Bi4Ti3O12 Films Research Articles

Enhanced energy storage performance in Bi4Ti3O12 thin films with oxygen depletion layers

The imprint effect in ferroelectric materials can significantly enhance the performance of energy storage devices. Bi4Ti3O12 (BTO) and oxygen-deficient Bi4Ti3O11.2 (DBTO) thin films were deposited on single-crystal Nb-doped SrTiO3 substrates using pulsed laser deposition. In stark contrast, multilayer DBTO/BTO thin films incorporating an oxygen-deficient layer were also fabricated, with the DBTO layers having thicknesses of 5 nm and 10 nm, respectively. Both BTO and DBTO thin films exhibited epitaxial growth and c-axis-oriented crystallinity. Regarding ferroelectric properties, DBTO/BTO capacitors with an oxygen-deficient layer demonstrated a pronounced imprint effect in their ferroelectric hysteresis loops, compared to the BTO and DBTO capacitors. This effect was particularly noticeable in the DBTO/BTO capacitor with a 10 nm oxygen-deficient layer, as opposed to the thin film with a 5 nm oxygen-deficient layer. We suggest that the imprint effect can be attributed to localized electric field variations and imbalances in charge transport at the interface between the oxygen-depleted layer and the remaining thin film. Consequently, the imprint effect observed in the DBTO/BTO capacitors contributed to an enhancement in energy storage density and efficiency, compared to BTO and DBTO capacitors. This study demonstrates the potential of multilayer DBTO/BTO thin films in advancing capacitive energy storage systems.

Preparation and Properties of Ce-Doped Bi4-xCexTi3O12 Thin Films

Ce-doped Bi4-XCexTi3O12 (BCT) films were prepared by the sol-gel method and the heat treatment process different from traditional ones. The experimental results show that the Bi4Ti3O12 (BTO) films still exhibit a single-phase bismuth-layered structure and can effectively improve the ferroelectric properties under the doping of Ce3+, and the maximum residual polarization intensity is 1.23μC/cm2 when the doping amount is x = 1.35. At the same time, the heat treatment process different from the traditional process was used to explore the influence of the preparation process on the surface morphology and ferroelectric properties of the films.

Ionizing radiation response of bismuth titanate-based metal-ferroelectric-semiconductor (MFS) type capacitor

In this study, the ionizing radiation (gamma-irradiation) response of the metal-ferroelectric-semiconductor (MFS) type capacitors whose interfacial layer is bismuth titanate (Bi4Ti3O12), which is from the Aurivillius family, was investigated. The capacitance-voltage (CV) and conductance-voltage (G/ω-V) characteristics and the device parameters derived from these characteristics provide essential information about the radiation response. Therefore, MFS-type capacitors in which their Schottky/rectifier contacts Au and interfacial layer Bi4Ti3O12 (BTO) were prepared and exposed to various doses of gamma-rays. The voltage-dependent C and G/ω data were obtained before irradiation and after some irradiation doses using the Hewlett Packard Impedance Analyzer. Experimental results showed that the CV curves exhibited multilayer capacitor behavior before and after radiation rather than conventional Schottky structures due to the very thin Silicon dioxide (SiO2) interlayer at the BTO/Si interface, which was assumed to form during the annealing of BTO films in air ambient. On the other hand, the C-G/ω-V curves exhibited asymmetric and hysteretic behavior because of the different electrical properties at the bottom and top interfaces and the natural ferroelectric behavior of the BTO. On the other hand, considering the radiation effects, the C-G/ω-V characteristics and other device parameters did not show any significant changes at the 5 kGy irradiation dose, but significant differences were observed in all parameters at the 22 kGy dose. Therefore, it can be concluded that these MFS-type structures with the BTO interlayer can be used as a capacitor or radiation sensor at doses around 5 kGy and below 22 kGy. However, it is not recommended to use them as capacitors at doses close to 22 kGy due to the large capacitive change (approximately tenfold decrease) at this dose. Besides, the tendency of the device to deteriorate at 22 kGy doses indicates that it is not suitable for use as a capacitor or radiation sensor above 22 kGy gamma-irradiation doses.

Structural and electrical study of BLT thin film deposited on different substrates by electron gun evaporation

Purpose The purpose of this study was to deposit Bi4Ti3O12 films by electron gun evaporation technique starting from Bi3.25La0.75Ti3O12 as a target without annealing. The films have been deposited on Si(100), on thin film buffer layer of Pt and glass substrates. X-ray diffraction (XRD) was used to analyze structure of the films, which possesses a good structure with (0010) preferred orientation. Electronic behavior of the samples has been studied. Design/methodology/approach The dependence of both the structure and quality of the BLT thin films on different substrates is studied using XRD. The electrical characteristics were determined using capacitance–voltage (C–V) and current–voltage (I–V) measurements at the frequency of 1 MHz. Optical properties of the grown films deposited on glass substrates were characterized by optical transmittance measurements (UV-Vis). Findings The XRD analysis approved the crystallographer structure of the prepared Bi4Ti3O12 films. The optical properties of deposited film (transmittance and the band gap value) are extracted by UV-Vis spectrum. Originality/value High crystalline quality Bi4Ti3O12 films have been obtained using different substrates at room temperature by means of electron gun deposition. The electrical and ferroelectric properties of thin films were studied using I–V and C–V measurements. The band gap has been found to be about 3.62 eV for the studied film deposited on glass substrate. Electron beam evaporation technique is the most interesting methods, once considering many advantages; such as its stability, reproducibility, high deposition rate and the compositions of the films are controlled.

Upconversion of Bi4Ti3O12:Er and its evaluation in silicon solar cell yield

Using the sol–gel method, Er3+-doped Bi4Ti3O12 films were prepared on monocrystalline silicon substrates. The current–voltage characteristics for the prepared structures were measured using a 300 W solar simulator at air mass 1.5 (AM1.5) and at a frequency of 50–60 Hz. In general, all samples maintained a clearly visible diode-like behavior. It was observed that the reference cell had a lower energy conversion efficiency compared to the silicon solar cell modified with the BIT:Er films adhered to its back contact in all cases under AM1.5 illumination. The 2 mol% sample reached an open circuit voltage of 544 mV and a short circuit current of 39.8 mA/cm2 with a 2.2% excess over the reference solar cell efficiency.

Controlling the crystallization of Nd-doped Bi4Ti3O12 thin-films for lead-free energy storage capacitors

Environmentally benign non-lead-based dielectric thin film capacitors with high electrostatic energy density, long-term stability, and fast charge/discharge capability are strongly demanded in advanced electrical and pulsed power devices. Here, we propose that insufficient crystallization is an effective method to achieve high energy storage performance. A high efficiency of 84.3%, together with a good energy density of 41.6J/cm3 and an excellent fatigue endurance, is obtained in a lead-free Nd-doped Bi4Ti3O12 film of low crystallization. An increase in the annealing temperature increases the crystallinity and grain size, which improves the ferroelectric polarization of a thin film. A narrow hysteresis loop with large maximum polarization and small remnant polarization is obtained in the insufficiently crystallized film, which is annealed in the intermediate temperature. This film also shows a lower leakage current compared with the fully crystallized counterpart due to the less defective microstructure. This work provides a straightforward and executable method to design ferroelectric materials for the applications of energy storage capacitors.

Enhanced and adjustable photoresponse performance in Bi4Ti3O12/TiO2 bilayer films

Bi4Ti3O12/TiO2 bilayer films were synthesized on general fluorine-doped tin oxide glass (FTO) substrates adopting a simple solution coating processing. The Bi4Ti3O12 film with different thickness was first prepared on FTO substrates, and then TiO2 layer with different thickness was spin-coated on the Bi4Ti3O12 layer, to obtain different Bi4Ti3O12/TiO2 bilayer films with almost the same total thickness. The morphology and crystal structure of Bi4Ti3O12/TiO2 bilayer films were characterized and analyzed by scanning electron microscopy and X-ray diffraction. The enhanced photoresponse was observed in bilayer films compared with the single-phase films of Bi4Ti3O12 or TiO2. Besides, it was also observed that the photoresponse of the bilayer films could drastically be adjusted by applied bias voltage and ferroelectric polarization states. The reasons for the above phenomena were also investigated.

Ferroelectric and photoluminescent properties of Eu3+-doped Bi4Ti3O12 films prepared via the spin-coating method

In this study, the films of Bi4−xEuxTi3O12 (x = 0, 0.2, 0.4, 0.6, 0.8) were prepared on substrates (Pt(111)/Ti/SiO2/Si(100)) by the spin-coating method. The results indicate that the films of Bi4−xEuxTi3O12 annealed at 700 °C consist of Bi4Ti3O12 and Pt from the substrates. This indicates that Eu3+ has dissolved into the lattice of Bi4Ti3O12. The doping of Eu3+ to the films of Bi4Ti3O12 enhances ferroelectric properties as well as photoluminescent properties. The films of Bi4−xEuxTi3O12 show two orange emission perks at 590 nm and 595 nm, and a red emission at 613 nm. These three peaks are ascribed to 5D0 → 7F0, 5D0 → 7F1, and 5D0 → 7F2, respectively. The bandgaps of the films of Bi4−xEuxTi3O12 range from 3.07 eV to 2.86 eV with the increase of the doping concentration of Eu3+. Bi3.4Eu0.6Ti3O12 film possesses maximal remanent polarization, minimal leakage current density, and greatest emission intensity.

Polarization Switching along a Substrate in Thin Bi4Ti3O12 Films under Different Deformation Stresses

Single-crystal Bi4Ti3O12 films deposited on a 4-nm-thick Ba0.4Sr0.6TiO3 sublayer covering an MgO(001) substrate have been investigated. It has been found that the unit cells of Bi4Ti3O12 films in the resulting heterostructures are turned by 45° relative to the cells of the MgO substrate in the interface plane. In addition, the unit cells of the films are strained, the amount of strain depending on the Bi4Ti3O12 film thickness. When the films become about 40 nm thick, strain changes sign. It has been shown that reversible spontaneous polarization in Bi4Ti3O12 films with a 180° domain structure in the interface plane arises at a thickness of 10 nm and grows with thickness to 54 μC/cm2. The anisotropy of the film’s properties in the interface plane and the influence of unit cell distortion on the properties of the heterostructures have been confirmed by studying the dielectric performance of the films.

Переключение поляризации вдоль подложки в тонких пленках Bi-=SUB=-4-=/SUB=-Ti-=SUB=-3-=/SUB=-O-=SUB=-12-=/SUB=- при различных деформационных напряжениях

A study of epitaxial Bi4Ti3O12 thin films with a pre-deposited 4 nm Ba0.4Sr0.6TiO3 sublay-er on (001) MgO substrates has been performed. In the obtained heterostructures, the rotation of the Bi4Ti3O12 film unit cells by an angle of 45° relative to the MgO substrate unit cell in the inter-face plane has been observed. The Bi4Ti3O12 films contain unit cell deformations depending on the thickness of the film and the sign of the deformation changes at a thickness of ~40 nm. The switchable in-plane spontaneous polarization of Bi4Ti3O12 film at the 180° domain structure oc-curs at a film thickness of 10 nm and increases with a thickness up to 54 µC/cm2. The study of the dielectric characteristics of the films confirmed the existence of properties anisotropy in the interface plane and the effect of deformation of the unit cell on the properties of heterostruc-tures.

Structure, lattice dynamics, properties of bismuth titanate heterostructures

We investigated the epitaxial Bi4Ti3O12 (BTO) thin films with different thicknesses on a (0 0 1) MgO substrate with a Ba0.4Sr0.6TiO3 buffer layer (4 nm). The crystallographic axis [1 0 0] of Bi4Ti3O12 film is rotated on degrees relative to [1 0 0] MgO axis. When the thickness of Bi4Ti3O12 film is less than 40 nm, the unit cell of the film is compressed in the normal to the interface plane direction. In cases of large film thicknesses, the sign of strain changes and correspondingly the unit cell is stretched. Switched spontaneous polarization in the interface plane with 180° domain structure occurs at 10 nm film thickness and increases with a thickness up to 55 C cm−2. Changes in the Raman spectra of the Bi4Ti3O12 films indicates to increase of monoclinic distortion of the film unit cell in comparison with the bulk unit cell. The anisotropy of in-plane dielectric properties and the effect of internal strain on dielectric properties for the Bi4Ti3O12 films are confirmed by the study of the dielectric characteristics of the films.

Lattice Structure and Dynamics of Two-Layer Heterostructures of Barium–Strontium Titanate and Layered Bismuth Titanate of Various Thicknesses on a Magnesium Oxide Substrate

The lattice structure and dynamics of single-crystal Bi4Ti3O12 with a thickness from 4 to 430 nm on a (001)MgO substrate with a previously deposited Ba0.4Sr0.6TiO3 sublayer (4 nm) have been studied. The two-layer structures were prepared by radio-frequency sputtering of ceramic targets of the corresponding compositions. The X-ray diffraction studies performed at room temperature have shown that, in this heterostructure, axis c of the Bi4Ti3O12 unit cell is perpendicular to the substrate, and direction [100] has an angle of ±45° to the [100]MgO direction. At the thicknesses of Bi4Ti3O12 to ~40 nm, the film unit cell is compressed in the direction of a normal to the substrate plane and extended in the conjugate plane, and the sign of the deformation is changed at large thicknesses. It is found that the phonon mode frequency in the Bi4Ti3O12 film shift and additional peaks appear in the Raman spectra, which demonstrates an increase in the degree of monoclinic distortion of the film crystal structure as compared to the crystal structure.

Enhancing Speed and Stability of Polarization Reversal in Predominantly a/b‐Axes‐Oriented Bi4Ti3O12 Thin Films Deposited on Pt/Ti/SiO2/Si

Bi4Ti3O12‐based thin films of a/b‐axes orientation grown on standard‐type Pt/Ti/SiO2/Si substrates are desirable for designing reliable ferroelectric devices. Herein, a/b‐axes‐oriented and randomly oriented Bi4Ti3O12 thin films are deposited on Pt/Ti/SiO2/Si through a sol–gel process. As expected, the a/b‐axes‐oriented films show a higher remnant polarization than that of randomly oriented films. The characteristic time τ of domain reversal in 74% a/b‐axes‐oriented Bi4Ti3O12 films is 1.1 ms, notably shorter than that (τ > 2.5 ms) for randomly oriented films. The dynamic hysteresis scaling relations of the randomly oriented 56% and 74% a/b‐axes‐oriented Bi4Ti3O12 films take the forms of ⟨A⟩ ∝ f−0.106E01.667, ⟨A⟩ ∝ f−0.035E00.952, and ⟨A⟩ ∝ f−0.026E00.600, respectively. The difference indicates that domain reversal in Bi4Ti3O12 films of predominant a/b‐axes orientation exhibits higher stability compared with randomly oriented Bi4Ti3O12 films under varying electric fields E(E0, f). The enhancing speed and stability of polarization reversal in predominantly a/b‐axes‐oriented Bi4Ti3O12 films can be understood in terms of their layered‐perovskite structure, large columnar grains, oxygen vacancies mobility, domain states, and polarization switching mechanism.

Structural and Electric Characteristics of Two-Layer Bi4Ti3O12/(Ba,Sr)TiO3 Thin Films Deposited on a Silicon Substrate by Radio-Frequency Sputtering at Increased Oxygen Pressures

The 400–450-nm-thick Bi4Ti3O12 thin films with various orientations of crystallites with respect to a normal to the (100)Si substrate plane have been studied. It is established that the crystallite orientation can be controlled by varying the composition of the 4-nm-thick BaxSr1 – xTiO3 sublayer. The use of Ba0.4Sr0.6TiO3 as a sublayer leads to the growth of the Bi4Ti3O12 film in the single-crystal state with plane (001) parallel to the substrate plane and with a monoclinic distortion of the crystal structure. The Ba0.8Sr0.2TiO3 sublayer is shown to lead to the formation of four crystallite orientations: (111), (117), (100), and (110) and two groups of domains in the Bi4Ti3O12 film; the first group with the polarization direction p-erpendicularly to the substrate and the second group with the polarization directed in the angular range 45.2°–57° with respect to a normal to the substrate. It is shown that, in the Bi4Ti3O12 film with the Ba0.8Sr0.2TiO3 sublayer, the polarization is directed to the substrate and is switched to new stable state with the polarization direction from the substrate when applying an external voltage higher than a critical one (4 V).

Структура и динамика решетки двухслойных гетероструктур титаната бария-стронция и слоистого титаната висмута разной толщины на подложке окcида магния

The results of the structure and lattice dynamics study of Bi4Ti3O12 films with a thickness varying from 4 to 430 nm on a (001) MgO substrate with a preliminarily deposited Ba0.4Sr0.6TiO3 (4 nm) sublayer are presented. Two-layer structures were fabricated by high-frequency sputtering of ceramic targets of the appropriate composition. X-ray diffraction studies at room temperature showed that in such heterostructures the c axis of the Bi4Ti3O12 film is perpendicular to the substrate plane, and the [100] axis makes an angle of ± 45◦ with the [100] MgO axis. Up to ≈ 40 nm of Bi4Ti3O12 film thickness, the unit cell have a compression strain in the normal to the substrate plane direction and tensile strain in the interface plane, for large thicknesses the sign of the deformation changes. The frequency shifts of phonon modes in the Bi4Ti3O12 film and the appearance of additional peaks in the Raman spectra were observed. It is indicating on increasement of monoclinic distortion of the films crystal structure compared to the bulk crystal.

Effect of thickness and crystalline morphology on electrical properties of rf-magnetron sputtering deposited Bi4Ti3O12 thin films

Bi4Ti3O12 (BiT) thin films with thicknesses ranging from 535 to 870 nm were deposited on Pt(111)/Ti/SiO2/Si substrates by rf-magnetron sputtering method. The films were crystallized by direct thermal annealing in air at 650 °C. The effects of film thicknesses and crystalline morphology on ferroelectric, leakage current conduction and dielectric properties were investigated. XRD and SEM analysises reveal that BiT thin film deposited at 400 °C is amorphous and exhibits a non-crystalline morphology. After annealing treatment, the amorphous film transformed into Aurivillius Bi4Ti3O12 crystalline phase with rodlike grains. The electrical properties show that the as-deposited thin film is a lossy dielectric with absence of ferroelectricity. The remnant polarization of crystalline BiT thin films increases slightly with decreasing thickness. The leakage currents of annealed BiT films exhibit ohmic behavior in low voltage region and Schottky emission conduction characteristic in high voltage region, respectively. For crystalline BiT films, the dielectric constant decreases significantly with increasing film thicknesses.

Some peculiarities at preparation of Bi4Ti3O12 films for bolometric applications

We discuss properties of Bi4Ti3O12 (BTO) films in BTO/CeO2/YSZ multilayered buffer system, on silicon and on silicon-on-insulator (SOI) substrates, employed for the preparation of epitaxial La0.67Sr0.33MnO3 (LSMO) films. We try to find a correlation between the parameters of film preparation and structural properties and morphology of the BTO films with the goal to eliminate defects. We also point out that many morphological defects observed on the final LSMO films prepared for bolometers originate in the bottom BTO layer. We have found out that the actual substrate temperature is a very important parameter during the process of BTO film deposition since deviations of only few degrees from the proper temperature can cause the growth of mixture crystal orientation.

Enhanced Ferroelectric Polarization in Laser-ablated Bi4Ti3O12 Thin Films by Controlling Preferred Orientation

Polycrystalline Bi4Ti3O12 thin films with various fractions of a-axis, c-axis and random orientations have been grown on Pt(111)/Ti/SiO2/Si substrates by laser-ablation under different kinetic growth conditions. The relationship between the structure and ferroelectric property of the films was investigated, so as to explore the possibility of enhancing ferroelectric polarization by controlling the preferred orientation. The structural characterization indicated that the large growth rate and high oxygen background pressure were both favorable for the growth of non-c-axis oriented grains in the Bi4Ti3O12 thin films. The films with high fractions of a-axis and random orientations, i e, f (a-sxis) = 28.3% and f (random) = 69.6%, could be obtained at the deposition temperature of 973 K, oxygen partial pressure of 15 Pa and laser fluence of 4.6 J/cm2, respectively. It was also noted that the variation of ferroelectric polarization was in accordance with the evolution non-c-axis orientation. A large value of remanent polarization (2Pr = 35.5 μC/cm2) was obtained for the Bi4Ti3O12 thin films with significant non-c-axis orientation, even higher than that of rare-earth-doped Bi4Ti3O12 films.

Integration of c-axis oriented Bi3.15Nd0.85Ti2.95Hf0.05O12/La0.67Sr0.33MnO3 ferromagnetic-ferroelectric composite film on Si substrate

A La0.67Sr0.33MnO3 (LSMO) ferromagnetic layer and a Nd3+/Hf4+ co-substituted Bi4Ti3O12 (Bi3.15Nd0.85Ti3-xHfxO12 (BNTHx, x = 0, 0.025, 0.05, 0.1 and 0.15)) ferroelectric layer were successively deposited onto the (00 l)-oriented LaNiO3 (LNO) layer buffered (001) Si substrate via all chemical solution deposition (CSD) method. As a result, the BNTHx/LSMO ferromagnetic-ferroelectric composite films integrated on Si substrate exhibit high c-axis orientation. The Nd3+/Hf4+ co-substituted BNTHx films have the lower leakage current and the better ferroelectric properties than the mono-substituted Bi4Ti3O12 (Bi3.15Nd0.85Ti3O12 and Bi4Ti2.95Hf0.05O12) films. In particular, the BNTH0.05/LSMO/LNO film has the lowest leakage current density of 2.5 × 10−7 A/cm2 at 200 kV/cm, and the highest remnant polarization (Pr) of 27.3 μC/cm2. The BNTH0.05/LSMO/LNO composite film also exhibits the soft ferromagnetism characteristics with a high saturated magnetization of 258 emu/cm3 at 300 K, and the excellent magnetoelectric (ME) effect. The variations of ME voltage coefficient αE values with DC bias magnetic field Hbias shows that the BNTH0.05/LSMO/LNO film has the high αE value at near zero Hbias. Moreover, at Hbias = 0 Oe, the αE value gradually increases from zero with the increasing of the AC magnetic field frequency, and eventually reaches about 18.9 V/cm·Oe at 100 kHz, suggesting the existence of self-biased ME effect.

Effect of starting powder morphology on film texture for bismuth layer-structured ferroelectrics prepared by aerosol deposition method

We report grain orientation control for bismuth layer-structured ferroelectrics (BLSFs) films deposited by aerosol deposition (AD) method at room temperature. Bi4Ti3O12 (BiT), SrBi2Ta2O9 (SBTa), and SrBi4Ti4O15 (SBTi) starting powders with particles of various shape (plate-like, spherical, and angular) were prepared by solid-state reaction and fused salt synthesis. Their AD films represented fine microstructures without pores, which agrees well with previous reports. Although the SBTa AD films deposited by using spherical particles exhibited an extremely low Lotgering factor (F), the BiT AD films deposited by using plate-like particles exhibited a marked c-axis orientation. The F of BiT and SBTi AD films decreased with increasing film thickness (t). We consider that the dispersion of agglomerated plate-like particles on the film surface and the densification of the compacted powder layer occurring while under particle impact are important in obtaining the grain-oriented AD films. These results of using the AD method with shape-controlled particles are expected to result in open up an innovative functional coating technique.