Lanthanum-modified Bismuth Titanate Research Articles

Tunable band gap of iron-doped lanthanum-modified bismuth titanate synthesized by using the thermal decomposition of a secondary phase

The photoelectric properties of complex oxides have prompted interest in materials with a tunable band gap, because the absorption The substitution of iron atoms in La-modified bismuth titanate (BLT) can lead to dramatic improvements in the band gap, however, the substitution of iron atoms in BLT without forming a BiFeO3 secondary phase is quite challenging. Therefore, a series of Fe-doped BLT (Fe-BLT) samples were characterized using a solid reaction at various calcination temperatures (300~900{\deg}C) to remove the secondary phase. The structural and optical properties were analyzed by X-ray diffraction and ultraviolet-visible absorption spectroscopy. This paper reports a new route to synthesize a pure Fe-BLT phase with a reduced optical band gap by high temperature calcination due to the thermal decomposition of BiFeO3 during high temperature calcination. This simple route to reduce the second phase can be adapted to other complex oxides for use in emerging oxide optoelectronic devices.

Synthesis, consolidation and dielectric properties of BLT ceramics processed by M-PVA gel ignition technique

Polycrystalline samples of lanthanum modified bismuth titanate ceramics with composition Bi3·25La0·75Ti3O12 (BLT) were prepared at three different sintering temperatures (1050, 1100 and 1150°C) using M-PVA gel ignition technique. The samples were characterised for structural and electrical properties. A significant amount of as burnt BLT powder was found to be crystalline in nature having a yellowish colour. Very fine BLT nanoparticles were characterised by TEM analysis. Single phase with orthorhombic crystal structure was observed for all the samples, confirmed by X-ray analysis. Dielectric properties were studied as a function of frequency and temperature. More than 95% relative density was observed for all the three sintered samples. The sample sintered at 1100°C showed highest dielectric constant and relative density and lowest loss.

Correlating structure, dielectric and impedance studies with lanthanum-ion substitution in bismuth titanate

Abstract A correlated study involving structure analysis, temperature dependence of dielectric functions and the complex impedance spectroscopy analysis was carried out in lanthanum modified bismuth titanate (BT, Bi4−xLaxTi3O12; x = 0, 0.75) ceramics. A series addition of three RQ circuits has been shown to remarkably fit the complex impedance data for both x = 0 and 0.75 samples, which were attributed to crystalline layer, plate boundary and grain boundary microelements. Temperature dependence DC-conductance of these microelements has revealed that lanthanum-ion substitution is mainly taking place in the perovskite blocks, which also led to decrease in orthorhombic splitting. This site preferred substitution was also consistent with shift in the oxygen ion-jump relaxation peak in the temperature dependence of the dissipation factor study.

LASER TRANSFER PROCESSING AND THE INTEGRATION OF FERROELECTRIC FILMS

ABSTRACT Thin films of barium strontium titanate (BST), lanthanum modified bismuth titanate (LBT) and lead zirconate titanate (PZT) were fabricated by sol-gel methods onto sapphire substrates. They were transferred to a second substrate by using UV eximer laser radiation to delaminate the films from the fabrication substrate. Scanning electron microscopy revealed melting at the interfacial region of the LBT and PZT films, thus providing the delamination mechanism, but melting was not observed in the BST films. Only the PZT film, after laser transfer, retained its ferroelectric properties, with remnant polarisation of ∼30 μC/cm2, and a high coercive field of 150 kV/cm.

Bi3.25La0.75Ti3O12 powders by the complex polymerization method: synthesis, characterization and morphology

Lanthanum-modified bismuth titanate (Bi3.25La0.75Ti3O12, BLTO) powders were prepared by the complex polymerization method. The structure and morphology of BLTO powders were investigated by X-ray diffraction and scanning electron microscopy. The complexation of citric acid with the metallic cations was detected by Fourier transformed infrared (FT-IR). The thermal analyses of obtained gels were investigated by differential thermal gravimetric (DTG). The pure and normally stoichiometric phase of BLTO powders could be obtained at relatively low temperature of 550–700 °C even if the bismuth content is not excess in the starting precursors, while the secondary phase could be detected at lower and higher calcination temperatures. The shape of the BLTO grains is similarly to platelet in Bi-layer structure and stoichiometry BLTO was detected by the analysis of energy dispersive spectrometry.

Effect of Calcination Temperature on the Microstructure and Morphology of Bi3.25La0.75Ti3O12 Powders Synthesized from Polymerized Precursors

Lanthanum-modified bismuth titanate, Bi3.25La0.75Ti3O12 (BLT), powders were prepared from polymerized precursors using citric acid as a chelating agent and ethylene glycol as an esterification agent. The effect of calcination temperature on the microstructure and morphology of powders were studied. It was shown that the pure and normally stoichiometric phase of BLT powders could be obtained at relative low temperature from 550o to 700oC even if the bismuth content is not excess in the starting precursors while the secondary phase could be detected at both lower and higher calcination temperatures. The shape of the BLT grains is also strongly influenced by the calcination temperature.

Effect of Annealing Temperature on Electrical Properties of Ferroelectric Bi3.25La0.75Ti3O12 Capacitors

Lanthanum-modified bismuth titanate (Bi3.25La0.75Ti3O12, BLT) thin films are fabricated on platinized Si wafers by the sol-gel method, and the effect of annealing temperatures ranging from 650°C to 800°C on the electrical properties of Pt/BLT/Pt capacitors are investigated. It is found that polarization and leakage current of BLT capacitors strongly depend on the annealing temperature although all the capacitors demonstrate very similar characteristics, except the value of polarization, in pulse-width dependence, retention, and fatigue. Remanent polarization increases with the increase of annealing temperature, and annealing temperature of 700°C can yield the largest remanent polarization, and then polarization decreases with increasing annealing temperature. For the 700°C annealed Pt/BLT/Pt capacitor, the remanent polarization 2Pr and the coercive field 2Ec, at an electric field of 226kV/cm, are 23.8μC/cm2 and 130kV/cm, respectively. Dielectric breakdown voltages of BLT films annealed at 750°C and 800°C are much lower than those annealed at 650°C and 700°C. At 100kV/cm, the leakage currents of BLT films prepared at 650°C and 700°C are only 1.5 × 10−6 A/cm2 and 8. 9 × 10−7 A/cm2, respectively. Moreover, all the Pt/BLT/Pt capacitors exhibit excellent retention properties after a cumulative time of 1 × 104 s and do not show any significant fatigue up to 1 × 1010 switching cycles at frequency of 1 MHz.

Size effects of polycrystalline lanthanum modified Bi 4Ti 3O 12 thin films

The film thickness dependence on the ferroelectric properties of lanthanum modified bismuth titanate Bi 3.25La 0.75Ti 3O 12 was investigated. Films with thicknesses ranging from 230 to 404 nm were grown on platinum-coated silicon substrates by the polymeric precursor method. The internal strain is strongly influenced by the film thickness. The morphology of the film changes as the number of layers increases indicating a thickness dependent grain size. The leakage current, remanent polarization and drive voltage were also affected by the film thickness.

Dielectric properties of pure and lanthanum modified bismuth titanate thin films

We investigated the dielectric properties of pure and lanthanum modified bismuth titanate thin films obtained by the polymeric precursor method. X-ray diffraction of the film annealed at 300 ° C for 2 h indicates a disordered structure. Lanthanum addition increases gradually the dielectric permittivity of films, keeping unchanged their loss tangent. From C– V curve we can see no hysteresis behavior indicating the absence of domain structure. The decrease in the conductivity for the heavily doped Bi 4Ti 3O 12 (BIT) must be associated to the unidentified crystal defects. For comparison, dielectric properties of crystalline BIT film were also investigated.

Impact of Pt bottom electrode on the properties of ferroelectric Bi 3.25La 0.75Ti 3O 12 capacitors

Lanthanum-modified bismuth titanate (Bi 3.25La 0.75Ti 3O 12) (BLT) films have been prepared on platinized silicon substrates using sol–gel method. Highly (001) or (111) oriented platinum layers are used as the bottom electrodes of ferroelectric BLT capacitors to study the effects of Pt orientation on the structural and physical properties of BLT films. X-ray diffraction (XRD) and atomic force microscopy (AFM) measurements demonstrate that the microstructure and morphology of BLT films are not strongly affected by the Pt films. BLT film on highly (111) oriented Pt possesses much higher remanent polarization (∼ 8 μC/cm 2) than that on highly (001) oriented Pt although the coercive fields of the two kinds of Bi 3.25La 0.75Ti 3O 12 films are almost equal.

Preparation of lanthanum doped Bi 4Ti 3O 12 ceramics by the polymeric precursor method

Lanthanum-modified bismuth titanate, Bi 4 − x La x Ti 3O 12 (BLT) ceramics, with x ranging from 0 to 0.75 were prepared by the polymeric precursor method. Orthorhombicity of the system is decreased with the increase of lanthanum content in the bismuth titanate (BIT) crystal lattice. No secondary phases were evident after lanthanum addition. Increasing lanthanum content causes a structural distortion in the bismuth titanate lattice. The shape of the grains is strongly influenced by the lanthanum added to the system.

Ferroelectric and Dielectric Properties of Lanthanum-Modified Bismuth Titanate Thin Films Obtained by the Polymeric Precursor Method

Lanthanum-modified bismuth titanate, Bi4 − xLaxTi3O12 (BLT), thin films with a La concentration of 0.75 was grown on Pt/Ti/SiO2/Si substrates by using the polymeric precursor solution and spin-coating method. The scanning electron microscopy (SEM) showed rounded grains, which is not typical for these system. The BLT films showed well-saturated polarization-electric field curve which 2Pr = 41.4 μC/cm2 and Vc = 0.99 V. The capacitance dependence on the voltage is strongly nonlinear, confirming the ferroelectric properties of the film resulting from the domains switching. These properties make BLT a promising material for FERAM applications.

Characterization of lanthanum-doped bismuth titanate thin films prepared by polymeric precursor method

Lanthanum-modified bismuth titanate, Bi 4− x La x Ti 3O 12 (BLT), with x ranging from 0 to 0.75 was grown on Pt/Ti/SiO 2/Si substrates using a polymeric precursor solution and spin-coating method. The dielectric constant of highly doped bismuth titanate was equal to 148 while dielectric losses remained low (tan δ=0.0018), and the films showed well-saturated polarization-electric field curves (2 P r=40.6 μC/cm 2 and V c=0.99 V). The leakage current densities improve for the lanthanum-doped system. For five-layered BLT films with x=0.75, a charge storage density of 35 fC/μm 2 and a thickness of 320 nm were found.

Electrical Characterization of Lanthanum-Modified Bismuth Titanate Thin Films Obtained by the Polymeric Precursor Method

Lanthanum-modified bismuth titanate, Bi− x LaxTi3O12 (BLT), thin films with a La concentration ranging from 0 to 0.75 were grown on Pt/Ti/SiO2/Si substrates using a polymeric precursor solution and spin-coating method. The scanning electron microscopy shows a change of morphology with increasing the lanthanum concentration. The BLT films show well-saturated polarization-electric field curves whit remnant polarizations of 14.7, 20.5, 21.5, and 20.4 μ C/cm2 for × = 0, 0.25, 0.50 and 0.75, respectively. The dielectric constant of BLT (x = 0.75 mol% La) is equal to 158 while dielectric loss remain low (tanδ = 0.0018).

Lanthanum modified bismuth titanate prepared by a hydrolysis method

La-modified bismuth titanate ceramics with compositions of Bi4−xLaxTi3O12 (x = 0, 0.2, 0.5, 0.75) were fabricated using powders prepared by a hydrolysis method. It was found that the partial substitution of lanthanum for bismuth in the Bi4Ti3O12 lattice significantly influenced the crystallization behavior, IR characteristics, and sinterability of the powder, as well as the microstructure and the dielectric properties of the sintered ceramics. The formation of the Bi2Ti2O7 intermediate phase during the powder crystallization process was suppressed, and the IR bands corresponding to Ti–O stretching vibrations were shifted to higher wavenumbers in La-modified bismuth titanate. With an increase in the doping level of lanthanum, the sintering temperature was steadily elevated, while the grain growth speed was reduced; and the dielectric property measurement indicated a continuous decrease in the Curie temperature and dielectric loss.

Ferroelectric properties of highly c-axis oriented Bi 4− xLa xTi 3O 12 film-based capacitors

Highly c-axis oriented lanthanum-modified bismuth titanate (Bi 4− x La x Ti 3O 12) films having a variety of lanthanum (La) contents were grown on Pt/TiO 2/SiO 2/Si(100) substrates using metal-organic sol deposition and subsequent annealing at 650 °C for 1 h. After systematically examining the ferroelectric properties of Bi 4− x La x Ti 3O 12 films as a function of the La-content, it was concluded that the film with x=0.85 had the largest remanent polarization in the direction parallel to the c-axis. The Pt/Bi 3.15La 0.85Ti 3O 12/Pt capacitor showed a well-saturated polarization–electric field (P–E) switching curve with the switching remanent polarization (2 P r) value of 33 μC/cm 2 and the coercive field ( E c) of 68 kV/cm at an applied voltage of 10 V. More importantly, the capacitor exhibited fatigue-free behavior up to 6.5×10 10 read/write switching cycles at a frequency of 1 MHz. The capacitor also demonstrated an excellent charge-retaining ability and a strong resistance against the imprinting failure.

Ferroelectric properties of alkoxy-derived lanthanum-modified bismuth titanate thin films

Lanthanum-modified bismuth titanate (BLT) thin films were fabricated by sol-gel processing with alkoxide solutions, and their physical and electrical properties were investigated as a first step to develop FeRAM devices for use in automobiles. Alkoxide solutions with compositions of Bi:La:Ti = 3.25:0.75:3 and 3.5:0.5:3 were prepared, and applied to formation of a Pt/BLT/Pt/Ti capacitor structure on oxidized Si wafers by spin coating, drying, calcining, and firing. The films have textured structures of polycrystalline BLT with c-planes parallel to the surface. The film prepared from the solution of 3.5:0.5:3 and fired three times at 750 °C for 20min showed a P-E hysteresis loop with remanent polarization (Pr) of 7.5 μC/cm2 and a coercive field (Ec) of 200 kV/cm. Fatigue tests up to 3 × 108 cycles were conducted both at room temperature (RT) and 150 °C. Only slight decrease of the switching polarization was observed at RT, whereas the polarization slightly decreased till 106 cycles, and then increased about three times compared to an initial value at 150 °C.

La-modified bismuth titanate thin films prepared by Sol-Gel process.

The lead-free ferroelectric lanthanum modified bismuth titanate (Bi4-xLaxTi3O12, BLT-10x) thin films were prepared by sol-gel method using tetrabyl titanate, bismuth nitrate and lanthanum nitrate. Polycrystalline BLT-5 thin films were obtained at relatively low annealing temperatures of 600-650°C. The intensity of XRD peaks of BLT-5 thin films was increased with the increase of annealing temperature. The typical cohesive electric field (Ec) and remnant polarization (Pr) for BLT-5 thin films annealed at 650°C were Ec=65kV/cm, Pr=11.2μC/cm2. The affection of heat treatment on the crystalline and electric properties of BLT-5 thin films was also discussed.