Epitaxial BiFeO3 Thin Films Research Articles

Electronic and Magnetic Properties of Chemical Solution Deposited BiFeO3 Thin Film: a Soft X-ray Magnetic Circular Dichroism Study

In this work, we report the electronic and magnetic properties of epitaxial BiFeO3 (BFO) thin film deposited on SrTiO3 (001) substrate via the chemical solution deposition technique. These properties of pure BFO thin films were characterized by X-ray absorption spectroscopy (XAS) and X-ray magnetic circular dichroism techniques (XMCD). The large magnetization at Fe-edge could be due to annealing in nitrogen. Annealing in nitrogen atmosphere increases the grain size and porosity which lead to create more oxygen vacancy. The presence of oxygen vacancies which compensate the bismuth vacancies created due to bismuth volatilization. The origin of large orbital momentum in BFO thin film is also discussed. The O K-edge spectra display large spectral variation depending on the polarization vector E. The structural distortion at oxygen octahedral induces large anisotropy due to Fe 3d–O 2p hybridization. The hybridized Fe 3d orbitals contribute a much larger orbital momentum in BFO thin film.

The role of intermediate S-polymorph towards high piezoelectricity in La-doped BiFeO3 epitaxial thin films

Mixed phases with stripe patterns are considered as a prerequisite for remarkable piezoelectric properties in epitaxial BiFeO3 (BFO) thin films. Among the mixed phases in BFO epitaxial thin films, the vital role in achieving high piezoelectric response is attributed to the intermediate S-polymorph that bridges the transition between rhombohedral and tetragonal phases. However, the correlation between the S-phase and the piezoelectric strain in mixed-phase BFO has not been understood well due to the lack of a probe technique that can investigate the structural and electromechanical variations simultaneously. Here, using the time-resolved X-ray microdiffraction technique, we investigate the electric field induced phase transformation dynamics of a mixed-phase epitaxial La-doped BFO (BLFO) thin film. The BLFO thin film exhibits phase transformations between monoclinic MC and tilted MC, S/tilted S (Stilt) phases under an applied electric field. Among the competing phases, we found that the S- or tilted S-phases exhibit the highest piezoelectric strain variation up to 0.7% at an electric field pulse of strength 1.3 MV/cm and width 5 μs. The present study correlates the high piezoelectric response in mixed-phase BLFO thin films with the presence of S/Stilt -phases, which offers a useful pathway into designing strain-engineered ultra-high piezoelectric BFO thin films.

Structural phase instability, mixed-phase, and energy band gap change in BiFeO3 under lattice strain effect from first-principles investigation

Lattice strain effects drive a variety of novel functional responses in epitaxial BiFeO3 thin films and have attracted significant interest and attention from researchers in experimental and theoretical studies. However, the difficulty in designing experimental techniques in addition to facing problems in the first principles approach, such as output accuracy and high computational costs, constitute the discovery of new functional responses in epitaxial BiFeO3 thin films not entirely understood. Therefore, in this study, we perform a first principles calculation based on the less expensive LDA+U method to investigate the structural phase instability and electronic properties change in BiFeO3 under the lattice strain effect. The structural phase transformation of BiFeO3 under volumetric and compressive/tensile lattice strain was examined established on the calculated lower energy phases. Importantly, we demonstrated that the change of crystal structure phases of BiFeO3 was extremely sensitive to the volumetric and compressive/tensile lattice strain, comparable with various experiment data, as reported in the literature. Moreover, we revealed for the first time from the first principles prediction the coexistence of mixed R-T phases in the region of moderate compressive ζin-plane of −2.9% (e.g. LaAlO3 substrates with ɑ = 3.79 Å). From the prediction of electronic properties obtained by the LDA+U and PBE0 methods, we found that the energy band gap increased when the compressive in-plane lattice strain is increased while, in contrast, the energy band gap decreased when BiFeO3 was under the tensile in-plane lattice strain effect. We also demonstrate that our computational technique based on the first principles study was sufficiently accurate enough, helping to speed up the process of designing new materials having an excellent multifunctional response (piezoelectric, magnetic, photovoltaic, spintronic).

Robust domain variants and ferroelectric property in epitaxial BiFeO3 films

Lead-free ferroelectric thin films have great potential for electric devices owing to the dramatically expanding information epoch. Exploring the domain engineering and temperature stability of ferroelectric films are still urgent since these polarization dipoles are sensitive to external effects. In this work, the temperature dependence of domain dynamic and local piezoelectric response were investigated in epitaxial BiFeO3 (BFO) thin films prepared by pulse leaser deposition (PLD) technology, which exhibiting well-defined polarization switchable intrinsic property. Regarding to temperature endurance, domain variants remain quite stable state regarding to both the vertical and lateral direction, which is in consistent with the free-energy minimum competition theory. Besides, the piezoelectric response along vertical and lateral direction exhibit good temperature endurance. This work demonstrates thin films with rather stable ferroelectric states, which show not only exploration on domain dynamic corresponding to temperature influence, but also promising potential application in electrical storage devices especially working under high temperature.

Electrically driven transient and permanent phase transformations in highly strained epitaxial BiFeO3 thin films

Electric-field-driven phase transformation phenomena in multiferroic BiFeO3 are directly linked to the functionalities of electronic devices based on multiferroic materials. Understanding how the transformation evolves at the nanoscale under the influence of an electric field will provide fascinating insights into key parameters that utilize the transformation features. Here, we report both the electric-field-driven transient and permanent phase transformations in highly strained BiFeO3 thin films and their transformation dynamics at the nanoscale. We found that two distinct transient and permanent phase transformations were triggered below and above a coercive voltage of the polymorphic phase, indicating that ferroelectric polarization switching could promote permanent phase transformations. We also found that the transient transformations evolve via complex phase boundary motions between the coexisting phases, whereas permanent transformations occurred via nucleation of the other phases.

Robust in-plane polarization switching in epitaxial BiFeO3 films

The enrichment domain state and its stability consist an imperative part regarding to the high density non-volatile ferroelectric random access memory (FRAM) devices. In this work, epitaxial BiFeO3 (BFO) thin films were prepared by pulsed-leaser deposition (PLD) technology, and the morphology, local polarization state and piezoelectric response were characterized by scanning probe microscopy (SPM) and piezoresponse force microscopy (PFM). The robust in-plane domain dynamic process is observed under the external electric fields. Furthermore, good retention and repeatability are observed especially under high temperature. The new-developed domain patterns are nearly stable, which is quite different from the existed research. The formation of these domains is probably induced by the combination of built-in electric fields and distribution of tip fields, both of which affect the polarization dynamic process. All the obtained results pave a great way for application corresponding to the FRAM devices especially under high temperature circumstance.

All chemical solution deposition of epitaxial porous BiFe0.93Mn0.07O3 thin films

BiFeO3 as a type of single-phase room temperature multiferroic materials has aroused intensive investigations in recent years. Construction of porous BiFeO3 thin films is helpful for integration of other materials with desirable functionalities. Here, epitaxial BiFe0.93Mn0.07O3 (001) thin films were prepared on SrRuO3-buffered LaAlO3 (001) single crystal substrates by an all-solution chemical solution deposition, which is beneficial for large area thin film fabrication with low cost. Through addition of F-127 copolymer into the precursor solution, pores can be effectively introduced in BiFe0.93Mn0.07O3 (001) thin films while the epitaxy can be maintained. The amount of pores is gradually increased with increasing of the F-127 addition. The leakage current density is not obviously changed and the conduction mechanism can be well fitted through Ohmic behavior. Electric field dependent polarization hysteresis loops can be effectively maintained and the room temperature magnetic moment is obviously improved with the introduction of pores in epitaxial BiFe0.93Mn0.07O3 (001) thin films. The results will provide a facile route to fabricate large area epitaxial porous BiFeO3 thin films as an effective template for integration of other materials to realize desirable functionalities.

Enhanced photovoltaic effect in Ca and Mn co-doped BiFeO3 epitaxial thin films

In this work, we proposed modification of BiFeO3 (BFO) by Mn and Ca co-doping, and realized the epitaxial growth of Bi0.98Ca0.02Fe0.95Mn0.05O3 (BCFMO) thin films on (100)-oriented SrTiO3 (STO) substrates with a SrRuO3 (SRO) buffer layer via pulsed laser deposition (PLD). A decreased bandgap (Eg = 2.41 eV) and enhanced remanent polarization (Pr = 92.5 μC/cm2) were simultaneously achieved in the BCFMO thin film. A tunable photovoltaic (PV) effect by changing the direction of the poling electric field was observed in the BCFMO-based device. We investigated the PV mechanism for the device by analyzing its band structure and attribute it to the coupling effects of the ferroelectric depolarization field and the interface Schottky barriers. The BCFMO-based device under favorable upward poling shows the best PV performance with an open-circuit voltage (Voc) of −0.29 V and short-circuit current density (Jsc) of 0.26 mA/cm2, which are much higher than those for the BFO-based device (Voc = -0.11 V, Jsc = 0.08 mA/cm2), resulting from the narrower bandgap and larger Pr in the BCFMO film. The approach of simultaneously introducing Ca and Mn into the BFO, enabling both a higher polarization and narrower bandgap, opens a new way for improving the performance of BFO-based PV devices.

Evolution of structure and electrical properties of epitaxial BiFeO3 thin films through solution and annealing atmosphere

Abstract Solution derived high-quality epitaxial BiFeO3 (BFO) thin films, with robust spontaneous polarization, are still desired because of their advantages. Herein, epitaxial BFO thin films and SrRuO3 bottom electrodes are prepared on LaAlO3 substrates via chemical solution deposition (CSD) technique. The influences of precursor solution on the microstructure and electrical properties are investigated by using nitrate salt and acetate salt system. The XRD and P-E hysteresis loops results demonstrate that the BFO thin film prepared by acetate salt system as a relatively green CSD route shows good epitaxial characteristic and ferroelectric performances. In addition, the effect of the annealing atmosphere on the microstructure and electrical properties are studied for the acetate salt derived BFO thin films. The microstructure and defects chemistry are analyzed by SEM and XPS, the BFO thin film annealed in air shows better comprehensive ferroelectric performances with the lowest leakage current and enhanced ferroelectric polarization due to the improved microstructure and decreased defect concentrations. The results here thus provide an instructive approach to optimize the growth of solution-derived epitaxial BFO-based thin films.

Self-assembled line network in BiFeO3 thin films

In this work we report on the controlled fabrication of a self-assembled line network in highly epitaxial BiFeO3 thin films on top of LaAlO3 in the kinetically limited grown region by RF sputtering. As previously shown in the case of manganite thin films, the remarkable degree of ordering is achieved using vicinal substrates with well-defined step-terrace morphology. Nanostructured BiFeO3 thin films show mixed-phase morphology. Besides typical formation following (100) and (010) axes, some mixed phase nanodomains are detected also in-between the regular line network. These particular microstructures open a playground for future applications in multiferroic nanomaterials.

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.

Enabling the multifunctionality of thin films through complex engineered nanoscale phases/nanodomains

BiFeO3 is one of the most promising multiferroic material exhibiting, however, drawbacks as low dielectric susceptibility, poor chemical stability and high dielectric loss. Recently, we have demonstrated that joining doping and epitaxial strain engineering a nanoscale stripe structure was induced, resulting in a significant improvement of the dielectric characteristics. By high resolution transmission electron microscopy (HR-TEM) we have evidenced nanostripe domains with alternating compressive and tensile strain in the Y-doped BiFeO3 epitaxial thin films. Small band gap values of bismuth ferrite (BFO) have triggered the interest due to the photovoltaic and photocatalytic potential, for pure or cation doped BFO. However, energy related properties of BiFeO3 can be improved by coupling the strong ferroelectricity of BFO with chemical stability and small band gap value of LaFeO3 within thickness graded heterostructures. Thin films of BFO on 5-20 nm thick epitaxial LFO films deposited on different strain-induced substrates, have been obtained by RHEED- assisted Pulsed Laser Deposition. Their functional dielectric, optical and photolytic behaviour has been unravelled in connexion with complex ensembles of nanoscale phase/nanodomain fluctuations within the epitaxial films.

Effect of mechanical force on domain switching in BiFeO3 ultrathin films

Ferroelectric polarization can be switched by an external applied electric field and may also be reversed by a mechanical force via flexoelectricity from the strain gradient. In this study, we report the mechanical writing of an epitaxial BiFeO3 (BFO) thin film and the combined action of an applied mechanical force and electric field on domain switching, where the mechanical force and electric field are applied using the tip of atomic force microscopy. When the applied force exceeds the threshold value, the upward polarization of the BFO thin film can be reversed by pure mechanical force via flexoelectricity; when an electric field is simultaneously applied, the mechanical force can reduce the coercive electric field because both the piezoelectricity from the homogeneous strain and the flexoelectricity from strain gradient contribute to the internal electric field in the film. The mechanically switched domains exhibit a slightly lower surface potential when compared with that exhibited by the electrically switched domains due to no charge injection in the mechanical method. Furthermore, both the mechanically and electrically switched domains exhibit a tunneling electroresistance in the BFO ferroelectric tunnel junction.

Orientation dependent leakage current behaviors and ferroelectric polarizations of off-axis sputtered BiFeO3 thin films

Epitaxial BiFeO3 thin films with (100), (110) and (111) orientations were grown on the SrRuO3-buffered SrTiO3 substrates by using an off-axis magnetron sputtering. Unlike the BiFeO3(110) and BiFeO3(111) thin films that exhibited a single rhombohedral phase structure, a dominant rhombohedral phase accompanying with a small amount of tetragonal phase was identified in the BiFeO3(100) thin film. In particular, the leakage currents and ferroelectric polarizations of sputtered BiFeO3 thin films were focused on and these films showed utterly different current density-electric field (J-E) behaviors whether in the positive or negative electric field. Among the three films, the ferroelectric polarization of the BiFeO3(100) thin film presented a good frequency stability and had the maximum remnant polarization of Pr ∼ 78 μC/cm2 @ 10 kHz, which could be further demonstrated by pulsed polarizations of films. The distinct differences in electrical properties of orientation-engineered BiFeO3 thin films in present case can be attributed to their different crystallographic orientations and microstructures.

Deterministic optical control of room temperature multiferroicity in BiFeO3 thin films.

Controlling ferroic orders (ferroelectricity, ferromagnetism and ferroelasticity) by optical methods is a significant challenge due to the large mismatch in energy scales between the order parameter coupling strengths and the incident photons. Here, we demonstrate an approach to manipulate multiple ferroic orders in an epitaxial mixed-phase BiFeO3 thin film at ambient temperature via laser illumination. Phase-field simulations indicate that a light-driven flexoelectric effect allows the targeted formation of ordered domains. We also achieved precise sequential laser writing and erasure of different domain patterns, which demonstrates a deterministic optical control of multiferroicity at room temperature. As ferroic orders directly influence susceptibility and conductivity in complex materials, our results not only shed light on the optical control of multiple functionalities, but also suggest possible developments for optoelectronics and related applications.

Expansion of the spin cycloid in multiferroic BiFeO3 thin films

Understanding and manipulating complex spin texture in multiferroics can offer new perspectives for electric field-controlled spin manipulation. In BiFeO3, a well-known room temperature multiferroic, the competition between various exchange interactions manifests itself as non-collinear spin order, i.e., an incommensurate spin cycloid with period 64 nm. We report on the stability and systematic expansion of the length of the spin cycloid in (110)-oriented epitaxial Co-doped BiFeO3 thin films. Neutron diffraction shows (i) this cycloid, despite its partly out-of-plane canted propagation vector, can be stabilized in thinnest films; (ii) the cycloid length expands significantly with decreasing film thickness; (iii) theory confirms a unique [11bar 2] cycloid propagation direction; and (iv) in the temperature dependence the cycloid length expands significantly close to TN. These observations are supported by Monte Carlo simulations based on a first-principles effective Hamiltonian method. Our results therefore offer new opportunities for nanoscale magnonic devices based on complex spin textures.

Composition control and introduction of an Fe2O3 seed layer in metalorganic chemical vapor deposition of epitaxial BiFeO3 thin films

We investigated the compositional self-regulation and seeding effects of an Fe2O3 layer during the metalorganic chemical vapor deposition of epitaxial BiFeO3 thin films. The growth of the BiFeO3 films was explained in terms of the prior deposition of Fe2O3 and the subsequent incorporation of Bi2O3 into Fe2O3. The self-regulation of film composition was achieved by supplying excess Bi at high growth temperature. The introduction of an Fe2O3 seed layer as thin as 5 nm promoted homogeneous thin film growth and consequently reduced the leakage and its fluctuation as well as the compositional fluctuation. The seed layer also served to control the interfacial composition between BiFeO3 and SrRuO3.

Role of rapid and slow cooling on leakage mechanism and ferroelectric polarization of sputtered epitaxial BiFeO3 thin films

Epitaxial BiFeO3 thin films with a (00l) orientation were grown on SrRuO3-coated LaAlO3 substrates via magnetron sputtering. A coexistence of dominant rhombohedral (R) phase with a small amount of tetragonal (T) phase was identified by XRD. The cooling speed did not affect the film's growth orientation and phase structure, while it showed a notable influence on ferroelectric polarization and leakage mechanism. It was shown that the improved electrical properties, e.g. a low dielectric loss (tanδ < 0.11) and a reduced leakage current (J < 8.0 × 10−4 A/cm2), as well as an enhanced polarization of 2Pr ∼ 135 μC/cm2 with a reduced coercive field of 2Ec ∼ 373 kV/cm, were achieved in the BiFeO3 thin film prepared via a slow cooling speed (∼6 °C/min in present case), which could be attributed to its fully crystallization without any impurity phase and well-developed microstructure at a slow cooling environment.

The Observation of Domain‐Wall Current Transients Along with Charge Injection at Elevated Temperatures

AbstractErasable conductive domain walls (DWs) in insulating ferroelectrics are key components to reconfigurable nanocircuitries, nanosensors, and memories, but the wall currents are found to decay with time, especially at high temperatures. In this study, DW currents are induced upon partial domain switching against the peripheral unswitched bulk domain under the application of in‐plane electric field between two top electrodes patterned on epitaxial BiFeO3 (BFO) thin films. The “on” currents are followed by a drop at higher bias above the coercive voltage, which becomes more significant at elevated temperature from 298 to 408 K. An opposite “on” current is observed in the back sweeping when the above walls are erased to restore the as‐grown, stable domain structure. The phenomenon is explained by a film defect‐related model that free charges are injected and locally trapped at the defect levels to help screen the uncompensated polarizations in the conducting DWs, resulting in the wall current reduction. Once the walls are erased at opposite coercive voltages, the trapped free charges at the conducting DWs are expelled to form a discharge current. This finding provides the fundamental physics of the wall conduction in correlation with the film defects and reversible charge injection.

Strain induced enhancement of erasable domain wall current in epitaxial BiFeO3 thin films

The characteristic of electronic transport at the ferroelectric domain boundary is intensively studied for the potential application in random access memory due to its unique resistance switching mechanism along with polarization reversal. Such high conductivity in artificially created domain walls is not only affected by the material defect chemistry, such as oxygen vacancies, but also pertinent to the multiple polarization states of the sample. Here, we show the enhanced domain wall current in BiFeO3 thin films that could be obtained by the optimization of epitaxial strains from substrates. The leakage current analysis reveals the electronic transport of domain wall current in line with the space-charge-limited conduction mechanism. It is believed that the uncompensated polarization charge arouses the band bending at the domain boundary, which profoundly affects the wall current. Free carriers are easily concentrated in the domain boundary region for the compensation of the enhanced polarization by the strain, resulting in an abrupt increase of the conductivity.