Ferroelectric Bi4Ti3O12 Thin Films Research Articles

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.

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.

Effect of Tb content on microstructure and ferroelectric properties of Bi4−x Tb x Ti3O12 thin films grown by sol–gel method

The effects of Tb substitution on the structural and electrical properties of ferroelectric Bi4Ti3O12 (BTO) thin films grown on Pt/TiO2/SiO2/Si substrates by a sol–gel process have been reported. X-ray diffraction indicated A-site Tb substitutions did not change the polycrystalline bi-layered Aurivillius structure of the BTO, but a lattice distortion was observed. The leakage current behavior at room temperature of the films was studied and it was found that the leakage current density decreased from 10−2 to 10−4 A/cm2 with the increase of x under 150 kV/cm. The remnant polarization (2P r ) and dielectric constant (e r) increase firstly and then decreases with the increase of the Tb content. We observed a substantial increase in the remnant polarization (2P r ) with Tb substitution and obtained a maximum value of~60 μC/cm2 at an applied electric field of 500 kV/cm for x = 0.4. Moreover, this BTT-0.4 capacitor did not show fatigue behaviors after 1.0 × 1010 switching cycles, suggesting an anti-fatigue character.

Preparation and characterization of transparent Bi3.6Ho0.4Ti3O12/ZnO:Al ferroelectric-semiconductor heterostructure by pulsed laser deposition

Ho-doped Bi4Ti3O12 ferroelectric thin films with the composition of Bi3.6Ho0.4Ti3O12 (BHT) were integrated with the epitaxial (0001)-oriented Al-doped ZnO (AZO) semiconductor thin films grown on c-sapphire substrates by pulsed laser deposition using a pure phase BHT ceramic target. The structure characterizations indicated that the BHT film grown on AZO (0001) was a single phase structure of Bi-layered Aurivillius phase bismuth titanate and showed (100)-preferred orientation. The BHT/AZO heterostructure was with a smooth interface and a uniform microstructure. All samples with various thickness were transparent and the average transmittance of (500nm)BHT/(300nm)AZO was higher than 89% in the range of 400–800nm.

Electrical and Physical Characteristics of Vanadium-Doped Bi4Ti3O12 Ferroelectric Thin Films after Rapid Thermal Annealing

The V2O5-doped Bi4Ti3O12 of Bi3.9Ti2.9V0.08O12 (BTV) ceramic was used as the target, and the BTV thin films were deposited on the Pt/Ti/SiO2/silicon substrate by radio frequency (RF) magnetron sputtering. The physical characteristics of BTV thin films were prepared under different conditions to find the optimal deposited parameters. The electrical characteristics of BTV thin films under the optimal sputtering and annealing parameters were prepared by using metal-ferroelectric-metal structure. Additionally, the remnant polarization of deposited BTV thin films would be improved by rapid thermal annealing process from our study, and 2Pr was about 40μC/cm2. Finally, the nonvolatile and non-destructive characteristics of one transistor-capacitor (1TC) structure FRAM device, using the annealed BTV films as the gate oxide, were be fabricated and discussed.

Bright up-conversion green photoluminescence in Ho3+-Yb3+ co-doped Bi4Ti3O12 ferroelectric thin films

Ho3+-Yb3+ co-doped bismuth titanate ferroelectric thin films were prepared by a chemical solution deposition method on fused silica substrates and their up-conversion luminescence characteristics excited by a 980 nm diode laser were investigated. The two emission bands centered at 546 and 656 nm in the emission spectra can be assigned to 5F4 + 5S2 → 5I8 and 5F5 → 5I8 transitions of Ho3+ ions, respectively. A bright green emission was observed even when the laser pumping power was relatively low. The dependence of the emission intensity on the pumping power indicated that the up-conversion emission in the thin films was a two-photon process. The up-conversion emission mechanism is discussed in detail. This study suggests that Ho3+ and Yb3+ co-doped Bi4Ti3O12 thin films can be applied to the fabrication of new multifunctional photoluminescence ferroelectric thin-film devices.

Structural and Electrical Properties of Direct-Patternable Bi4-XNdXTi3O12 Ferroelectric Thin Films

Variations in the structural and electrical properties of Bi4Ti3O12 ferroelectric thin films in the Aurivillius family were investigated by substituting Nd3+ for Bi as Bi4-xNdxTi3O12 (x = 0, 0.35, 0.65, and 0.85) using a direct-patternable photochemical metal-organic deposition method. Photoemission spectroscopic and X-ray absorption spectroscopic analyses revealed that as the substitution of Nd3+ for Bi increased until x = 0.65, the orthorhombicity decreased followed by an increase in covalent mixing and centro-symmetry of the Ti-O octahedrons. However, an increase in polarization of the Ti-O octahedron due to Jahn-Teller distortion was observed due to the generation of oxygen vacancies at x = 0.85.

Strong upconversion luminescence properties of Yb3+ and Er3+ codoped Bi4Ti3O12 ferroelectric thin films

Strong upconversion green luminescence was observed in Er3+ and Yb3+ codoped Bi4Ti3O12 ferroelectric thin films. The thin films were prepared on fused silica and Pt/TiO2/SiO2/Si substrates by chemical solution deposition method. There are two strong green emission bands centered at 524 and 545 nm and a weak red emission band centered at 667 nm in the upconversion luminescence spectra pumped by 980 nm at room temperature. They correspond to the radiative relaxation of Er3+ from H211/2, S43/2, and F49/2 levels to the ground level I415/2, respectively. Bi3.65Yb0.3Er0.05Ti3O12 thin films exhibit higher green emission intensity by a factor of about 30 compared with Bi3.95Er0.05Ti3O12 thin films. The upconversion emission mechanism of the thin films has been discussed. In addition, Bi3.65Yb0.3Er0.05Ti3O12 ferroelectric thin films on Pt/TiO2/SiO2/Si substrates also show well-defined hysteresis loops with a remnant polarization (2Pr) of 28.2 μC/cm2. This study opens the possibility of constructing new integrated photoluminescent ferroelectric thin film devices.

Electrical Characteristics of Bi4Ti3O12 Ferroelectric Thin Films Annealed under Different Temperature for Applications in Nonvolatile Memory Devices

Ferroelectric Bi4Ti3O12 thin films are deposited on Pt/Ti/Si(100) (MFM structure) and SiO2/Pt/Ti/Si (100) (MFIS) substrates using RF magnetron sputtering at room temperature. The Bi4Ti3O12 thin films are then annealed by a rapid thermal annealing (RTA) process conducted in oxygen atmosphere ranging from 500°C–700°C. X-ray diffraction examination reveals that the crystalline orientation of 700°C-annealed Bi4Ti3O12 thin films is better than that of 500°C-annealed ones. Finally, the top view and cross-sectional images of SEM, memory windows, leakage currents and polarization characteristics of the Bi4Ti3O12 thin films are well developed

Ferroelectric and dielectric properties of SrBi4Ti4O15 thin films grown on Bi4Ti3O12 film layer

Ferroelectric and dielectric properties of bilayered ferroelectric thin films, SrBi4Ti4O15 grown on Bi4Ti3O12, were investigated. The thin films were annealed at 700°C under oxygen atmosphere. The bilayered thin films were prepared on a Pt(111)/Ti/SiO2/Si substrate by a chemical solution deposition method. The dielectric constant and dielectric loss of the bilayered thin films were 645 and 0.09, respectively, at 100 kHz. The value of remnant polarization (2Pr) measured from the ferroelectric thin film capacitors was 60.5 μC/cm2 at electric field of 200 kV/cm. The remnant polarization was reduced by 22% of the initial value after 1010 switching cycles. The results showed that the ferroelectric and dielectric properties of the SrBi4Ti4O15 on Bi4Ti3O12 ferroelectric thin films were better than those of the SrBi4Ti4O15 grown on a Pt-coated Si substrate suggesting that the improved properties may be due to the different nucleation and growth kinetics of SrBi4Ti4O15 on the c-axis-oriented Bi4Ti3O12 layer or on the Pt-coated Si substrate.

Polarization offsets of compositionally graded Nd-substituted Bi4Ti3O12 ferroelectric thin films

The vertical offsets of hysteresis loops of compositionally graded Nd-substituted Bi4Ti3O12 (BNdT) ferroelectric films were studied as functions of the Nd gradient, the temperature, and the amplitude of driving voltage. We show that graded BNdT films may exhibit no offsets at room temperature and both graded and nongraded BNdT films exhibit pronounced offsets at elevated temperatures. The offset does not have a direct relation with the composition gradient but increases as the maximum voltage is increased. It is verified that the offset increases with the increase in asymmetry of leakage currents at positive and negative biases.

The Microstructure and Ferroelectric Properties of Ce-Doped Bi4Ti3O12Thin Films Fabricated by Liquid Delivery MOCVD

Ferroelectric Ce-doped Bi₄Ti₃O₁₂ (BCT) thin films were deposited by liquid delivery metal organic chemical vapor deposition (MOCVD) onto a Pt(111)/Ti/SiO₂/Si(100) substrate. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to identify the crystal structure, the surface, and the cross-section morphology of the deposited ferroelectric films. After annealing above 640℃, the BCT films exhibited a polycrystalline structure with preferred (00l) and (117) orientations. The BCT film capacitor with a top Pt electrode showed a large remnant polarization (2P r ) of 44.56 μC/㎠ at an applied voltage of 5 V and exhibited fatigue-free behavior up to 1.0×10¹¹ switching cycles at a frequency of 1 ㎒. This study clearly reveals that BCT thin film has potential for application in non-volatile ferroelectric random access memories and dynamic random access memories.

Effects of Lanthanides-Substitution on the Ferroelectric Properties of Bismuth Titanate Thin Films Prepared by MOCVD Process

Ferroelectric lanthanides-substituted Bi₄Ti₃O₁₂ (Bi 4-X Ln X Ti₃O₁₂, BLnT) thin films approximately 200㎚ in thickness were deposited by metal organic chemical vapor deposition onto Pt(111)/Ti/SiO₂/Si(100) substrates. Many researchers reported that the lanthanides substitution for Bi in the pseudo-perovskite layer caused the distortion of TiO6 octahedron in the a-b plane accompanied with a shift of the octahedron along the a-axis. In this study, the effect of lanthanides (Ln=Pr, Eu, Gd, Dy)-substitution and crystallization temperature on their ferroelectric properties of bismuth titanate (Bi₄Ti₃O₁₂, BIT) thin films were investigated. As BLnT thin films were substituted to lanthanide elements (Pr, Eu, Gd, Dy) with a smaller ionic radius, the remnant polarization (2P r ) values had a tendency to increase and made an exception of the Eu-substituted case because Bi 4-x Eu x Ti₃O₁₂ (BET) thin films had the smaller grain sizes than the others. In this study, we confirmed that better ferroelectric properties can be expected for films composed of larger grains in bismuth layered peroskite materials. The crystallinity of the thin films was improved and the average grain size increased as the crystallization temperature increased from 600 to 720℃. Moreover, the BLnT thin film capacitor is characterized by well-saturated polarization-electric field (P-E) curves with an increase in annealing temperature. The BLnT thin films exhibited no significant degradation of switching charge for at least up to 1.0×10¹¹ switching cycles at a frequency of 1㎒. From these results, we can suggest that the BLnT thin films are the suitable dielectric materials for ferroelectric random access memory applications.

Effect of La doping on ferroelectric properties of Bi4Ti3O12 thin film

The Bi4Ti3O12 and Bi3.25La0.75Ti3O12 thin films are prepared on the Pt/Ti/SiO2/Si substrate using sol-gel method. The structures Pt/Bi3.25La0.75Ti3O12/Pt and Pt/Bi4Ti3O12/Pt are fabricated. The effects of La doping on the microstructures, and ferroelectric properties of Bi4Ti3O12 films are investigated. The 2Pr with test voltage 6V for the sample annealed at 700℃ increased from 12.5 μC/cm2 to 18.6 μC/cm2, and 2Vc is still 2.8 V when the La3+ ions occupy part of Bi3+ sites. The Bi3.25La0.75Ti3O12 films showed fatigue-free behavior. The mechanism of improvement of La-doped Bi4Ti3O12 ferroelectric thin films is discussed.

Microstructure and ferroelectric properties of Nb-doped Bi4Ti3O12 thin films prepared by sol–gel method

The effects of Nb doping on the ferroelectric properties of Bi4Ti3O12 (BIT) thin films were investigated. Ferroelectric Nb-doped Bi4Ti3O12 (BTN) thin films were prepared by a sol–gel spin-coating method and annealed at several temperatures in an oxygen atmosphere. From analyzing X-ray diffraction patterns, it could be determined that a randomly oriented film was obtained by doping Nb ion into BIT thin film. The BTN film consisted of more disordered plate-like grains whereas the plate-like grains in the BIT film were more parallel to the substrate. The remanent polarization (2Pr) and coercive field (2Ec) of the BTN thin film that annealed at 700°C were about 39.8μC/cm2 and 136.7kV/cm at a sweep electric field of 175kV/cm. The pulse polarization (Psw−Pns) and the shape of the hysteresis loops did not change significantly before and after the 1.5×1010 switching cycles.

Enhanced Ferroelectric Properties of Nitrogen‐Doped Bi4Ti3O12 Thin Films

Ferroelectric nitrogen-doped Bi4Ti3O12 (BIT) and nitrogen-substituted BIT (N-BIT) thin films are prepared by an radiofrequency-magnetron-sputtering method such that nitrogen atoms are incorporated into oxygen sites. The remnant polarization Pr and the coercive field Ec of the nitrogen-doped BIT are enhanced relative to those of the undoped BIT (Figure). The leakage current of the nitrogen-doped BIT decreases and the fatigue properties improves compared to the undoped BIT.

Studies on the preparation and characterization of Bi4Ti3O12 thin films on p-Si substrates

High-quality Bi4Ti3O12 ferroelectric thin films on p-Si substrates were prepared by using the sol-gel technique. The growth behavior, ferroelectric properties, C-V characteristic and fatigue characteristic of Bi4Ti3O12 were investigated. The results show that the growth of Bi4Ti3O12 on bare p-Si substrates is c-axis-oriented with the increase of annealing temperature, and the apparent dependence of the ferroelectric properties of Bi4Ti3O12 films on the grain size, growth behavior and carrier concentration stems from the temperature effects. The C-V hysteresis curves with clockwise loops proved that the Ag/Bi4Ti3O12/p-Si heterostructure can realize a memory effect due to the ferroelectric polarization of Bi4Ti3O12 films and the remnant polarization of the Bi4Ti3O12 films reduced by 12% of the initial value after 109 bipolar switching cycles, which indicated that these Bi4Ti3O12 films deposited on p-Si substrates would be qualified for ferroelectric memories.

Growth of Bi4Ti3O12 Thin Film by Two-Dimensional RF Magnetron Sputtering with Bi2O3 and TiO2 Targets

Ferroelectric Bi4Ti3O12 (BIT) thin films were grown on Pt/Ti/SiO2/Si substrate by a two-dimensional RF magnetron sputtering method. When the RF powers of Bi2O3 and TiO2 were fixed at 100 W and 200 W, respectively, the BIT film exhibited c-axis orientation. The prepared BIT thin film consisted of well-developed grains and exhibited a good polarization–voltage hysteresis loop. The remanent polarization and coercive field were 4.09 µC/cm2 and Ec=56.1 kV/cm, respectively. The dielectric constant was about 330 and the loss factor was 2.18%.

Domain rearrangement in ferroelectric Bi4Ti3O12 thin films studied by in situ optical second harmonic generation

Electric-field induced rearrangement of domain microstructure in an epitaxial thin film of Bi4Ti3O12 on a SrTiO3 (001) substrate is studied by optical second harmonic generation measurements. The input polarization dependence of the second harmonic signal exhibits spatial symmetries that reflect the presence of eight different domain variants present in the film. Changes in these symmetries with the application of electric field are experimentally studied at 23° C and 60 °C, and theoretically modeled to extract the hysteresis loops that reflect quantitative changes in the fraction of domain variants. A strong correlation is observed between the dc electrical conductance (as distinct from transient currents) and the ferroelectric domain state of the film, which is proposed to arise from the creation and destruction of charged domain walls within the film with applied field.

Probing domain microstructure in ferroelectric Bi4Ti3O12 thin films by optical second harmonic generation

The domain microstructure in an epitaxial thin film of Bi4Ti3O12 on a SrTiO3(001) substrate is studied by second harmonic generation measurements. The input polarization dependence of the second harmonic signal exhibits spatial symmetries that reflect the presence of eight different domain variants present in the film. A theoretical model is presented that explains the observed symmetries and extracts quantitative information on the nonlinear optical coefficients of the material and statistics of domain variants present in the film area being probed. The following ratios of nonlinear coefficients and birefringence was determined: d12/d11=−3.498±0.171, |d26/d12|=0.365±0.010, |d26/d11|=1.273±0.036, and |nb−na|=0.101±0.018 (at 532 nm).