BiFeO3 Thin Films Research Articles

Bipolar Resistive Switching Behaviour of Polycrystalline BiFeO3 Thin Films Synthesized via Sol-gel Assisted Spin Coating Technique

Polycrystalline BiFeO3 thin films have been grown on glass substrates using a simple but efficient method commonly known as the spin coating technique. When used in a Cu / BiFeO3 / Cu configuration, the annealed BiFeO3 film (at 350 °C) exhibits bipolar resistive switching behaviour. The device shows stable resistive switching behaviour, where a stable hysteresis in the current–voltage curve was well developed by applying +/- 10 V at room temperature. The ratio of resistance in the high resistance state to the low resistance state of the device is ~ 104 with a good retention time of more than 106 min. The Poole–Frenkel emission at the Cu / BiFeO3 interface is proposed, and a redistribution of oxygen vacancies along the grain boundaries is found to play a key role in the resistance switching in the polycrystalline pure BiFeO3 films.

Origin of the switchable photocurrent direction in BiFeO3 thin films.

We report external bias driven switchable photocurrent (anodic and cathodic) in 2.3 eV indirect band gap perovskite (BiFeO3) photoactive thin films. Depending on the applied bias our BiFeO3 films exhibit photocurrents more usually found in p- or n-type semiconductor photoelectrodes. In order to understand the anomalous behaviour ambient photoemission spectroscopy and Kelvin-probe techniques have been used to determine the band structure of the BiFeO3. We found that the Fermi level (Ef) is at -4.96 eV (vs. vacuum) with a mid-gap at -4.93 eV (vs. vacuum). Our photochemically determined flat band potential (Efb) was found to be 0.3 V vs. NHE (-4.8 V vs. vacuum). These band positions indicate that Ef is close to mid-gap, and Efb is close to the equilibrium with the electrolyte enabling either cathodic or anodic band bending. We show an ability to control switching from n- to p-type behaviour through the application of external bias to the BiFeO3 thin film. This ability to control majority carrier dynamics at low applied bias opens a number of applications in novel optoelectronic switches, logic and energy conversion devices.

Anion-induced robust ferroelectricity in sulfurized pseudo-rhombohedral epitaxial BiFeO3 thin films via polarization rotation.

Polarization rotation caused by various strains, such as substrate and/or chemical strain, is essential to control the electronic structure and properties of ferroelectric materials. This study proposes anion-induced polarization rotation with chemical strain, which effectively improves ferroelectricity. A method for the sulfurization of BiFeO3 thin films by introducing sulfur anions is presented. The sulfurized films exhibited substantial enhancement in room-temperature ferroelectric polarization through polarization rotation and distortion, with a 170% increase in the remnant polarization from 58 to 100.7 μC cm-2. According to first-principles calculations and the results of X-ray absorption spectroscopy and high-angle annular dark-field scanning transmission electron microscopy, this enhancement arose from the introduction of S atoms driving the re-distribution of the lone-pair electrons of Bi, resulting in the rotation of the polarization state from the [001] direction to the [110] or [111] one. The presented method of anion-driven polarization rotation might enable the improvement of the properties of oxide materials.

Deterministic Manipulation of Multi‐State Polarization Switching in Multiferroic Thin Films

AbstractDeterministically controllable multi‐state polarizations in ferroelectric materials are promising for the application of next‐generation non‐volatile multi‐state memory devices. However, the achievement of multi‐state polarizations has been inhibited by the challenge of selective control of switching pathways. Herein, an approach to selectively control 71° ferroelastic and 180° ferroelectric switching paths by combining the out‐of‐plane electric field and in‐plane trailing field in multiferroic BiFeO3 thin films with periodically ordered 71° domain wall is reported. Four‐state polarization states can be deterministically achieved and reversibly controlled through precisely selecting different switching paths. These studies reveal the ability to obtain multiple polarization states for the realization of multi‐state memories and magnetoelectric coupling‐based devices.

Large increase in photo-induced conductivity of two-dimensional electron gas at SrTiO3 surface with BiFeO3 topping layer

In this work, we study and compare the photo-induced conductivity of a two-dimensional electron gas (2DEG) at the bare surface of SrTiO3 (STO) and in the heterostructure of BiFeO3 (BFO) and STO, where BFO was deposited by radio frequency magnetron sputtering. The photo-induced conductance of the BFO/STO interface shows a large increase which is 20.62 times more than the sum of photo-induced conductance from each individual BFO thin film and STO crystal. Since this photo-induced conductance of the BFO/STO heterostructure can be adjusted to become higher and lower by applying an electric field to the top surface, we attribute this large increase to the strong photo-induced electrical polarization of BFO. With the two-point setup of positive bias and negative bias, the conductivity also exhibits diode-like behavior where the forward and backward resistances are different. This work provides methods to interplay between light irradiation, electric field, and conductivity in all-oxide electronics.

Multiferroic and energy-storage characteristics of polycrystalline Ca-doped BiFeO3 thin films on Si substrates

Multiferroic and energy-storage characteristics of polycrystalline Ca-doped BiFeO3 thin films on Si substrates

Ferroelastic Domain Walls in BiFeO3as Memristive Networks

Electronic conduction along individual domain walls (DWs) is reported in BiFeO3(BFO) and other nominally insulating ferroelectrics. DWs in these materials separate regions of differently oriented electrical polarization (domains) and are just a few atoms wide, providing self‐assembled nanometric conduction paths. Herein, it is shown that electronic transport is possible also from wall‐to‐wall through the dense network of as‐grown DWs in BFO thin films. Electric field cycling at different points of the network, performed locally by conducting atomic force microscopy (cAFM), induces resistive switching selectively at the DWs, both for vertical (single wall) and lateral (wall‐to‐wall) conduction. These findings are the first step toward investigating DWs as memristive networks for information processing andin-materiocomputing.

Tunable Optical and Multiferroic Properties of Zirconium and Dysprosium Substituted Bismuth Ferrite Thin Films.

This work presents optical and multiferroic properties of bismuth ferrite thin films that are affected by zirconium and dysprosium substitution. Non-centrosymmetric BiFeO3,Bi0.95Zr0.05FeO3, and Bi0.95Dy0.05FeO3 thin films were coated on Pt/TiO2/SiO2/Si substrates using the spin coating method. The crystal structure, optical properties, microstructural, ferromagnetic, and ferroelectric properties of doped bismuth ferrite thin films were systematically investigated. From the XRD patterns, all the prepared thin films matched well with the rhombohedral structure with R3c space group with no observed impurity phases. The average crystallite size of the bismuth ferrite thin films were between 35 and 47 nm, and the size depended on the type of dopant. The determined energy band gap values of BiFeO3, Bi0.95Dy0.05FeO3, and Bi0.95Zr0.05FeO3 thin films were 2.32 eV, 2.3 eV, and 2 eV, respectively. Doping of Dy and Zr at the Bi site led to reduced surface roughness. The prepared thin films exhibited enhanced ferromagnetic and ferroelectric properties. The remnant magnetization of Zr-doped BiFeO3 was greater than that of the BiFeO3 and Dy-doped BiFeO3 thin films. From the obtained results, it was concluded that Zr-doped BiFeO3 thin films are suitable for solar cell fabrication.

Effect of growth condition on the electrical behavior of BiFeO3/Si thin film devices

BiFeO3 thin films were deposited onto single crystalline Si (100) substrate using Pulsed Laser Deposition technique. The effect of different oxygen partial pressure on the BiFeO3 film growth, structural and transport properties has been investigated. Structural characterization shows that film growth under lower partial pressure exhibits a minor impurity phase which gets suppressed under higher oxygen partial pressure (OP2). The effect of OP2 during the deposition on the leakage current and the role of oxygen vacancies have been understood in the context of various charge transport mechanism. RBS study confirms the presence of oxygen content in the film studied which are responsible for the modification in the Electrical behavior of BFO/Si thin film devices.

Impact of laser energy on resistive switching properties of BiFeO3 thin films

In this report, BFO thin film samples were deposited using Pulsed Laser Deposition (PLD) technique by altering the PLD laser energy from 150 to 250 mJ, to study the role of laser energy on resistance switching property of pure BFO thin film. Efficient resistive switching properties are essentially required for realization of a memristor device. An appreciably high resistance ratio (∼100) between HRS i.e. high resistance state (“OFF” state) and LRS i.e. low resistance state (“ON” state) was obtained for the sample fabricated at 200 mJ PLD laser energy with a stable retention period of 106 s with reproducible endurance cycles of 108. From the study of charge transport mechanism, it has been observed that the Ohmic conduction is the dominating mechanism in the lower applied biased region while space charge limited conduction (SCLC) and Child's law are the dominating mechanisms in the higher applied biased region for all the deposited BFO thin film samples. Current study extends the application of multiferroic BFO thin films for memory devices.

Three-state resistive switching effect in BiFeO3 thin films

Resistive switching (RS) memristor has been widely used in the in-memory computation systems. Due to the strong information processing capability and low area cost of the ternary logic, the development of the three-state RS memristor was promoted. Here, we demonstrate a three-state RS phenomenon on Pt/BiFeO3/SrRuO3 structure. After applying a positive voltage to the thin film for a period, an abrupt RS effect occurs, where the three-state RS behavior can be obtained. By analyzing the conduction mechanisms of the current-voltage curves and the behavior of the capacitance-voltage curves, the three-state storage capability of the memristor can be ascribed to the movement of oxygen vacancies and the trapping/detrapping of charge carriers at the interface. The ternary OR logic gate was also designed with three steps by using only one memristor.

High piezoelectricity in epitaxial BiFeO3 microcantilevers

The large switchable ferroelectric polarization and lead-free composition of BiFeO3 make it a promising candidate as an active material in numerous applications, in particular, in micro-electro-mechanical systems (MEMS) when BiFeO3 is integrated in a thin film form on a silicon substrate. Here, 200-nm-thick Mn-doped BiFeO3 thin films have been epitaxially grown on a SrRuO3/SrTiO3/Si substrate and patterned into microcantilevers as prototype device structures for piezoelectric actuation. The devices demonstrate excellent ferroelectric response with a remanent polarization of 55 μC/cm2. The epitaxial BiFeO3 MEMS exhibit very high piezoelectric response with transverse piezoelectric coefficient d31 reaching 83 pm/V. The BiFeO3 cantilevers show larger electromechanical performance (the ratio of curvature/electric field) than that of state-of-art piezoelectric cantilevers, including well-known PZT (Pb(Zr,Ti)O3) and the hyper-active PMN–PT (Pb(Mg1/3Nb2/3)O3-PbTiO3). In addition, the piezoelectricity in BiFeO3 MEMS is found to depend on the ferroelectric polarization direction, which could originate from the flexoelectric effect and be exploited to further enhance the electromechanical performance of the devices. These results could potentially lead to a replacement of lead-based piezoelectrics by BiFeO3 in many microdevices.

Effects of 120 MeV Ag9+ swift heavy ion irradiation on the structural, optical and electrical properties of pristine and Ni doped BiFeO3 thin films grown by pulsed laser deposition

In this work, the effects of swift heavy ion (SHI) irradiation on the structure, morphology, optical and electrical properties of pristine and Ni-doped BiFeO3 thin films have been explored. The thin films with composition of BiFe1-xNixO3 (x = 0, 0.01, 0.03 and 0.05) were deposited on the n-type single crystal Si(100) substrates through pulsed laser deposition technique. Further, irradiation on the thin films was performed with 120 MeV Ag9+ ions at different ion fluences, i.e., 1 × 1013, 5 × 1013 and 1 × 1014 ions/cm2. To investigate the effects of SHI irradiation on the aforementioned properties, all the films have been characterized by various analytical techniques, such as X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy, scanning electron microscopy, energy dispersive X-ray spectroscopy, UV-visible diffuse reflectance spectroscopy and two-probe resistivity setup. The effects of the irradiation on the crystal structure and phase of the thin films were analyzed by the XRD and Raman measurements, and it has been observed that irradiation tends to amorphize the thin films. A decrease in the band gap has been observed for the irradiated thin films with the increase of ion fluence. The current-voltage plots of all the unirradiated and irradiated thin films demonstrate the rectifying diode type characteristics.

Co‐Polarized Second Harmonic Generation Induced by Ferroelectric Domains and Domain Wall Arrays

AbstractOptical second harmonic generation (SHG) is a sensitive and powerful technique to probe various symmetry breaking, especially ferroelectric/magnetic domains, in artificial thin films or heterostructures. In a wide range of ferroelectric crystals, a non‐trivial broken symmetry will emerge at domain boundaries, however, the contribution of such symmetry breaking to SHG is often neglected due to its random orientation. Here, a co‐polarized SHG effect from the well‐aligned domain wall arrays is observed in BiFeO3 thin films. This SHG component is highly dependent on the local orientation of the wall arrays, and its polarization feature reveals the Néel‐like evolution of electric polar vectors in the wall region. Based on this intriguing effect, mixed‐phase patches with four possible orientations in the quasi‐tetragonal stripes are identified by polarization‐sensitive SHG mapping. These findings not only clarify a critical SHG contribution in this multiferroic material, but also demonstrate an optical approach for exploring phase‐determined function unit.

Boosting the Photocurrent in BiFeO3 Thin Films via a Domain‐Wall‐Defect Interaction

Ferroelectric domain walls are of great interest due to their nontrivial electronic properties. Herein, a large photocurrent is detected only in the stripe domain structure of BiFeO3 thin films. By the temperature‐resolved time‐dependent photocurrent measurements, the significant trapping effect is revealed to be associated with the abnormal photocurrent. Further optoelectronic measurements with the defect density tuned unveiled that the interaction between the defects and domain walls is essential to give a rise to a large photocurrent in BiFeO3 thin films. The study reveals a novel domain‐wall‐defect photovoltaic effect and provides a simple thermal annealing‐treatment strategy to boost the photocurrents and power output by several folds.

Bipolar resistive switching in Mn-doped BiFeO3 thin films synthesized via sol–gel-assisted spin coating technique

Bipolar resistive switching in Mn-doped BiFeO3 thin films synthesized via sol–gel-assisted spin coating technique

Synergistic Optimization in a 0.90BaTiO3–0.08Bi(Ni0.5Zr0.5)O3–0.02BiFeO3 Thin Film with High Breakdown Strength and Energy Density

Amorphous thin films have attracted wide attention owing to their high breakdown strength in the field of dielectric ceramic thin-film energy storage. However, improving the polarization strength has been a great challenge for amorphous films due to the inverse relationship between polarization and breakdown strength. Herein, a concept of ternary synergistic optimization design is proposed and proved to be an effective strategy to improve this challenge. BaTiO3–Bi(Ni0.5Zr0.5)O3–BiFeO3 amorphous films are prepared by a sol–gel method. BaTiO3 is chosen as the main component, taking advantage of its large spontaneous polarization. The introduction of Bi(Ni0.5Zr0.5)O3 facilitates the formation of nanoscale crystalline regions. For BiFeO3, Fe2+ and Fe3+ incorporation into thin films bound oxygen vacancy defects and enhanced the polarization, which further improved the energy storage performance. As a result, 0.90BaTiO3–0.08Bi(Ni0.5Zr0.5)O3–0.02BiFeO3 thin film achieves an energy storage density of 114.3 J cm–3 and energy storage efficiency of 87.0%, together with excellent thermal stability in a temperature range of 20–150 °C. This work provides a universal method to improve the polarization and energy storage properties of amorphous films.

Growth of heteroepitaxial La and Mn co-substituted BiFeO3 thin films on Si (100) substrate by pulsed laser deposition

Growth of heteroepitaxial La and Mn co-substituted BiFeO3 thin films on Si (100) substrate by pulsed laser deposition

Synthesis and characterization of Bi1−x La x FeO3 multiferroic thin films

Lanthanum modified BiFeO3 thin films were prepared via the sol-gel method and deposited through the dip-coating technique on ITO coated substrates. The structural properties were investigated at room temperature from X-ray diffraction as well as Raman spectroscopy. Results confirm the formation of the perovskite structure without secondary phases, thus corroborating the high-quality of the obtained thin films. Well-defined and nanometric scale grains have been obtained from atomic force microscopy, revealing indeed crack-free surfaces. From the technological point of view, this result is very important since residual stresses promoted by surface cracks led to additional conduction behavior, which could affect the real electric response of the sample to be used in electronic devices.

Surface and Electrical Characterization of Bilayers Based on BiFeO3 and VO2.

Thin films of BiFeO3, VO2, and BiFeO3/VO2 were grown on SrTiO3(100) and Al2O3(0001) monocrystalline substrates using radio frequency and direct current sputtering techniques. To observe the effect of the coupling between these materials, the surface of the films was characterized by profilometry, atomic force microscopy, and X-ray photoelectron spectroscopy. The heterostructures, monolayers, and bilayers based on BiFeO3 and VO2 grew with good adhesion and without delamination or signs of incompatibility between the layers. A good granular arrangement and RMS roughness between 1 and 5 nm for the individual layers (VO2 and BiFeO3) and between 6 and 18 nm for the bilayers (BiFeO3/VO2) were observed. Their grain size is between 20 nm and 26 nm for the individual layers and between 63 nm and 67 nm for the bilayers. X-ray photoelectron spectroscopy measurements show a higher proportion of V4+, Bi3+, and Fe3+ in the films obtained. The homogeneous ordering, low roughness, and oxidation states on the obtained surface show a good coupling in these films. The I-V curves show ohmic behavior at room temperature and change with increasing temperature. The effect of coupling these materials in a thin film shows the appearance of hysteresis cycles, I-V and R-T, which is typical of materials with high potential in applications, such as resistive memories and solar cells.