Ferroelectric Bismuth Titanate Thin Films Research Articles

Down-conversion luminescence and its temperature-sensing properties from Er3+-doped sodium bismuth titanate ferroelectric thin films

Here, we demonstrate outstanding temperature-sensing properties from Na0.5Bi0.49Er0.01TiO3 (NBT:Er) thin films. The perovskite phase for them is stable in the temperature range from 80 to 440 K. Interestingly, the Er doping enhances the ferroelectric polarization and introduces local dipolar, which are positive for temperature sensing. Pumped by a 488-nm laser, the NBT:Er thin films show strong green luminescence with two bands around 525 and 548 nm. The intensity ratio I 525/I 548 can be used for temperature sensing, and the maximum sensitivity is about 2.3 × 10−3 K−1, higher than that from Er-doped silicon oxide. These suggest NBT:Er thin film is a promising candidate for temperature sensor.

White upconversion photoluminescence for Er3+–Tm3+–Yb3+ tri-codoped bismuth titanate ferroelectric thin films

The upconversion (UC) photoluminescence and ferroelectric properties of Er3+–Tm3+–Yb3+ tri-codoped bismuth titanate (BETYT) thin films were studied. The thin films were prepared by chemical solution deposition method. There are four emission bands in the visible UC luminescence spectra excited by 980nm at room temperature. The 478nm blue emission band corresponds to 1G4→3H6 transition of Tm3+, and the 527nm, 548nm green emission bands correspond to 2H11/2→4I15/2 and 4S3/2→4I15/2 transitions of Er3+, and the 657nm red emission band corresponds to 4F9/2→4I15/2 transition of Er3+ and 1G4→3F4 transition of Tm3+. The fluorescent color can be tuned by adjusting Er3+, Tm3+ and Yb3+ concentrations and the optimal color coordinate in these films is (0.31, 0.34), very close to the standard white-light coordinate of (0.33, 0.33) when pumped under power intensity about 2.8W/cm2. The UC emission mechanism was discussed and there exist significant energy transfers from Er3+ to Tm3+, which will affect the relative intensity of blue, green and red emissions and their quenching concentrations. The remanent polarization (2Pr) value of BEr0.0085TYT thin film prepared on Pt/Ti/SiO2/Si substrate is 27.8μC/cm2. The combination of UC photoluminescence and ferroelectric properties in BETYT thin films would open the possibility of inventing novel multifunctional optoelectronic integration devices.

The microstructure and ferroelectric properties of Sm and Ta-doped bismuth titanate ferroelectric thin films

Sm–Ta co-doped Bi4Ti3O12 (Bi3.96Sm0.04Ti2.92Ta0.08O12, BSTTO) thin films were fabricated on Pt(111)/Ti/SiO2/Si(100) substrates by sol–gel technology, and the microstructure, dielectric, ferroelectric and leakage current properties of the thin films were investigated. BSTTO thin film exhibits a high remanent polarization (2Pr) of 46.2μC/cm2 and a low coercive field (2Ec) of 102kV/cm, fatigue-free characteristics up to ≧108 switching cycles. The 2Pr of the BSTTO thin film is larger than that of the Bi4Ti3O12 thin film (2Pr=26μC/cm2). In addition, the leakage current density of Sm–Ta co-doped Bi4Ti3O12 thin film is 2.56×10−8A/cm2, which is lower than that of the Bi4Ti3O12 thin film (1.0×10−5A/cm2). These improvements in the ferroelectric and leakage current properties can be attributed to the enhanced stability of the oxygen in the Ti–O octahedron layer by the substitution with Sm and Ta for Bi3+ and Ti4+ in A-site and B-site of the BTO thin film, respectively.

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.

Preparation and Photoluminescence of Praseodymium-Doped Bismuth Titanate Ferroelectric Thin Films

Bi4-xPrxTi3O12 thin films were prepared by a chemical solution deposition method and their photoluminescence properties are investigated. The layered perovskite structure of the films is not destroyed by substituting Bi with Pr. The emission lines at 493, 533 and 612 nm are observed, and they can be assigned to the transitions of 3P0→3H4, 3P0→3H5 and 1D2→3H4 of Pr3+ ions. The quenching concentrations are identified to be x = 0.09 and 0.03 for the emissions at 493 and 612 nm. The crystal field plays a crucial role in the luminescence properties of Bi4-xPrxTi3O12 thin films.

Oxygen Stability and Leakage Current Properties of La-Modified Bismuth Titanate Ferroelectric Thin Films

The effects of oxygen annealing on the structural properties and leakage current behaviors of sol–gel-derived Bi3.25La0.75Ti3O12-y ferroelectric thin films have been studied. The thin films were pyrolyzed in different atmospheric conditions including a fully oxygenized ambient of 1 bar and an evacuated ambient of less than 10 mbar. X-ray diffraction patterns of the deoxygenated films showed measurable shifts to a higher angle, indicating that each deoxygenated film had a smaller lattice constant than the oxygenated one. Scanning probe analysis showed well-developed grains and inhomogeneous and asymmetric bound charges in both samples. The leakage current behavior of the films was measured as a function of voltage and temperature and was explained on the basis of the Schottky–Simmons thermionic emission model. X-ray photoemission spectra showed that oxygen stability in Bi2O2 layers plays an important role in determining the level of leakage current in the films.

Low temperature preparation of ferroelectric bismuth titanate thin films

The low temperature metal organic decomposition techniques of ferroelectric bismuth titanate (BIT) thin films were investigated. BIT was found to be crystallized by rapid thermal processing at 450 °C. The stoichiometric Bi4Ti3O12 sample exhibited (117) orientation, while the Bi4.8Ti3O13.2 sample, with 20% excess bismuth, possessed a/b axes orientation with (117) component. Pt/Bi4.8Ti3O13.2/Pt ferroelectric capacitors were fabricated with temperature confined below 450 °C. The saturated 2Pr value was 31.1 μC/cm2. Such method is valuable for ferroelectric memories at 65 nm technology node and beyond because low temperature processes are required for the stability of interconnect material nickel silicide.

Improved photoluminescence and electrical properties of Eu- and Gd-codoped bismuth titanate ferroelectric thin films

Eu- and Gd-codoped bismuth titanate [Bi3.15Eu0.425Gd0.425Ti3O12], thin films as well as Bi3.15Eu0.85Ti3O12 (BEuT) and Bi3.15Gd0.85Ti3O12 (BGdT) thin films were prepared on indium-tin-oxide-coated glass substrates by chemical solution deposition and the photoluminescence, ferroelectric, and dielectric properties of the thin films were investigated. All the thin films exhibited a polycrystalline bismuth-layered perovskite structure. Photoluminescence studies showed that the enhancement of emission intensities for two Eu3+ emission transitions of D05→F17 (594nm) and D05→F27 (617nm) was observed for the BEGT thin films as compared to BEuT thin films. This photoluminescence improvement can be attributed to Eu content of BEGT thin films close to quenching concentration of BEuT thin films and local distortion of crystal field surrounding the Eu3+ activator induced by different ionic radii of Eu3+ and Gd3+ ions. Electrical measurements indicated that BEGT thin films had larger remanent polarization and higher dielectric constant than BGdT and BEuT thin films prepared under the same experimental conditions. These results suggest that codoping of rare earth ions such as Eu and Gd in bismuth titanate thin films is an effective way to improve photoluminescence and electrical properties of the thin films.

Preparation and electric characteristics of gadolinium-substituted bismuth titanate ferroelectric thin films

Gadolinium-substituted bismuth titanate (Bi 4 − x Gd x Ti 3O 12, BGT) thin films were deposited on the (111)Pt/Ti/SiO 2/(100)Si substrates by a modified Sol–Gel process. With the aid of X-ray diffraction (XRD), the structure and appropriate Gd-substituted content x of BGT thin films were detected. The surface quality of the films was measured by means of an atomic force microscope (AFM) in tapping mode. The surface of BGT thin films was smooth and the phase was homogeneous. The ferroelectric properties of the films were investigated. When the Gd-substituted content x was equal to 0.75, the remanent polarization was the largest. The remanent polarization 2Pr value was equal to 15.4 μC/cm 2 and the coercive field Ec value was 54.17 kV/cm.

Effects of oxygen-plasma treatment on lanthanum-substituted bismuth titanate ferroelectric thin films

Oxygen-plasma treatment has been done on Bi3.25La0.85Ti3O12 (BLT) films for lower crystallization temperatures and improved ferroelectric properties. The films, prepared by metal organic decomposition method on Pt and IrO2 electrodes sputtered on Al2O3/SiO2/Si substrates, were pre-annealed at the relatively low temperatures of 550 and 600 °C, followed by room temperature microwave oxygen-plasma treatment. The oxygen-plasma-treated BLT films showed very good ferroelectric characteristics after a final furnace annealing at 600 °C.

Priority of domain wall pinning during the fatigue period in bismuth titanate ferroelectric thin films

The switching current densities of pure Bi4Ti3O12 thin films were measured during a fatigue process. The peak switching current density and switching time decrease with the increase of switching cycles, which indicates a domain wall pinning effect by defects inside thin films. Through the fitting of the switching current density using the modified Kolmogorov-Avrami theory, it is found that the percentage of two-dimensional (2D) domain growth decreases during the fatigue period, which reveals that 2D domain growth has an absolute priority to be pinned compared with one-dimensional domain growth. The mechanism of the increased activation field during the fatigue process is briefly discussed based on the domain wall pinning effect. These results are of great importance in the switching kinetics and fatigue mechanisms in ferroelectric films.

Structural and electrical properties of samarium-substituted bismuth titanate ferroelectric thin films on Pt/TiO x/SiO 2/Si substrates

Polycrystalline samarium-substituted ferroelectric bismuth titanate films (Bi 3.45Sm 0.55Ti 3O 12) were prepared on Pt/TiO x /SiO 2/Si substrates by chemical solution deposition (CSD) and were annealed at various temperatures. For comparison, films with similar composition were grown by pulsed laser deposition (PLD) and annealed at 700 °C. X-ray diffraction and Raman spectroscopy results showed similar crystallographic orientations and structures for the films fabricated using these different deposition methods. It was found that film crystallinity, structural, dielectric, ferroelectric and leakage current properties were strongly dependent upon the annealing temperature for CSD-derived films. Atomic force microscopy showed that film surface roughness values are comparable for both deposition methods following annealing at the same temperature, although PLD-grown films had a more uniform grain size. Cross-sectional images obtained by scanning electron microscopy revealed distinct grain shapes and structures: dense, columnar grains for PLD-grown films, spherical grains and a more porous structure for CSD-derived films. The remanent polarization of CSD-derived films increased with increasing annealing temperature. The PLD-grown films annealed at 700 °C showed higher remanent polarization (2 P r = 41.8 μC/cm 2) and lower coercive field ( E c = 91 kV/cm), than CSD-derived films annealed at the same temperature (27.4 μC/cm 2 and 121 kV/cm). The former also had a lower leakage current density (6.7 × 10 −7 A/cm 2) than the latter (4.6 × 10 −6 A/cm 2) at a dc electric field of 100 kV/cm (∼10 V), and both film types demonstrated fatigue-free behaviour up to 10 9 read/write switching cycles with 1 MHz bipolar pulses.

Effects of niobium doping on microstructures and ferroelectric properties of bismuth titanate ferroelectric thin films

Bi 4Ti 3O 12 thin films doped with 3 mol.% niobium were prepared on Pt/Ti/SiO 2/Si substrates by sol–gel method. The niobium-doped Bi 4Ti 3O 12 (Nb–BiTiO) films annealed at 700 °C for 30 min in oxygen showed randomly oriented layered perovskite structures and was composed of plate-like and rod-like grains with no crack. The remanent polarization (2 P r) and coercive field (2 E c) of the Nb–BiTiO film annealed at 700 °C were 28 μC/cm 2 and 110 kV/cm, respectively. In addition, the film showed good switching endurance under bipolar pulse at least up to 4.5×10 10 cycles. Substitution of Nb in BiTiO thin films was effective for reducing the oxygen vacancies and generating good ferroelectric properties.

Structural, morphology and electrical studies on ferroelectric bismuth titanate thin films prepared by sol–gel technique

Crystal structure, surface morphology, compositional homogeneity and electrical properties of layered perovskite bismuth titanate (BTO) thin films have been investigated. BTO thin films were deposited on silicon and platinum-coated silicon substrates by spin coating. X-ray diffraction analysis confirms that the crystallinity of the films increases with increasing annealing temperature and the optimum temperature is found to be 600°C. Morphology studies by AFM showed that the surface of the films were smooth, dense and crack free. Composition analysis on the surface and in-depth confirms the stoichiometry of the films. C– V measurements show a counter-clockwise dielectric hysteresis, indicating that the ferroelectric property sufficiently controls the silicon potential with a memory width of 2 V. The leakage current density of the films is measured to be 2×10 −7 A/cm 2 from I– V characteristics at an applied voltage of 1 V.

MICROSTRUCTURE AND PROPERTIES OF FERROELECTRIC Bi4Ti3O12 THIN FILMS

The film morphology and defect structure of ferroelectric bismuth titanate thin films are studied by high resolution transmission electron microscopy. As-grown and RTA-processed thin films have similar defect structures, consisting of stacking faults and complex intergrowth defect structures. The as-grown specimens prepared at low temperature had smaller particle size with higher density of these defects compared to RTA-processed samples. Detailed atomic structure models for the stacking faults and intergrowth defect structures are proposed and the computer-simulated images are compared with experiment.

Photoelectric effects in chemical solution deposited Bi4Ti3O12 thin films

Bismuth titanate ferroelectric thin films were deposited on platinum coated silicon using chemical solution deposition. Photoconductive and photovoltaic effects were observed both in continuous and modulated light. With the increasing of the annealing temperature the maximum of the photoconductive signal spectral distribution is moving toward longer wavelengths, from 340 nm to 370 nm. The existence of the photovoltaic effect could be due to some potential barriers existing at the grain boundaries and the enlargement of its spectral distribution toward longer wavelengths could be due to the presence of some traps at the grain boundaries.

Structural and electrical properties of ferroelectric bismuth titanate thin films prepared by the sol gel method

Crystalline bismuth titanate films were fabricated using a modified sol gel method. The process was simplified using stock titanium and bismuth solutions. Films were fabricated using rapid thermal processing with a ramp rate of 100 °C/s to 700 °C for 30 s. The average grain size of films was 175 nm. A dielectric constant in the typical range of 220 and tanδ of 0.01 were measured. P- E hysteresis loops were observed with the ferroelectric properties of P s = 22 μC/cm 2, P r = 10 μC/cm 2 and the coercive force E c = 165 kV/cm. The leakage current density was of the order of 10 nA/cm 2 at 60 kV/cm. An almost flat C- V characteristic and a charge storage density of 42 fC/μm 2 at 220 kV/cm were measured.

Preparation of Bismuth Titanate Thin Films by Laser Ablation

Ferroelectric bismuth titanate thin films have been prepared on Si substrates at low temperature by the laser ablation method using an ArF laser. Dependences of crystallographic property of the film on substrate temperature, O2 gas pressure, laser repetition frequency and annealing temperature are presented. The preferentially c-axis-oriented films have been obtained on Si substrates at a substrate temperature of 400° C with annealing at 700° C, and on Pt substrates at a low substrate temperature of 450° C.

Studies on structural and electro-optic properties of ferroelectric bismuth titanate thin films

Abstract Ferroelectric bismuth titanate thin films have been deposited on LaAlO3, SrTiO3, MgO, A12O3, and ZrO2/SiO2/Si substrates using a Q-switched Nd:YAG laser. X-ray diffraction studies show that most films on single crystal substrates have preferential crystallographic orientation. The film on MgO(110) shows large electro-optic characteristics with an effective electro-optic coefficient of about 3.8×10−-15 m2/V2. Using a lens coupling method, it is demonstrated that light can propagate along the Bi4Ti3O12 layer in a Bi4Ti3O12/ZrO2/SiO2/Si(100) heterostructure.

Preparation and Use of Mosaic Target for the Deposition of Ferroelectric Bismuth Titanate Thin Films

ABSTRACTIon beam sputter deposition using a Kaufman source is a well-established technique for fabricating thin films. This method has the capability for yielding smooth surfaces and depositing uniform films over a large area. We have used this technique, in the past, with a 5″ diameter target to deposit smooth ferroelectric bismuth titanate (Bi4Ti3O12) films. One of the major obstacles was fabricating the large target with the desired composition. To reduce cost by simplifying the target preparation process we developed a ‘mosaic’ target to deposit ferroelectric Bi4Ti3O12films. Films were deposited at different substrate temperatures. Measurements show that the post-annealing of the deposited films results in measurable ferroelectric properties and enhanced low field dielectric behavior in the films. The discussion will include the target fabrication, the observed properties of the resultant films, and the effect of the deposition and post-annealing temperatures on the film properties.