BiFeO3 Thin Films Research Articles

Enhanced Mechanical Hardness of Mixed-Phase BiFeO3 Films through Quenching

Due to epitaxial strain, BiFeO3 (BFO) thin films exhibit a morphotropic phase boundary with coexisting rhombohedral-like (R-like) and tetragonal-like (T-like) phases. The T-like phase, distinguished by its large c/a ratio and giant polarization, has garnered extensive interest. In this work, by quenching an epitaxial mixed-phase BFO thin film grown on a LaAlO3 substrate, a pronounced transition from the R-like to the T-like phase is observed. This transition is concomitant with improved phase structure stability under the force field induced by an atomic force microscope tip. PeakForce Quantitative NanoMechanics mapping reveals that the T-like phase exhibits a higher Young's modulus than the R-like phase, signifying an overall enhancement in the mechanical hardness of the BFO film. This work introduces a simple but powerful approach to manipulating the fraction of the T-like phase in the mixed-phase BFO films, presenting prospects for enhancing their performance and expanding their application range in advanced techniques.

Structure and ferroelectric photovoltaic effect modulation in the epitaxial BiFeO3/La0.5Sr0.5CO3 heterostructures

Square-shaped BiFeO3 (BFO) thin films were prepared on SrTiO3 (STO) substrates by off-axis magnetron sputtering using La0.5Sr0.5CoO3 (LSCO) as the bottom electrode, and both LSCO and BFO are perovskite structure. The BFO crystalline quality improves and the grain size becomes larger as the thickness H increases, and both have smaller mean square roughness. The effects of BFO thickness H, light intensity Ip and ambient temperature AT on the ferroelectric photovoltaic effect (FPE) for the Pt/BFO/LSCO heterostructures are significant. With the H increase, the open-circuit voltage Voc and short-circuit current Jsc increase linearly, with Voc up to 0.44 V. The short-circuit current modulation intensity ΔJsc follows the ΔJsc-300>ΔJsc-200>ΔJsc-100 rule. With the Ip increase, Pt/BFO(300nm)/LSCO' Jsc increases linearly, and Voc first increases and then tends to saturation, which satisfies Glass' law. With the AT increases, Voc is linearly decreasing and Jsc shows the law: decreasing→increasing→decreasing. The polarization-modulated FPE mechanism is investigated by analyzing the Pt/BFO/LSCO energy band structure, which is attributed to the coupling effect between the BFO depolarization field (Edp) and the built-in electric field at the interface (EPt/BFO & EBFO/LSCO), and the FPE mechanism is shifted from the interface barrier domination to the ferroelectric polarization domination as H increases.

Exploring current conduction dynamics in multiferroic BiFeO3 thin films prepared via modified chemical solution method

The current study describes current conduction mechanisms in BiFeO3 thin films prepared by using a modified chemical solution-based technique. In particular, in these films, X-ray photoelectron spectroscopy revealed that the defect causing Fe2+ ions reduced by about 18% compared to the solution-based synthesis methods reported before, indicating better film quality. The leakage current density was measured to be 1.7 × 10–6 A/cm2 at an applied voltage of 1.5 V, which is one order of magnitude less than the previously reported work in a similar system. Oxygen annealing was found to be effective in further reducing the current conduction, making these films a suitable choice for device applications. The current–voltage curves exhibited three different behaviours of current conduction mechanisms in these films. In particular, when the applied electric field was increased, a transition from Ohmic conduction to trap-filled space charge limited conduction was observed. The presented investigations demonstrate the importance of deposition methods in determining the suitability of thin films for device applications.

Individual domain characteristics determined by second-harmonic generation diffractometer for multi-domain multiferroics

We investigated the multi-domain states of a multiferroic La-doped BiFeO3 (BLFO) thin film by examining diffraction patterns in optical second-harmonic generation (SHG) measurement. By directing a laser onto the domain wall within the domain-patterned sample, we observed clear diffraction signatures of SHG waves generated from two ferroelectric domains. We explained the experimental results of the diffraction patterns, including the intensity distribution and the polarization characteristics, using Fresnel propagation of SHG waves. From this, we could determine the amplitude and phase of the SHG waves generated from each domain, and figure out not only the polarization direction of each ferroelectric domain but also the phase related to the complex-valued second-order susceptibility tensor. Consequently, we could present SHG diffractometry as an effective measurement method to reveal the phase details of electric polarization of the multi-domain states of ferroic materials.

Large resistive switching in ultrathin BiFeO3 thin films

Electrically induced resistive switching (RS) effects have been proposed as the basis for future non-volatile memories. In this work, 9 nm-thick BiFeO3 (BFO) epitaxial thin films were deposited on (001)-oriented SrTiO3 substrates by pulsed laser deposition and their resistive switching (RS) behaviors were investigated. A large resistive switching with ON/OFF ratio of ∼106 is observed, surpassing the performance of most resistive random access memories ever reported. The conducting filament is proposed to dominate the RS behavior in the positive voltage region, while the modulation of ferroelectric polarization is suggested to play a significant role in the negative voltage region. Our study significantly deepens the understanding of the physical origin of RS and could provide a reference for designing high-performance memories and memristors based on ultrathin ferroelectric films.

Structural properties and electrical characteristics of BiFeO3 thin films with RE2O3 buffer layers

Structural properties and electrical characteristics of BiFeO3 thin films with RE2O3 buffer layers

High ferroelectric polarization and bandgap modulation in Sol-gel-Synthesis BiFe1–2xAxCoxO3 (A=Mn2+, Ti4+) polycrystalline films

Co-doping of Fe sites in BiFeO3 polycrystalline films effectively suppresses leakage current. Polycrystalline BiFe1–2xMnxCoxO3 and BiFe1–2xTixCoxO3 (x = 0, 0.01, 0.02, 0.03) films were prepared using the sol-gel spin-coating method. A systematic investigation was conducted to determine the impact of (Mn2+/Ti4+, Co2+) co-doping on the structural, leakage current, ferroelectric, dielectric, and optical properties of BiFeO3 films. Results reveal that BiFe1–2xAxCoxO3 (A=Mn2+, Ti4+) films maintain a stable tripartite structure, with (Mn2+/Ti4+, Co2+) co-doping inducing lattice distortion. The leakage current density of BiFe0.98Mn0.01Co0.01O3 (3.45×10−7 A/cm2) and BiFe0.96Ti0.02Co0.02O3 (2.05×10−6 A/cm2) is reduced by 2–3 orders of magnitude relative to pure BiFeO3. Additionally, these films exhibit high remanent polarization values of 188.4 μC/cm2 and 186.8 μC/cm2, respectively. Optical band gaps decrease with (Mn, Co) co-doping and increase with (Ti, Co) co-doping. The magnetic properties are enhanced with increased (Mn, Co) doping but are reduced as (Ti, Co) doping increases. This study provides enhanced understanding of Fe-site co-doping's role in reducing leakage current and improving ferroelectricity in BiFeO3 films, offering valuable insights for photovoltaic applications involving BiFeO3 thin films.

Study of the physical properties of BiFeO3 films obtained by RF sputtering using a homemade target

Ferroelectric materials have attracted significant attention in recent decades because they can be used in various electronic devices. BiFeO3 (BFO) has a perovskite-type ABO3 structure with antiferromagnetic and ferroelectric properties and a band gap within the visible light range, making it a promising candidate for use in sensors and solar cells. In this study, BiFeO3 thin films were grown by radio frequency (RF) sputtering using a homemade target and sintered at different times to evaluate the influence of sintering time on their physical properties. Its physical properties were evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and Raman spectroscopy. These results revealed that all films maintained a perovskite-like structure with rhombohedral and bismuth-rich secondary phases (Bi2O2.33). Atomic force microscopy (AFM) and piezoresponse force micrography (PFM) images depicted the impact of sintering time on surface roughness and ferroelectric domain reorientation. The optical properties, such as n, k, and the band gap of all samples, were obtained using reflection-transmission spectroscopy. The photoresponse under dark, visible and UV illumination were evaluated on BFO films grown over glass-FTO substrate.

Oxygen vacancy induced electrical conductivity enhancement in Ca-doped BiFeO3 thin films

Bismuth ferrite, recognized for its exceptional physical properties as a multiferroic material, has recently attracted considerable attention due to its high domain wall conduction. However, challenges in further improving domain wall conductivity and magnetoelectric coupling hamper the practical application of pure-phase BFO thin films. Addressing these limitations, we successfully synthesized Bi1-xCaxFeO3 thin films on LaAlO₃ (001) substrates with varied calcium (Ca) doping levels (x = 0–0.3) through pulsed laser deposition. Our experimental data demonstrate a direct relationship between the films' electrical conductivity and the prevalence of oxygen vacancies (OV). An upward trend in both OV density and electrical conductivity emerges with incremental Ca doping, peaking at x = 0.2. Beyond this doping threshold, a further increase in Ca concentration inversely affects OV density and electrical conductivity. These insights suggest that fine-tuning OV concentration in Ca-doped BFO thin films can significantly modulate their electrical properties, paving the way for the advancement of bismuth ferrite-based electronic devices.

Tuning ferroelectric domains and polarization properties of flexible BiFeO3 thin films by phase transition engineering

In the present investigation, thin films of flexible (Bi1-xSmx)(Fe0.95Mn0.05)O3 (denoted as BSFM0, BSFM4, BSFM8, and BSFM12 for x=0 %, 4 %, 8 %, and 12 %, respectively) were synthesized on metallic Ni-Cr substrates via the sol-gel technique. A comprehensive examination was conducted to elucidate the influence of varying concentrations of Sm3+ substitution on the phase constitution, oxygen vacancy concentration, and leakage behavior of the (Bi1-xSmx)(Fe0.95Mn0.05)O3 films. Utilizing X-ray diffraction (XRD) and Raman spectrometry, it was ascertained that all films exhibited a composite structure comprising orthorhombic (Pnma) and rhombohedral (R3c) phases. An augmentation in Sm3+ doping led to a non-monotonic variation in the R3c phase content, initially increasing and subsequently decreasing, while the nonpolar Pnma phase demonstrated a progressive enhancement. The distorted R3c phase was found to promote the flipping of the domain architecture, thereby augmenting the polarization attributes. Notably, at a Sm doping level of 4 %, the maximum polarization (Pmax) attained was 95 μC/cm2, with a remnant polarization (Pr) of 78 μC/cm2. The incorporation of a judicious amount of Sm was observed to curtail film leakage and decelerate the aging phenomenon. The compositional stability of this doping level was evaluated across a frequency spectrum of 0.1–12 kHz and a temperature gradient of 25–200 °C, revealing no propensity for degradation under conditions of polarization, a holding duration of 104 s, or fatigue scenarios involving a curvature of 5 mm and 108 switching cycles subjected to 103 instances of bending. These findings provide innovative insights for the development of next-generation lead-free transducer apparatus and flexible information storage mediums.

Influence of RE2O3 (Dy2O3, Yb2O3, and Lu2O3) buffer layers on the structural and ferroelectric characteristics of BiFeO3 thin films

This study investigates the impact of RE2O3 (Dy2O3, Yb2O3, and Lu2O3) buffer layers on the structural and ferroelectric properties of BiFeO3 thin films grown using a spin-coating method. BiFeO3 thin films with various RE2O3 buffer layers were analyzed using x-ray diffraction, atomic force microscopy, secondary ion mass spectrometry, and x-ray photoelectron spectroscopy to determine their crystalline structures, surface topographies, depth profiles, and chemical compositions, respectively. The RE2O3-buffered BiFeO3 film showed better electrical properties compared to the control BiFeO3 film. The buffer layer composed of Yb2O3 showed the lowest leakage current of 6.82 × 10−6 A cm−2, highest remnant polarization of 44.1 μC cm−2, and smallest coercive field of 189 kV cm−1 because of the incorporation of Yb3+ ions into the BiFeO3 film, high degree of (110) preferred orientation, high Fe3+ content, low surface roughness, reduction of Fe3+ valence fluctuation to Fe2+ ions, and decrease in oxygen vacancies. Such BiFeO3 thin films with various RE2O3 buffer layers using spin-coating method pave a pathway toward practical applications of spintronic, sensor and memory.

Electrical conduction of ferroelectric domains and domain walls in polycrystalline BiFeO3 and Bi5Ti3FeO15 thin films

The unique properties of domain wall conductivity have garnered significant interest for their potential application in non-volatile ferroelectric domain wall memory. In this study, we investigated the electrical conduction within ferroelectric domains and domain walls of polycrystalline BiFeO3 (BFO) and Bi5Ti3FeO15 (BTFO) thin films, which were deposited on Pt/Ta/glass substrates via pulsed laser deposition. BFO thin film consistently demonstrated a (111) orientation, while BTFO thin film exhibited mixed crystallinity, featuring both c-axis and a-axis orientations. This mixed crystallinity in BTFO thin film contributed to a higher remanent polarization of 38.2 μC/cm2 compared to 20.3 μC/cm2 in BFO thin film, which is attributed to the a-oriented crystallinity within the Bi-layered perovskite structure of BTFO thin film. Additionally, BTFO thin film displayed a greater prevalence of 90° domain walls, which enhanced electrical conduction due to charge accumulation, particularly when compared to 180° domain walls. A significant change in resistance was observed when the domain wall was present versus absent, with a more pronounced effect in the BTFO capacitor compared to the BFO capacitor. This is attributed to the higher domain wall conductivity in BTFO thin film, confirming their superiority for use in ferroelectric capacitor devices that leverage domain wall conductivity.

Synthesis, characterization and photoelectrochemical performance of Bi2Fe4O9 thin films

Mullite-type Bi2Fe4O9 has been little explored in thin film form as a photoanode for photoelectrochemical water splitting. In this study, Bi2Fe4O9 thin films have been prepared using the sol-gel technique from a simple precursor solution based on the corresponding metal salts, acetic acid, and polyvinyl alcohol. The films were deposited by dip-coating onto fluorine-doped tin oxide substrates and dried at 350 °C, repeating the dipping-drying cycle six times, and finally sintered at 600 °C. The films were characterized by GIXRD, revealing the formation of the material in its orthorhombic phase. Raman spectroscopy showed the Ag and B1g vibrational modes, validating the formation of the bismuth iron oxide. UV–Vis transmittance measurements revealed that the material exhibits two optical transitions: a direct band gap of 2.86 eV and an indirect band gap of 1.98 eV. FESEM micrographs and AFM images showed a uniform nanostructured surface morphology. The photoelectrochemical properties of the Bi2Fe4O9 films were studied using cyclic voltammetry and chronoamperometry with front side illumination, demonstrating the stability of the material in aqueous media and the generation of photocurrent in the presence of H2O2. Furthermore, results from intensity-modulated photocurrent spectroscopy (IMPS) revealed that the photocurrent is limited by both bulk and surface recombination and a short hole diffusion length.

Exploring the Possibility of Thermally Assisted Creation and Annihilation of Anti‐Frenkel Defects in a Multiferroic Oxide for Tuning Interfacial Ferroelectricity

AbstractLattice defects such as oxygen vacancies, interstitials, and their complexes are present in crystalline oxide materials. In particular, anti‐Frenkel defects, which refer to charge‐neutral anion vacancy‐interstitial pairs, are strongly coupled with ferroelectric and dielectric properties as electric dipoles. However, in order to observe their macroscopic manifestation, delicate defect controls are required to the extent that electronic and ionic charges are almost completely suppressed. Here, the thermal cycle dependence of dielectric and piezoelectric properties is scrutinized in the strain‐driven morphotropic phase boundaries of multiferroic La‐substituted BiFeO3 thin films. Electrochemical impedance spectroscopy provides the Warburg feature that is considered evidence of the ionic origin. The observations are discussed based on anti‐Frenkel defects that are created or annihilated reversibly by thermal cycles through high‐temperature structural phase transition temperature or magnetic Néel temperature. The defect dipoles are spontaneously aligned by the flexoelectric effect in the phase boundaries inducing a metastable interfacial ferroelectric phase. The findings offer useful insight into defect dipoles.

Multiphase cooperation for multilevel strain accommodation in a single-crystalline BiFeO3 thin film

Abstract The functionalities and diverse metastable phases of multiferroic BiFeO3 (BFO) thin films depend on the misfit strain. Although mixed-phase-induced strain relaxation in multiphase BFO thin films is well known, it is unclear whether a single-crystalline BFO thin film can accommodate misfit strain without the involvement of its polymorphs. Thus, understanding the strain relaxation behavior is key to elucidating the lattice strain–property relationship. In this study, a correlative strain analysis based on dark-field inline electron holography (DIH) and quantitative scanning transmission electron microscopy (STEM) was performed to reveal the structural mechanism for strain accommodation of a single-crystalline BFO thin film. The nanoscale DIH strain analysis results indicated a random combination of multiple strain states that acted as a primary strain relief, forming irregularly strained nanodomains. The STEM-based bond length measurement of the corresponding strained nanodomains revealed a unique strain accommodation behavior achieved by a statistical combination of multiple modes of distorted structures on the unit-cell scale. The globally integrated strain for each nanodomain was estimated to be close to -1.5%, irrespective of the nanoscale strain states, which was consistent with the fully strained BFO film on the SrTiO3 substrate. Density functional theory calculations suggested that strain accommodation by the combination of metastable phases was energetically favored in relation to single-phase-mediated relaxation. This discovery allows a comprehensive understanding of strain accommodation behavior in ferroelectric oxide films, such as BFO, with various low-symmetry polymorphs.

Transfer of High-Temperature-Sputtered BiFeO3 Thin Films onto Flexible Substrates Using α-MoO3 Layers.

Inducing external strains on highly oriented thin films transferred onto mechanically deformable substrates enables a drastic enhancement of their ferroelectric, magnetic, and electronic performances, which cannot be achieved in films on rigid single crystals. Herein, the growth and diffusion behaviors of BiFeO3 thin films grown at various temperatures is reported on α-MoO3 layers of different thicknesses using sputtering. When the BiFeO3 thin films are deposited at a high temperature, significant diffusion of Fe into α-MoO3 occurs, producing the Fe1.89Mo4.11O7 phase and suppressing the maintenance of the 2D structure of the α-MoO3 layers. Although lowering the deposition temperature alleviates the diffusion yielding the survival of the α-MoO3 layer, enabling exfoliation, the BiFeO3 is amorphous and the formation of the Fe1.89Mo4.11O7 phase cannot be suppressed at the crystallization temperature. High-temperature-grown BiFeO3 thin films are successfully transferred onto flexible substrates via mechanical exfoliation by introducing a blocking layer of Au and measured the ferroelectric properties of the transferred films.

Four dimensional-scanning transmission electron microscopy study on relationship between crystallographic orientation and spontaneous polarization in epitaxial BiFeO3

Spontaneous polarization and crystallographic orientations within ferroelectric domains are investigated using an epitaxially grown BiFeO3 thin film under bi-axial tensile strain. Four dimensional-scanning transmission electron microscopy (4D-STEM) and atomic resolution STEM techniques revealed that the tensile strain applied is not enough to cause breakdown of equilibrium BiFeO3 symmetry (rhombohedral with space group: R3c). 4D-STEM data exhibit two types of BiFeO3 ferroelectric domains: one with projected polarization vector possessing out-of-plane component only, and the other with that consisting of both in-plane and out-of-plane components. For domains with only out-of-plane polarization, convergent beam electron diffraction (CBED) patterns exhibit “extra” Bragg’s reflections (compared to CBED of cubic-perovskite) that indicate rhombohedral symmetry. In addition, beam damage effects on ferroelectric property measurements were investigated by systematically changing electron energy from 60 to 300 keV.

Non-contact intelligent sensor for recognizing transparent and naked-eye indistinguishable materials based on ferroelectric BiFeO3 thin films

At present, the research on ferroelectric photovoltaic materials mainly focuses on photoelectric detection. In the context of the rapid development of the Internet of Things (IoT), it is particularly important to use smaller thin-film devices as sensors. In this work, an indium tin oxide/bismuth ferrite (BFO)/lanthanum nickelate device has been fabricated on an F-doped tin oxide glass substrate using the sol–gel method. The sensor can continuously output photoelectric signals with little environmental impact. Compared to other types of sensors, this photoelectric sensor has an ultra-low response time of 1.25 ms and ultra-high sensitivity. Furthermore, a material recognition system based on a BFO sensor is developed. It can effectively identify eight kinds of materials that are difficult for human eyes to distinguish. This provides new ideas and methods for developing the IoT in material identification.

Intermediate multidomain state in single-crystalline Mn-doped BiFeO3 thin films during ferroelectric polarization switching

A intermediate multidomain state and large crystallographic tilting of 1.78° for the (hh0)pc planes of a (001)pc-oriented single-domain Mn-doped BiFeO3 (BFMO) thin film were found when an electric field was applied along the [110]pc direction. The anomalous crystallographic tilting was caused by ferroelastic domain switching of the 109° domain switching. In addition, ferroelastic domain switching occurred via an intermediate multidomain state. To investigate these switching dynamics under an electric field, we used in situ fluorescent X-ray induced Kossel line pattern measurements with synchrotron radiation. In addition, in situ inverse X-ray fluorescence holography (XFH) experiments revealed that atomic displacement occurred under an applied electric field. We attributed the atomic displacement to crystallographic tilting induced by a converse piezoelectric effect. Our findings provide important insights for the design of piezoelectric and ferroelectric materials and devices.

Electrical and structural properties of BiFeO3 thin films with four distinct buffer layers: La2O3, Pr2O3, Sm2O3, and Tm2O3

In this study, BiFeO3 thin films were fabricated using a spin-coating technique on SrRuO3/n+-Si substrates, employing four different RE2O3 (La2O3, Pr2O3, Sm2O3, and Tm2O3) buffer layers. The investigation focused on assessing the structural and ferroelectric characteristics of these films. X-ray diffraction analysis revealed a (110) preferred orientation with a perovskite-rhombohedral structure in films with RE2O3 buffer layers. Atomic force microscopy analysis indicated smooth surfaces for these layers. Secondary ion mass spectrometry demonstrated high levels of Bi ions in films with RE2O3 buffer layers, while X-ray photoelectron spectroscopy revealed a low Fe2+/Fe3+ ratio in these layers. Notably, the film with a Sm2O3 buffer layer showed a lower leakage current (2.05 × 10−6 A/cm2) and a higher remnant polarization (43.65 μC/cm2). This improvement is attributed to Sm3+ ions fostering (110) orientation, reducing surface roughness, increasing Fe3+ content, thereby minimizing oxygen vacancies, and suppressing Fe3+ to Fe2+ valence fluctuation.