BiFeO3 Films Research Articles

Designer defect stabilization of the super tetragonal phase in >70-nm-thick BiFeO3 films on LaAlO3 substrates

The stability of the “T-like” (T′) phase in BiFeO3 films grown on LaAlO3(001) is investigated. We show that the T′ phase can be stabilized for thicknesses >70 nm under ultralow incident flux conditions in pulsed laser ablation growth. This low flux results in a low growth rate; thus, the sample is held at high temperatures (>600 °C) for much longer than is typical. Transmission electron microscopy and X-ray diffraction analysis suggest that such growth conditions favor the formation of nanoscale “defect pockets”, which apply a local compressive strain of ∼1.8%. We propose that the cumulative effect of local stresses induced by such “designer defects” maintains macroscale strain coherence mechanical boundary conditions, which then preserves the T′ phase to thicknesses beyond conventional wisdom. Finally, by intentionally introducing an amorphous phase at the film-substrate interface, it is shown that the mixed-phase proportion can be tuned for a given thickness.

Hierarchical Domain Structure and Extremely Large Wall Current in Epitaxial BiFeO3 Thin Films

AbstractErasable electrical conductive domain walls in an insulating ferroelectric matrix provide novel functionalities for applications in logic and memory devices. The crux of such success requires sufficiently high wall currents to drive high‐speed and high‐power nanodevices. This work provides an appealing strategy to increase the current by two orders of magnitude through the careful selection of current flowing paths along the charged walls. The dense walls come into form through the hierarchical evolution of the 71°, 109°, and 180° domains of epitaxial BiFeO3 films in a planar‐geometry ferroelectric resistance‐switching memory cell. The engineered films grown on SrTiO3 and GdScO3 substrates allow the observation of detailed local configurations and the evolution of the different domain types using vector piezo‐force microscopy. The higher local electrical conductivity near the charged domain walls is identified by conductive atomic‐force microscopy. It is shown that 180° domain reversal proceeds by three‐step 71° rotations of the pristine domains. Surprisingly, a maximum current of ≈300 nA is observed for current paths along charge‐uncompensated head‐to‐head hierarchical domain walls connecting the two electrodes on the film surface. Furthermore, the achievable current level can be conveniently controlled by varying the relative directions of the initial polarization and the applied field.

Influence of nickel doping on structural, morphological and mechanical properties of BiFeO3thin films

This paper presents the effect of incorporation of Ni-substitution (0–20 at %) on structural, morphological and mechanical properties of BiFeO3 films. The films are found to be in polycrystalline perovskite phase with no additional NiO phase. The (012) and (110) x-ray diffraction peaks of the films are found to be gradually shifting towards the lower diffraction angle side due to the generation of higher biaxial strain with the increase in Ni substitution in BiFeO3 lattice. Average grain size of the films is increased from 55 nm (pristine) to 90 nm (20 at % Ni). Measuredelastic modulus (51–80 GPa) and hardness (7.5–14.5 GPa) of the BiFeO3 films are found to be increasing with the Ni incorporation. The improved mechanical properties of the films are seems to be due to the collective effects of the bi-axial tensile strain, nano-structured grains and substitution of Fe atoms with high elastic material like Ni.

Resistance switching behavior and ferroelectric properties of the Bi0.89Ho0.08Sr0.03Fe0.97−xMn0.03Zn x O3 thin films

Bi0.89Ho0.08Sr0.03Fe0.97−xMn0.03Zn x O3 (BHSFMZn x O) thin films were prepared by a chemical solution deposition method on the fluorine doped tin oxide (FTO) substrates. The effects of Sr, Ho, Mn and Zn co-doping on the crystal structure, defects, leakage current, resistance switching behavior and ferroelectric properties of the BiFeO3 films were investigated. The results show that Zn2+ doped BHSFMO films lead to the transformation of the preferred orientation from (110) to (100). The oxygen vacancies, $${(Zn{\prime _{Fe}} - V_{O}^{{ \cdot \cdot }})^ \cdot },$$ leakage current density and the Schottky barrier of BHSFMZn x O films were increased with the increase of Zn2+ doping. The BHSFMZn0.04O film shows the highest resistance switching ratio (18.6) at 200 kV/cm. The BHSFMZn0.01O film have larger remanent polarization and switching current (P r ~ 135 µC/cm2 and I S ~ 1.5 mA), and the relatively low coercive field and the polarization leakage current (E c ~ 350 kV/cm and I L ~ 0.14 mA). Therefore, the resistance switching behavior or ferroelectric properties can be obtained by controlling the doping amount of Zn2+.

Leakage mechanisms of partially self-polarized BiFeO3 film

Partially self-polarized BiFeO3 (BFO) film was fabricated on Pt(111)/Ti/SiO2/Si substrate by a metal organic decomposition process at 480 °C. The influence of soaking times on leakage properties was discussed. The soaking times considered were those that could keep the leakage currents being measured in a steady state condition during the leakage tests. The length of soaking time during leakage measurement should be greater than or equal to 500 ms to rule out the contribution of backswitching to leakage properties. Different leakage characteristics induced by the built-in negative field occurred under positive and negative fields. The "flat-band" feature of the leakage current curves may result from competition between domain switching and the switching of defect complexes. It is suggested that the "unswitched" mode should be adopted during leakage measurement in BFO films. Additionally, negative bias should be adopted in negatively polarized BFO films and positive bias should be adopted in positively polarized BFO films.

Perfection of leakage and ferroelectric properties of Ni‐doped BiFeO 3 thin films

BiFe1− x Ni x O3 (x = 0%, 1, 2 and 3%) films were deposited on ITO/glass substrate using sol–gel process. The work reports the impacts of Ni doped on the crystal microstructure, leakage current, conduction mechanism and ferroelectric behaviour systematically. From the XRD analysis, all samples match well with the perovskite structure without impurity phase. Polarisation-electric filed hysteresis loop demonstrates that the optimal Ni-doped content of BiFeO3 films is x = 2%, of which the remnant double polarisation (2P r) is 141.4 μC/cm2 at the test electric field of 1067 kV/cm. Leakage current density curves show that Ni doping has a great contribution in reducing leakage. The value of leakage is 4.79 × 10−7 A/cm2 at tested electric field of 300 kV/cm. In addition, the leakage conduction mechanism transforms from the Ohmic conduction under the low electric field into the space charge limited conduction under high electric field. Ni doped does not cause significant change in the conduction mechanism.

Pure phase BiFeO3 thin films sputtered over Si: A new route towards high magnetization

We have investigated the structural and magnetic properties of BiFeO3 (BFO) thin films grown over (100)-oriented Si substrates by rf magnetron sputtering in a new route under O2 free low pressure Ar atmosphere. Single-phase BFO films were deposited in a heated substrate and post-annealed in situ. The new routed allows high deposition rate and produce polycrystalline BFO pure phase, confirmed by high resolution X-ray diffraction. Scanning electron and atomic force microscopy reveal very low surface roughness and mean particle size of 33 nm. The BFO phase and composition were confirmed by transmission electron microscopy and line scanning energy-dispersive X-ray spectroscopy in transmission electron microscopy mode. The surface chemistry of the thin film, analyzed by X-ray photoelectron spectroscopy, reveals the presence of Fe3+ and Fe2+ in a 2:1 ratio, a strong indication that the film contains oxygen vacancies. An hysteretic ferromagnetic behavior with room temperature high saturation magnetization ∼165 × 103 A/m was measured along the film perpendicular and parallel directions. Such high magnetization, deriving from this new route, is explained in the scope of oxygen vacancies, the break of the antiferromagnetic cycloidal order and the increase of spin canting by change in the surface/volume ratio. Understanding the magnetic behavior of a multiferroic thin films is a key for the development of heterogeneous layered structures and multilayered devices and the production of multiferroic materials over Si substrates opens new possibilities in the development of materials that can be directly integrated into the existent semiconductor and spintronic technologies.

Optimizing the electromechanical response in morphotropic BiFeO3

sMorphotropic phase boundaries (MPBs) in ferroelectrics are highly desired phase transition regions, where crystal structure becomes extremely susceptible to external stimuli leading to very large electromechanical responses with applications in high-performance piezoelectric actuators and sensors. Since the recent discovery of a strain-driven MPB in chemically simple oxides, especially in thin films of BiFeO3 (BFO), much of the research emphasis has been on the physical structural MPBs which spatially separate intimately mixed phases in corrugated nanoscale regions. Very large electromechanical responses and other coupled novel functionalities, i.e. magnetism and electronic conductivity have been observed at such physical structural MPBs. In this report, we investigate and identify a region in the thickness-dependent phase diagram of the morphotropic BFO thin films at which a superior piezoelectric performance is achieved. In this region, an electrically recoverable strain approximately 40% higher than the previously reported values for the morphotropic BFO is observed. This enhanced electromechanical coupling has been attributed to the degeneracy of the mixed-phase states and concurrent increased mechanical softening.

Temperature controlled evolution of monoclinic to super-tetragonal phase of epitaxial BiFeO3 thin films on La0.67Sr0.33MnO3 buffered SrTiO3 substrate

Epitaxial BiFeO3 thin films of 130nm were deposited by pulsed laser deposition (PLD) technique on La0.67Sr0.33MnO3 buffered SrTiO3 (001) substrate at various temperatures under different ambient oxygen pressures. Reciprocal space mapping reveals that, with decreasing temperature and oxygen pressure, the broadly reported monoclinic phase (MA) of BiFeO3 thin film initially transforms to a tetragonal phase (T1) with c/a =1.05 (1) in a narrow girth of deposition condition and then to a super-tetragonal phase (T2) with giant c/a = 1.24 (1), as confirmed by reciprocal space mapping using high resolution x-ray diffraction. The surface morphology of the films reveals the island growth of the BiFeO3 films deposited at low temperatures. We propose that the transformation from monoclinic to the super-tetragonal phase is essentially due to the manifestation of excess local strain as a result of the island growth. This study offers a recipe to grow the super-tetragonal phase of BiFeO3, with giant c/a =1.24 (1) which exhibits exceptionally large ferroelectric polarization, on ferromagnetic layer La0.67Sr0.33MnO3. This phase of BiFeO3 can be utilized for the ferroelectric control of magnetism at the interface of BiFeO3 and La0.67Sr0.33MnO3.

Microstructure, enhanced piezoelectric, optical and magnetic properties of Mn substituted BiFeO3 film synthesized by chemical method

Very thin and high transparent manganese (Mn) substituted bismuth ferrite (BiFe1−xMn x O3—BFMO) (x = 0, 0.02, 0.04, 0.06, 0.08, 0.10) films deposited on the SnO2:F (FTO)/glass substrate were synthesized by chemical solution deposition technique. The effect of Mn substitution on the microstructure, piezoelectric, optical and magnetic properties was systematically investigated. X-ray diffraction and Raman spectra analysis revealed that the structure transition and lattice distortion of BiFeO3 (BFO) thin film occurred with Mn substitution. SEM images demonstrated that the average grain size of Mn substituted BFO thin film increased with increase of Mn substitution and EDS results confirmed the element composition with presence of Mn in the synthesized thin film. The piezoresponse of BFMO thin film increased with increase of Mn substitution and when Mn substitution x = 0.10, the maximum d33,eff reached to 41.81 pm/V, which is almost 125% higher than the pure BFO thin film. The transmittance and optical band gap decreased with increase of Mn substitution. The BFO thin film with Mn substitution x = 0.10 also showed a lower band gap of 2.64 eV. Furthermore, the Mn substituted BFO thin film show a much higher saturated magnetization compared to pure BFO thin film due to enhanced magnetically driven distortion of spiral spin cycloid induced by Mn substitution.

Structural, magnetic, and ferroelectric properties of T-like cobalt-doped BiFeO3 thin films

We present a comprehensive study of the physical properties of epitaxial cobalt-doped BiFeO3 films ∼50 nm thick grown on (001) LaAlO3 substrates. X-ray diffraction and magnetic characterization demonstrate high quality purely tetragonal-like (T′) phase films with no parasitic impurities. Remarkably, the step-and-terrace film surface morphology can be fully recovered following a local electric-field-induced rhombohedral-like to T′ phase transformation. Local switching spectroscopy experiments confirm the ferroelectric switching to follow previously reported transition pathways. Critically, we show unequivocal evidence for conduction at domain walls between polarization variants in T′-like BFO, making this material system an attractive candidate for domain wall-based nanoelectronics.

Stripe domains in epitaxial BiFeO3 thin films on (100) SrTiO3 substrates

Highly crystallized ferroelectric BiFeO3 thin films were deposited on various miscut (100) SrTiO3 substrates. The domain structure of the films varies with substrate miscut angles and atomic-layer termination at the surface. The BiFeO3 thin films grown on 4° miscut substrates exhibit 71° periodic stripe domains consisting of two downward polarization variants. In contrast, four downward variants coexist in the films deposited on 0.2° miscut substrates, regardless of atomic-layer termination at the surface of the substrate. The introduction of an additional SrTiO3 repair layer on the same substrate results in a step-flow growth mode of the film. It is believed that the improved mobility of the BiFeO3 atomic species during the film growth leads to the appearance of preferred ferroelectric variants at the step edges. These preferred variants eventually form stripe domains. Our results reveal that both the miscut angle and the step-flow growth promote the formation of the two variant striped domains in BiFeO3 films deposited on decorated isotropic SrTiO3 substrates.

Tensile stress effect on epitaxial BiFeO3 thin film grown on KTaO3

Comprehensive crystal structural study is performed for BiFeO3 (BFO) film grown on KTaO3 (KTO) substrate using transmission electron microscopy (TEM) and x-ray diffraction (XRD). Nano-beam electron diffraction (NBED) combined with structure factor calculation and high resolution TEM images clearly reveal that the crystal structure within BFO thin film is rhombohedral BFO, i.e., bulk BFO phase. Epitaxial relationship found by NBED indicates the BFO film grows in a manner that minimizes lattice mismatch with KTO. It further suggests BFO film is under slight biaxial tensile stress (~0.35%) along in-plane direction. XRD reveals BFO lattice is under compressive stress (~1.6%), along out-of-plane direction as a result of the biaxial tensile strain applied along in-plane direction. This leads to Poisson’s ratio of ~0.68. In addition, we demonstrate (1) why hexagonal notation rather than pseudocubic one is required for accurate BFO phase evaluation and (2) a new XRD method that shows how rhombohedral BFO can readily be identified among other phases by measuring a rhombohedral specific Bragg’s reflection.

Dipole pinning effect on photovoltaic characteristics of ferroelectric BiFeO3 films

Ferroelectric bismuth ferrite is an attractive candidate for switchable devices. The effect of dipole pinning due to the oxygen vacancy layer on the switching behavior of the BiFeO3 thin film fabricated by the chemical solution deposition method was studied after annealing under air, O2, and N2 environment. The air annealed film showed well defined and dense grains leading to a lower leakage current and superior electrical properties compared to the other two films. The photovoltage and transient photocurrent measured under positive and negative poling elucidated the switching nature of the films. Though the air and O2 annealed films showed a switchable photovoltaic response, the response was severely affected by oxygen vacancies in the N2 annealed film. In addition, the open circuit voltage was found to be mostly dependent on the polarization of BiFeO3 rather than the Schottky barriers at the interface. This work provides an important insight into the effect of dipole pinning caused by oxygen vacancies on the switchable photovoltaic effect of BiFeO3 thin films along with the importance of stoichiometric, defect free, and phase pure samples to facilitate meaningful practical applications.

Ferroelectric Domain Studies of Patterned (001) BiFeO3 by Angle-Resolved Piezoresponse Force Microscopy

We have studied the ferroelectric domains in (001) BiFeO3 (BFO) films patterned into mesas with various aspect ratios, using angle-resolved piezoresponse force microscope (AR-PFM), which can image the in-plane polarization component with an angular resolution of 30°. We observed not only stable polarization variants, but also meta-stable polarization variants, which can reduce the charge accumulated at domain boundaries. We considered the number of neighboring domains that are in contact, in order to analyze the complexity of the ferroelectric domain structure. Comparison of the ferroelectric domains from the patterned and unpatterned regions showed that the elastic relaxation induced by removal of the film surrounding the mesas led to a reduction of the average number of neighboring domains, indicative of a decrease in domain complexity. We also found that the rectangular BFO patterns with high aspect ratio had a simpler domain configuration and enhanced piezoelectric characteristics than square-shaped mesas. Manipulation of the ferroelectric domains by controlling the aspect ratio of the patterned BFO thin film mesas can be useful for nanoelectronic applications.

Direct and converse piezoelectric responses at the nanoscale from epitaxial BiFeO3 thin films grown by polymer assisted deposition.

We use an original water-based chemical method to grow pure epitaxial BiFeO3 (BFO) ultra-thin films with excellent piezoelectric properties. Particularly, we show that this novel chemical route produces higher natural ferroelectric domain size distribution and coercive field compared to similar BFO films grown by physical methods. Moreover, we measured the d33 piezoelectric coefficient of 60 nm thick BFO films by a direct approach, using Direct Piezoelectric Force Microscopy (DPFM). As a result, first piezo-generated charge maps of a very thin BFO layer were obtained applying this novel technology. We also performed a comparative study of the d33 coefficients between standard PFM analysis and DPFM microscopy showing similar values i.e. 17 pm V-1 and 22 pC N-1, respectively. Finally, we proved that the directionality of the piezoelectric effect in BFO ferroelectric thin films is preserved at low thickness dimensions demonstrating the potential of chemical processes for the development of low cost functional ferroelectric and piezoelectric devices.

Electrical properties of calcium doped BiFeO3 films on LaNiO3 coated Pt substrates

Pure and calcium-modified (CaxBi1-xFeO3, x = 0.0, 0.1, 0.2, 0.30) thin films were fabricated on Pt(111)/Ti/SiO2/Si substrates by the soft chemical method using LaNiO3 as the bottom electrode. Highly (200)- oriented BFO film was coherently grown on LNO at 500?C. Ca-doped BiFeO3 films have a dense microstructure and rounded grains. The conventional problem of the leakage current for the highest doped film was reduced from 10-5 to 10-10 with remarkable improvement in the film/electrode interface, chemical homogeneity, crystallinity, and morphology of the BFO film. Enhanced ferroelectricity was observed at room temperature due to the bottom electrode. Fatigue-free films were grown on LaNiO3 bottom electrodes with no degradation after 1?1010 switching cycles at an applied voltage of 5 V with a frequency of 1MHz. After several tests the capacitors retained 77% of its polarization upon a retention time of 104 s. Room temperature magnetic coercive field measurements indicate that the magnetic behaviour is influenced by the nature of the bottom electrode.

Coexistence of rhombohedral and orthorhombic phases in ultrathin BiFeO3 films driven by interfacial oxygen octahedral coupling

Coexistence of two phases creates a morphotropic phase boundary in perovskite oxides, which can provide large piezoelectric response, generating it a well suited system for probe-based memories and actuator applications. The coexistence of two phases in thin films is proposed to be induced by epitaxial constraints from substrates or chemical compositional modifications by substitution. In this work, we found a new formation mechanism of two-phase coexistence driven by interfacial oxygen octahedral coupling (OOC) in oxide heterostructures. We fabricated a series of BiFeO3 (BFO) ultrathin films on various orthorhombic substrates exerting from tensile to compressive strains by Pulsed Laser Deposition (PLD) techniques. Aberration-corrected transmission electron microscopy demonstrates that the lattice rotation and oxygen octahedral rotation (OOR) patterns transfer from these substrates to BFO films in about 3 unit cells while an orthorhombic (Pnma) phase forms at the interface due to OOC. This Pnma phase is non-polar, which differs from polar phases of Ima2 or Pmc21 when a large tensile strain is imposed onto BFO. First-principles calculations reproduce these experimental results perfectly. This phase transition occurs when BFO films are under both tensile and compressive strains suggesting that OOC alone can induce phase transition in ultrathin BFO films. Such coexistence of two phases may have many potential applications in the field of electronics, such as ferroelectric sensors and actuators.

Mechanisms of resistive switching in BiFeO3 thin films modulated by bottom electrode

BiFeO3 thin films were fabricated by pulsed laser deposition on Nb–SrTiO3 (0 0 1), La2/3Sr1/3MnO3/SrTiO3 (0 0 1), and SrRuO3/SrTiO3 (0 0 1) electrode surfaces. All films grew epitaxially along the substrates and were fully strained. The current–voltage curves of the structures revealed that the current level depended on the barrier height at the ferroelectric/bottom electrode interface. Resistive switching behavior was observed in the BiFeO3 films grown on all electrodes, which was rationalized in terms of conductive filaments and the variation of the Schottky barrier at the ferroelectric/electrode interfaces due to ferroelectric polarization. The conductive filament model and modulation of the Schottky barriers at film/electrode interfaces due to ferroelectric polarization applied to BiFeO3 films on SrRuO3 and La2/3Sr1/3MnO3 electrodes, respectively, while a combination of these two mechanisms was suggested to be responsible for the observed resistive switching of BiFeO3 film on Nb–SrTiO3.

Multiferroic properties of a YCrO3/BiFeO3 bilayered thin film prepared by a sol-gel method

YCrO3 (YCO), BiFeO3 (BFO), and YCrO3/BiFeO3 (YCO-BFO) thin films were prepared on quartz substrates using spin coating by a sol-gel method. X-ray diffraction demonstrated that YCO and BFO thin films had a perovskite orthorhombic, and rhombohedral structure, respectively. The stronger and sharper diffraction intensity of YCO-BFO bilayered thin film indicated that the bottom YCO layer was able to promote the grain growth of BFO film, which was further verified by scanning electron microscope. The ferroelectric test demonstrated that the leakage current density of YCO-BFO bilayered film was reduced by one order of magnitude compared to that of BFO film, which had a better ferroelectric property. Optical absorption spectra indicated that the band gap of YCO-BFO thin film was lower than that of BFO film, suggesting their potential application as UV and blue-green-driven photocatalysts. The magnetic test verified that the ferromagnetic property of YCO-BFO film was obviously enhanced compared to those of BFO and YCO thin films. The results revealed that YCO layer played an important role for improving multiferroic properties of BFO thin film. The mechanisms of the effects of bottom YCO layer on optical and multiferroic properties of BFO thin film in the YCO-BFO bilayered film were discussed in detail.