Polycrystalline BiFeO3 Films Research Articles

Photovoltaic properties of ferroelectric solar cells based on polycrystalline BiFeO3 films sputtered on indium tin oxide substrates

To study the ferroelectric photovoltaic effect based on polycrystalline films, preparation of high-quality polycrystalline films with low leakage and high remnant polarization is essential. Polycrystalline BiFeO3 (BFO) thin films with extremely large remnant polarization (2Pr = 180 C/cm2) were successfully deposited on glass substrates coated with indium tin oxide using a modified radio frequency magnetron sputtering method. Symmetric and asymmetric cells were constructed to investigate the ferroelectric photovoltaic effect in order to understand the relationship between polarization and photovoltaic response. All examined cells showed polarization-induced photovoltaic effect. Our findings also showed that the ferroelectric photovoltaic effect is highly dependent on the material used for the top electrode and the thickness of the polycrystalline film.

Columnar Growth of BiFeO3 Films Prepared by Magnetic-field-assisted Pulsed Laser Deposition

Polycrystalline BiFeO3 (BFO) films have been prepared by using magnetic-field-assisted pulsed laser deposition (PLD), and the microstructure and ferroelectric properties have been investigated. A high deposition rate has been achieved, and a columnar structure has been observed in comparison with that of a BFO thin film prepared without a magnetic field. Trajectories of the charged particles in the plume under a magnetic field have been calculated. As a result, the angle between the substrate normal and the incident particle on the substrate becomes large in magnetic-field-assisted PLD. The evolution of film growth in PLD under a magnetic field has been analyzed by the calculation using the facet growth model. The calculation results show the film with the columnar structure normal to the substrate is obtained. In the experiments, the plume is confined by a magnetic field, and its emission intensity becomes high in PLD under a magnetic field. Moreover, the evaporated ions are excited more strongly under a magnetic field than without a magnetic field. The BFO films show the columnar structure that coincides well with the calculated image, and the deposition rate is enhanced. The polarization versus electric field (P – E) hysteresis loops of BFO films is obtained at room temperature (RT), and the polarization is reduced in the film deposited under a magnetic field.

Study on pure and Nd-doped BiFeO3 thin films prepared by chemical solution deposition method

Polycrystalline BiFeO3 (BFO) films with varying precursor composition and Bi1−xNdxFeO3 (BNFO, x=0.06–0.20) films were deposited on FTO substrates by using a chemical solution deposition method. Present data indicated that the precursor played a crucial role, i.e., microstructure, morphology and ferroelectric properties of the BFO film are closely related to the proportion of acetic anhydride. XRD analysis and Raman spectra revealed a structural transition from rhombohedral to tetragonal as the amount of acetic anhydride was increased. The effect of Nd on microstructure, chemical state, dielectric, ferroelectric, magnetic and optical properties has been investigated. A structural transition with increasing Nd content was proved by using XRD and Raman scattering spectra. X-ray photoelectron spectroscopy confirmed the formation of Fe2+ and the valence fluctuation of Bi of the BNFO films. The strongest ferroelectric polarization and magnetocapacitance were obtained in the sample with 15% Nd. A dramatic reduction in ferroelectric polarization and the strongest magnetization were observed in the BNFO (x=0.20) film due to its paraelectric nature and orthorhombic structure. With increasing Nd content, the band gap of the BNFO films decreased with Eg=(2.45−0.024x)eV.

Defect-mediated ferroelectric domain depinning of polycrystalline BiFeO3 multiferroic thin films

The ferroelectric domain depinning in a polycrystalline BiFeO3 film is studied by a defect-mediated diffusion mechanism driven by a secondary re-oxidation anneal. The presence of defect complexes (oxygen-vacancy-associated dipoles) responsible for pinning is discussed from the current-voltage (I-V) characteristics of the film. Dissociation of these complexes by re-oxidation anneal produces the effective depinning of domains in the material. The released oxygen vacancies would be compensated at the re-oxidized state due to the valence change of Fe2+ to Fe3+. Improvement on domain mobility results in a larger contribution to ferroelectric switching, showing a room-temperature remanent polarization of 67 μC cm−2.

Effects of Thickness and Polarization Field on the Photovoltaic Properties of BiFeO3 Thin Films

We study the effects of film thickness on lattice parameters, direct band gap and photovoltaic outputs in the sol-gel derived BiFeO3 thin films. With the change of the film thickness, the great transitions will take place in the preferred orientation and lattice parameters. Furthermore, the photovoltaic outputs are significantly dependent on the film thickness. The results show that the open circuit voltage gradually increases and the short circuit current reciprocally decreases with the increase of film thickness. In particular, we demonstrate for the first time that there are tunable photovoltaic outputs with external electric field polarization switching in the polycrystalline BiFeO3 film, which is critical for the future device applications based on the photovoltaic properties of BiFeO3 films.

Repetition Rate Dependence of Ferroelectric Properties of Polycrystalline BiFeO3 Films Prepared by Pulsed Laser Deposition Method

Polycrystalline BiFeO3 (BFO) films on Pt/TiO2/SiO2/Si substrate have been prepared by a pulsed laser deposition (PLD) method with various repetition rates. The X-ray diffraction (XRD) pattern observes single-phase perovskite structure with a small secondary phase. While the film-thickness is increased with increasing repetition rates, the grain size increases, and the leakage current reduces. Also, good polarization versus electric field (P-E) hysteresis loops of all polycrystalline BFO films were obtained at room temperature (RT). It is seen that the polarization value (Pr ) practically remains almost constant for all the BFO films, and the coercive field (Ec ) decreases with increasing the film thickness.

Electric and Magnetic Properties of Sputter Deposited BiFeO3Films

Polycrystalline BiFeO3films have been magnetron sputter deposited at room temperature and subsequently heat-treated ex situ at temperatures between 400 and 700°C. The deposition was done in pure Ar atmosphere, as the use of oxygen-argon mixture was found to lead to nonstoichiometric films due to resputtering effects. At a target-to-substrate distanced=2′′the BiFeO3structure can be obtained in larger range process gas pressures (2–7 mTorr) but the films do not show a specific texture. Atd=6′′codeposition from BiFeO3and Bi2O3has been used. Films sputtered at low rate tend to grow with the (001) texture of the pseudo-cubic BiFeO3structure. As the film structure does not depend on epitaxy similar results are obtained on different substrates. A result of the volatility of Bi, Bi rich oxide phases occur after heat treatment at high temperatures. A Bi2SiO5impurity phase forms on the substrate side, and does not affect the properties of the main phase. Despite the deposition on amorphous silicon oxide substrate weak ferromagnetism phenomena and displaced loops have been observed at low temperatures showing that their origin is not strain. Ba, La, Ca, and Sr doping suppress the formation of impurity phases and leakage currents.

Structural dependent ultrafast electron-phonon coupling in multiferroic BiFeO3 films

The electronic energy relaxation of polycrystalline BiFeO3 films is studied using ultrafast pump-probe spectroscopy. After photo-excitation with femtosecond laser pulses, the relaxation of hot electrons is identified to decay with two different characteristic times. The fast process is attributed to scattering of electrons with lattice-vibration modes, and the slow one is corresponding to the spin-lattice thermalization. The electron-phonon coupling is characterized by the second moment of the Eliashberg function, λ〈ω2〉. Due to the structural strain and symmetry breaking, the electron-phonon interaction strength of tetragonal BiFeO3 films is larger than that of rhombohedral counterparts.

Thickness-dependent ferroelectric behaviors of (111)-textured polycrystalline pseudo-cubic BiFeO3 thin films

Preferentially (111)-textured polycrystalline BiFeO3 (BFO) thin films with different thicknesses were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by using a pulsed laser deposition method. The structure of the BFO films was found to be pseudo-cubic with a lattice constant of 3.96 A based on the X-ray diffraction analysis. The (111)-textured polycrystalline BFO films exhibited good square-type P-E loops with a relatively large remnant polarization (2Pr) of 41 ∼ 54 µC/cm2 and a coercive field (2Ec) of 470 ∼ 510 kV/cm at an applied electric field strength of ±460 kV/cm. Even though the thicknesses of the films were changed, the changes in 2Pr and 2Ec were relatively small. The enhanced leakage current densities of the polycrystalline BFO films were observed to be at least comparable with those of other epitaxial BFO films. Furthermore, the leakage current densities of the films improved with increasing thickness.

The Bipolar Resistive Switching in BiFeO3 Films

Pure-phase polycrystalline BiFeO3 films have been successfully prepared by pulsed-laser deposition on surface oxidized Si substrates using LaNiO3 buffer layer with substrate temperature (Ts) ranging from 550 °C to 800 °C and a laser frequency of 5 Hz and 10 Hz. Bipolar resistive switching has been observed in all the films using LaNiO3 as bottom electrodes and silver glue dots as top electrodes, the resistivity switches from a high-resistance state (HRS) to a low-resistance state (LRS) with positive voltage applied on the top Ag electrodes, and from LRS to HRS with positive voltage applied on the bottom LaNiO3 electrodes. The mechanism of the resistive switching has been confirmed to be due to the voltage polarity dependent formation/rupture of the conducting filaments formed by the O vacancies. The highest resistive ratio of HRS to LRS, of more than 2 orders of magnitude, has been achieved in the highest resistive BiFeO3 film prepared at Ts of 650 °C and laser frequency of 10 Hz.

Local leakage current behaviours of BiFeO3 films

The leakage current behaviours of polycrystalline BiFeO3 thin films are investigated by using both conductive atomic force microscopy and current-voltage characteristic measurements. The local charge transport pathways are found to be located mainly at the grain boundaries of the films. The leakage current density can be tuned by changing the post-annealing temperature, the annealing time, the bias voltage and the light illumination, which can be used to improve the performances of the ferroelectric devices based on the BiFeO3 films. A possible leakage mechanism is proposed to interpret the charge transports in the polycrystalline BiFeO3 films.

Thermooptic Property of Polycrystalline BiFeO3 Film

We have fabricated a polycrystalline BiFeO3 (BFO) film with a thickness of 650 nm on a Pt/Ti/SiO2/Si substrate by a chemical solution deposition method, and its optical and thermooptic properties were evaluated by spectroscopic ellipsometry. The optical band gap energy of the BFO film was 2.95 eV at 50 °C and monotonically decreased with increasing temperature to 2.79 eV at 530 °C. The extinction coefficient was significantly low at a wavelength longer than 600 nm in the temperature range from 50 to 530 °C. The refractive index of the BFO film was estimated to be 2.96 at 600 nm and 2.68 at 1550 nm at 50 °C using a single Gaussian oscillator, and a thermooptic coefficient of 0.8×10-4 K-1 was obtained at a wavelength of 1550 nm. Although the refractive index decreased with increasing temperature at all wavelengths, the variation was larger at shorter wavelengths. This seems to be caused by the combination of broadening in the Gaussian oscillator vibration and thermal expansion.

Thermooptic Property of Polycrystalline BiFeO3Film

We have fabricated a polycrystalline BiFeO3 (BFO) film with a thickness of 650 nm on a Pt/Ti/SiO2/Si substrate by a chemical solution deposition method, and its optical and thermooptic properties were evaluated by spectroscopic ellipsometry. The optical band gap energy of the BFO film was 2.95 eV at 50 °C and monotonically decreased with increasing temperature to 2.79 eV at 530 °C. The extinction coefficient was significantly low at a wavelength longer than 600 nm in the temperature range from 50 to 530 °C. The refractive index of the BFO film was estimated to be 2.96 at 600 nm and 2.68 at 1550 nm at 50 °C using a single Gaussian oscillator, and a thermooptic coefficient of 0.8×10-4 K-1 was obtained at a wavelength of 1550 nm. Although the refractive index decreased with increasing temperature at all wavelengths, the variation was larger at shorter wavelengths. This seems to be caused by the combination of broadening in the Gaussian oscillator vibration and thermal expansion.

Microwave dielectric properties of BiFeO3 multiferoic films deposited on conductive layers

Nondoped BiFeO3 (BFO) and doped Bi0.9La0.1Fe0.9Mn0.1O3 (BLFMO) thin films (d = 200–350 nm) were grown at 650–750 °C by RF sputtering on Si and SrTiO3(100), coated by conductive LaNiO3 films and La2/3Ca1/3MnO3/SrRuO3 bilayers. The complex dielectric permittivity of the films was measured at room temperature in the frequency range from 10 MHz to 10 GHz using parallel plate capacitor structures. Dielectric properties of the polycrystalline BFO films were compared with those of the epitaxial quality BLFMO films, and it was seen that the latter has better microwave performance than the former. The dielectric losses were below 0.05 at 1 GHz frequency, which may be acceptable for microwave applications.

Enhancement of Multiferroic Properties in BiFeO3–Ba(Cu1/3Nb2/3)O3: Film Fabricated by Aerosol Deposition

Lead‐free tetragonal Ba(Cu1/3Nb2/3)O3 (BCN) was chosen to enhance the multiferroic characteristics of rhombohedral BiFeO3 (BFO). When the aerosol‐deposited film with a composition of 0.9BFO–0.1BCN, chosen to be near that of the morphotropic phase boundary, was annealed at 600°C for 1 h, the film showed superior dielectric, ferroelectric, and magnetic properties to those of pure polycrystalline BFO films. TEM revealed that BCN addition and subsequent annealing process enlarged the lattice constant of the BFO film along the 〈111〉 direction. The greatly enhanced multiferroic properties were attributed to the distorted lattice structure induced by the BCN addition and the high residual stress effect.

THE OPTICAL PROPERTY OF MULTIFERROIC BiFeO3 FILMS

ABSTRACT We fabricated a multiferroic BiFeO3 (BFO) film and evaluated its optical properties. A polycrystalline BFO film with a thickness of 650 nm was formed on a Pt/Ti/SiO2/Si substrate. The refractive index of the BFO film was estimated to be approximately 3.0 at a wavelength of 600 nm. This value is the largest in the visible light range for oxide films such as rutile-type TiO2. Furthermore, the extinction coefficient was estimated to be zero in the wavelength range of 600 to 1670 nm. These results suggest that the produced BFO film is a promising material for optical communication devices.

Enhanced multiferroic properties of (1 1 0)-oriented BiFeO3 film deposited on Bi3.5Nd0.5Ti3O12-buffered indium tin oxide/Si substrate

BiFeO3 (BFO) films with and without a Bi3.5Nd0.5Ti3O12 (BNT) buffer layer were fabricated on indium tin oxide (ITO)/Si substrates using a metal organic decomposition process. X-ray diffraction measurements reveal that a BNT buffer layer can favour the growth of (1 1 0)-oriented grains in the BFO film. BFO film with a BNT buffer layer exhibits well saturated P–E hysteresis loops with good rectangularity as well as large remanent polarization (∼70.2 µC cm−2) owing to its higher volume fraction of (1 1 0)-oriented grains, lower leakage current and lower coercive field in comparison with those of the BFO film deposited directly on the ITO/Si substrate. In addition, the magnetization of the (1 1 0)-oriented BFO film is more easily saturated compared with that of the polycrystalline BFO film without a BNT buffer layer.

Ferroelectric, electrical and magnetic properties of Cr, Mn, Co, Ni, Cu added polycrystalline BiFeO3 films

Cr, Mn, Co, Ni, and Cu were added to polycrystalline BiFeO3 films, and their influence on the ferroelectric, electrical, and magnetic properties was investigated. All the additives except Ni reduced the leakage current density in the high electric field region. The addition of Cu and Co decreased the coercive field without reducing remanent polarization. The addition of Co caused spontaneous magnetization at room temperature, which exhibited a large coercive field of 16kOe at 10K. It was revealed that Co addition suppressed the leakage current density, decreased the electric coercive field, and induced spontaneous magnetization and large magnetic coercivity.

Estimation of Leakage Current Density and Remanent Polarization of BiFeO3 Films with Low Resistivity by Positive, Up, Negative, and Down Measurements

Polycrystalline BiFeO3 film was fabricated by chemical solution deposition (CSD) on 111-textured Pt/Ti/SiO2/Si(100) substrates followed by post-annealing at 923 K in air. The leakage current density was estimated by pulse response of a positive, up, negative, and down (PUND) measurement. It was revealed that the leakage current density estimated by PUND measurements is useful for high leakage current materials because the dielectric breakdown could be extended to the high electric field region when compared to a conventional measurement way using a pico-ampere meter. Therefore, the changing point of the leakage current mechanism at the higher electric field of 0.3 MV/cm could be observed by using the PUND method in the present specimen. When the temperature was decreased, it was revealed that the leakage current component did not affect the ferroelectric polarization below 150 K in the case of present specimen. At 93 K, where leakage current was suppressed, the remanent polarizations were evaluated by ferroelectric hysteresis loops and PUND, and it was revealed that the remanent polarization did not saturate until an electric field of 1.4 MV/cm was applied (Pr = 89 µC/cm2 at P–E loop, 2Pr = 164 µC/cm2 at PUND). This result indicates the potential of BiFeO3 to have quite high remanent polarization.

Dependence of ferroelectric and magnetic properties on measuring temperatures for polycrystalline BiFeO3 films

A multiferroic BiFeO(3) film was fabricated on a Pt/Ti/SiO(3)/Si(100) substrate by a chemical solution deposition (CSD) method, and this was followed by postdeposition annealing at 923 K for 10 min in air. X-ray diffraction analysis indicated the formation of the polycrystalline single phase of the BiFeO(3) film. A high remanent polarization of 89 microC/cm(2) was observed at 90 K together with a relatively low electric coercive field of 0.32 MV/cm, although the ferroelectric hysteresis loops could not be observed at room temperature due to a high leakage current density. The temperature dependence of the ferroelectric hysteresis loops indicated that these hysteresis loops lose their shape above 165 K, and the nominal remanent polarization drastically increased due to the leakage current. Magnetic measurements indicated that the saturation magnetization was less than 1 emu/cm(3) at room temperature and increased to approximately 2 emu/cm(3) at 100 K, although the spontaneous magnetization could not appear. The magnetization curves of polycrystalline BiFeO3 film were nonlinear at both temperatures, which is different with BiFeO3 single crystal.