Bi4Ti3O12 Thin Films Research Articles

Microstructure modulation of a Bi4Ti3O12 thin film system by combining the effect of a simple processing methodology with a co-doping strategy involving Nd3+ and Nb5+

A microstructure based on plate-like grains is expected in the obtaining of Bi4Ti3O12 ferroelectric ceramics, promoted by the Aurivillius crystalline structure that this material exhibit. This grain morphology would lead to conductivity making the functional response unpractical. In this frame, we address the control of the crystal growth which leads to this grain morphology by combining the effect of a simple thin film obtaining procedure with doping strategies. On the one hand, an aqueous solution-gel plus spin-coating methodology is performed here for the obtaining of these thin films. On the other hand, a simultaneous incorporation of Nd3+ and Nb5+ in the crystal lattice replacing Bi3+ and Ti4+, respectively, is conducted in this contribution. The results obtained by X-ray diffraction, UV–visible measurements and FESEM confirm that the incorporated dopants are able to block (or at least control) the mentioned crystal growth.

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.

Excellent Bipolar Resistive Switching Characteristics of Bi4Ti3O12 Thin Films Prepared via Sol-Gel Process.

Herein, Bi4Ti3O12 (BIT) ferroelectric thin films were fabricated into Au/BIT/LaNiO3/Si structures to demonstrate their memristor properties. Repeatable and stable bipolar resistive switching (RS) characteristics of the device are first reported in this work. The switching ratio of the device annealed in air reached approximately 102 at 0.1 and −0.1 V. The RS performance was not significantly degraded after 100 consecutive cycles of testing. We also explored the factors affecting the RS behavior of the device. By investigating the RS characteristics of the devices annealed in O2, and in combination with XPS analysis, we found that the RS properties were closely related to the presence of oxygen vacancies. The devices annealed in air exhibited a markedly improved RS effect over those annealed in O2. According to the slope fitting, the conduction mechanism of the device was the ohmic conduction and space charge limited current (SCLC). This study is the first to successfully apply BIT ferroelectric films to the RS layers of memristors. Additionally, a theory of conductive filaments is proposed to adequately explain the relationship between RS behavior and oxygen vacancies, providing meaningful inspiration for designing high-quality resistive random access memory devices.

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.

Microstructure and ferroelectric properties of bi-excess Bi4Ti3O12 thin films grown on Si and Pt/Ti/SiO2/Si substrates

Bismuth layer-structured ferroelectric Bi4Ti3O12 thin films have been deposited on n-type Si(111) and Pt(111)/Ti/SiO2/Si substrates using rf-magnetron sputtering process. The effects of substrates and annealing treatments on microstructure and ferroelectric properties were investigated. Scanning electron microscopy and X-ray diffraction patterns exhibited that the as-deposited thin films were noncystalline and transformed to crystalline Bi4Ti3O12 phase with a second phase of Bi12TiO20 after annealed at 700 °C. Ferroelectric properties showed that as-deposited thin film in metal-ferroelectric-semiconductor capacitor form and annealed thin film in metal-ferroelectric-metal form exhibited typical ferroelectric behaviors, wherein the values of remnant polarization were around 0.8 µC/cm2 and 4.8 µC/cm2, respectively.

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.

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.

Effect of SmFeO3 content on structure and multiferroic properties of mSmFeO3–Bi4Ti3O12 thin films

mSmFeO3–Bi4Ti3O12 (m = 0.5, 1.0, 1.5, 2.0) thin films were prepared on silicon wafer by sol–gel progress. Their structure, leakage current density, ferroelectricity, magnetism, and dielectric property were investigated. All the samples have a single-phase Aurivillius structure. The interplanar spacing and volume of the unit cell decrease with the increase of SmFeO3 content. Moreover, the multiferroic properties have been significantly improved, with 2Prmax ~ 62 μC/cm2 (m = 0.5) and Msmax ~ 5.8 emu/cm3 (m = 2) at room-temperature. The samples with m = 0.5, 1, 1.5 exhibit Ohmic mechanism-dominated conductive behavior, however, for m = 2 sample, the space-charge-limited current mechanism becomes dominant. The dielectric constant er of the thin films at 1 MHz for m = 0.5, 1.0, 1.5 and 2.0 are 498, 485, 272, and 217 respectively.

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).

Effects of deposition temperatures of Nd-doped Bi4Ti3O12 thin films prepared by pulsed laser deposition

Recently the deleterious environmental impact of the lead used in many ferroelectric and piezoelectric devices has stimulated research into lead-free alternatives with comparable properties. Bismuth titanate-based materials are considered to be candidates for use as lead-free piezoelectrics. In this study, we prepared the Bi3.15Nd0.85Ti3O12 (BNdT) thin films by using Pulsed Laser Deposition (PLD), which is a relatively easy method for the fabrication of thin films. The samples were grown on Pt/Ti/SiO2/Si substrates using a KrF laser (248 nm). To find an optimal fabrication condition of this sample, we varied the PLD deposition conditions, including the substrate temperature, oxygen gas pressure, repetition frequency, target-substrate distance and pulse energy density. We found that the deposition temperature is an important parameter which should be optimized to get high-quality BNdT films. The BNdT thin films were studied by X-ray diffraction (XRD), atomic force microscopy (AFM) surface images, measurements of frequency-dependent dielectric constant and dielectric loss, polarization–electric field (P–E) curves, and fatigue tests. With increasing the deposition temperature the (117) peak of XRD increased, and the intensity ratio of (117)/{(117) + (006) + (200)} peak increased and its value was 68.3% at 650 °C and 44.3% at 700 °C. We note that the phase formation was strongly dependent on the deposition temperature in our BNdT films.

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.

Dielectric, ferroelectric, and photoluminescent properties of Sm-doped Bi4Ti3O12 thin films synthesized by sol-gel method

We report on the structure, dielectric, ferroelectric, and photoluminescent properties of Sm3+-doped Bi4Ti3O12 thin films which were prepared on fused silica and Pt/Ti/SiO2/Si substrates by sol-gel method. The X-ray diffraction analysis confirmed that the Bi4-xSmxTi3O12 (BSmT) thin films were well crystallized in layered perovskite structure without any secondary phase. Raman spectra indicated that the structure of BSmT thin films was significantly distorted because of the Sm3+ doping. An appropriate doping amount of Sm3+ ions leads to obvious enhancement in ferroelectric and dielectric properties of BSmT thin films due to structure distortion and reduction in defects. In addition, the BSmT thin films also show orange-red color emission at 601 nm and long florescence lifetime (> 0.6 ms). This study indicated that lead-free BSmT thin films, which are featuring good electrical and photoluminescent properties, may have potential applications in integrated optoelectronic devices.

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.

Effects of Nb Content on the Ferroelectric and Dielectric Properties of Nb/Nd-Co-doped Bi4Ti3O12 Thin Films

Nd/Nb-co-substituted Bi3.15Nd0.85Ti3−x Nb x O12 (BNTN x , x = 0.01, 0.03, 0.05 and 0.07) thin films were grown on Pt/Ti/SiO2/Si (100) substrates by chemical solution deposition. The effects of Nb content on the micro-structural, dielectric, ferroelectric, leakage current and capacitive properties of the BNTN x thin films were investigated. A low-concentration substitution with Nb ions in BNTN x can greatly enhance its remanent polarization (2P r) and reduce the coercive field (2E c) compared with those of Bi4Ti3O12 (BIT) thin film. The highest 2P r (71.4 μC/cm2) was observed in the BNTN0.03 thin film when the 2E c was 202 kV/cm. Leakage currents of all the films were on the order of 10−6 to 10−5 A/cm2, and the BNTN0.03 thin film has a minimum leakage current (2.1 × 10−6 A/cm2) under the high electric field (267 kV/cm). Besides, the C–V curve of the BNTN0.03 thin film is the most symmetrical, and the maximum tunability (21.0%) was also observed in this film. The BNTN0.03 thin film shows the largest dielectric constant and the lowest dielectric loss and its maximum Curie temperature is 410 ± 5°C.

Ferroelectric properties of highly a-oriented polycrystalline Bi4Ti3O12 thin films grown on glass substrates

Thin films of highly a-oriented polycrystalline Bi4Ti3O12 (BTO) were deposited on Pt/Ta/glass substrates via pulsed laser deposition. X-ray diffraction experiments confirmed that the BTO thin films were preferentially oriented along the (104) direction. As a ferroelectric thin film, the BTO thin films exhibited good ferroelectric properties with a remanent polarization of 13.8 μC/cm2. The fatigue resistance test revealed that owing to their crystal orientation along the (104) direction, the BTO thin films significantly degrade after being subjected to ferroelectric polarization after around 108 switching cycles.

Room-temperature multiferroic properties in 1.5LaFeO3–Bi4Ti3O12 thin films

Random-oriented 1.5LaFeO3–Bi4Ti3O12 thin films were fabricated on (111)Pt/Ti/SiO2/Si(100) substrates by a sol–gel method. Both X-ray diffraction and Raman spectroscopy characterizations confirm that the films have a single-phase Aurivillius structure with 4–5 successive perovskite blocks. The films exhibit room-temperature multiferroic properties with the remanent polarization 2P r ~ 27.20 µC/cm2 and saturated magnetization M s ~ 4.28 emu/cm3. Moreover, room-temperature magneto-dielectric effect with magneto-dielectric coefficient of ~ 1.03% was observed, which arises from the charge ordering of Fe2+ and Fe3+. Theoretical analysis of temperature-dependent dielectric spectrum and X-ray photoelectron spectroscopy data reveals a major contribution of charge ordering of Fe2+ and Fe3+ to the magneto-dielectric effect in 1.5LaFeO3–Bi4Ti3O12 thin films.

Effect of controlled Mn doping on transition of oxygen vacancies in Bi2Ti2O7 thin films: An electrochemical study

Crystalline pyrochlore Bi2Ti2O7 (B2T2) thin films were well formed at 300°C under 740mTorr of oxygen partial pressure using pulsed laser deposition. In order to improve the electrical properties of the dielectric B2T2 films, Mn ions were doped into the films and their influence was investigated. Improvement in electrical behavior, especially leakage current density were revealed via impedance spectroscopy and electrochemical studies. Mn doping at an appropriate level improved the electrical properties of the films by affording extrinsic oxygen vacancies that reduced the number of intrinsic oxygen vacancies acting as electron trap sites at the interface between the Pt electrode and the B2T2 film. Schottky emission was posited as the leakage current mechanism in the 10 mol% Mn doped B2T2 (Mn:B2T2) films. The barrier height between the Pt electrode and the Mn:B2T2 film was approximately 1.46eV, but decreased to 0.51eV for the non-doped film due to large numbers of intrinsic oxygen vacancies.

A controllable photoresponse and photovoltaic performance in Bi4Ti3O12 ferroelectric thin films

Polycrystalline Bi4Ti3O12(BIT) thin films were prepared on the fluorine-doped tin oxide glass substrates by a simple sol-gel method. Structural, ferroelectric and optical properties as well as photovoltaic properties and photocurrent responses of the BIT films in different conditions were investigated. The unpoled BIT film revealed an obvious photovoltaic effect with Jsc of about 0.18 μA/cm2 and Voc of about 0.02 V. The photoresponse of the film was increased by applying positive biases, while decreased by negative biases. On the other hand, its photoresponse was markedly increased to almost three times of that of the unpoled film through negative poling. Positive poling not only decreased the photocurrent, but also reversed the photocurrent direction. The wide tunability of photovoltaic performance and photoresponse proved BIT to be a promising alternative material for ferroelectric photovoltaics for applications in ferroelectric optoelectronic and energy fields.

Influence of deposition temperature on structural and ferroelectric properties of Bi4Ti3O12 thin films

Bismuth titanate thin films were deposited using in situ layer-by-layer reactive DC magnetron sputtering. Films were deposited on platinized silicon (Pt/Ti/SiO2/Si) substrates at the temperature of 400–500°C. The study focused on the dependency of structural, morphological and ferroelectric properties on the parameters of material growth. Thin films, which were formed at the temperature of 450–500°C, have dense columnar structure and flat surface. Hysteresis measurements revealed the ferroelectric nature of some of the studied films, except for the films deposited at the temperature lower than 450°C. It was demonstrated that the volatility of bismuth oxide is affected by the temperature starting at 475°C. The highest coercive field and remnant polarization of Ec=130kV/cm and of Pr=73μC/cm2 was obtained for film deposited at 450°C. It appeared that the shape was clearly influenced by a certain extrinsic contribution (other than leakage) which produces an overestimation of the remnant polarization. Based on the results of the current leakage data investigation, it was established that films exhibit the space charge limited conduction.