Oriented BiFeO3 Thin Films Research Articles

Large-Scale Epitaxial Growth of Ultralong Stripe BiFeO3 Films and Anisotropic Optical Properties.

The controlled synthesis of large-scale ferroelectric domains with high uniformity is crucial for practical applications in next-generation nanoelectronics on the basis of their intriguing properties. Here, ultralong and highly uniform stripe domains in (110)-oriented BiFeO3 thin films are large-area synthesized through a pulsed laser deposition technique. Utilizing scanning transmission electron microscopy and piezoresponse force microscopy, we verified that the ferroelectric domains have one-dimensional 109° domains and the length of a domain is up to centimeter scale. More importantly, the ferroelectric displacement is directly determined on atomic-scale precision, further confirming the domain structure. We find that the unique one-dimensional ferroelectric domain significantly enhances the optical anisotropy. Furthermore, we demonstrate that the purely parallel domain patterns can be used to control photovoltaic current. These ultralong ferroelectric domains can be patterned into various functional devices, which may inspire research efforts to explore their properties and various applications.

Atomic-Scale Tunable Flexoelectric Couplings in Oxide Multiferroics.

Flexoelectricity is an effective tool in modulating the crystallographic structures and properties of oxides for multifunctional applications. However, engineering the nonuniform strain to obtain tunable flexoelectric behaviors at the atomic scale remains an ongoing challenge in conventional substrate-imposed ferroelectric films. Here, the regulatable flexoelectric behaviors are demonstrated at atomic scale in [110]-oriented BiFeO3 thin films, which are triggered by the strain-field coupling of high-density interfacial dislocations. Using aberration-corrected scanning transmission electron microscopy, the asymmetric polarization rotation around the single dislocation is revealed, which is induced by the gradient strain fields of the single dislocation. These strain fields are highly correlated to generate huge strain gradients between neighboring dislocations, and thereby, serial flexoelectric responses are engineered as a function of dislocation spacings in thicker BiFeO3 films. This work opens a pathway for the modulation of flexoelectric responses in ferroelectrics, which could be extended to other functional materials to create exotic phenomena.

Antiphase-Boundary-Engineered Domain Switching in a (110)-Oriented BiFeO3 Film

A ferroelectric domain wall memory device exploits the presence (or absence) of domain walls as the basis for information storage. Although structural defects strongly influence ferroelectric domain wall nucleation and growth, this correlation is far from being fully understood. For example, a single defect can play opposing roles: it can act as a nucleation site to initiate domain switching or, conversely, serve as a pinning center inhibiting domain wall motion. This is particularly the case for antiphase boundaries (APBs), whose influence on switching remains relatively unexplored despite being ubiquitous in oxide perovskite ferroelectrics. Here, we use aberration-corrected scanning transmission microscopy and in situ transmission electron microscopy under an applied bias to extensively characterize the influence of an APB on the switching behavior in a (110)-oriented BiFeO3 thin film. We demonstrate that deterministic nucleation occurs in the proximity of a vertically oriented APB, following which domain growth occurs laterally in a symmetric fashion. Atomic-scale structural analysis reveals that the shear operation vector for the APB is 1/2[110]; this half-unit-cell shift extends over a few nanometers, implying that the APB plane is inclined toward the imaging direction. The symmetric switching observation is attributed to local inhomogeneous lattice strain and a reduction of polarization caused by the APB. These results demonstrate that an extended structural defect can be used as an active switching element rather than a stumbling block in reprogrammable domain-wall-based devices. Ultimately, this will expand the capabilities of ferroelectric thin films for future electronic devices based on ferroelectric domain walls.

Influence of chemical solution growth and vacuum annealing on the properties of (100) pseudocubic oriented BiFeO3 thin films

BiFeO3 (BFO), a Pb-free perovskite oxide, is being explored for its potential use in a multitude of applications. We report on the oriented growth of BFO thin films using a facile metal-organic chemical solution deposition. Unlike the growth characteristics observed in Si/SiO2 and glass/FTO substrates, the solution growth process on sapphire (0001) is found to yield highly oriented thin films along (100)pc planes. Furthermore, annealing in air (BFO-A) and high-vacuum (BFO-V) ambients are done to explore the tunable limits of its physical properties. Temperature-dependent Raman studies highlight the high quality of thin films with sharp changes in Raman modes around transition temperatures. In addition, the films exhibit a hitherto unreported anomalous shift in A1(TO) and E(TO) modes around 450 K. The bandgap of BFO-V (Eg = 2 eV) is lower than that of BFO-A (Eg = 2.12 eV) and exhibits an increased defect photoluminescence emission. The magnetization (M) is twofold higher for BFO-V [M ≈ 42 (67) emu/cm3 at 300 K (5 K)]. In-plane and out-of-plane M vs H plots show larger anisotropy and hard hysteresis for BFO-A compared to BFO-V. Piezoelectric switching with d33 values of 5–10 pm/V is the characteristic of BFO ferroelectric materials. Photoconductivity measurements show a one order increase due to vacuum annealing. Carrier generation and recombination lifetimes are twofold faster in BFO-V as compared to BFO-A thin films. The controllable physical properties of oriented BiFeO3 thin films will be useful in magnetoelectrics and photoferroelectrics applications.

Dielectric relaxation and electrical conductivity of random oriented BiFeO3 thin films

Complex impedance and electric modulus spectroscopies were used to investigate the dielectric relaxation and conductivity of random oriented BiFeO3 thin films. Thermally activated charge transport models yielded activation energies of eV, which is consistent with an electrical conduction dominated by oxygen vacancies. The non-Debye behavior of impedance and electric modulus relaxations were modeled by Cole-Cole functions. Results suggest a coexistence of components from both long-range and localized relaxation in the studied BiFeO3 films dominated by grain boundaries. The dielectric relaxation induced by electric field does not follow the Arrhenius formalism.

Bipolar Resistive Switching Effect in BiFeO3/Nb:SrTiO3 Heterostructure by RF Sputtering at Room Temperature

The (001) oriented BiFeO3 thin film was deposited on the Nb: SrTiO3 substrate by radio frequency magnetron sputtering technology, and the bipolar resistive switching effect was observed in the BiFeO3/Nb: SrTiO3 heterostructure. The results showed that the ratio between the high resistance and low resistance was more than two orders at a reading pulse of -0.5 V and it exhibited excellent retention over 3600 s. The current density-voltage characteristic was dominated by the space-charge-limited conduction. The resistive switching effect of the structure was attributed to the trapping/detrapping of the charge carriers.

Sol–gel-processed (001)-textured BiFeO3 thin films on Pt(111)/Ti/SiO2/Si substrates with PbO seeding nanocrystals

Highly (001)-oriented BiFeO3 thin films were grown on Pt(111)/Ti/SiO2/Si substrates with PbO seeding nanocrystals by a layer-by-layer annealing method.

180° Ferroelectric Stripe Nanodomains in BiFeO3 Thin Films.

There is growing evidence that domain walls in ferroics can possess emergent properties that are absent in the bulk. For example, 180° ferroelectric domain walls in the ferroelectric-antiferromagnetic BiFeO3 are particularly interesting because they have been predicted to possess a range of intriguing behaviors, including electronic conduction and enhanced magnetization. To date, however, ordered arrays of such domain structures have not been reported. Here, we report the observation of 180° stripe nanodomains in (110)-oriented BiFeO3 thin films grown on orthorhombic GdScO3 (010)O substrates and their impact on exchange coupling to metallic ferromagnets. Nanoscale ferroelectric 180° stripe domains with {112̅} domain walls were observed in films <32 nm thick. With increasing film thickness, we observed a domain structure crossover from the depolarization field-driven 180° stripe nanodomains to 71° ferroelastic domains determined by the elastic energy. These 180° domain walls (which are typically cylindrical or meandering in nature due to a lack of strong anisotropy associated with the energy of such walls) are found to be highly ordered. Additional studies of Co0.9Fe0.1/BiFeO3 heterostructures reveal exchange bias and exchange enhancement in heterostructures based on BiFeO3 with 180° domain walls and an absence of exchange bias in heterostructures based on BiFeO3 with 71° domain walls; suggesting that the 180° domain walls could be the possible source for pinned uncompensated spins that give rise to exchange bias. This is further confirmed by X-ray circular magnetic dichroism studies, which demonstrate that films with predominantly 180° domain walls have larger magnetization than those with primarily 71° domain walls. Our results could be useful to extract the structure of domain walls and to explore domain wall functionalities in BiFeO3.

Surface double-layer structure in (110) oriented BiFeO3 thin film

Surface double-layer structure different from the interior was found in BiFeO3 thin film grown on SrRuO3 covered SrTiO3 (110) substrate by pulsed laser deposition. It was shown that BiFeO3 film exhibits epitaxial phase with single domain. X-ray reflectivity and X-ray photoelectron spectroscopy results revealed a skin layer of less than 1 nm with a reduced electron density and different surface state. Grazing incidence x-ray diffraction convinced a surface multi-domain structure of several nm beneath the surface skin layer. The double-layer near surface structure would be originated from the large depolarization field produced by the single-domain structure with strain.

Effect of Poling on Photovoltaic Properties in Highly Oriented BiFeO3 Thin Films

We report enhancement of photovoltaic properties of highly oriented BiFeO3 (BFO) thin films. The single phase rhombohedral R3c space group structure was confirmed by XRD and Raman spectra in the films grown on SrTiO3 (111) substrates using RF magnetron sputtering. The films of BFO were characterized in planar geometry with top Pt electrodes having 200μm diameter, ∼40nm thickness and at a regular spacing of ∼150μm deposited by DC sputtering process. For photovoltaic studies the current voltage characteristic of as grown and poled samples were studied under the white light illumination having energy density of ∼1kW/m2. The as grown films showed open circuit voltage (Voc) of ∼3.9V which increased up to ∼17.8V on poling with 200V. This enhancement in Voc by poling was attributed to increase in net polarization due to alignment of domains in the direction of applied electric field and hence potential barriers between the domains.

Fabrication and polarization-modulated resistive switching behavior of predominantly (110)-oriented BiFeO3 thin films on indium tin oxide/glass substrates

Predominantly (110)-oriented BiFeO3 (BFO) thin films were deposited on indium tin oxide/glass substrates using a metal organic decomposition process by controlling the preheating temperature. The structure evolution with film thickness at different preheating temperatures was investigated to clarify the growth mode of (110)-predominant BFO film. The formation of the (110)-oriented BFO film is due to the low-temperature nucleation of (110)-oriented grains preheated at 425 °C. In the Au/BFO(110)/ITO heterostructure, a polarization-modulated bistable resistive switching behavior with high ratio of resistance and large diode current characteristics was observed, which makes the heterostructure attractive for application in resistive ferroelectric memory.

Epitaxial growth of multiferroic BiFeO3 thin films with (101) and (111) orientations on (100) Si substrates

Multiferroic (101)- and (111)-oriented BiFeO3 thin films were grown on heteroepitaxial substrates of (110) SrRuO3/(100) YSZ/(100) Si and (111) SrRuO3/(111) Pt/(100) YSZ/(100) Si, respectively, by pulsed laser deposition. X-ray diffraction θ–2θ scans, ϕ scans, and pole figures showed the well-developed out-of-plane and in-plane orientations of the epitaxial BiFeO3 films. The (111) BiFeO3 films showed ∼3/2 times higher remanent polarization (58.9 μC/cm2) than that of the (101) BiFeO3 films (44.3 μC/cm2), thus revealing the ferroelectric anisotropy. The (101) BiFeO3 films exhibited a good fatigue resistance up to 4.6 × 107 cycles, whereas the (111) BiFeO3 films proved to be vulnerable to fatigue, which is associated with the polarization switching path.

Highly (111)-oriented multiferroic BiFeO3 thin film on a glass substrate

BiFeO3 (BFO) thin films were deposited on Pt/Ta/glass substrates by pulsed laser deposition. To grow highly (111)-oriented and good adhesive Pt bottom electrodes, Ta adhesion layers were used between Pt bottom electrodes and glass substrates. On the highly (111)-oriented Pt bottom electrodes, highly (111)-oriented polycrystalline BFO thin films were obtained as confirmed by X-ray diffraction experiments. The BFO thin film exhibited a high ferroelectric polarization (2Pr≈56μC/cm2). The highly (111)-oriented BFO thin film also showed a ferromagnetic hysteresis loop and a ferroelectric domain size of approximately 30–40nm, which is equivalent to 0.25Tbit/in2.

Ultrafast Dynamics of BiFeO3 Thin Films Studied by Dual-Color Femtosecond Spectroscopy

The ultrafast dynamics in (100)-oriented BiFeO3 thin films were studied from 20 K to 300 K by using the dual-color femtosecond pump-probe spectroscopy, which were performed by the 400 nm (3.1 eV) pump pulses above the energy gap of 2.67 eV and the 800 nm (1.55 eV) probe pulses below the energy gap of 2.67 eV. From the temperature-dependent transient reflectivity changes (ΔR/R), the anomalous changes were clearly observed below 200 K caused by magnons. Thus, the femtosecond pump-probe spectroscopy could unambiguously reveal the magnetization dynamics in BiFeO3 with strong magnetoelectric coupling.

Switching kinetics in epitaxial BiFeO3 thin films

The switching kinetics in epitaxial (001)-, (110)-, and (111)-oriented BiFeO3 thin films were investigated as a function of applied field and time. It was found that the ferroelectric switching behavior obeys the Kolmogorov–Avrami–Ishibashi theory only in the high field range. The detailed behavior depends on the film orientation. A comparison with standard systems, such as epitaxial Pb(Zr0.2Ti0.8)O3 films, reveals some similarities as well as some differences. For instance, the presence of 109° and 71° ferroelastic domain walls might be ruled out as the source of the decrease in switched polarization at low applied fields, in contrast to what is the case for a/c domain walls in tetragonal Pb(Zr0.2Ti0.8)O3.

Improved ferroelectric behavior in (110) oriented BiFeO3 thin films

Multiferroic BiFeO3 thin films were grown on LaNiO3-buffered Pt/TiO2/SiO2/Si substrates by off-axis radio frequency magnetic sputtering, where (110) oriented texture was obtained. The LaNiO3 buffer layer promoted the growth of BiFeO3 film, resulting in a dense texture and columnar growth of BiFeO3 thin films. The BiFeO3 thin film exhibits 2Pr∼145.8 μC/cm2 and 2Ec∼573.5 kV/cm, together with a very low dielectric loss (0.80% at 10 kHz) and an almost fatigue-free behavior at 500 kHz up to 1010 switching cycles. The 2Pr value is comparable to the reported value of the BiFeO3 deposited on the SrTiO3(110) substrate. While a weak ferromagnetism is demonstrated for the multiferroic thin film at room temperature, the low leakage current and high (110) orientation contribute toward the improved ferroelectric behavior. The BFO thin film with a giant remanent polarization and a fatigue-free behavior promises a candidate material for high-temperature ferroelectric random access memory.

Orientation dependence of ferroelectric behavior of BiFeO3 thin films

Multiferroic BiFeO3 (BFO) thin films with (111), (100), (110) preferred, and random orientations were deposited by radio frequency magnetron sputtering on SrRuO3-buffered SrTiO3(111), SrTiO3(100), SrTiO3(110), and Pt(111)/Ti/SiO2/Si(100) substrates, respectively. The orientation dependences of ferroelectric and fatigue behavior of the BFO thin films were investigated. As expected, the (111)-oriented BFO thin film exhibits the highest, giant ferroelectric polarization (2Pr=196.9 μC/cm2) at 1 kHz and room temperature using positive up negative down measurement, while the (100)-oriented BFO thin film possesses an almost fatigue-free behavior up to 5.25×107 switching cycles when measured at 100 kHz and room temperature. The observed behavior confirms that the largest spontaneous polarization direction and the fatigue endurance are (111) and (100) for BFO thin films, respectively. Regardless of the film orientation, the charge carriers that are responsible for dielectric relaxation and conductivity are oxygen vacancies (VO••) .

Adsorption-Controlled Growth of BiFeO3 by MBE and Integration with Wide Band Gap Semiconductors

BiFeO3 thin films have been deposited on (001) SrTiO3, (101) DyScO3, (011) DyScO3, (0001) AlGaN/GaN, and (0001) 6H-SiC single crystal substrates by reactive molecular beam epitaxy in an adsorption-controlled growth regime. This is achieved by supplying a bismuth over-pressure and utilizing the differential vapor pressures between bismuth oxides and BiFeO3 to control stoichiometry in accordance with thermodynamic calculations. Four-circle x-ray diffraction and transmission electron microscopy reveal phase-pure, epitaxial films with rocking curve full width at half maximum values as narrow as 7.2 arc seconds (0.002 degrees). Epitaxial growth of (0001)-oriented BiFeO3 thin films on (0001) GaN, including AlGaN HEMT structures, and (0001) SiC has been realized using intervening epitaxial (111) SrTiO3 / (100) TiO2 buffer layers. The epitaxial BiFeO3 thin films have 2 in-plane orientations: [1120] BiFeO3 || [1120] GaN (SiC) plus a twin variant related by a 180 degrees in-plane rotation. This epitaxial integration of the ferroelectric with the highest known polarization, BiFeO3, with high bandgap semiconductors is an important step toward novel field-effect devices.

Computer simulation of ferroelectric domain structures in epitaxial BiFeO3 thin films

Ferroelectric domain structures of (001)c, (101)c, and (111)c oriented epitaxial BiFeO3 thin films were studied by using the phase-field approach. Long-range elastic and electrostatic interactions were taken into account. The effects of various types of substrate constraint on the domain morphologies were systematically analyzed. It is demonstrated that domain structures of BiFeO3 thin films could be controlled by selecting proper film orientations and substrate constraint. The dependence of the {110}c-type domain wall orientation on substrate constraint for the (001)c oriented BiFeO3 thin film was also discussed.

Epitaxial integration of (0001) BiFeO3 with (0001) GaN

Epitaxial growth of (0001)-oriented BiFeO3 thin films on the (0001) surface of GaN has been realized using intervening epitaxial (111) SrTiO3∕(100) TiO2 buffer layers. The epitaxial BiFeO3 thin films have two in-plane orientations: [112¯0]BiFeO3‖[112¯0]GaN plus a twin variant related by a 180° in-plane rotation. BiFeO3 shows an out-of-plane remanent polarization of ∼90μC∕cm2, which is comparable to the remanent polarization of BiFeO3 prepared on (111) SrTiO3 single crystal substrates. The orientation of BiFeO3 realized on GaN provides the maximal out-of-plane polarization of BiFeO3, which is equivalent to a surface charge of 5×1014electrons∕cm2.