Ferroelectric Strontium Bismuth Tantalate Research Articles

Optical and X-ray characterization of ferroelectric strontium–bismuth–tantalate (SBT) thin films

Metal-organic chemical vapor deposition (MOCVD) made layers of strontium–bismuth–tantalate (SBT) were characterized by spectroscopic ellipsometry (SE) using the Adachi model [S. Adachi, Phys. Rev. B 35 (1987) 7454–7463]. The evaluated optical parameters were correlated with the physical and chemical behavior examined by X-ray diffraction (XRD). As a result, it was possible to fit the measured spectra with the Adachi model in a wide range covering the region of the band gap. The Adachi model provides electronic layer parameters like the transition energy E 0 and broadening Γ. Our investigations established a correlation between XRD-determined average grain size and the electronic layer parameters.

Structural and electrical characterization of strontium bismuth tantalate (SBT) thin films

Ferroelectric strontium bismuth tantalate (SBT) thin films were deposited by thermal metalorganic chemical vapour deposition (MOCVD) onto a complex layered Pt/IrO 2 /Ir/Ti(Al)N substrate. A study of ultra-violet (UV)-assisted rapid thermal processing (RTP) annealing strategies of the SBT thin films was performed. The influence of UV irradiation temperature and annealing atmosphere on the crystallinity of the deposited films was evaluated using both microstructural and electrical analysis techniques. A UV-RTP strategy in an oxygen atmosphere above 400 °C, followed by a furnace treatment at 700 °C, provided an optimum remnant polarization figure of merit.

Crystallization, Heterostructure, Microstructure, and Properties of Ferroelectric Strontium Bismuth Tantalate Films Derived from Tantalum Glycolate Solutions.

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Crystallization, Heterostructure, Microstructure, and Properties of Ferroelectric Strontium Bismuth Tantalate Films Derived from Tantalum Glycolate Solutions

Strontium bismuth tantalate (SBT) films were prepared by chemical solution deposition (CSD) onto Pt/TiO2/SiO2/(100)Si substrates. Nominal chemical compositions of the precursor solutions of SrBi2Ta2O9, SrBi2.2Ta2O9, Sr0.8Bi2Ta2O9, and Sr0.8Bi2.2Ta2O9 were tested for the deposition of the films. Crystallization of the films was carried out by a two-step process (TS) or a single-step process (SS). The TS treatment consisted of thermal treatment of the films in oxygen atmosphere at 550 °C for 7200 s, with a heating rate of 8 °C/s, followed by a rapid thermal processing (RTP) in oxygen at a temperature of 650 °C for 3600 s, using a heating rate of 200 °C/s. The SS process was just the RTP treatment of the films. The X-ray diffraction (XRD) patterns of the crystalline films showed the formation of the layered perovskite together with a second phase. Analysis of the films by means of Rutherford backscattering spectroscopy (RBS) showed that this second phase was placed at the interface and it was formed by the r...

Nanoscale properties of ferroelectric ultrathin SBT films

Strontium bismuth tantalate (SBT) ultrathin films with thickness about 40 nm have been prepared by a chemical solution deposition (CSD) method based on the synthesis of a Ta-glycolate derivative. Layers of the precursor solution diluted with 2-ethyl-1-hexanol were spin-coated onto Pt/TiO 2/SiO 2/(100)Si substrates. Crystallisation was carried out in oxygen at 650 °C by Rapid Thermal Processing (RTP). Topography of the films was observed by Scanning Force Microscopy (SFM). The piezoresponse mode of the SFM equipment was used to perform the nanoscale observation of the ferroelectric domain structure of the SBT ultrathin films and to measure piezoelectric hysteresis loops. The macroscopic ferroelectric response was measured on capacitors prepared after the deposition on the film surfaces of top platinum electrodes of ∼0.05 mm 2 of area.

Structural and electronic properties of SBT in FEDREAM devices

In the present study, we performed first-principles modeling of calculation on electricity behavior of ferroelectric strontium bismuth tantalate (SBT). Different cluster models were chosen for the discussion of the influence of atomic coordination on the atomic valence configuration, atomic charge, and bond order etc. in the DV-Xα calculations. This allowed us to understand size effects of cluster on the electronic structure of SBT. The results of first-principles calculations of the electronic structure in SrBi 2Ta 2O 9 (SBT) were reported. The electronic density distribution indicates density of states induced local levels within the gap, and finds significant hybridization between metal d states and the O 2p–Ta 5d and Ta 6s state. The charge of Sr is transferred to the oxygen atom, the bonding between them was nearly ionic bonding. The calculated electronic structure of SBT indicates that the absence of an energy gap in the vicinity of the fermi level stems from lowering of the Sr orbital energies due to its lower electro-negativity relative to Ta and Bi ions, and significant hybridization between Ta, Bi and O ions. The calculated total densities of states were in agreement with the reported X-ray photoemission spectroscopy data.

Control of bismuth volatility in SBT by vanadium doping

The volatility of bismuth (Bi) presents difficulty in the integration of ferroelectric strontium bismuth tantalate (SrBi2Ta2O9 or SBT) thin films for FeRAM applications. This paper presents preliminary results indicating that the Bi volatility problem may be controlled by the addition of vanadium (V) to SBT. A series of V-doped SBT films (∼150 nm), of a nominal composition SrBi2.2(Ta1-xVx)2O9 with x = 0, 0.01, and 0.07, were deposited on Pt/SiO2/Si wafers by sol-gel processing. XRD and SEM revealed a polycrystalline morphology and a vanadium solubility in SBT of at least 7%. SBT with 7% V-doping exhibited little structural damage from a forming gas anneal. 7% V-doping also limited Bi diffusion into Pt bottom electrodes and increased the surface Bi 4f7/2 electron binding energies. The ferroelectric and polarization fatigue properties did not improve with V-doping, which was attributed to oxygen vacancy formation due to acceptor doping. Vanadium stabilized Bi in the SBT matrix by reducing from the pentavalent to the tetravalent state, which provided the free energy to either oxidize bismuth or prevent the reduction of bismuth.

Synthesis of ferroelectric nanocrystalline SrBi 2Ta 2O 9 powder by the colloid-gel process

An innovative colloid-gel process using inorganic salts as starting materials has been successfully developed for preparing ferroelectric strontium bismuth tantalate (SrBi 2Ta 2O 9) powder. This new technique began with the preparation of a mixture of Ta(OH) 5 colloid with the aqueous solutions of strontium and bismuth cations, followed by the addition of ethylene glycol and citric acid as gel-forming reagents. During the calcination process the fluorite phase was formed as an intermediate that was subsequently converted into the SrBi 2Ta 2O 9 phase. After calcination at 750°C, pure ultrafine SrBi 2Ta 2O 9 polycrystalline powder with a narrow particle size distribution was obtained. The primary particle size of the obtained SrBi 2Ta 2O 9 powder was around 40 nm. In comparison with the solid-state reaction, the newly developed colloid-gel process significantly reduces the heating temperature for synthesizing SrBi 2Ta 2O 9, as well as the particle size of the obtained powder.

In situ Observation of the Crystallization Process of Ferroelectric Thin Films by Raman Microspectroscopy

Crystallization processes of ferroelectric strontium bismuth tantalate (SrBi2Ta2O9; SBT) films with a Bi-layered structure and lead zirconate titanate [Pb(Zr0.5Ti0.5)O3; PZT] films with a perovskite structure have been investigated by Raman microspectroscopy. Amorphous films were prepared by rf-magnetron sputtering at room temperature using nearly stoichiometric ceramic targets. In situ Raman observations were performed at intervals with increasing sample temperatures. Under a heating rate of 20°C/min, the SBT films began to crystallize at 710°C, and completed crystallization at 850°C. It was found that SBT film crystallization advanced rapidly in the temperature region from 750 to 800°C. Raman mapping measurements of the SBT films at room temperature after annealing at 700 and 800°C clearly indicated the nucleation and growth processes of the crystal grains, respectively. The Raman peaks of the PZT films, which were observed in situ in spite of the cubic symmetry at the annealing temperatures, showed perovskite structure formation at temperatures as low as 500°C.

Influence of the presence of titanium oxide on the formation of pyrochlore phase in ferroelectric strontium bismuth tantalate ceramics

The effects of addition of TiO2 on the formation of pyrochlore phase and dielectric properties of ferroelectric SrBi2Ta2O9 have been investigated. When the content of Ti02 is less than 10 mol%, no secondary phases are produced, and SrBi2Ta2O9-TiO2 solid solutions are formed. However, with TiO2 content ⩾20 mol%, a pyrochlore phase is produced. Raising the content of TiO2 in starting materials significantly increases the amounts of pyrochlore phase, and reduces the initial temperature for the formation of pyrochlore phase. The existence of TiO2 also decreases the thermal stability of SrBi2Ta2O9, causing the decomposition of SrBi2Ta2O9to take place at lower temperatures. Furthermore, the presence of TiO2 reduces the dielectric constants as well as remnant polarization of formed ceramics.

Effect of Film Composition on Low Temperature Processing of SBT Deposited by MOCVD

AbstractBi-layered ferroelectric strontium bismuth tantalate (SBT) thin films of various film compositions were deposited on Pt/Ti/SiO2/Si substrates by metalorganic chemical vapor deposition (MOCVD) and crystallized at 700°C in oxygen ambient. Phase transition, orientation, second phases and remanent polarization were investigated with respect to film stoichiometry. X-ray diffraction (XRD) measurements revealed that excess Bi lowers the transition temperature from fluorite-type to ferroelectric phase. However, SBT films with Bi-excess of 15% or higher exhibit pronounced Bi-loss during crystallization and a decrease in the relative intensity of the (200) peak. Highly Sr-deficient films are not fully crystallized but support pyrochlore formation. The maximum remanent polarization is obtained at a Sr-deficiency of 15–25% and a Bi-excess of 10% (0.85/2.20/2.00).

Dielectric anomaly in strontium bismuth tantalate thin films

The authors have succeeded in observing the dielectric anomaly in ferroelectric strontium bismuth tantalate (SBT) thin films, and studied how it varies with film composition. For a stoichiometric SBT film, the dielectric anomaly was diffused, and the Curie temperature (Tc) was approximately 260 °C, which is lower than that for the bulk ceramic. A SBT film with excess Bi composition showed Tc of approximately 300 °C, and Tc for Sr deficient SBT films was higher than 340 °C. The composition dependence of remanent polarization and coercive field near room temperature for the SBT films reflects the composition dependence of the Tc values.

Dielectric breakdown, negative resistivity, depletion widths and the role of Bi in strontium bismuth tantalate (SBT) devices

A discussion is presented on details of dielectric breakdown in ferroelectric thin-film memories, including BST and SBT. The origin of negative differential resistivity is revisited. And subtleties of depletion widths in ferroelectric films and of elemental bismuth at the SBT/Pt electrode interface are considered.

Preparation of SrBi2Ta2O9 thin films with a single alkoxide sol–gel precursor

Y. Kim, H. K. Chae, K. S. Lee and W. I. Lee, J. Mater. Chem., 1998, 8, 2317 DOI: 10.1039/A806275G

Strontium bismuth tantalate layered ferroelectric ceramics: Reaction kinetics and thermal stability

Using BiTaO 4 as the precursor, ferroelectric strontium bismuth tantalate SrBi 2Ta 2O 9 was successfully synthesized. Heating the mixtures of BiTaO 4 and SrCO 3 at 800 °C resulted in the entire formation of a single phase SrBi 2. Ta 2O 9. The formation process of this compound was verified to be a direct reaction occurring between the reactants without the presence of intermediates. The conversion of SrBi 2Ta 2O 9 was isothermally analyzed for exploring the reaction kinetics. The results revealed that the formation process was controlled by the diffusion process. Based on the Ginstling-Brounshtein and Jander models, the activation energy of the diffusion process was estimated to be 194.3 kJ mol −1 and 210.7 kJ mol −1, respectively. SrBi 2Ta 2O 9 was found to become unstable when heated at above 1200 °C. Once the decomposition process started, this compound was dissociated into SrTa 2O 6 and Bi 2O 3 from surface to interior.

Crystallization Mechanism of Ferroelectric SBT Thin Films

ABSTRCTWe characterize, via selected area electron diffraction and high resolution TEM, the multistage crystallization process by which strontium bismuth tantalate(SBT) thin films are transformed from initial amorphous phase to the fully formed layer-structured perovskite. During the annealing process at 800°C, we found that an intermediate fluorite-like stage is formed after 3min as the SBT precursor crystallizing into the final SBT structure. This new information will be very important for formation and crystallization of ferroelectric SBT films.