BaTiO3 Films Research Articles

Characterization of BaTiO3/LaSrMnO3 heterostructures on sapphire for ferroelectric tunnel junctions

The BaTiO3 (BTO)/La0.7Sr0.3MnO3 (LSMO) heterostructure has been fabricated by radio-frequency deposition on a sapphire single crystal substrate. Structure quality of LSMO layer was optimized by the proper choice of a substrate temperature during the deposition. It was found that the highly oriented LSMO bottom layer shows a marked effect on the structure properties of the BTO film in comparison with BTO on sapphire grown by similar process. 10-nm-thick BTO film on LSMO/sapphire demonstrated ferroelectric behavior according to C–V characteristics.

Modelling domain switching of ferroelectric BaTiO3 integrated in silicon photonic waveguides

We present a model to investigate the local change of the refractive index of a ferroelectric material employed as upper cladding of silicon photonic waveguides. The model is used to predict the saturation voltage required to achieve complete domain switching in case of a BaTiO3 film and to evaluate the performance of non-volatile phase-actuators integrated in silicon waveguides. Results show that the refractive index change associated with the domain switching of the BaTiO3 cladding enables the realization of compact phase actuators with a length of few tens of micron. The proposed model has a general validity and can be used to other ferroelectric materials as well as to other semiconductor or dielectric waveguides.

Influence of clamping effect in BaTiO3 film on the magnetoelectric behavior of layered multiferroic heterostructures

The experimental observation and analysis for the influence of clamping effect on the magnetoelectric properties of the bilayer La0.7Sr0.3MnO3/BaTiO3 (LSMO/BTO) and trilayer LSMO/BTO/LSMO heterostructures were reported. Because the significant two-dimensional clamping effect caused by the lattice mismatch of the constituting layers, the relative dielectric constant and the ferroelectric coercive field of the trilayer heterostructure was lower than that of the bilayer one. Furthermore, the variation in ME voltage coefficient (αE) as a function of frequency for the two heterostructures was found to exhibit a monotonous increasing tendency. The value of αE for the trilayer heterostructure was little lower than that of the bilayer one, that is because the clamping effect suppressed both the ferroelectric properties of BTO and the transferred strain at the hetero interface.

First Observation of Ferroelectricity in ∼1 nm Ultrathin Semiconducting BaTiO3 Films.

The requirements of multifunctionality in thin-film systems have led to the discovery of unique physical properties and degrees of freedom, which exist only in film forms. With progress in growth techniques, one can decrease the film thickness to the scale of a few nanometers (∼nm), where its unique physical properties are still pronounced. Among advanced ultrathin film systems, ferroelectrics have generated tremendous interest. As a prototype ferroelectric, the electrical properties of BaTiO3 (BTO) films have been extensively studied, and it has been theoretically predicted that ferroelectricity sustains down to ∼nm thick films. However, efforts toward determining the minimum thickness for ferroelectric films have been hindered by practical issues surrounding large leakage currents. In this study, we used ∼nm thick BTO films, exhibiting semiconducting characteristics, grown on a LaAlO3/SrTiO3 (LAO/STO) heterostructure. In particular, we utilized two-dimensional electron gas at the LAO/STO heterointerface as the bottom electrode in these capacitor junctions. We demonstrate that the BTO film exhibits ferroelectricity at room temperature, even when it is only ∼2 unit-cells thick, and the total thickness of the capacitor junction can be reduced to less than ∼4 nm. Observation of ferroelectricity in ultrathin semiconducting films and the resulting shrunken capacitor thickness will expand the applicability of ferroelectrics in the next generation of functional devices.

Ferroelectric polarization-controlled resistive switching in BaTiO3/SmNiO3 epitaxial heterostructures

As an emerging phenomenon in oxide heterostructures, the ferroelectric control of electronic transport is attracting considerable research attention. However, the effect of ferroelectric polarization on resistive switching (RS) remains controversial. In this study, to determine the effects of ferroelectric polarization on memory characteristics, ferroelectric and non-ferroelectric BaTiO3 (BTO) films were fabricated using different oxygen partial pressures (Po2) during film growth. A correlated electron oxide, SmNiO3 (SNO), was selected as the bottom electrode. Based on piezoelectric force microscopy, ferroelectricity was verified in the BTO films prepared at Po2 ≥ 3 Pa, whereas the films fabricated at lower Po2 did not exhibit ferroelectricity. Remarkable RS with an ON/OFF ratio of 104% was clearly observed in heterostructures containing ferroelectric BTO, while RS was negligible in structures with non-ferroelectric BTO. The strong ferroelectricity dependence of RS behavior on the BTO/SNO heterostructures was attributed to the ferroelectric control of device transport between bulk-limited current in the low-resistance state and interface-limited conductance in the high-resistance state, which results from the modulation of the potential barrier at the BTO/SNO interface. The findings provide strong evidence for the dominant effect of ferroelectric polarization of BTO on the transport properties of BTO/SNO heterostructures.

Spin coating of epitaxial BaTiO3/CoFe2O4 thin films on single crystalline (001)-SrTiO3

Abstract Composite thin films of BaTiO3 (BTO) and CoFe2O4 (CFO) on (001)-oriented SrTiO3 (STO) substrates were prepared by the spin coating technique using solutions of the corresponding metal cations in N,N-dimethylformamide/acetic acid. Ten and five individual coating steps for the bottom perovskite layer and the top ferrite layer resulted in thicknesses of 250 nm and 200 nm, respectively. X-ray diffraction (XRD) and Raman spectroscopy proved the formation of BaTiO3 and CoFe2O4 without any detectable impurities. Rocking curve scans, reciprocal space mapping (RSM) and pole figures clearly demonstrated the epitaxial growth of both the perovskite and the ferrite layer, i.e. STO(001)[100]||BTO(001)[100]||CFO(001)[100]. Furthermore, scanning electron microscopy (SEM) investigations revealed smooth interfaces and dense films. From atomic force microscopy (AFM) surface scans, a roughness of about 8 nm was deduced. A saturation magnetization of about 350 emu·cm−3 was found and the samples showed only a small magnetic anisotropy.

Magnetic-field control of ionic bonds on ferroelectric surfaces

It is shown theoretically that applying magnetic fields can modulate the strength of ionic bonds on the surface of a ferroelectric film via field-induced strains from a magnetostrictive substrate. The ionic bonds form between oppositely charged polyelectrolytes that are bonded to the ferroelectric surface via the Coulomb interaction. The calculations show that applying a magnetic field can tune the activation energy of bond dissociation up by ∼50% or down to zero. This large modulation results from the high sensitivity of ferroelectric surface polarization to strain variation near ferroelectric phase transitions, which occur in epitaxial BaTiO3 films at experimentally accessible residual strains. Our results suggest an opportunity for tailoring the ionic bond strength on ferroelectric surfaces both statically and dynamically via strain engineering.

Synthesis and characterization of (111)-oriented BaTiO3 thin films

The synthesis of (111)-oriented BaTiO3 thin films on Nb-doped SrTiO3 and bilayers of BaTiO3 and La0.7Sr0.3MnO3 on SrTiO3 are investigated. With increasing thickness the films are found to exhibit a decreasing out-of-plane lattice parameter and increased surface roughness. The BaTiO3 films on doped SrTiO3 are found to be relaxed with indications of increasing defect density with increasing thickness. Through piezoresponse force microscopy, pyroelectric measurements, and tunneling electroresistance measurements, indications of ferroelectric behavior are found in (111)-oriented BaTiO3 down to a thickness of 5 nm.

Inter-digital capacitors with aerosol-deposited high-K dielectric layer for highest capacitance value in capacitive super-sensing applications

Inter-digital capacitors (IDCs) with aerosol-deposition (AD) high-k dielectric layer were compared via simulation and measurements of bare IDCs and AD IDCs at room temperature and subjected to a post-annealing process for realizing capacitive super-sensing applications. IDCs with thin AD films can provide higher capacitive intensity and improvements for other dielectric performances. Therefore, IDC patterns with AD high-k dielectric layers were fabricated by varying the finger widths and gap. Moreover, we analyzed the layer microstructure design patterns using simulations and experiments with AD BaTiO3 as-deposited IDCs and IDCs subjected to annealing at 500 °C. These three different IDCs were measured using an impedance analyzer; furthermore, the AD BaTiO3 films were evaluated using X-ray diffraction, atomic force microscopy, and traveling electron microscopy. The results for the IDCs with the AD BaTiO3 film show the highest capacitance when compared with other thin layer capacitors, which is expected to be useful in realizing super-sensing applications in the future.

Van der Waals epitaxy of highly (111)-oriented BaTiO3 on MXene.

We report on the high temperature thin film growth of BaTiO3 on Ti3C2Tx MXene flakes using van der Waals epitaxy on a degradable template layer. MXene was deposited on amorphous and crystalline substrates by spray- and dip-coating techniques, while the growth of BaTiO3 at 700 °C was accomplished using pulsed laser deposition in an oxygen rich environment. We demonstrate that the MXene flakes act as a temporary seed layer, which promotes highly oriented BaTiO3 growth along the (111) direction independent of the underlying substrate. The lattice parameters of the BaTiO3 films are close to the bulk value suggesting that the BaTiO3 films remains unstrained, as expected for van der Waals epitaxy. The initial size of the MXene flakes has an impact on the orientation of the BaTiO3 films with larger flake sizes promoting a higher fraction of the polycrystalline film to grow along the (111) direction. The deposited BaTiO3 film adopts the same morphology as the original flakes and piezoresponse force microscopy shows a robust ferroelectric behavior for individual grains. Transmission electron microscopy results indicate that the Ti3C2Tx MXene fully decomposes during the BaTiO3 deposition and the surplus Ti atoms are readily incorporated into the BaTiO3 film. Electrical measurements show a similar dielectric constant as a BaTiO3 film grown without the MXene seed layer. The demonstrated process has the potential to overcome the longstanding issue of integrating highly oriented complex oxide thin films directly on any desired substrate.

Copper-nanoparticle-dispersed amorphous BaTiO3 thin films as hole-trapping centers: enhanced photocatalytic activity and stability.

Nobel metal (Au and Ag) nanoparticles are often used in semiconductor photocatalysis to enhance the photocatalytic activity, while inexpensive Cu attracts less attention due to its easy oxidization. Herein, an elaborate study was conducted using Cu-nanoparticle-dispersed amorphous BaTiO3 films as photocatalysts. Photocatalytic and photoelectrochemical measurements demonstrated that the degradation efficiency and photocurrent density of the nanocomposite films are approximately 3.5 and 10 times as high as the pristine BaTiO3 film, respectively, which can be ascribed to a synergetic effect of the surface plasmon resonance and interband excitation. In addition, a good stability was also demonstrated by cyclic tests for the degradation of rhodamine B, which may be due to the amorphous nature of the BaTiO3 matrix providing hole-trapping centers. The high photocatalytic stability suggests that Cu is a promising alternative metal to replace Au and Ag for the development of cost-effective photocatalysts. Our work demonstrates a simple and promising strategy for improving the photostability of Cu nanomaterials and may provide a useful guideline for designing Cu-based composite materials toward various photocatalytic applications such as water pollution treatment.

Atomic-Scale Insight into the Reversibility of Polar Order in Ultrathin Epitaxial Nb:SrTiO <sub>3</sub>/BaTiO <sub>3</sub> Heterostructure and Its Implication to Resistive Switching

Ferroelectric heterostructures with bi-stable state of polarization are appealing for data storage as well as tunable functionalities such as memristor behavior. While an increasing number of experimental and theoretical studies suggest that polarization persists in ultrathin epitaxial heterostructures approaching just a couple of unit cells, the switching of such polar order is much less well understood, and whether polarization can be reversed in ultrathin ferroelectric heterostructures remains to be answered. Here we fabricate high-quality 7-unit cell thick BaTiO3 (BTO) films on Nb-doped single crystalline SrTiO3 (NSTO) substrate, and demonstrate their apparent yet unambiguously false polarization reversal due to charge injection using comprehensive piezoresponse force microscopy (PFM) studies. The presence of weak polar order consistent with linear piezoelectricity is confirmed at the atomic scale by high resolution integrated differential phase contrast (IDPC) of transmission electron microscopy (TEM) as well as macroscopic second harmonic generation (SHG), while the lack of polarization reversal under the voltage applied is supported by density functional theory calculation showing the persistence of dead layer on the surface. Nevertheless, poling-induced electric conduction differing by two orders of magnitude is observed, demonstrating resistive switching in ferroelectric heterostructure in the absence of polarization reversal, even with weak polar order. Our finding has technological implications on emerging memristor applications with potentially more accessible states than bi-stable polarization modulated mechanism, and raises technical challenges to unambiguously demonstrate polarization switching in ultrathin films at their critical size limit.

Effect of induced strain by substrate orientation on magnetic properties in BaTiO3/La0.7Sr0.3MnO3 bilayers

Abstract Hybrid multiferroics composed of ferromagnetic (FM) / ferroelectric (FE) heterostructures offer a possible route to design artificial multiferroic material exhibiting, for example, magnetoelectric coupling. We have grown BaTiO3 (100 nm)/ La2/3Sr1/3MnO3 (25 nm) bilayers on (001)-, (110)-, and (111)-oriented SrTiO3 substrate to study the induced strain influence in their magnetic properties. From structural characterization by x-ray analysis we found that La2/3Sr1/3MnO3 (LSMO) layer grew epitaxially strained, whereas BaTiO3 (BTO) film grew relaxed with lattice parameter close to that for single film. The magnetic measurements show a ferromagnetic order for all bilayers at room temperature with a Curie temperature TC∼350 K exhibiting a slight decrease of the magnetic properties (MS, HC and TC) of the bilayers in comparison with those for the La2/3Sr1/3MnO3 film. These variations could be associated to strained/relaxed regions induced by substrate orientation, and dislocation formation close to BTO/LSMO interface, among others phenomena. The magnetization mechanism was studied using the Berger model, where a critical β exponent ∼ 0.25 was found for all LSMO films in different STO orientations, which agrees with the tricritical point model. However, we found a β=0.283 only for the bilayer deposited on (001)STO substrate that coincides with previous reports for YMnO3 multiferroic materials. This behavior is not observed n either in the bilayer grown on (110) STO or (111) STO and displays indirectly a possible magnetoelectrical coupling in this configuration.

Optical revelation of defects in epitaxial barium titanate films

Allying epitaxial strain and synthesis conditions allows for the introduction of specific point defects in perovskite oxide films. In ferroelectric films, such defects lead to essential polar and electronic properties, which can enable advanced applications. Here, to elucidate the nature of the defects, optical constants are investigated in the spectral range of 0.7-8.8 eV in epitaxial ferroelectric BaTiO3 films, which are synthesized under different conditions. It is demonstrated that oxygen-vacancy-related defects are responsible for a peculiar transition below the bandgap at ∼2.7-2.9 eV and significant blueshifts of ∼0.3-0.4 eV of the gap and the main interband transitions. These observations suggest that the defects are dipolar complexes comprising titanium cations and oxygen vacancies (Ti3+-VO).

Mechanisms of electron transport in BaTiO3 ultrathin epitaxial films in the temperature range 40 K–295 K

In this paper, we report the results of experimental study of electron transport mechanisms in ferroelectric BaTiO3 ultrathin epitaxial films in a broad temperature range. For this purpose ferroelectric tunnel junctions based on STO/LSMO/BTO/Au structures were fabricated with different thicknesses of ferroelectric films (from 3 to 12 nm). Ferroelectric and electrical properties of heterostructures were measured at temperatures ranging from 40 K to 300 K. Based on experimental results, we find that prevailing transport mechanisms in BaTiO3 epitaxial thin film is determined by the film thickness. Below the certain thickness of the BaTiO3 film, a dominant contribution in the current through the structure is due to elastic tunneling. Increasing the thickness of the ferroelectric film leads to an impact from inelastic tunneling, mainly due to an interaction with traps caused by cation and anion vacancies in oxide sub-lattices. We suppose that the specificity of transport mechanisms in BaTiO3 ultrathin films is defined by the ratio of film thickness and the mean free path of injected electrons, along with repolarization processes synchronized with the ferroelectric coercive field. The latter point has been confirmed by the results of experimental study in a broad temperature range.

Tuning optical and magnetic properties of nanocrystalline BaTiO3 films by Fe doping

Fe-doped BaTiO3 is a promising multiferroics material due to its appealing physical properties and potential applications in sensors, transducers and non-volatile memories and multilayer capacitors. Herein, Fe-doped BaTiO3 nanocrystalline films were deposited on silicon substrates by chemical solution route. The relation between Fe content and the optical and magnetic properties of the samples was analyzed. X-ray diffraction data show that the films are tetragonal with P4mm space group. The atomic force microscopy measurement indicates that the film surface roughness increases with Fe composition. The optical response behavior of the films was investigated by Adachi’s dielectric function model with fitting ellipsometric spectra in the photon energy 1.5–4.13 eV. With increasing Fe content, the real part of dielectric functions gradually decreases in the transparent region. Moreover, EOBG (the optical band gap) is reduced with increasing Fe composition x, and the relationship between them is EOBG = (3.817–1.440 × x) eV. All Fe-doped films show weak ferromagnetism coming from the bound magnetic polarons and exchange interaction between oxygen vacancies and Fe3+ in the films. When iron content x exceeds 0.06, the magnetization values of the samples exhibit an unsaturated characteristic at more than 3000 Oe magnetic fields, which is due to the competition between the ferromagnetism and the antiferromagnetism of two Fe3+ ions superexchange. These results suggest that optical and magnetic properties of Fe-doped BaTiO3 nanocrystalline films can be changed by Fe composition.

High-performance supercapacitors based on the carbon nanotubes, graphene and graphite nanoparticles electrodes

In this study, the three structures of the symmetric paper supercapacitors based on the carbon nanotubes (CNTs), graphite nanoparticles (GNPs) and graphene electrodes have been fabricated. In the supercapacitors was used of polyvinyl alcohol (PVA)/phosphoric acid (H3PO4) as a gel electrolyte and the BaTiO3 film as a separator film. The carbon nanomaterials, gel electrolyte surface, and electrode films were characterized by scanning electron microscopy (SEM) and transmission electron microscope (TEM). The specific capacitance of the symmetric paper supercapacitors using charge-discharge technique and C-V curves at the voltage scan rates 20 mV/s and 150 mV/s have been investigated. The symmetric paper supercapacitor based on the CNTs electrode showed higher specific capacitance 411 F g−1, compared to GNPs and graphene supercapacitors. Also by electrochemical impedance spectroscopy, the Nyquist curves of the symmetric paper supercapacitors have been plotted. For the symmetric paper supercapacitors based on the GNPs, graphene and CNTs electrodes the equivalent series resistance (ESR) resistance was 210 Ω, 96 Ω and 101 Ω respectively. The flexible symmetric paper supercapacitor based on BaTiO3/PVA/CNTs structure denotes a new type of the flexible supercapacitor that can be applied to the soft electronic.

Thickness-dependent thermal conductivity of ultrathin (< 100 nm) barium titanate films

Thin film barium titanate (BaTiO3) is a promising material in the electronics and ceramics industry owing to its compelling dielectric properties. A number of works have investigated its dielectric and structural properties, but less studied are its thermal properties particularly at sub-100 nm thicknesses. Here, we measure the room-temperature thermal conductivity of ultrathin (< 100 nm), pulsed laser deposited BaTiO3 films. The measured thermal conductivities are thickness-dependent, and this trend is consistent with the thickness-dependent crystallinity of the films. Transmission electron microscopy analysis of the films reveals the presence of an initial amorphous layer ~ 60 nm thick from the growth interface and the subsequent formation of columnar grains of width ~ 12 nm that are embedded within an amorphous matrix. For a region that incorporates grains with columnar morphology, we find that cross-plane heat conduction may be favored by 30–40% over in-plane heat conduction due to the columnar morphology of grains.

Synchrotron x-ray microdiffraction study of residual stresses in BaTiO3 films deposited at room temperature by aerosol deposition

The residual stress through the thickness of a BaTiO3 ceramic film deposited on steel substrate at room temperature with aerosol deposition has been analyzed using synchrotron x-ray microdiffraction. A gradient in stress distribution was evident through the film and the maximum biaxial compressive stress of −800 MPa was observed at the film-substrate interface. Heat-treatment was found to relax the internal compressive stress, due to thermal expansion mismatch between the film and substrate. Variation in ferroelectric response was correlated to the change in stress state by thermal treatment. This analysis is crucial for development of micro-and nanoelectronic devices with AD films.

BaTiO&lt;sub&gt;3&lt;/sub&gt;/ Teflon Nanocomposite Ferroelectric Thin Films for Low Voltage Electrowetting Systems

We present a simple and cost effective method for the design and fabrication of electrowetting devices using a nanocomposite thin film of BaTiO3 and Teflon-AF as the dielectric layer to achieve low voltage operation. The nanocomposite film is prepared by using Teflon-AF as matrix and BaTiO3 nanoparticles as the filler material. The solution is spin coated to deposit thin film on metal electrodes. The characterization results show that the nanocomposite thin film can serve as the dielectric for EWOD with a high dielectric constant and a crack free hydrophobic film. To test the electrowetting effect, the variation of droplet contact angle achieved with DC voltage, AC voltage and AC frequency change are fully experimented. The EWOD device with nanocomposite dielectric layer also manipulates water droplet at low driving voltages. This study shows the potential of using ferroelectric nanocomposite film as the dielectric layer in high-performance EWOD devices.