Polycrystalline BiFeO3 Thin Films Research Articles

Tailoring of charge carriers with deposition temperature in pulsed laser deposited BiFeO3 thin films

This research investigates the structural, topological, electrical and optical properties of pulsed laser deposited polycrystalline BiFeO3 thin films on silicon and glass substrate at varying deposition temperatures ranging from 400 °C to 700 °C. X-ray diffraction confirm rhombohedral phase and X-ray photoelectron spectroscopy reveals stoichiometric BiFeO3 films. The optical bandgap of thin films obtained from absorption spectra increases with the substrate temperature. Photoluminescence emission spectrum reveals the defects and Atomic Force Microscopy analysis bring out the surface topography and crystallinity improvement with temperature. The charge transport studies reveal a transition in conductivity from n-type at lower deposition temperature to p-type at higher deposition temperature, attributed to oxygen and bismuth vacancies respectively. The intricate understanding of conductivity tuning and the leaky nature of BiFeO3 thin film opens avenues for applications in non-volatile memories, particularly neuromorphic devices.

Reliable ferroelectricity in sol–gel‐derived BiFeO3 thin films below 200 nm

AbstractTo evaluate optimizing processing conditions for bismuth ferrite BiFeO3 (BFO) thin films with thicknesses below 200 nm on Pt(111)/Ti/SiO2/Si substrates. The impact of the amorphous components, as well as defects and elemental composition changes and microstructure evolution (i.e., grain size) as regulated by the treatment temperatures (475–600°C) on the ferroelectric and dielectric properties are investigated. The current work shows that with the smallest grain size and the least defects amount achieved in the film annealed at 550°C, the conductivity is compressed greatly. Such a temperature will also allow adequate ferroelectric switching by reducing the amorphous components, which remain the majority below 500°C and thus reinforce the polarization switching. The leakage path is deduced via crystallized grains in BFO thin films instead of the remaining including grain boundaries and amorphous components. The best remanent polarization of ∼60.4 μC·cm‐2 from PUND measurements excluding the leakage contribution at 500 kV·cm−1 and an excellent fatigue property of 106 cycles are achieved. In addition, broadened fabrication temperatures for amorphous‐protected polycrystalline BFO thin films from 525 to 575°C, exhibiting superior ferroelectricity than most previously reported BFO thin films below 200 nm, either derived by chemical or physical methods. Ultimately, it is suggested that for leaky ferroelectrics such as BFO, optimal annealing temperature should balance the amorphous components by reducing the annealing temperature while maintaining adequate crystallization for ferroelectric switching.

Leakage current characteristics of polycrystalline BiFeO3 thin films affected by thickness-dependent domain wall currents

Polycrystalline BiFeO3 (BFO) thin films with a mixed crystallinity of c-oriented crystallinity and a-oriented crystallinity were deposited on a (200) Pt/TiO2/SiO2/Si substrate to have a thickness of 50, 100, and 150 nm. The ferroelectric domains observed with vertical piezoresponse force microscope (VPFM) and lateral PFM (LPFM) showed a mixed domain structure of c-domains and a-domains, and the ratio of a-domains increased as the thickness increased. The domain wall currents at the a-domains and c-domain boundaries of the BFO thin films were measured by conduction atomic force microscopy (CAFM) experiments and they were larger than those at the c-domains boundaries. In particular, the leakage currents, ferroelectric polarizations, and piezoelectric coefficients of the BFO thin films according to their thickness were significantly affected by the formation of the a-domains.

Polarization-independent enhancement in UV photoconductivity of BiFeO3/Sn:In2O3 heterostructure

Currently, polarization-dependent transport in ferroelectric materials under optical illumination is gaining a lot of attention in optoelectronics. This idea is implemented on BiFeO3 to illustrate its UV–Visible light photodetection property, however, naturally occurring bismuth or oxygen vacancies serve a major disadvantage as they interfere with its polarization ability. It is very difficult to overcome this defect issue in bismuth-based perovskites, therefore, based on this herein an enhancement in the photoconductive property of polycrystalline BiFeO3 thin film deposited via spray pyrolysis technique on Sn:In2O3 coated glass substrate is demonstrated without polarizing it. An extraordinary photodetection behavior with higher photoresponsivity (1.01 A/W) and external quantum efficiency (364) as well as faster response speed (6 ms) even under low UV illumination (340 nm) and lower applied bias of 2 V is observed. The roles of the BiFeO3–Sn:In2O3 interface, as well as the adsorption/desorption of oxygen molecules on the surface of BiFeO3 layers, were highlighted for the ferroelectric material's UV photodetector application.

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.

Leakage Current Characteristics of Polycrystalline Bifeo3 Thin Films Affected by Thickness-Dependent Domain Wall Currents

Leakage Current Characteristics of Polycrystalline Bifeo3 Thin Films Affected by Thickness-Dependent Domain Wall Currents

Magnetic-Electric Behaviors and Physical Properties of The Thin Films on ITO-Glass Substrate

Polycrystalline BiFeO3 thin films on ITO glass substrates were prepared by radio frequency magnetron sputtering using a Bi1.1FeO3 target. The samples which were annealed with different annealing conditions are pure without impurities. We measured the magnetic properties and ferroelectricity of the BiFeO3 films. The measurement results show that the magnetic and electrical properties of the BiFeO3 films are significantly different under different annealing conditions.

Photovoltaic effect in Sr-doped BiFeO3 thin films

We present the structural and photovoltaic properties of Sr-doped polycrystalline BiFeO3 thin films prepared by sol-gel method. The result shows that all the BiFeO3 thin films doped with 0 ∼ 20 at.% Sr are rhombohedral distortion perovskite structure and the lattice constant decreases with increasing Sr amount. The addition of Sr can effectively inhibit the grain growth and enhance the uniformity of BFO films, besides, the band gap can be tuned availably as well. As Sr content increases, both the short circuit photocurrent density and the open circuit photovoltage begins to increase and then decrease, the maximum values achieved when 15 at.% Sr-doped. The power conversion efficiency accordingly increases first and then drops with Sr content increases, reaching 0.011% when the Sr-doped content is 15%, which results from the decreased band gap and oxygen vacancies. Highlights All the BiFeO3 thin films are rhombohedral distortion perovskite structure. Addition of Sr decreases the band gap of BFO thin films. The power conversion efficiency increases as Sr content increases. The maximal power conversion efficiency is 0.011%

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.

Maximizing Short Circuit Current Density and Open Circuit Voltage in Oxygen Vacancy-Controlled Bi1-xCaxFe1-yTiyO3-δ Thin-Film Solar Cells.

Designing solid-state perovskite oxide solar cells with large short circuit current (JSC) and open circuit voltage (VOC) has been a challenging problem. Epitaxial BiFeO3 (BFO) films are known to exhibit large VOC (>50 V). However, they exhibit low JSC (≪μA/cm2) under 1 Sun illumination. In this work, taking polycrystalline BiFeO3 thin films, we demonstrate that oxygen vacancies (VO) present within the lattice and at grain boundary (GB) can explicitly be controlled to achieve high JSC and VOC simultaneously. While aliovalent substitution (Ca2+ at Bi3+ site) is used to control the lattice VO, Ca and Ti cosubstitution is used to bring out only GB-VO. Fluorine-doped tin oxide (FTO)/Bi1-xCaxFe1-yTiyO3-δ/Au devices are tested for photovoltaic characteristics. Introducing VO increases the photocurrent by four orders (JSC ∼ 3 mA/cm2). On the contrary, VOC is found to be <0.5 V, as against 0.5-3 V observed for the pristine BiFeO3. Ca and Ti cosubstitution facilitate the formation of smaller crystallites, which in turn increase the GB area and thereby the GB-VO. This creates defect bands occupying the bulk band gap, as inferred from the diffused reflection spectra and band structure calculations, leading to a three-order increase in JSC. The cosubstitution, following a charge compensation mechanism, decreases the lattice VO concentration significantly to retain the ferroelectric nature with enhanced polarization. It helps to achieve VOC (3-8 V) much larger than that of BiFeO3 (0.5-3 V). It is noteworthy that as Ca substitution maintains moderate crystallite size, the lattice VO concentration dominates GB-VO concentration. Notwithstanding, both lattice and GB-VO contribute to the increase in JSC; the former weakens ferroelectricity, and as a consequence, undesirably, VOC is lowered well below 0.5 V. Using optimum JSC and VOC, we demonstrate that the efficiency ∼0.22% can be achieved in solid-state BFO solar cells under AM 1.5 one Sun illumination.

Switchable ferroelectric diode and photovoltaic effects in polycrystalline BiFeO3 thin films grown on transparent substrates

Switchable ferroelectric diode and photovoltaic effects in epitaxial BiFeO3 (BFO) thin films have drawn intense attention for its potential applications in resistive memory and solar cell. However, both of these two effects were observed almost entirely in epitaxial BFO thin films or bulk crystal. In this paper, we demonstrate these effects in a polycrystalline BFO thin film grown on a transparent Indium Tin Oxide (ITO) covered quartz plate by the pulsed laser deposition technique. The BFO thin film shows good ferroelectricity with remanent polarization of around 60 μC/cm2 and has a direct band gap of 2.64 eV. The current-voltage curves measured at an Au/BFO/ITO configuration show diode-like rectifying characteristics, and the diode polarity can be reversed with polarization switching. The polycrystalline BFO thin film also exhibits a switchable photovoltaic effect with an open-circuit voltage of approximately 0.2 V and a short-circuit current of nearly 60 μA/cm2 under illumination. This study demonstrates that the polycrystalline BFO thin film on a transparent substrate also has the ferroelectric switchable diode and photovoltaic effects. Therefore, these results will be useful to expand the application of BFO thin films in optoelectronic devices.

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.

Photo‐Induced Enhancement in Ferroelectric Polarization and Abnormal Photo‐Conductivity in Polycrystalline Pt‐Sandwiched BiFeO3 Thin Film Capacitor

Polycrystalline BiFeO3 (BFO) thin film is directly deposited on Pt(111)/Ti/SiO2/Si(001) substrate at 550 °C by radio frequency (rf) magnetron sputtering. Metal Pt is used as top electrode to construct the symmetric Pt/BFO/Pt capacitor. The Pt/BFO/Pt capacitor exhibits photo‐induced enhancement in both remanent and switchable polarization when the capacitor is illuminated, which can be explained by the reorientation of ferroelectric polarization because of the trapping of photo‐induced carriers. The steady current density both in dark and in violet shows a diode‐like characteristic which is mainly dominated by the top Pt/BFO interface. In addition, an abnormal observation of the decrease regarding the illuminated current density with illumination time is acquired, indicating a difference from the conventional Si‐based p–n junction. The achieved results provide the evidence for the application of BFO‐based photo‐electric devices.

Morphology-induced spin frustration in granular BiFeO3 thin films: Origin of the magnetic vertical shift

Pronounced room temperature vertical shifts in the magnetic hysteresis loops of granular, highly polycrystalline and ferromagnetic-like BiFeO3 thin films are observed upon field-cooling from a temperature above the Néel temperature of bulk BiFeO3. This is ascribed to the interplay between the preferential alignment, established by the field-cooling process, of the net magnetic moment, which arises from uncompensated antiferromagnetic spins, and the pinning of a fraction of these spins at the particle boundaries. Conversely, field-cooling of an epitaxially grown BiFeO3 film results in no vertical shift, confirming the effective role played by the particle boundaries (i.e., morphology) of the granular-like BiFeO3 films in the process of spin frustration.

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.

Decreased oxygen vacancies and improved ferroelectric properties of the BiFeO3 thin films with high magnetic field annealing

High magnetic field annealing (MFA) effects on polycrystalline BiFeO3 (BFO) thin films deposited on ITO buffered glass substrates by the chemical solution deposition (CSD) method have been investigated. Magnetic force induced by magnetic field leads to the increased crystallite size and the decrease in activation energy for boundary motion, especially for the low temperature annealed films. More Fe3+ ions are induced with increasing magnetic field to lower the free energy of the BFO films, which also leads to a decreased oxygen vacancy, resulting in low leakage current density. The increased remanent polarization and the decreased coercive field initially with the increase in magnetic field are attributed to the reduced domain wall pinning resulted from decreased oxygen vacancies and grain boundaries. Generally, the decreased coercivity and the improved ferroelectric polarization suggest high MFA is an effective way to enhance ferroelectric properties of the CSD-derived BiFeO3 thin films.

Effect of substrates on the structure and optical properties of polycrystalline BiFeO3 thin films

Multiferroic BiFeO3 thin films were prepared on Pt/TiO2/SiO2/Si and ITO/glass substrates by sol–gel preparation method. Pure perovskite phase and uniform surface topography have been verified by X-ray diffraction and atomic force microscopy, respectively. In addition, optical property measured by spectroscopic ellipsometer indicated that the absorptivity of both BiFeO3 thin films was enhanced with increasing photo energy after an illumination among 250–1000nm. Tauc calculation indicated that the direct band gap was 2.92eV for the film on Pt/TiO2/SiO2/Si, which is about 0.4eV lower than that of the film on ITO/glass.

Magnetic behaviour of sol–gel driven BiFeO3 thin films with different grain size distribution

BiFeO3 (BFO) thin films with uniform thickness of ~200nm were prepared by the sol–gel assisted spin coating method. Different grain size distributions in the as-grown BFO films were then induced by varying the annealing temperature between 525 and 600°C. It is found that the grain size distribution become wider as the annealing temperature increases. All the films showed a well-saturated magnetization (M) versus magnetic field (H) hysteresis loops at 300K. A strong dependence of M on the grain size distribution is observed. An optimal grain size distribution with average grain size ~90nm is responsible for high M in the BFO films. The non-saturated M–H loops obtained at 10K suggest the spin glass behaviour of BFO films. The zero field cooled (ZFC) and field cooled (FC) magnetization curves shows split at 300K and a cusp at ~50K in the ZFC curve, which further confirms the spin glass state of polycrystalline BFO thin films.

Structural and electrical characterizations of BiFeO3 capacitors deposited by sol–gel dip coating technique

Bismuth ferrite (BiFeO3) thin films were deposited by sol–gel dip coating (SGDC) technique on Si-P(100) and glass substrates to investigate the structural and electrical characteristics. The aluminum (Al) metal contacts were formed on the samples deposited on the Si-P(100) to fabricate metal-oxide-semiconductor (MOS) capacitors. The fabricated MOS structures were characterized electrically by capacitance–voltage (C–V) and conductance–voltage (G/ω–V) measurements. The structural characterizations were performed by X-ray diffraction technique and scanning electron microscopy. The compositions of the films were investigated by energy-dispersive X-ray spectroscopy. The results exhibit that pure rhombohedral perovskite phase films were fabricated without any elemental contamination. Average grain sizes of the BiFeO3 deposited on silicon and glass wafers were found to be about 34,50 and 30,00nm, respectively. In addition, while the thin films deposited on glass substrate exhibit porous surface, those deposited on Si-P(100) wafers exhibit dense microstructure with a homogenous surface. Moreover, the C–V and G/ω–V characteristics are sensitive to applied voltage frequency due to frequency dependent charges (Nss) and series resistance (Rs). The peak values of Rs have been decreased from 2,6kΩ to 40Ω, while Nss is varied from 6,57×1012 to 3,68×1012eV−1cm−2 with increasing in frequency. Consequently, pure phase polycrystalline BiFeO3 thin films were fabricated successfully by SGDC technique and BiFeO3 dielectric layer exhibits stable insulation characteristics.

Lead Free Metal-Ferroelectric-Insulator-Semiconductor Devices for Non-Volatile Memory Appliactions

The polarization bistability of the ferroelectric material offers the possibility to develop dense, energy efficient, fast and non volatile ferroelctric random access memory (FeRAM) with long retention and high endurance, as a promising replacement to both DRAM and Flash for the continued scaling. With this motivation, metal-ferroelectric-insulator-semiconductor (MFIS) structures were fabricated on p-Si <100> semiconductor substrate, by first depositing a 6 nm thick amorphous DyScO3 high-k insulator layer using metal organic chemical vapor deposition (MOCVD), followed by a 265 nm thick polycrystalline BiFeO3 (BFO) ferroelectric layer employing radio frequency (RF) sputtering, and finally a 40 nm thick platinum metal layer by DC magnetron sputtering. Electrical properties of the developed MFIS (Pt/BiFeO3/DyScO3/SiOx/Si) stacks were characterized as a function of thermal agitation (200 K to 400 K) to evaluate their device performance. Fat hysteresis loop was observed in the frequency dependent (1 kHz-1 MHz) capacitance-voltage (C-V) measurement with a memory window (ΔVFB) of 0.8 V at an exceptionally very low sweep voltage of ± 2 V. The memory window increased to 2.2 V as the sweep voltage was raised to ± 4 V and beyond up to ± 8 V, indicating a highly stable device. Low leakage current (1.2 μA/cm2 at 4 V) and excellent data retention characteristics with distinguishable high and low capacitance values over the long time period of 106 sec, revealed strong charge storage potential of the fabricated device architecture. The non-linear hysteretic J-E curves of the MFIS structure were also investigated in detail as a function of temperature to understand the device reliability and the charge transport mechanisms. Piezoelectric force microscopy (PFM) and P-E hysteresis loop (domains reversal) studies were carried out to confirm the ferroelectric nature of the polycrystalline BFO thin films. The encouraging results achieved reveals that Pt/BiFeO3/DyScO3/SiOx/Si MFIS device architecture is a promising one for realization of FeRAM devices.