Bi3.25La0.75Ti3O12 Thin 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.

Reversible magnetoelectric switching in multiferroic three-dimensional nanocup heterostructure films

Self-assembled nanocomposite films containing ferroelectric and ferromagnetic phases have attracted enormous research interest because they are the most promising candidates for practical multiferroic applications. However, obtaining a genuine magnetoelectric (ME) coupling effect is still challenging in this research area. To substantially improve the ME effect, new heterostructure designs with efficient strain control between two phases are urgently needed. Herein, a novel three-dimensional (3D) nanocup architecture of a heterostructure film is developed. To establish the unique architecture, a heavily Co, Fe-doped ferroelectric Bi3.25La0.75Ti3O12 (BLT) target was used during the growth of BLT thin films via pulsed laser deposition. Consequently, 3D nanocup-structured CoFe2O4 (CFO) particles formed inside the BLT via spontaneous nucleation and agglomeration. The 3D nanocup BLT-CFO film exhibited magnetically controlled reversible dielectric switching, which is direct evidence of strong ME coupling caused by the efficient interfacial strain coupling and low leakage of the novel nanocup architecture. The obtained results strongly suggest that the 3D nanocup heterostructure film significantly improves the ME coupling effect. In addition, we propose a new paradigm in the architecture design of self-assembled nanocomposite films for diverse multifunctional devices.

Characterization of highly (117)-oriented Bi3.25La0.75Ti3O12 thin films prepared by rf-magnetron sputtering technique

Ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin films were deposited on Pt(111)/Ti/SiO2/Si substrates at 400 °C using rf-magnetron sputtering method. The microstructures were studied by X-ray diffraction, scanning electron microscopy and energy dispersive spectrometer. The as-deposited thin film is amorphous, while transformed to microcrystalline and well crystalline states after annealing at 650 °C and 750 °C, respectively. After annealing at 750 °C, the polycrystalline BLT thin film showed plated-like grains all with (117)-preferred orientation. Analyses of ferroelectric properties indicated that, comparing with the as-deposited thin film, the highly (117)-oriented crystalline thin film exhibited well-saturated hysteresis loops with a superior remnant polarization (2Pr) of 30.7 μC/cm2.

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.

Mechanical stress effect on leakage current in Bi3.25La0.75Ti3O12 thin films

Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films were prepared on Pt/Ti/SiO2/Si substrates by metalorganic decomposition method. Mechanical stress effect on leakage current was investigated. The results demonstrate that the leakage current increases with increasing electric field. Under mechanical tensile or compressive stress, the increase trend of the leakage current with electric field shows negligible change in low field region (below 75kV/cm), while that gets weakened in high field region (above 75kV/cm) compared with that at free state (zero stress). At a certain electric field (above 75kV/cm), the leakage current decreases with increasing tensile or compressive stress. The effect of domain reorientation induced by stress combined with the contribution from charge injected into films is found to be reasonable for the explanation of the leakage current in BLT thin films under different stress.

Dielectric functions and energy band gap variation studies of manganese doped Bi3.25La0.75Ti3O12 thin films using spectroscopic ellipsometry

Single phase polycrystalline Mn-modified Bi3.25La0.75Ti3O12 (BLT) thin films were prepared by chemical solution deposition method using spin coating technique on Pt/Ti/SiO2/Si (100) substrates. Raman spectroscopy of these films shows that Mn3+ is well substituted at Ti4+ site. The optical properties of BLT and Mn modified BLT thin films were investigated at room temperature by using spectroscopic ellipsometry (SE) in the energy range 0.72–6.2eV. A double Tauc–Lorentz (DTL) dispersion relation was successfully used to model the dielectric functions of these films where a shift to the lower energy side with Mn doping is seen. The full width at half maxima (FWHM) (Г) of dielectric function is found to increase with Mn doping. This increase in FWHM may be attributed to the increase in the trap density in forbidden band which consequently decreases the value of direct optical band gap (Egd). The direct optical band gap (Egd) is found to decrease with increase in Mn content in the studied composition range. This decrease in Egd with doping may be attributed to the variation in the defect concentration present in the structure.

Mechanism for leakage current conduction in manganese doped Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films

Bi3.25La0.75Ti3O12 (BLT) is one of the prospective candidate materials for data storage devices application. In order to understand the leakage current conduction mechanism in BLT we have studied Mn doped Bi3.25La0.75Ti3O12 (BLT) thin films with varying concentration of Mn. These films were prepared by chemical solution deposition technique. The structure and phase analysis of these films was performed at room temperature by X-ray diffraction, and scanning electron microscope was used to investigate the surface morphology. Different leakage current conduction mechanisms, e.g., Schottky emission, Modified Schottky, Poole–Frenkel emission and space charge limited (Lampert’s theory) conduction were examined to explain the true nature of charge transport phenomena in BLT and Mn doped BLT thin films. It was found that BLT and Mn doped BLT thin films leakage current conduction mechanism follows the Lampert’s theory of space charge limited conduction in an insulator with traps.

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.

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.

The crystalline orientation and ferroelectric properties of Bi3.25La0.75Ti3O12 thin films mediated by the intermediate layer of LaNiO3

c-Axis-oriented and (117)-oriented Bi3.25La0.75Ti3O12 (BLT) thin films are successfully controlled by the intermediate layer of LaNiO3 (LNO) with chemical solution deposition (CSD), respectively. X-ray diffraction (XRD) demonstrates that the structure and orientation of LNO thin films have a strong effect on the orientation of BLT thin films. Scanning electron microscopy suggests that BLT thin films on LNO electrode exhibit crack-free, uniform size grains and dense microstructure. A crystalline orientation dependent remanent polarization is observed in BLT thin films, and it is found that the remanent polarization (2Pr) of (117)-oriented films is larger than that of c-axis-oriented films. Our research directly demonstrates that the vector of the main spontaneous polarization in these layered perovskite materials (BLT) is along a-axis.

Dielectric Functions of Niobium Doped Bi3.25La0.75Ti3O12 Thin Films Using Spectroscopic Ellipsometry

Ferroelectric thin films of Nb doped Bi3.25La0.75Ti3O12 (BLT) were prepared by chemical solution deposition method onto Pt/Ti/SiO2/Si(100) substrates using spin coating technique. From the X-ray diffraction analysis it was found that films were polycrystalline with single phase. The optical properties of the Nb doped BLT thin films were investigated at room temperature in the 0.72–6.2 eV energy range using spectroscopic ellipsometry (SE). Double Tauc–Lorentz (DTL) dispersion relation was successfully fitted in the uv/visible region. Value of the full width at half maxima (Г) in dielectric function plots increased with Nb doping. This increase in FWHM may be attributed to the increase in the trap density in forbidden band which consequently decreases the value of Eg. The optical band gap energy (E g d) was found to decrease with increase in Nb doping concentration. This decrease in E g d with doping may be due to the changes in the defect concentration present in the structure.

Effect of annealing pressure on structure and properties of ferroelectric Bi3.25La0.75Ti3O12 thin films prepared by sol–gel method

The effect of annealing pressure was investigated for Bi3.25La0.75Ti3O12 (BLT) thin films prepared on Pt/TiO2/SiO2/p-Si(100) substrates by sol–gel method. The amorphous films were annealed at 750 °C for 30 min under different oxygen pressures varying from 10−4 to 3 atm. The largest Pr of 17.8 μC/cm2 with the Ec of 73.6 kV/cm was obtained for the film annealed under 0.1 atm PO2. 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 ferroelectric properties. The XRD and Raman spectroscopy results indicated a clear decreasing of the crystallization degree of the films annealed under 10−4 and 3 atm PO2. FSEM results showed the different growth orientation of grains under different oxygen pressures. This study indicated some important effects of annealing pressure on the physical properties of BLT thin films.

Bi<sub>4</sub>Ti<sub>3</sub>O<sub>12</sub> and Bi<sub>3.25</sub>La<sub>0.75</sub>Ti<sub>3</sub>O<sub>12</sub> Thin Films Prepared by RF Magnetron Sputtering

Bi4Ti3O12 (BTO) and Bi3.25La0.75Ti3O12 (BLT) ferroelectric thin films were deposited on Pt/Si substrates by RF magnetron sputtering with Bi4Ti3O12 (BTO) and Bi3.25La0.75Ti3O12 (BLT) targets with 50-mm diameter and 5-mm thickness. The microstructure and ferroelectric properties of thin films were investigated. The grain growth behavior and ferroelectric properties such as remanent polarization were different in these two kinds of film, the effects of La doping in the BLT thin film were very obvious.

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.

Ferroelectric properties of Bi3.25La0.75Ti3O12 films using HfO2 as buffer layers for nonvolatile-memory field-effect transistors

The ferroelectric Bi3.25La0.75Ti3O12 (BLT) thin film and HfO2 layer were fabricated using both metal-organic decomposition and atomic-layer deposition methods. The HfO2 thin film was deposited as a buffer layer between Si substrate and BLT thin films. The electrical and structural properties of the metal-ferroelectric-insulator-semiconductor (MFIS) structure were investigated by varying the HfO2 layer thickness. Transmission electron microscopy showed no interdiffusion and reaction occurring when the HfO2 film is used as a buffer layer. The width of the memory window in the capacitance-voltage curves for the MFIS structure was decreased with increasing thickness of the HfO2 buffer layer. The experimental results showed that the BLT-based MFIS structure is suitable for nonvolatile-memory field-effect transistors with a large memory window.

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.

FERROELECTRIC PROPERTIES OF Zr-DOPED Bi3.25La0.75Ti3O12 THIN FILM DEPOSITED BY RF MAGNETRON SPUTTERING

ABSTRACT Thin films of Bi3.25La0.75Ti3O12 (BLT) and B-site substituted BLT by Zr, i.e. Bi3.25La0.75Ti3−x Zr x O12 (BLTZx, x = 0.20, 0.50, 0.75, 1.00 and 1.50) were fabricated on Pt/TiO2/SiO2/Si substrates by RF magnetron sputtering method. Effect of Zr4+ amount on the microstructure and ferroelectric characteristics of the thin film BLTZx were investigated. X-ray diffraction shows that A-site La3+ and B-site Zr4+ co-substitution do not destroy the layered perovskite structure. Compared with the well-known BLT thin films, appropriate Zr4+ added, such as the BLTZ0.20 thin film, has larger remnant polarization (2Pr) and better fatigue resistance. However, with further increasing Zr4+-doping concentration, the remnant polarization (2Pr) tends to decrease. When x > 0.75 the remnant polarization become lower than that of BLT thin film. The remnant polarization (2Pr) of the BLTZx thin films under the 12 V were 17.8, 25.6, 22.4, 17.2, 11.6, 10.2 μC/cm2, respectively, for x = 0, 0.20, 0.50, 0.75, 1.00 and 1.50, whereas there are almost no obvious difference in the Vc values.

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.

Characteristics of Ag/Bi3.25La0.75Ti3O12/p-Si heterostructure prepared by sol-gel processing

The metal-ferroelectric-semiconductor (MFS) heterostructure has been fabricated using Bi3.25La0.75Ti3O12 (BLT) as a ferroelectric layer by sol-gel processing. The effect of annealing temperature on phase formation and electrical characteristics of Ag/BLT/p-Si heterostructure were investigated. The BLT thin films annealed at from 500°C to 650°C are polycrystalline, with no pyrochlore or other second phases. The C-V curves of Ag/BLT/p-Si heterostructure annealed at 600°C show a clockwise C-V ferroelectric hysteresis loops and obtain good electrical properties with low current density of below 2×10−8 A/cm2 within ±4 V, a memory window of over 0.7 V for a thickness of 400 nm BLT films. The memory window enlarges and the current density reduces with the increase of annealing temperature, but a annealing temperature over 600°C is disadvantageous for good electrical properties.

Effect of vanadium doping on ferroelectric and electrical properties of Bi3.25La0.75Ti3O12 thin film

Bi3.25La0.75Ti3O12 (BLT) and V-doped BLT (BLTV) thin films were prepared on Pt/Ti/SiO2/Si substrates by a pulsed laser deposition method. The effects of V doping on ferroelectric and electrical properties were investigated by polarization-electric field hysteresis loops and leakage current-voltage measurements. BLTV single phases were confirmed by X-ray diffraction. Remnant polarization was increased and the leakage current density was decreased by V doping. The leakage current density of BLT thin films suddenly increased at 100 kV/cm while that of BLTV thin films increased at the higher electric field of 160 kV/cm. The power law relationship J α En of current density vs. applied electric field is estimated to be J αE2.0 for BLT and J αE1.0 for BLTV thin films. The leakage current of the BLT/Pt junction can be explained by space-charge-limited current. However, that of the BLTV/Pt junction was characterized by the Schottky emission behavior.