Polycrystalline BiFeO3 Films Research Articles

Characterization of polycrystalline BiFeO3 films prepared by magnetic-field-assisted 90° off-axis pulsed laser deposition

Polycrystalline BiFeO3 (BFO) films were prepared on a Pt/TiO2/SiO2/Si substate using magnetic field-assisted 90° off-axis pulsed laser deposition (PLD). We have successfully obtained polycrystalline BFO films owing to a high deposition rate derived by the confinement of the plume under a magnetic field. The obtained polycrystalline BFO films have a droplet-free surface morphology and a columnar-like microstructure. A RT ferroelectric hysteresis loop is obtained, and at the same time, the remanent polarization of 90 μC cm−2 and the reduced coercive field of 178 kV cm−1 are confirmed. Also, an evolution of the polarization switching is observed by the piezoresponse force microscopy. In this study, we provide a possible route to realize the polycrystalline film growth which has a good quality in a 90° off-axis deposition system using magnetic field-assisted PLD.

Broadband and High‐Sensitivity Photodetector Based on BiFeO3/Si Heterojunction

AbstractPhotovoltaic devices based on ferroelectric materials have broad application prospects; however, there are also problems of narrow bandwidth and low sensitivity. In this paper, polycrystalline BiFeO3 (BFO) films are prepared directly on n‐Si (100) substrates to form ferroelectric/semiconductor heterostructures, the electrical response of which under weak light intensity (0.8 W m−2) is systematically investigated. Accompanied by the dielectric relaxation originated from photocarriers, the polarization of BFO increases while the coercive voltage decreases. Especially, current–voltage (I–V) curve under negative bias voltage changes obviously with illumination, and cyclically switchable photocurrent is detected at 0 V bias, with the on‐off ratio increasing for lower temperature. Although the photoresponse is detectable in the broad wavelength range of 275–1100 nm, it is more pronounced for visible light. It is considered that the built‐in electric field of the PN junction at the BFO/n‐Si interface drives the separation and migration of photocarriers, which in turn affects the interface barrier and the distribution of monovalent oxygen vacancies () in BFO while producing the photovoltaic effect. This work provides an effective route for developing photodetectors with high sensitivity and broadband detection.

Polarization driven self-biased and enhanced UV–visible photodetector characteristics of ferroelectric thin film

BiFeO3 is a promising multifunctional material in terms of its intriguing physics and diverse application potential. In this work, self-powered UV–visible photodetector characteristics of the polycrystalline BiFeO3 thin film exhibiting pronounced photo-response under both UV and visible light are demonstrated. Interestingly, the film displays switchable photodetector characteristics in accordance with the polarization switching properties and thereby confirms the role of polarization driven depolarization field on the observed properties. Importantly, the photodetector performance parameters such as responsivity , detectivity , photoconductive gain , and external quantum efficiency (5.3 %) observed under UV light are 2 to 4 order of magnitude higher than the values reported in similar oxide ferroelectric systems. These studies establish the application of polycrystalline BiFeO3 film as a self-powered UV–visible photodetector.

Improved Photovoltaic Effect of p-i-n Structured BiFeO₃ Film Deposited by Radio-Frequency Magnetron Sputtering.

Pure phase polycrystalline BiFeO₃ film was deposited onto FTO substrate by RF magnetron sputtering method. SEM result shows that BiFeO₃ film has the obvious porosity and large clusters which lead to the poor ferroelectric and photovoltaic properties in FTO/BiFeO₃/Ag device. However, these properties are improved in p-i-n structured FTO/TiO₂/BiFeO₃/HTM/Ag device by incorporating the electron and hole transport materials. The hysteresis loop measurement demonstrates the excellent ferroelectric property with large remnant polarization (2Pr = 180 μC/cm²) and low leakage current. The J-V curve shows the short-circuit current density is dozens of times larger than that of FTO/BiFeO₃/Ag device. Moreover, the photovoltaic output depends on the poling field where the positive poling improves the short-circuit current density to -85 μA/cm₂ and the negative poling reduces both the photocurrent and photovoltage. It is believed that the ferroelectric polarization plays a dominant role in the photovoltaic effect.

Study of BiFeO3 thin film obtained by a simple chemical method for the heterojunction-type solar cell design

Polycrystalline BiFeO3 (BFO) films of different thickness were synthesized by a 2-methoxyethanol (acid)-free simple chemical method. A rhombohedral-type pure phase with a crystallite size less than 18.8 nm was obtained. SEM micrographs showed that BFO has a flat and homogeneous morphology when it was deposited on different semiconductor substrates (ZnO, Ni doped ZnO, and CdS). The highest roughness value (8.6 nm) was observed when BFO was deposited on CdS. The optical response showed that the optical band gap slightly changes as thickness increases. The photovoltaic response of the BFO film was assessed employing different solar cell architectures (p-BFO-n and BFO-n). The results showed that the solar cell based on the Ag/PbS/BFO/CdS/FTO/glass structure presented a short-circuit current density of JSC=239.6μA/cm2 and a power conversion efficiency of η=7.65×10−3%. Photoelectrochemical and ferroelectric measurements were employed to explain the photovoltaic response.

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.

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.

Magnetization and magnetic resonance in sol-gel derived polycrystalline BiFeO3 film

Abstract The structure and magnetic properties of polycrystalline BiFeO3 films grown on glass substrate using Sol-Gel technique have been investigated. X-ray diffraction (XRD) analysis revealed the average grain size of about 21 nm. The root mean square (RMS) surface roughness of about 50 nm has been obtained for the film by using scanning electron microscope (SEM) technique. Ferromagnetic resonance (FMR) studies showed the presence of macroscopic ferromagnetic magnetization and uniaxial out-of-plane magnetic anisotropy. The temperature dependences of the magnetization of BiFeO3 films registered in zero field cooling (ZFC) and field cooling (FC) regimes revealed a spin glass state. The average magnetic crystallite sizes obtained from the magnetizations studies coincide with the values revealed from the structural studies.

Microstructure and improved electrical properties of Ti-substituted BiFeO3 thin films

This paper discussed about the effects of Ti-substitution (0–10 atom%) at Fe-site on the structure and electrical properties of BiFeO3 (BFO) films deposited by sol-gel technique. The lattice parameters of the phase pure (perovskite) polycrystalline BFO films are found to be reduced up-to 0.38% and 1.43% in a and c directions respectively due to the Ti-substitution. Grain sizes of the BFO films are found to be increasing with the Ti-substitution. The maximum remnant polarization of 70μC/cm2is observed in 2.5% Ti-substituted BFO film. The leakage current density (∼10−4A/cm2 at 0.6 MV/cm applied electric field) of the Ti substituted BFO film (10%) is found to be three orders lower than the pristine BFO film. Reason for the improved electrical properties seems to be due to replacement of Fe3+ withTi4+ions which in turn might help in reducing the effect of oxygen vacancies defects.

Investigation of multilevel data storage in silicon-based polycrystalline ferroelectric tunnel junction

Multilevel data ferroelectric tunnel junction is a breakthrough for further improving the storage density of ferroelectric random access memories. However, the application of these ferroelectric tunnel junctions is limited by high cost of epitaxial perovskite heterostructures, unsatisfactory retention and difficulty of exactly controlling the middle polarization states. In order to overcome the issues, we develop a ferroelectric tunnel junction with smooth ultrathin polycrystalline BiFeO3 (BFO) film. Through controlling the polarization state and oxygen vacancy migration using voltage pulses, we demonstrate that voltage-controlled barrier yields a memristive behavior in the device, in which the resistance variations exceed over two orders of magnitude. And we achieve multi logic states written and read easily using voltage pulses in the device. Especially the device is integrated with the silicon technology in modern microelectronics. Our results suggest new opportunity for ferroelectrics as high storage density nonvolatile memories.

Formation of BiFeO3 from a Binary Oxide Superlattice Grown by Atomic Layer Deposition.

We report on the growth of polycrystalline BiFeO3 thin films on SiO2 /Si(001) and Pt(111) substrates by atomic layer deposition using the precursors ferrocene, triphenyl-bismuth, and ozone. By growing alternating layers of Fe2 O3 and Bi2 O3 , we employ a superlattice approach and demonstrate an efficient control of the cation stoichiometry. The superlattice decay and the resulting formation of polycrystalline BiFeO3 films are studied by in situ X-ray diffraction, in situ X-ray photoelectron spectroscopy, and transmission electron microscopy. No intermediate ternary phases are formed and BiFeO3 crystallization is initiated in the Bi2 O3 layers at 450 °C following the diffusion-driven intermixing of the cations. Our study of the BiFeO3 formation provides an insight into the complex interplay between microstructural evolution, grain growth, and bismuth oxide evaporation, with implications for optimization of ferroelectric properties.

Photovoltaic and photo-capacitance effects in ferroelectric BiFeO3 thin film

A polycrystalline BiFeO3 film on Pt/Ti/SiO2/Si was fabricated using the spin coating technique. The film shows diode-like characteristics with and without poling measured under dark conditions. However, it exhibits a switchable photovoltaic effect with light illumination under poled conditions. The measured photovoltaic effect revealed an open circuit voltage of ∼0.47 V and a short circuit current of 3.82 μA/cm2 under the illumination of 165 mW/cm2 irradiance. The studies clarified the dominant role of the depolarization field rather than the interface in the photovoltaic characteristics of the BiFeO3 film. Significantly, the photo-capacitance effect was demonstrated with a substantial enhancement in capacitance (∼45%) in Au/BiFeO3/Pt geometry, which could open up a new window for BiFeO3 applications.

Coexistence of analog and digital resistive switching in BiFeO3-based memristive devices

The resistive switching behavior of polycrystalline BiFeO3 films was investigated, and the coexistence of analog and digital resistive switching behaviors in a single Pt/BiFeO3/Pt device was discovered. Without electroforming, the Pt/BiFeO3/Pt device showed analog switching characteristics with a resistance ratio greater than 10; however, after electroforming, the device exhibited digital bipolar resistive switching features. It is suggested that the charge trapping/detrapping is crucial in the analog resistive switching, while the conductive filament can account for the digital switching. Additionally, a physical model is proposed to disclose the diffusion kinetics of oxygen vacancies in the memristive behaviors of the device. This work opens up a way in designing multi-functional memristive devices, which could be an ideal solution for the neuromorphic computation, non-volatile information storage and logic operation.

Enhanced ferroelectric photoelectrochemical properties of polycrystalline BiFeO3 film by decorating with Ag nanoparticles

Polycrystalline BiFeO3 (BFO) films are fabricated on Pt/Ti/SiO2/Si(100) substrate as photoelectrode using sol-gel method. The microstructure, optical, and photoelectrochemical (PEC) properties of the films are characterized and optimized by controlling the film thickness. Moreover, the PEC properties of the BFO films are dependent on ferroelectric polarization, which is mainly ascribed to the modulation of band structure at the BFO/electrolyte interface by the polarization. Further enhancement of PEC properties is obtained by decorating the samples with appropriate amounts of Ag nanoparticles, which is attributed to the reduced electron-hole recombination, and localized surface plasmon resonance effect of Ag nanoparticles.

Au纳米颗粒/BFO/ZnO复合结构的光电化学性质的提高

本文通过磁控溅射技术在ITO导电玻璃上制备了多晶的铁酸铋(BFO)薄膜，并负载了适量的Au纳米颗粒来提高样品的光电流。Au纳米颗粒的表面等离子共振效应以及金属/半导体界面处形成的肖特基势垒均有利于提高电子-空穴对的分离。10 nm ZnO过渡层的引入使得BFO薄膜表面的平整度，致密性，结晶度得到了很好的改善，从而减小了BFO薄膜的漏电流，提高了BFO薄膜的剩余极化。BFO薄膜经过极化后产生的退极化场也促进了电子-空穴对的分离，减少了光生载流子的复合，从而可以获得更好的光电化学性能。 Polycrystalline BiFeO3 (BFO) film was synthesized on the ITO conductive glass by magnetron sputtering technique, and the appropriate amount of gold nanoparticle (Au Nps) was successfully loaded to improve the photocurrent of the sample. The surface plasma resonance (LSPR) effect of gold nanoparticles, and the formation of the Schottky barrier of metal/semiconductor interface are beneficial to accelerate the separation of electronic-hole. The BFO/10 nm ZnO films show denser and smoother surface and higher degree crystallization, which is beneficial to reduce the leakage current density and achieve larger double-remant-polarization value. The generation of depolarization field by polaring BFO film also promotes the separation of the electronic-hole, and prevents the rapid recombination of photogenic charge carrier, which can obtain better photoelectrochemical properties.

Studying the Polarization Switching in Polycrystalline BiFeO3 Films by 2D Piezoresponse Force Microscopy.

For rhombohedral multiferroelectrics, non-180° ferroelectric domain switching may induce ferroelastic and/or (anti-)ferromagnetic effect. So the determination and control of ferroelectric domain switching angles is crucial for nonvolatile information storage and exchange-coupled magnetoelectric devices. We try to study the intrinsic characters of polarization switching in BiFeO3 by introducing a special data processing method to determine the switching angle from 2D PFM (Piezoresponse Force Microscopy) images of randomly oriented samples. The response surface of BiFeO3 is first plotted using the piezoelectric tensor got from first principles calculations. Then from the normalized 2D PFM signals before and after switching, the switching angles of randomly oriented BiFeO3 grains can be determined through numerical calculations. In the polycrystalline BiFeO3 films, up to 34% of all switched area is that with original out-of-plane (OP) polarization parallel to the poling field. 71° polarization switching is more favorable, with the area percentages of 71°, 109° and 180° domain switching being about 42%, 29% and 29%, respectively. Our analysis further reveals that IP stress and charge migration have comparable effect on switching, and they are sensitive to the geometric arrangements. This work helps exploring a route to control polarization switching in BiFeO3, so as to realize desirable magnetoelectric coupling.

Structural, optical and ferroelectric behavior of pure BiFeO3 thin films synthesized by the sol–gel method

In this paper, BiFeO3 (BFO) thin films were synthesized by the sol–gel spin-coating technique followed by annealing at different temperatures. The effect of annealing temperature on structural, optical, dielectric and ferroelectric properties of BFO thin film has been investigated. Single phase polycrystalline BFO films are confirmed by X-ray diffraction patterns. A strong dependence of grain size and shape on annealing temperature was observed. The optical energy band gap values found decrease from 2.42 to 2.27 eV with raise annealing temperature from 500–600 °C, respectively. The film annealed at 500 °C showed a well saturated polarization–electric field loop. The remnant polarization and saturation polarization values are decreased with increasing annealing temperature. The anomaly in temperature dependent dielectric permittivity was observed at magnetic transition point.

Magnetic Properties of Polycrystalline Bismuth Ferrite Thin Films Grown by Atomic Layer Deposition.

The atomic layer deposition (ALD) method was applied to grow thin polycrystalline BiFeO3 (BFO) films on Pt/SiO2/Si substrates. The 50 nm thick films were found to exhibit high resistivity, good morphological integrity, and homogeneity achieved by the applied ALD technique. Magnetic characterization revealed saturated magnetization of 25 emu/cm(3) with temperature-dependent coercivity varying from 5 to 530 Oe within the temperature range from 300 to 2 K. Magnetism observed in the films was found to change gradually from ferromagnetic spin ordering to pinned magnetic domain interactions mixed with weak spin-glass-like behavior of magnetically frustrated antiferromagnetic/ferromagnetic (AFM-FM) spin ordering depending on the temperature and magnitude of the applied magnetic field. Antiferromagnetic order of spin cycloids was broken in polycrystalline films by crystal sizes smaller than the cycloid length (∼60 nm). Uncompensated spincycloids and magnetic domain walls were found to be the cause of the high magnetization of the BFO films.

Effect of film thickness and bottom electrode material on the ferroelectric and photovoltaic properties of sputtered polycrystalline BiFeO3 films

Abstract High-quality polycrystalline BiFeO 3 (BFO) films have been of great interest for many electrical and optical applications, however the fabrication of polycrystalline BFO films offering a large remnant polarization and low leakage current are difficult. In this work, polycrystalline BFO films were deposited using a radio frequency magnetron sputtering method. It was found that the remnant polarization of BFO deposited on ITO is about three and seventy times larger than films deposited on Pt and FTO, respectively. Film quality was highly dependent on the film thickness, with a thinner film displaying greater leakage. Despite the degradation in ferroelectric performance, however, the photovoltaic response was enhanced. Ultrathin BFO films therefore broaden the potential applications for ferroelectric photovoltaic devices by improving their carrier mobility, and allow for fine-tuning of their band structure when combined with semiconductors and electrodes.

Switchable photovoltaic effect in bilayer graphene/BiFeO3/Pt heterostructures

We report the switchable photovoltaic effects in graphene/BiFeO3/Pt heterostructures. Pure phase polycrystalline BiFeO3 films were deposited on Pt/TiO2/SiO2/Si substrates by pulse laser deposition. A bilayer graphene was transferred onto the BiFeO3 film which serves as transparent conducting electrodes. The heterostructures showed switchable photovoltaic effect depending on ferroelectric polarization directions indicating depolarization field induced separation of photo-generated carriers. The open circuit voltage (VOC) and short circuit current density (JSC) were measured to be ∼110 mV, ∼92 μA/cm2 in positive polarity and similar values were obtained when the polarity was reversed. The JSC and VOC also showed rapid response (<100 ms) as a function of light exposure time.