BiFeO3 thin films on Si(001) substrates by pulsed electron deposition

To achieve the epitaxial thin films deposition, it is necessary to use properly oriented substrates, with or without buffer layers, matching the lattice parameters of the epitaxial thin film we want to grow. In this work, the deposition of epitaxial Bi2SiO5(200) and BiFeO3(001) thin films on Si(001) substrates by pulsed electron deposition (PED) technique is reported without special substrate preparation. The deposition of epitaxial BSO(200) and T-BFO(001) directly onto Si(001) substrates during a single target deposition process is relevant and presents enormous potential to reduce costs and improve practicality, interface quality and BFO integration efficiency with Si(001) substrates.

Photoelectrocatalytic Degradation of Methylene Blue on Electrodeposited Bismuth Ferrite Perovskite Films.

Electrodeposited bismuth ferrite (BiFeO3) thin films on fluorine-doped tin oxide (FTO) substrate were employed as photoanodes in the photoelectrocatalytic degradation of methylene blue. The BiFeO3 thin films electrodeposited for 300 s, 600 s, 1200 s, 1800 s and 3600 s were characterised with XRD, field emission scanning electron microscopy (FESEM) and UV-vis diffuse reflectance spectroscopy. SEM images displayed different morphology at different electrodeposition times which affects the photoelectrocatalytic (PEC) performances. The FESEM cross-sectional area was used to measure the thickness of the film. The optical properties showed that the band gaps of the photoanodes were increasing as the electrodeposition time increased. The photocurrent response obtained showed that all thin film photoanodes responded to visible light and lower charge transfer resistance (from electrochemical impedance spectroscopy studies) was observed with photoanodes electrodeposited at a shorter time compared to those at a longer time. The PEC application of the photoanode for the removal of methylene blue (MB) dye in water showed that the percentage degradation decreased with an increase in electrodeposition time with removal rates of 97.6% and 70% observed in 300 s and 3600 s electrodeposition time, respectively. The extent of mineralisation was measured by total organic carbon and reusability studies were carried out. Control experiments such as adsorption, photolysis, photocatalysis and electrocatalysis processes were also investigated in comparison with PEC. The electrodeposition approach with citric acid exhibited improved electrode stability while mitigating the problem of catalyst leaching or peeling off during the PEC process.

Correlation between the unit cell parameters and electronic transitions in Bi1-xEuxFeO3 thin films

Among AFeO3 (A being divalent or trivalent cation) perovskites, BiFeO3 (BFO) exhibits unique as well as complex electronic structure. In the present study local structure of BFO have been manipulated systematically and correlation with the electronic structure has been presented in detail. An electronic energy-level diagram of polycrystalline Bi1-xEuxFeO3 (0 ≤ x ≤ 0.225) has been presented. Using detailed quantitative structural analysis, diffuse reflectance spectrum and the Kubelka-Munk model, the effect of local structural modification on the electronic structure has been investigated. Systematic structural modification has been achieved by Eu3+ substitution which affects bond length, bond angle, and octahedral rotation, consequently, drives the system towards a higher symmetry configuration. Changes in the multiple electronic transitions in pure BFO thin films as a function of Eu3+ substitution have been investigated in detail. An additional change is the emergence of the 4f levels of the rare earth substituent in the valence and conduction bands, Charge-transfer, and doubly degenerated d-d transitions (6A1g→4T2g and 6A1g→4T1g) have been traced as a function of local changes at the unit cell level. In centrosymmetric configuration, 6A1g→4T1g electronic transition is found to be forbidden by the Laporte Rule.

Physical properties of Cr-doped epitaxial BiFeO3 thin films influenced by ferroelectric domain structures

We investigated the physical properties of Cr-doped epitaxial BiFeO3 (CBFO) thin films influenced by ferroelectric domain structures. The epitaxial CBFO thin films with Cr concentrations of 0, 2, 4, 6, and 8% were deposited on Nb-doped single crystal SrTiO3 substrates by pulsed laser deposition. The epitaxial CBFO thin films with Cr concentrations of 0, 2, and 4% had highly c-oriented crystallinity and the epitaxial CBFO thin films with Cr concentrations of 6% and 8% exhibited mixed crystallinity of c-oriented crystallinity and (101)-oriented crystallinity. The CBFO thin film with a Cr concentration of 4% showed the highest ferroelectric polarization, and the ferroelectric polarizations of the CBFO thin films with a Cr concentration of more than 4% were reduced by the formation of (101)-oriented grains. On the other hand, the piezoelectric properties, leakage current characteristics, and ferromagnetic properties of CBFO thin films were simultaneously improved by increasing the Cr concentration. Terraces indicating layer-by-layer growth were observed on the surface of the CBFO thin films, and the edges of the terraces formed on the surface of the CBFO thin films were blurred when the Cr concentration increased. The ferroelectric domain structures of the CBFO thin films observed with a piezoelectric response microscope changed from a stripe domain structure to a mosaic domain structure when the Cr concentration exceeded 6%.

Facile Control of Ferroelastic Domain Patterns in Multiferroic Thin Films by a Scanning Tip Bias.

BiFeO3, known as the "holy grail of all multiferroics", provides an appealing platform for exploration of multifield coupling physics and design of functional devices. Many fantastic properties of BiFeO3 are regulated by its ferroelastic domain structure. However, a facile programable control on the ferroelastic domain structure in BiFeO3 remains challenging and our understanding on the existing control strategies is also far from complete. This work reports a facile control of ferroelastic domain patterns in BiFeO3 thin films under area scanning poling by exploiting the tip bias as the control parameter. Combining scanning probe microscopy experiments and simulations, we found that BiFeO3 thin films with pristine 71° rhombohedral-phase stripe domains exhibit at least four switching pathways solely by controlling the scanning tip bias. As a result, one can readily write mesoscopic topological defects into the films without the necessity to change the tip motion. The correlation between conductance of the scanned region and the switching pathway is further investigated. Our results extend the current understanding on the domain switching kinetics and the coupled electronic transport properties in BiFeO3 thin films. The facile voltage control of ferroelastic domains should facilitate the development of configurable electronic and spintronic devices.

Imaging and control of periodic array of stripe domain and domain wall in epitaxial BiFeO3 thin films

Ferroelectric stripe domain structure and domain walls were investigated by vector PFM on epitaxial BiFeO3 thin films. Measurements of topography of film versus distance between spikes, we identify the [Formula: see text] domain in the film, were supported by XRD and AFM characterization. [Formula: see text] domain can be switched under the electric field engendered by the biased PFM tip, and their controllable [Formula: see text] rotation can be maintained by electron injection by the PFM tip. These stripe domain walls are conductive, provide an opportunity to further study their new properties in high-density memory devices.

Reduced Leakage Current and Enhanced Photovoltaic Effect in Zn-Doped BiFeO3 Thin Films

Zn-doped BiFeO3 (BFO) thin films with compositional formula BiFe1–xZnxO3 (x = 0, 0.1, and 0.2) have been epitaxially grown on SrRuO3 buffered SrTiO3 substrates by pulsed laser deposition. The high-concentration Zn doping does not suppress the ferroelectric polarization of the BFO films, and the Zn-doped BFO thin films also show reduced leakage current and an enhanced photovoltaic effect. By optical and photoelectron spectroscopy measurements, with Zn doping, the BFO thin films show more oxygen vacancies and a structural evolution, and moreover, a blue-shift of optical bandgap and an increase of work function are demonstrated. The reduction of leakage current and the enhancement of photovoltaic effect are related to the variation of interfacial Schottky junctions and oxygen vacancies with doping. This study reveals systematic insights into the effects of Zn doping on the physical properties of BFO thin films.

Connecting electrical current conduction and Urbach energy in doped BiFeO3 thin-films

The present study explores the influence of Co2+ and Mn4+ ions on the optical and electrical properties of bismuth ferrite thin films prepared on indium tin oxide-coated glass substrates. In particular, a correlation between the Urbach energy tails and the current conduction is established. Doping-dependent variations in Urbach energy tail widths, optical bandgap energies and J-V curves of the fabricated films are investigated. The Urbach energy tail width of Co-doped BiFeO3 is found to increase by 36%, while that of Mn-doped one is found to reduce by 15%. The optical bandgap of Co-doped and Mn-doped BiFeO3 was found to reduce by 44% and 40% respectively compared to undoped BiFeO3. A correlation between the induced defect states and the optical parameters has been observed. Analysis of J - V characteristics reveals the dependence of the current conduction upon the valence state of the dopant. In particular, the dark current density decreased by almost one order of magnitude with Mn-doping and increased by more than one order with Co-doping. A defect-dependent space-charge-limited conduction (SCLC) mechanism is proposed to understand the J - V curve in the Co-doped and Mn-doped BFO samples. A direct variation of the leakage current density with the Urbach energy suggests that Urbach energy studies could be considered as an alternative indicator of the leakage current density in thin-film BiFeO3-based devices.

Write-once-read-many-times memory device based on Pt/BiFeO3/LaNiO3 heterostructures

Nonvolatile memory devices based on resistive switching effects are promising candidates for next generation high performance nonvolatile data storage. Here, we report the write-once-read-many-times (WORM) resistive switching behavior for the first time in Pt/BiFeO3/LaNiO3 heterostructures. BiFeO3 thin films with (100) preferred orientation have been prepared by RF magnetron sputtering combined with annealing at temperature higher than 600 ℃, and show excellent WORM resistive switching behaviors with data retention for>3600 s and reliable endurance for>1000 cycles. The devices with BiFeO3 annealed at 650 ℃ exhibit extremely high ON/OFF ratio (>3 × 104) and low switching voltage (∼3 V) due to the high quality of BiFeO3 thin films. The observed resistive switching behavior is attributed to the formation of permanent conducting filaments that composed of oxygen vacancies. These results demonstrate that BiFeO3 is promising for novel WORM memory device applications.

Enhanced responsivity of Zr-doped BiFeO3 based self-powered UV–visible heterojunction photodetector fabricated via spray pyrolysis technique

Herein, we report enhanced responsivity of Zr-substituted BiFeO3 thin film fabricated through spray pyrolysis technique in ultraviolet & visible region of the solar spectrum. Optimum substitution of Zr4+ ions at Fe site reduced the oxygen vacancies and optical bandgap from 2.3 eV to ∼2.08 eV. Both X-ray Diffraction and Raman results confirm that BiFe1-xZrxO3 (x = 0.0, 0.05, 0.10) films belong to rhombohedral phase. Besides, X-ray photoelectron spectroscopy and Electron paramagnetic resonance analysis confirms optimum substitution of Zr4+ ions reduce the leakage current and enhance the remnant polarization. The AZO/BiFe1-xZrxO3 (x = 0.0, 0.05, 0.10) interface junction confines the optical absorption within the ferroelectric layer, expands the space charge region and reduces charge carrier recombination. The PEDOT: PSS thin film deposited upon photoactive ferroelectric layer facilitates hole transport. The achieved maximum responsivity, detectivity, and response time of the AZO/BiFe0.95Zr0.05O3/PEDOT: PSS device for 365 nm (50 μW/cm2) light is 18 mA/W, 8.72 × 1010 Jones, and ∼0.2 s. The device responsivity for 100 mW/cm2 white light is 0.14 mA/W; and LEDs (10 mW/cm2) of wavelengths of 405 and 450 nm are 2.38 mA/W and 0.67 mA/W, respectively. This work confirms that optimum Zr substitution at Fe-site enhances the optoelectronic properties of BiFeO3 based UV–vis self-powered photodetector heterojunction.

Nd dopant effect on structural properties of BiFeO3 thin films and application in a negative capacitance transistor

When Nd doping in BiNdFeO (BNFO) film exceeds 10%, the two diffraction peaks (104)/(110) merge into one sharp single peak with increasing addition of Nd. It seems that a phase structural distortion can be obtained due to the substitution of Nd3+ ion for Bi3+ ion. For ferroelectric properties, among all Nd-doped (0%–20%) BiFeO3 capacitors, the BNFO capacitor with 10% Nd doping shows a higher Pr value due to a phase structural distortion and a lower leakage current. In a resistive-capacitive circuit diagram, the transient response of the voltage (V FE) across the BNFO capacitor can be measured. We demonstrate that the capacitance value is negative in the spike interval of V FE(t), with the result that the negative capacitance of BNFO with a 10% Nd doping capacitor can obviously improve the I DS–V GS characteristics of a control thin-film transistor.

Deterministic Manipulation of Multi‐State Polarization Switching in Multiferroic Thin Films (Adv. Funct. Mater. 8/2023)

Four-State Nonvolatile Memories In article number 2208244, Deyang Chen and co-workers report that four-state polarization states in multiferroic BiFeO3 thin films are deterministically achieved and reversibly controlled through precisely selecting 71° ferroelastic and 180° ferroelectric switching paths. This study provides the platform for the realization of multi-state memories and magnetoelectric coupling based devices.

A Ferroelectric Domain-Wall Transistor

On the basis of novel properties of ferroelectric conducting domain walls, the domain wall nanoelectronics emerges and provides a brand-new dimension for the development of high-density, high-speed and energy-efficient nanodevices. For in-memory computing, three-terminal devices with both logic and memory functions such as transistors purely based on ferroelectric domain walls are urgently required. Here, a prototype ferroelectric domain-wall transistor with a well-designed coplanar electrode geometry is demonstrated on epitaxial BiFeO3 thin films. For the logic function, the current switching between on/off states of the transistor depends on the creation or elimination of conducting domain walls between drain and source electrodes. For the data storage, the transistor can maintain nonvolatile on/off states after the write/erase operations, providing an innovative approach for the development of the domain wall nanoelectronics.