Bi3.25La0.75Ti3O12 Films Research Articles

Optoelectronic Properties of Ferroelectric Composites of Bi3.25La0.75Ti3O12 (BLT) and Co-Doped BLT Thin Films Modified by FeCo-Doped BLT

Driven by the growing demand for renewable and clean energy, the photovoltaic effect of various solar cells and materials was investigated for the conversion of light energy into electricity. We modified the Bi3.25La0.75Ti3O12 (BLT) and Co-doped BLT (Co–BLT) composites with Fe and Co-doped BLT (FeCo–BLT) films to narrow the bandgap and increase visible light absorption, thereby improving the efficiency of the photovoltaic reaction. In this study, BLT and Co–BLT thin films were fabricated by off-axis sputtering and then modified with FeCo–BLT thin films to produce dual-ferroelectric, thin-film composite materials that improved the photovoltaic power generation performance. Photoelectric test results showed that the modified double-ferroelectric, thin-film composites had superior optoelectronic properties. The current density was significantly enhanced by modifying the BLT films with doped Fe and Co. Therefore, this modification improved the efficiency of ferroelectric thin-film photovoltaic reactions.

Ferroelectric B-Site Modified Bismuth Lanthanum Titanate Thin Films for High-Efficiency PV Systems

Over the past decades, ferroelectric photovoltaic (FE-PV) systems, which use a homogenous ferroelectric material as a light-absorbing layer, have been studied using ferroelectric oxides. The PV activity of materials can be enhanced by adjusting the bandgap of materials, and it would have a large effect on the ferroelectric complex oxides. This phenomenon in epitaxial thin films of ferroelectric complex oxide, Bi3.25La0.75Ti3O12 (BLT), Fe- and Co-doped films were observed. Compared with undoped BLT, Co-(BLCT) doping and Fe and Co combined (BLFCT) doping resulted in the gradual reduction in the bandgap and efficient visible light absorption. The reduction in the bandgap to 11.4% and 18.1% smaller than the experimentally measured Eg of the bismuth titanate-based film using a simple Fe- and Co-doping method was performed, while maintaining ferroelectricity by analyzing the BLCT and BLFCT films based on polarization loops, and the temperature range of the out-of-plane lattice parameters and the photocurrent density of the BLFCT film was 32.2 times higher than that of the BLT film, which was caused by the decrease in the bandgap. This simple doping technique can be used to tune additional wide-bandgap complex oxides so that they can be used in photovoltaic energy conversion or optoelectronic devices.

Large enhancement of the photovoltaic effect in ferroelectric complex oxides through bandgap reduction.

Tuning the bandgap in ferroelectric complex oxides is a possible route for improving the photovoltaic activity of materials. Here, we report the realization of this effect in epitaxial thin films of the ferroelectric complex oxide Bi3.25La0.75Ti3O12 (BLT) suitably doped by Fe and Co. Our study shows that Co (BLCT) doping and combined Fe, Co (BLFCT) doping lead to a reduction of the bandgap by more than 1 eV compared to undoped BLT, accompanied by a surprisingly more efficient visible light absorption. Both BLCT and BLFCT films can absorb visible light with a wavelength of up to 500 nm while still exhibiting ferroelectricity, whereas undoped BLT only absorbs UV light with a wavelength of less than 350 nm. Correlated with its bandgap reduction, the BLFCT film shows a photocurrent density enhanced by 25 times compared to that of BLT films. Density functional theory calculations indicate that the bandgap contraction is caused by the formation of new energy states below the conduction bands due to intermixed transition metal dopants (Fe, Co) in BLT. This mechanism of tuning the bandgap by simple doping can be applied to other wide-bandgap complex oxides, thereby enabling their use in solar energy conversion or optoelectronic applications.

Effects of Bi2O3, TiO2, and Bi4Ti3O12 Seeding Layers on the Structural and Electrical Properties of Bi3.25La0.75Ti3O12 Thin Films Grown by a Sol–Gel Method

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films with Bi2O3, TiO2, and Bi4 Ti3O12 (BTO) seeding layers were prepared on Pt/TiO2/SiO2/Si (100) substrate by a sol–gel technique. The effects of the different seeding layers on the structural and electrical properties of the BLT films were investigated. X-ray diffraction indicated that the predominant orientation of the BLT thin film on the BTO seeding layer was (117). Growth of films with the Pt/BLT/Bi2O3/Pt structure was oriented differently from that of films with the Pt/BLT/TiO2/Pt and Pt/BLT/Pt structures. In addition, in comparison with the BLT film deposited directly on Pt, the TiO2 layer substantially enhanced the leakage current resistance of the BLT film. The ferroelectric nature of the BLT thin film was substantially improved by use of a BTO seeding layer. In an applied field of 750 kV/cm, the remnant polarization (2P r) of the Pt/BLT/BTO/Pt capacitor was 61.5 μC/cm2. After 1010 switching cycles, 2P r of the BLT, BLT/Bi2O3, BLT/TiO2, and BLT/BTO films was degraded by approximately 13, 11, 1, and 2%, respectively, indicating that all the capacitors with the different seeding layers had good polarization fatigue characteristics.

Effects of annealing ambient on ferroelectric properties and surface chemistry of sol–gel derived Bi3.25La0.75Ti3O12 thin films

In this work, Bi3.25La0.75Ti3O12 (BLT) thin films were prepared by sol-gel coatings followed by rapid thermal annealing in Ar or O-2 ambient. The correlation among annealing ambient, ferroelectric characteristics and surface chemistry of the BLT thin films were investigated. The BLT thin film annealed in Ar showed weaker crystallization, less dense surface and smaller polarization value than that annealed in O-2. After 10(9) cycles, the remnant polarization of the BLT film annealed in Ar decreased to 83.5 % of the initial value while it remained 89.5 % for the sample annealed in O-2. X-ray photoelectron spectroscopy results indicated the inferior fatigue characteristics of the sample annealed in Ar was the comprehensive result of oxygen vacancies vicinity to Bi and Ti ion in the thin film.

Mechanical stress modified ferroelectric aging behavior

Mechanical stress effect on aging behavior of Bi3.25La0.75Ti3O12 (BLT) and PbZr0.53Ti0.47O3 (PZT) films was investigated. It is found that the remnant polarization decreases with time while the coercive field increases in stress-free BLT films. For unconfined PZT films, both the remnant polarization and the coercive field decrease as time elapses. The applied tensile stress weakens the aging of remnant polarization of BLT films but strengthens the aging of coercive field, while the applied tensile stress possesses opposite effect. In contrary, the applied compressive stress simultaneously improves the aging behavior of both remnant polarization and coercive field of PZT films. Mechanical-stress-induced variation of domain wall mobility in different materials was suggested as the possible origin of these observations. This work indicates that the aging behavior modification using stress could be realized, and it is helpful for promoting the reliability of ferroelectric films for industrial applications.

Electrical and Physical Properties of the Bi3.25La0.75Ti3O12 Ferroelectric Thin Films Prepared by Conventional Temperature Annealing Process

We have investigated the structure and ferroelectric properties of the Bi3.25La0.75Ti3O12 (BLT) thin films on SiO2/Si substrate fabricated by sol-gel method. We used the BLT films were annealed at various temperatures of 600, 650, and 700°C for one hour by conventional furnace annealing (CTA). The temperature dependence of leakage currents densities of ferroelectric BLT thin films. The crystalline structure of the prepared BLT thin films was analyzed by X-ray diffraction (XRD). Field emission scanning electron microscopy (FESEM) was used to observe the film thickness and the surface morphology including grain size and porosity. The leakage current density and capacitance of thin film were measured by HP4156C.

MAXWELL—WAGNER MECHANISM INDUCED DIELECTRIC RELAXOR IN BiFeO3/Bi3.25La0.75Ti3O12 FILM

Maxwell—Wagner (M-W) theory was used to explain the dielectric relaxor behavior of BiFeO3(BFO)/Bi3.25La0.75Ti3O12(BLT) film grown on Pt/Ti/SiO2/Si substrate. The BFO film was considered to be ferroelectric film, and its temperature dependence of dielectric constant was calculated by Landau theory. BLT film was of dielectric-relaxor behavior and its temperature dependence of dielectric constant was given by Smolenski theory. The leakage currents for the BFO and BLT films were assumed to obey Poole–Frenkel emission and Schottky emission, respectively. The temperature dependent dielectric constant and loss of the BFO/BLT film were characterized, which agrees with the experiment. The enhancement of the polarization in BFO/BLT film may induced by the current leakage restraining in BFO film.

THE EFFECT OF A-SITE La AND Nd SUBSTITUTION ON THE RESIDUAL STRESS OF Bi4Ti3O12 THIN FILMS FABRICATED BY SOL-GEL METHOD

ABSTRACT In this paper, thin films of Bi4Ti3O12 (BTO), Bi3.25La0.75Ti3O12 (BLT) and Bi3.15Nd0.85Ti3O12 (BNT) were fabricated on Pt/Ti/SiO2/Si substrates by sol-gel method. The structures and ferroelectric properties of the polycrystalline films are studied and the correlation between substitution and residual stress are discussed in detail. The results show that, the ferroelectric and fatigue properties of the BLT and BNT films are improved greatly, compared with those of the BTO thin film. The Pr values of BLT and BNT are 13.14 and 21.23 μC/cm2, respectively. X-ray diffraction result shows that the measured residual stress is tensile stress for all films. After A-site substitution, the value of tensile stress for the BTO films has increased and the stress value is maximal for the BNT film. The calculated intrinsic residual stress values are 288.4, 418.4 and 608.2 MPa for BTO, BLT and BNT, respectively.

CO-EFFECT OF ANNEALING TEMPERATURE AND AMBIENT ON FERROELECTRIC PROPERTIES OF Bi3.25La0.75Ti3O12 FILMS

ABSTRACT Co-effect of crystallization ambients (O2 and N2 mixed gas) and temperatures (600–800°C) on crystal structures and ferroelectric properties of the sol-gel deposited Bi3.25La0.75Ti3O12 (BLT) films is studied in this work. As the annealing temperature increases, the films get full crystallization and show different orientation. The film annealed in N2 at 650°C is a-axis oriented, and get large Pr but bad fatigue property, which is caused by the charged oxygen vacancies in the films annealed in the oxygen-deficient ambients. The film annealed in 20% O2 at 750°C is random oriented and exhibits excellent ferroelectric properties in terms of the largest Pr value of 11.5 μC/cm2 and the lowest Ec value of 83.3 kV/cm under a voltage of 12 V. Moreover, the film shows good fatigue endurance after 109 switching cycles. The crystllization and growth of the BLT films are sensitive to annealing ambients and temperatures. Therefore, the appropriate annealing ambient and temperature can improve the properties of the ferroelectric films greatly.

Electrical Properties of Au/(Bi,La)4Ti3O12/Pt/SrTa2O6/Si Field-Effect Transistor

The metal-ferroelectric-metal-insulator-semiconductor field-effect transistor using the Au/(Bi,La)4Ti3O12(BLT)/Pt/SrTa2O6(STA)/Si structure is prepared. The STA and BLT films are spun-casted by the sol-gel method. The equivalent oxide thickness value of the STA thin film is 5.2 nm and negligibly small hysteresis loops have been observed for the Au/STA/Si structure. The leakage current density is lower than 10− 7 A/cm2 under 6 V. The remanent polarization of the 420 nm-thick BLT film was 35.2 μ C/cm2 at 450 kV/cm. The Au/BLT/Pt/STA/Si MFMIS-FET was fabricated with different area ratio (AI/AF) from 1 to 8. From the drain current–gate voltage characteristics at the drain voltage of 0.2 V, the memory window is only 0.5 V for the device with AI/AF = 1 but it is increased to 1.8 V as the AI/AF is increased to 8. For the AI/AF ratio of 4, the drain current of 1.2× 10− 5 A rapidly drops after 1.4× 104 s to 4× 10− 6 A. The retention time improves as the AI/AF ratio is increased to 8 and it has been found to drop after 8.5× 104 seconds.

Microstructures and Electrical Properties of Random-Oriented (Bi,La)<sub>4</sub>Ti<sub>3</sub>O<sub>12</sub> Thin Film Deposited by Pulsed-DC Sputtering Method

Ferroelectric properties of Lead-free (Bi,La)4Ti3O12 (BLT) films were evaluated on the newly developed MTP (Merged Top-electrode and Plate-line) cell structure. The BLT film was deposited by pulsed-DC sputtering method on a buried Pt/IrOx/Ir bottom electrode stack with W-plug. The BLT composition in the sintered sputtering target was Bi4.8La1.0Ti3.0O12. However, the deposited film composition was about Bi4.0La1.0Ti3.0O12 after the heat treatment of crystallization at 700°C/O2/30sec. And grains of the BLT film were randomly oriented and uniformly small ellipsoidal shape (long direction: ~100nm, short direction: ~20 nm). The remnant polarization (2Pr) and the leakage current density measured in the 100nm-thick BLT film were about 21 C/cm2 and 3 ×10-5 A/cm2 at 3 V, respectively. The fatigue loss was about 10% of the initial polarization value after 1×1011 fatigue cycles.

Ferroelectric Properties of Bi3.25La0.75 Ti3O12 Thin Films Crystallized in Different N2/O2 Ambients

Bi3.25La0.75 Ti3O12 (BLT) ferroelectric thin films are deposited by sol-gel method and annealed for crystallizaion in total 1sccm N2/O2 mixed gas with various ratio at 750°C for 30 min. The effect of crystallization ambient on the structural and ferroelectric properties of the BLT films is studied. The growth direction and grain size of BLT film are revealed to affect ferroelectric properties. After the BLT film is annealed in 20% O2, the largest Pr value is obtained, which is ascribed to an increase of random orientation and large grain size. The fatigue property is improved with the concentration of oxygen in the ambient increasing, which is ascribed to annealing in the ambient with high concentrated oxygen adequately decreasing the defects related to lack of oxygen.

Local Structural and Electrical Properties of Ferroelectric Bi<SUB>3.25</SUB>La<SUB>0.75</SUB>Ti<SUB>3</SUB>O<SUB>12</SUB> Thin Films on Pt

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films have been grown by a sol-gel method. Annealing conditions after the drying process have been explored over a wide range of temperature. BLT thin films prepared on Pt/TiO2 coated SiO2/Si(100) were investigated by Raman scattering spectroscopy, atomic force microscopy (AFM), and X-ray photoelectron spectroscopy. Raman spectra reveal the growth route of phase-formation of the BLT films. Ferroelectric domains in the thin films are observed by using AFM, registering the electrostatic force response of the thin films in the presence of a low ac field. It is found that the as-grown domain configuration and switching behavior are strongly dependent on growth temperature. Depth-profiling of the electronic states of Bi, Ti, and La atoms shows the oxidation during the growth.

Effect of annealing pressure on the structure and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films

The effect of annealing pressure on the structure and ferroelectric properties was investigated for Bi3.25La0.75Ti3O12(BLT) thin film prepared on Pt/TiO2/SiO2/p-Si(100) substrate by sol-gel method. The amorphous film samples were annealed at 750 ℃ for 30 min under oxygen pressures varying from 10-4 to 3 atm. Then the structure, crystallization degree, and morphology were characterized by X-ray diffraction (XRD), Raman spectroscopy, and field-emission scanning electron microscope (FSEM) to clarify the effect of annealing pressure on the structure of the film. The XRD and Raman spectroscopy results showed a clear decreasing of the crystallization degree of the film annealed under oxygen pressures of 10-4 and 3 atm. FSEM results showed different growth orientations of grains under different oxygen pressures. The structure of the BLT film was revealed to affect their ferroelectric properties. The largest remanent polarization of 17.8 μC/cm2 with the coercive field of 73.6 kV/cm and good fatigue property were obtained for the film annealed under oxygen pressure of 0.1 atm.

(Bi,La)4Ti3O12 as a ferroelectric layer and SrTa2O6 as a buffer layer for metal-ferroelectric-metal-insulator-semiconductor field-effect transistor

The metal-ferroelectric-metal-insulator-semiconductor field-effect transistor (MFMIS-FET) using the (Bi,La)4Ti3O12 (BLT) as a ferroelectric and SrTa2O6 (STA) as a buffer layer is prepared. The Au/STA/Si structure shows about 1 nF/cm2 of the accumulation capacitance value which is equivalent to about 6.2 nm of SiO2. The leakage current density is lower than 107 A/cm2 under 5 V. The remanent polarization of the 420 nm-thick BLT film was 35.2 μC/cm2 at 450 kV/cm. The MFMIS-FET was fabricated with different area ratio (AI/AF) from 1 to 8. From the drain current-gate voltage characteristics at the drain voltage of 0.2 V, the memory window is only 0.5 V for the device with AI/AF = 1 but it is increased to 1.8 V as the AI/AF is increased to 8. For the AI/AF ratio of 8, the on state of the drain current of 1.12 × 10-5 A rapidly drops after 105 s to 2 × 10-6 A and the off state current increase from 10-7 A to 10-6 A after 105 s. The on/off current ratio decrease from 3 × 102 to 8.

EFFECT OF BISMUTH EXCESS ON THE CRYSTALLIZATION OF Bi3.25La0.75Ti3O12 CERAMIC AND THIN FILM

ABSTRACT Varying amount of Bi2O3 excess Bi3.25La0.75Ti3O12 (BLT) ceramics were prepared by solid state reaction and BLT thin films were fabricated on Pt/TiO2/SiO2/Si using RF magnetron sputtering method. The effect of Bi2O3 excess content on the microstructure and ferroelectric properties of the BLT ceramic and thin film was investigated. The X-ray diffraction analysis reveals a layered peroviskite structure phase without pyrochlore phase in all the BLT ceramics with varying amounts of Bi2O3 excess. The SEM micrographs of the BLT ceramics show plate-like morphology. Compared with other BLT thin films with different amount of Bi2O3 excess, the BLT thin film with 10% Bi2O3 excess shows higher degree of squareness and larger remnant value (2Pr). The remnant polarization (2Pr) and coercive voltage (Vc) of the BLT thin film with 10% Bi2O3 excess are about 17.6 μ C/cm2 and 3.7 V, respectively. The BLT thin film with 10% Bi2O3 excess shows the best fatigue resistance characteristics among all the BLT films and just about 9% polarization degradation was observed in the film after 1× 109 read/write switching cycles.

Data retention characteristics of Bi3.25La0.75Ti3O12 thin films on conductive SrRuO3 electrodes

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films were deposited on SrRuO3 (SRO) and Pt electrodes by pulsed laser deposition. The data retention characteristics of the BLT films on different electrode materials were characterized. The experimental results are well fitted by a stretched exponential decay model, indicating that BLT films on both Pt and SRO exhibit the similar decay mechanisms. However, the decay behavior is quite different due to the use of different bottom electrodes. The superior data retention performance of BLT film on conductive SRO electrode is believed to be from the reduction of domain polarization backswitching originated from the oxygen vacancies in the depletion region of BLT films.

Direct observation of oxygen stabilization in layered ferroelectric Bi3.25La0.75Ti3O12

Electron charge density distributions in layered ferroelectrics Bi4Ti3O12 (BiT) and Bi3.25La0.75Ti3O12 (BLT) are investigated by analyzing high-energy synchrotron-radiation powder diffraction data using the maximum entropy method/Rietveld method. BiT shows that chemical bonding resulting from orbital hybridization is established between Bi–O in the perovskite layer only along the a axis, whereas BLT exhibits isotropic chemical bonding of Bi∕La–O with a high electron density both along the a and b axes. High endurance to polarization fatigue reported for BLT films is suggested to originate from the stabilization of oxygen in the perovskite layer due to the isotropic chemical bonding of Bi∕La–O.

Ferroelectric properties and leakage current mechanisms of Bi3.25La0.75Ti3O12 thin films with a-axis preferred orientation prepared by sol–gel method

The Bi3.25La0.75Ti3O12 (BLT) thin films were prepared on the Pt/Ti/SiO2/Si substrates using the sol–gel process and crystallized subsequently using the conventional (CTA) and rapid thermal annealing (RTA) processes. It was found that the crystalline, structure, ferroelectric and leakage current properties of BLT films strongly depend upon the heating rate of the sol–gel method. BLT thin films crystallized by the RTA method showed a-axis preferred orientation with smaller grain size comparable with those of films crystallized by the CTA process. The values of remanent polarization (2Pr) and the coercive field (2Ec) of film crystallized by the RTA process were 49 μC/cm2 and 105 kV/cm, respectively. This 2Pr value was larger than that of CTA-derived film (2Pr and 2Ec values were 36 μC/cm2 and 125 kV/cm, respectively). The RTA-derived film had a lower leakage current (4.85×10−8 A/cm2) than the latter (6.5×10−8 A/cm2) at 200 kV/cm, and both films demonstrated fatigue-free behavior up to 108 switching cycles.