BiFeO3 Films Research Articles

Photovoltaic Effect in BiFeO₃ Film Deposited by RF Magnetron Sputtering.

A polycrystalline BiFeO₃ film was deposited on ITO substrate by RF magnetron sputtering method. Small crystallite size and compact structure are obtained for BiFeO₃ film which has the excellent ferroelectric properties. The measured photovoltaic response reveals an open-circuit voltage of ~0.52 V and a short-circuit current density of ~10 μA/cm² under the illumination of 100 mW/cm² irradiance. Moreover, a tunable photovoltaic effect with light illumination is observed under different voltage sweep mode. High initial sweep voltage can enhance the photovoltaic effect largely, however, the photovoltaic response decreases with the increase of voltage sweep interval. The results indicate the ferroelectric polarization plays an important role in the photovoltaic effect.

Influence of Mn doping on structural, optical, and magnetic properties of BiFeO3 films fabricated by the sol-gel method

Influence of Mn doping on structural, optical, and magnetic properties of BiFeO3 films fabricated by the sol-gel method

Exploration of the intrinsic factors limiting the photocurrent density in ferroelectric BiFeO3 thin film

The limiting factor for low photocurrent density of polarization switchable ferroelectric BiFeO3 film is due to severe charge recombination at the interfaces of the domain walls rather than recombination inside the domains.

The composition and poling-dependent photovoltaic studies in ferroelectric (Bi1−xSrx)(Fe1−xTix)O3 thin films

The polycrystalline (Bi1−xSrx)(Fe1−xTix)O3 (x = 0.0, 0.05, 0.10, and 0.20) thin films were grown on Pt(111)/TiO2/SiO2/Si(100) substrates using chemical solution deposition technique. X-ray diffraction pattern revealed that pure BiFeO3 film is crystallized in rhombohedrally distorted crystal structure. However, the patterns for doped samples displayed a mixed-phase (rhombohedral + tetragonal) structure. The Raman studies revealed the emergence of B mode related to the tetragonal phase along with the modes corresponding to the rhombohedral phase in doped systems. The photovoltaic studies displayed a large photovoltaic response for the parent compound with an open-circuit voltage of 0.47 V. However, it showed a decrease in the photovoltaic response with an increase in composition x. The composition-dependent photovoltaic response could be correlated to the evolution of tetragonal phase fractions and the polarization. Additionally, the poling-dependent photovoltaic studies revealed the dominant role played by the polarization in comparison with the interface Schottky effect. This work gives an improved understanding of the ferroelectric photovoltaic mechanism and, therefore, may offer guidelines to design and optimize photovoltaic materials.

Strain control of phase transition and magnetic property in multiferroic BiFeO3 thin films

BiFeO3 (BFO), a room-temperature antiferromagnetic-ferroelectric multiferroic, is widely researched due to its potential applications for electric-field control of the magnetism. In this work, the strain control of the phase transition and magnetic properties in the BFO/LaAlO3 heterostructures were investigated. The O K edge polarization-dependent X-ray absorption spectroscopy (XAS) spectra show that the Fe 3d level splits into five levels, which proves that the FeO5 pyramid is asymmetric in the highly strained tetragonal-like BFO. The spin canting induced by the asymmetric structure leads to the magnetic moment. Thus, an obvious magnetic signal in the 17-nm-thick BFO thin films was observed by the Quantum Design magnetic property measurement system. With the increase of the BFO film thickness, the clamping effect induced by the substrate becomes weak, further leading to the BFO phase transition. The O K edge polarization-dependent XAS spectra demonstrate that the orbital reconstruction exists at the mixed BFO phase boundaries. Since the orbital reconstructions can induce the strong magnetic coupling, the magnetic order of the different BFO phases will be coupled with each other. It causes a variation of the magnetic property at the phase boundaries or in the BFO phases.

Unveiling multiferroic proximity effect in graphene

We demonstrate that electronic and magnetic properties of graphene can be tuned via proximity of multiferroic substrate. Our first-principles calculations performed both with and without spin–orbit coupling clearly show that by contacting graphene with bismuth ferrite BiFeO3 (BFO) film, the spin-dependent electronic structure of graphene is strongly impacted both by the magnetic order and by electric polarization in the underlying BFO. Based on extracted Hamiltonian parameters obtained from the graphene band structure, we propose a concept of six-resistance device based on exploring multiferroic proximity effect giving rise to significant proximity electro- (PER), magneto- (PMR), and multiferroic (PMER) resistance effects. This finding paves a way towards multiferroic control of magnetic properties in two dimensional materials.

Quantitative analysis of the direct piezoelectric response of bismuth ferrite films by scanning probe microscopy

Polarisation domain structure is a microstructure specific to ferroelectrics and plays a role in their various fascinating characteristics. The piezoelectric properties of ferroelectrics are influenced by the domain wall contribution. This study provides a direct observation of the contribution of domain walls to the direct piezoelectric response of bismuth ferrite (BiFeO3) films, which have been widely studied as lead-free piezoelectrics. To achieve this purpose, we developed a scanning probe microscopy-based measurement technique, termed direct piezoelectric response microscopy (DPRM), to observe the domain structure of BiFeO3 films via the direct piezoelectric response. Quantitative analysis of the direct piezoelectric response obtained by DPRM, detailed analysis of the domain structure by conventional piezoelectric force microscopy, and microscopic characterisation of the direct piezoelectric properties of BiFeO3 films with different domain structures revealed that their direct piezoelectric response is enhanced by the walls between the domains of spontaneous polarisation in the same out-of-plane direction.

BiFeO3/CH3NH3PbI3 Perovskite Heterojunction Based Near-Infrared Photodetector

The Letter reports a hybrid (i.e. organic-inorganic) perovskite CH3NH3PbI3 and inorganic only perovskite BiFeO3 based heterojunction diode for near-infrared detection applications. The hybrid halide CH3NH3PbI3 is synthesized by using sol-gel chemical route while the BiFeO3 is synthesized by using a solid-state route. A layer of uniformly distributed BiFeO3 thin film is first grown on an indium doped tin oxide (ITO) coated glass substrate using BiFeO3 nanoparticles with an average size of ~65 nm. Then, CH3NH3PbI3 nanoparticles of average size ~45 nm with a tetragonal phase are deposited BiFeO3 film for fabricating the heterojunction device under study. The photoresponse is measured by using a monochromatic light over the wavelengths from 400 to 900 nm. The device shows a dual-band photoresponsivity originated from the individual absorption characteristics of BiFeO3 and CH3NH3PbI3 perovskites used for the heterojunction. The proposed photodetector shows the maximum responsivity of ~2 A/W at 800 nm for 2 V bias while the detectivity is estimated as $\sim {7.8}\times {10}^{{12}}$ cmHz1/2/W. The photodetector has a reasonably good photoresponse with a rise time and fall time of 0.74 s and 0.088 s, respectively.

Activation energy mediated band structure in strained multiferroic BiFeO3 thin films

Although improved magnetic and piezoelectric performances were discovered in BiFeO3 rhombohedral-tetragonal mixed-phase, the associated electronic conduction mechanisms are less investigated. In the present work, we systematically investigated the leakage current and dielectric response as a function of temperature, frequency and bias field for pure rhombohedral and rhombohedral-tetragonal mixed-phase BiFeO3 films grown by pulse laser deposition. The multiferroic properties are demonstrated from their polarization and magnetization hysteresis. Both current-voltage and dielectric relaxation measurements indicate a lower activation energy observed for the mixed-phase BiFeO3 as compared to the rhombohedral-phase one. The present work for the first time indicates the distinguished transport properties as governed by different electronic conduction mechanisms for BiFeO3 at various phases, and improves the understandings associated to the strain engineering related electrical properties.

Light-Induced Currents at Domain Walls in Multiferroic BiFeO3.

Multiferroic BiFeO3 (BFO) films with spontaneously formed periodic stripe domains can generate above-gap open circuit voltages under visible light illumination; nevertheless the underlying mechanism behind this intriguing optoelectronic response has not been understood to date. Here, we make contact-free measurements of light-induced currents in epitaxial BFO films via detecting terahertz radiation emanated by these currents, enabling a direct probe of the intrinsic charge separation mechanisms along with quantitative measurements of the current amplitudes and their directions. In the periodic stripe samples, we find that the net photocurrent is dominated by the charge separation across the domain walls, whereas in the monodomain samples the photovoltaic response arises from a bulk shift current associated with the non-centrosymmetry of the crystal. The peak current amplitude driven by the charge separation at the domain walls is found to be 2 orders of magnitude higher than the bulk shift current response, indicating the prominent role of domain walls acting as nanoscale junctions to efficiently separate photogenerated charges in the stripe domain BFO films. These findings show that domain-wall-engineered BFO thin films offer exciting prospects for ferroelectric-based optoelectronics, as well as bias-free strong terahertz emitters.

A magnetic phase diagram for nanoscale epitaxial BiFeO3 films

BiFeO3 thin films have attracted considerable attention by virtue of their potential application in low-energy spintronic and magnonic devices. BiFeO3 possesses an intricate magnetic structure, characterized by a spin cycloid with period ∼62 nm that governs the functional magnonic response, and which can be modulated or even destroyed by strain, magnetic and electric fields, or chemical doping. The literature on (110)-oriented BiFeO3 films is not explicit in defining the conditions under which this cycloid persists, as its presence depends on synthesis method and thin-film boundary conditions, especially in the sub-100 nm thickness regime. This report aims to end “trial and error” approaches in determining the conditions under which this cycloid and its associated functional magnonic response exist. We show that in specific crystallographic orientations of epitaxial BiFeO3, an unexplored strain parameter—the distortion in the ab plane of the monoclinic unit cell—significantly influences the spin structure. Combining Mössbauer spectroscopy and low-energy Raman spectroscopy with first-principles-based effective Hamiltonian calculations, we show that both average strain and this distortion destroy the cycloid. For films grown on (110)-oriented SrTiO3 substrates, if the BiFeO3 lattice parameters a and b differ by more than about 1.2%, the cycloid is destabilized, resulting in a pseudocollinear magnetic order ground state. We are thereby able to construct a phase diagram of the spin structure for nanoscale epitaxial BiFeO3 films, which aims to resolve long-standing literature inconsistencies and provide powerful guidelines for the design of future magnonic and spintronic devices.

Dynamics and manipulation of ferroelectric domain walls in bismuth ferrite thin films.

Ferroelectric domain walls differ from domains not only in their crystalline and discrete symmetry, but also in their electronic, magnetic, and mechanical properties. Although domain walls provide a degree of freedom to regulate the physical properties at the nanoscale, the relatively lower controllability prevents their practical applications in nano-devices. In this work, with the advantages of 3D domain configuration detection based on piezoresponse force microscopy, we find that the mobility of three types of domain walls (tail-to-tail, head-to-tail, head-to-head) in (001) BiFeO3 films varies with the applied electrical field. Under low voltages, head-to-tail domain walls are more mobile than other domain walls, while, under high voltages, tail-to-tail domain walls become rather active and possess relatively long average lengths. This is due to the high nucleation energy and relatively low growth energy for charged domain walls. Finally, we demonstrate the manipulation of domain walls through successive electric writings, resulting in well-aligned conduction paths as designed, paving the way for their application in advanced spintronic, memory and communication nano-devices.

Presence of a purely tetragonal phase in ultrathin BiFeO3 films: Thermodynamics and phase-field simulations

The stability of a purely tetragonal phase relative to the nominal rhombohedral phase in ultrathin BiFeO3 films is investigated using thermodynamics and phase-field simulations. The thermodynamic analysis demonstrates the possible presence of a purely tetragonal state primarily due to the interfacial effect from the constraint of the adjacent layer although the built-in potential and compressive in-plane strain also play a role. Phase-field simulations of the corresponding ultrathin films reveal the coexistence of tetragonal and rhombohedral phases at certain film thickness arising from strain phase separation. It is shown that the piezoelectric coefficient d33 of the two-phase mixture is up to 200% higher than that of the rhombohedral single phase.

Influence of two-dimensional oxide nanosheets seed layers on the growth of (100)BiFeO3 thin films synthesized by chemical solution deposition

Complex oxide thin films of high quality, such as the multiferroic BiFeO3 (BFO) material, are required for integrated oxides electronics. However, the properties of these materials are strongly dependent on their crystalline orientation and their optimal performances are usually obtained on expensive single-crystal substrates. A lower cost alternative approach is the use of oxide nanosheets seed layers that can be deposited on any substrate, including silicon (Si) used in CMOS technology, for the growth of complex oxides with the desired crystallographic orientation. In this work, BFO films were deposited by chemical solution deposition using polymeric precursor method. First, the optimization of synthesis parameters was performed on amorphous silica and on (100)Si substrates. The structural and microstructural characterizations revealed a polycrystalline growth of the BFO films on both substrates and showed the influence of the synthesis temperature, in particular to obtain the pure perovskite phase. The influence of a two-dimensional nanosheets seed layer was then investigated. Ca2Nb3O10− nanosheets were prepared by the exfoliation process of HCa2Nb3O10 phase that is obtained by cation exchange of the layered KCa2Nb3O10 niobate in an acidic solution. The nanosheets were deposited directly on both substrates by Langmuir–Blodgett process, followed by the subsequent deposition of BFO by chemical solution deposition. X-ray diffraction and transmission electronic microscopy experiments showed a highly oriented (100) growth of the BFO thin films confirming the great interest of two-dimensional nanosheets seed layers.

Polycrystalline BiFeO3 thin films prepared by magnetic controlling sputtering method

Polycrystalline BiFeO3 films were fabricated by radio frequency magnetron sputtering using a Bi1.1FeO3 target onto the glass and Si/SiO2/Ti/Pt substrates respectively. The samples which were annealed with different annealing conditions are pure without impurities. Our results indicate that there are relatively wider growth conditions for pure polycrystalline BFO phases.

Domain and Switching Control of the Bulk Photovoltaic Effect in Epitaxial BiFeO3 Thin Films

Absence of inversion symmetry is the underlying origin of ferroelectricity, piezoelectricity, and the bulk photovoltaic (BPV) effect, as a result of which they are inextricably linked. However, till now, only the piezoelectric effects (inverse) have been commonly utilized for probing ferroelectric characteristics such as domain arrangements and resultant polarization orientation. The bulk photovoltaic effect, despite sharing same relation with the symmetry as piezoelectricity, has been mostly perceived as an outcome of ferroelectricity and not as a possible analytical method. In this work, we investigate the development of BPV characteristics, i.e. amplitude and angular dependency of short-circuit current, as the ferroelastic domain arrangement is varied by applying electric fields in planar devices of BiFeO3 films. A rather sensitive co-dependency was observed from measurements on sample with ordered and disordered domain arrangements. Analysis of the photovoltaic response manifested in a mathematical model to estimate the proportion of switched and un-switched regions. The results unravel the potential utility of BPV effect to trace the orientation of the polarization vectors (direction and amplitude) in areas much larger than that can be accommodated in probe-based techniques.

Enhancement of dielectric, ferromagnetic and electrochemical properties of BiFeO3 nanostructured films through rare earth metal doping

In this present work first assessment of enhanced electrochemical properties of Bismuth Ferrite (BiFeO3)thin films through Samarium(Sm) doping are delivered. Apart from this enhancement of structural, dielectric and magnetic properties with increasing samarium concentration is discussed. The pure phase BiFeO3 films and Sm-doped BiFeO3(Bi1-xSmxFeO3 where x = 0.05 & x = 0.1) films were synthesized using 2methoxy aided sol-gel process and were deposited on platinum substrates through spin coating technique. X-ray diffraction confirmed the formation of pure phase BiFeO3 with Rhombohedral (R3c) structure. Morphological characterization through SEM presented the formation of nanostructures and its structural transformation through doping variant. AFM confirmed the smoothness of the film with a maximum grain size of 172.72 nm for the measured films. The elemental analysis and elemental purity was confirmed through EDAX. Mechanistic aspects of the prepared films were analyzed through Thermogravimetric, Differential Thermal Analysis and Fourier Transform Infrared spectroscopy. The variation of dielectric constant with frequency was measured until 1 MHz and remains almost constant due to the independent nature of polarization with frequency. The magnetic coercivity of the film improved from 77.7G to 240G with samarium doping. The Bi0.9Sm0.1FeO3 films deposited on platinum substrates enhanced the specific capacity to about 184Fg-1 along with its retention capability enabling it to be used as electrode material for supercapacitors.

Photovoltaic, photo-impedance, and photo-capacitance effects of the flexible (111) BiFeO3 film

Ferroelectric polarization allows a depolarization electric field to separate electron-hole pairs excited by lights, and thus, the photovoltaic properties of ABO3-type films on a hard SrTiO3 or Si substrate have been extensively studied recently. However, there are a few reports on the photocapacitance and photoimpedance of these oxide films, especially on flexible substrates. In this work, the strong photovoltaic, giant photocapacitance, and photoimpedance effects were observed in the flexible (111) BiFeO3 films with natural downward polarization. The flexible mica/SrRuO3/BiFeO3/Au cells show a maximum photovoltaic efficiency at 150 °C, a 95.5% decrease in photoimpedance effects, and a 316% increase in photocapacitance effects under 405-nm-wavelength light illumination at 25 °C. Most importantly, these properties do not show obvious degradation when the device is bent to 3 mm radius for 104 times. This work is of vital importance for us to develop new flexible photoelectronic devices.

Growth of (001) preferentially oriented BiFeO3 films on Si substrate by sol-gel method

Obtaining high-quality oriented/textured BFO thin films on traditional Si substrates by low-cost manageable sol-gel method is still challenging and currently relevant. The paper presents a comprehensive set of experimental investigations on the role of the buffer-layer (None, Pt/Ti/SiO2, and LNO) and the thickness of LNO & BFO films on the crystalline structure and microstructure involution of sol-gel based spin-coated BFO films on Si substrates. All BFO and LNO films exhibit good crystallinity and pure perovskite phase. No obvious preferential orientation was found in BFO films fabricated on bare Si(100) and Pt(111)/Ti/SiO2/Si substrates. While highly (001) preferred orientation was obtained for BFO film spin-coated on LNO(00l)/Si substrate. It demonstrated that highly oriented LNO buffer-layer with a certain thickness can promise the growth of high quality oriented /textured BFO thin films. In addition, the crystallization, orientation, phase, grain size, strain state and dislocation are all dependent on the film thickness. The XRD, SEM and W-H equation theoretical analysis results illuminate that the thickness of the BFO films may affect the stress relaxation state, and then the grain size, dislocation and orientation. We demonstrate that suitable buffer-layer materials, optimized thickness of buffer-layer and ferroelectric film have relevant role on the development of high-quality oriented BFO thin films on Si prepared by sol-gel and derived silicon based devices.

Acidic aqueous solution switching of magnetism in BiFeO3/La1 − xSrxMnO3 heterostructures

Control of magnetism in multiferroic heterostructures using low-energy consumption is significant both in fundamental physics and for practical applications such as storage or logic devices. Through switching the ferroelectric polarization of the whole BiFeO3 films using an acidic aqueous solution, we studied the deterministic control of exchange coupling and magnetization in BiFeO3/La1 − xSrxMnO3 heterostructures. It is found that the hole concentration of La1 − xSrxMnO3 is closely coupled with BiFeO3 polarization, which can be used to switch the interfacial magnetism. The polarization-determined exchange coupling for the Sr concentration of x = 0.2 is contrary to that of x = 0.33 and 0.5 due to different double exchange strengths, while for x = 0.5, the coupling effects are more sensitive to polarization. This work provides a pathway to control interfacial magnetism and multiferroicity at a large scale using green energy.