Ferroelectric SrBi2Ta2O9 Thin Films Research Articles

Ferroelectric SrBi2Ta2O9 thin film deposition at 550 °C by plasma-enhanced metalorganic chemical vapor deposition onto a metalorganic chemical vapor deposition platinum bottom electrode

Ferroelectric bismuth-layer SrBi2Ta2O9 (SBT) thin films were prepared on Pt/SiO2/Si substrates by plasma-enhanced metalorganic chemical vapor deposition. The platinum bottom electrode deposited by metalorganic chemical vapor deposition shows a dense smooth state after deposition of SBT films and prevents bismuth diffusion into the platinum layer. The c-axis oriented SBT films were crystallized at a deposition temperature of 550 °C. The dielectric constant and dissipation factor of SBT films are 310 and 0.08, respectively, at 100 kHz. The remanent polarization and the coercive field obtained for 180 nm thick Sr0.9Bi2Ta2O9 films deposited at 550 °C were 15 μ/cm2 and 50 kV/cm, respectively, at an applied voltage of 5 V. The leakage current density was about 5.0×10−7 A/cm2 at 250 kV/cm. The films showed fatigue-free characteristics up to 7.0×109 switching bipolar pulses under 5 V.

Preparation of a New SrBi2Ta2O9 (SBT) Chemical Solution Using Crown Ether and its Thin Film Deposition

AbstractA new chemical solution for the deposition of SrBi2Ta2O9 (SBT) thin films using crown ether is proposed. Crown ether enhances the solubility of bismuth acetate in acetic acid, which makes a solvent solution possible. A metal acetate-based SBT precursor solution was prepared in acetic acid using 18-crown-6, strontium acetate, bismuth acetate and tantalum ethoxide. (115)-preferentially oriented ferroelectric SBT thin films were formed on Pt/TiO2/SiO2/Si substrates by spin-coating with the new solution. After post-annealing at 800°C, a hysteresis loop obtained from a 2000Å thick SBT film with Pt electrodes showed a remanent polarization of ∼3μC/cm2 and a coercive voltage of ∼50kV/cm. It was shown from SIMS depth profiles that excess bismuth atoms in the SBT thin film diffused towards the Pt electrodes during post-annealing.

Retention characteristics of SrBi2Ta2O9 thin films prepared by metalorganic decomposition

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. Electric measurements demonstrate that the polarization decay increases with increasing the write/read voltage within the first second. This could be attributed to the depolarization fields, which increases with increasing the retained polarization. However, we found that the polarization loss is insignificant with different write/read voltages over a range of 1–30 000 S. Furthermore, experiment indicates that there is weak pinning of domain walls existing in SBT, which plays an important role for SBT thin film over a range of 1–30 000 S with a low write/read voltage.

Ferroelectric Properties of SrBi2Ta2O9 Thin Films Deposited on Various Bottom Electrodes by a Modified Radio-Frequency Magnetron Sputtering Technique

Ferroelectric layered-oxide SrBi2Ta2O9 thin films were successfully deposited on metalorganic chemical vapor deposited (MOCVD) Pt/Ta/SiO2/Si (MPTSS), MOCVD-RuO2/Si (MRS), and Pt/Ti/SiO2/Si (PTSS) substrates using a sintered Sr1.2Bi2.4Ta2.0O9 ceramic and Bi metal targets by modified rf magnetron sputtering. The 190 nm thick Sr1.0Bi2.7Ta2.0O9 films deposited on PTSS substrates have a remanent polarization (Pr) of 4.3 µC/cm2 and a coercive field (Ec) of 36.6 kV/cm at an applied voltage of 5 V. The SBT films deposited on PTSS showed a fatigue-free characteristic up to 1.0×1010 cycles under a 5 V bipolar pulse and a leakage current density of 8.0×10-9 A/cm2 at 150 kV/cm.

Low-Temperature Synthesis of SrBi2Ta2O9 Ferroelectric Thin Films through the Complex Alkoxide Method: Effects of Functional Group, Hydrolysis and Water Vapor Treatment

SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized at lower temperatures by the sol–gel method using complex alkoxides. The effects of the complex alkoxide functional group, the amount of water used in hydrolysis, and a mixture of water vapor and oxygen flow during calcination, on the crystallization, the microstructural development, and the ferroelectric properties of the SBT thin films were addressed. The film which was deposited from complex alkoxide solutions and then calcined at 250°C in the mixture, contained a relatively smaller amount of the remaining alkoxy groups, and therefore, crystallized to SBT via rapid thermal annealing at temperatures below 500°C in an oxygen flow. Crystallinity improved significantly and the microstructure was able to develop up to 650°C. Remarkable grain growth was observed particularly in the SBT thin film deposited from the ethanolic solution of the complex ethoxide. Film annealed at 650°C exhibited the saturated P–E hysteresis loop and improved endurance behavior.

Low temperature preparation of ferroelectric SrBi2Ta2O9 thin films by a modified RF magnetron sputtering technique

Bi–layered ferroelectric SrBi2Ta2O9 (SBT) films were successfully prepared on Pt/Ti/SiO2/Si substrates at 650°C by a modified rf magnetron sputtering technique. The SBT films annealed for 1 h in O2 (760 torr) and again for 30 min in O2 (5 torr) at 650°C show a average grain size of about 49 nm. The SBT films annealed at 65 0°C have a remanent polarization (Pr) of 6.0 μC/cm2 and coercive field (Ec) of 36 kV/cm at an excitation voltage of 5 V. The films showed fatigue–free characteristics up to 4.0 × 1010 switching cycles under 5 V bipolar pulse. The retention characteristics of SBT films looked very promosing up to 1.0 × 105 s.

Electronic characteristics of the SrBi2Ta2O9–Pt junction

The voltage and film composition dependence of leakage currents of ferroelectric SrBi2Ta2O9 thin films, sandwiched between Pt has been studied. Schottky emission dominated the leakage current at voltages above the ohmic conduction regime, while space charge limited currents (SCLC), for which the observed temperature dependence is correctly predicted in Rose’s theory, appeared to dominate the leakage current in high conductivity SrBi2Ta2O9 thin films including bismuth-excess samples. A consequence of the latter was the observation of negative differential resistivity in high conductivity SrBi2Ta2O9 thin films. X-ray photoemission spectroscopy depth profiling indicated that Bi has diffused into the ferroelectric–metal interface and also influenced the electronic conduction mechanism of the ferroelectric capacitors. Confirmation of this was found through the current–voltage dependence of Pt/SrBi2Ta2O9/Bi, which is compatible with space charge limited currents. The theory of Rose was successfully applied to the temperature and voltage dependencies of leakage currents in Bi-excess SBT; this is the first application of such theories to ferroelectric films and results in a generalization of the Child’s Law approximation for space-charge-limited currents. A single fitting parameter T*=315±20 K satisfied data at the voltages 3<V<4 and temperatures 300 K<T<400 K most useful for engineering device applications.

Fatigue characteristics of SrBi2Ta2O9 thin films prepared by metalorganic decomposition

Polycrystalline SrBi2Ta2O9 (SBT) ferroelectric thin films were synthesized on Pt/Ti/SiO2/Si substrates by metalorganic decomposition. Electric measurements demonstrate that fatigue increases with decreasing switching voltage and frequency, and the suppressed polarization caused at a lower switching voltage can be recovered by switching at a higher voltage. This suggests that the domain walls of SBT thin films are weakly pinned and easily depinned by a higher external field. The polarization of SBT thin films annealed in air shows more degradation than that annealed in oxygen, which indicates that the oxygen vacancy also plays an important role in fatigue behavior of SBT thin films.

Control of epitaxial growth for SrBi2Ta2O9 thin films

We report the (a00) oriented as well as the (00l) oriented epitaxial SrBi2Ta2O9 thin film grown on (100) LaAlO3 and (200) yttria-stabilized zirconia single crystal substrates. On (100) SrTiO3 single crystal substrate, we only observe the (00l) oriented epitaxial growth of SrBi2Ta2O9 thin film. We infer that the (a00) oriented epitaxial growth results from the strain by the lattice mismatch. The band gaps of the (00l) and the (a00) oriented SrBi2Ta2O9 films are ∼3.9 and ∼3.5 eV, respectively. The two epitaxial orientations of ferroelectric SrBi2Ta2O9 thin film can provide an opportunity to study the effects on microwave and optical waveguides as well as underlying anisotropic physical properties.

Phase Transition in Ferroelectric SrBi2Ta2O9 Thin Films with Change of Heat-treatment Temperature

Changes in the composition and the crystal structure of micro-grains have been studied using transmission electron microscopy (TEM) and energy dispersive X-ray microanalysis in order to clarify the phase formation of ferroelectric SrBi2Ta2O9 (SBT) with heat-treatment at 650–750°C. Micrograins crystallized at 650°C begin to change to bismuth-layer-structure-family (BLSF) grains at temperatures exceeding 700°C, and form pure-BLSF phase grains at 800°C. Hence, a micrograins phase exists in thin films at 650–750°C and we determined that the bismuth composition in these grains decreases. Using selected diffraction and high-resolution TEM, we found that a pyrochlore phase was formed in the bismuth-poor micrograins that were heat treated at 750°C. The formation of pyrochlore made pure-phase BLSF difficult to obtain.

Pulsed laser ablation synthesis and ferroelectric properties of SrBi2Ta2O9 thin films

Ferroelectric SrBi2Ta2O9 (SBT) thin films were synthesized on Pt/Ti/SiO2/Si substrates using pulsed laser deposition combined with annealing technique, and structural and electrical properties were investigated. The SBT thin films with fine grain size and well-saturated square hysteresis loop was obtained. The films showed high diffraction peaks of (115) and (008). A dense structure with around 200 nm grain size was observed using the scanning electron microscope (SEM). Good ferroelectric properties were obtained from the films; Pr and E C were 8.4μC/cm2 and 57kV/cm, respectively. No fatigue was observed up to 1010 switching cycles. I-V characteristic showed the two peaks at the coercive voltage. It has a 560 pF capacitance and a static dielectric constant of 600 at zero applied voltage from the I-V characteristic. These properties are very attractive for nonvolatile memory application.

Ferroelectric SrBi2Ta2O9 Thin Films Deposited on Si(100) by Pulsed Laser Deposition

We report on the successful deposition of SrBi 2 Ta 2 O 9 (SBT) thin films directly on Si(100) substrates by pulsed laser deposition. The 200 nm thick SBT film deposited at a laser energy of 700 mJ and a temperature of 780 °C consisted of grains of less than 200 nm in diameter and exhibited a random crystallographic structure. The effective small signal dielectric constant and dissipation factor at 100 kHz were 177 and 0.014, respectively, and were relatively unchanged with increasing frequency up to 1 MHz. The SBT film showed good ferroelectric properties. The measured P r and E c were 10.6 μC/cm 2 and 182 kV/cm, respectively, at an applied electric field of 450 kV/cm. No significant fatigue was observed up to 10 10 switching cycles.

Electromechanical properties of SrBi2Ta2O9 thin films

Weak field piezoelectric coefficient and strain were investigated in ferroelectric SrBi2Ta2O9 (SBT) thin films by means of optical interferometry. Though the maximum polarization and dielectric constant were small enough (7 μC/cm2 and 150, respectively), the piezoelectric coefficient attained 17 pm/V under the dc electric field. This value is comparable with the piezoelectric coefficients reported previously on poled SBT ceramics. Electrically induced strain of 5⋅10−4 was observed using bipolar driving field of 300 kV/cm. No piezoelectric fatigue was found until 109 switching pulses, in agreement with polarization data. The piezoelectric properties and strains were successfully described by using a simple electrostriction equation with the effective electrostriction coefficient of 0.1 m4/C2. The electromechanical behavior of SBT films suggested no or weak contribution of non-180° domain walls to the strain response.

Preparation and Basic Properties of SrBi 2Ta 2O 9 Films

Ferroelectric SrBi2Ta2O9 thin films have been prepared on Pt and SiO2/n-Si(100) by laser ablation using an ArF excimer laser below 560°C. Crystallographic properties of the films are characterized as a function of substrate temperature and ambient pressure. Preferentially (105)-oriented SrBi2Ta2O9 thin films are obtained on SiO2/Si and Pt sheets. D–E hysteresis loops are observed in SrBi2Ta2O9 thin films deposited on Pt sheets. The remnant polarization is 2.5 µC/cm2 and the coercive force is 34 kV/cm. The films deposited on SiO2/Si consist of grains about 100 nm in diameter. Depth profiles from X-ray photoelectron spectra reveal homogeneous compositions, and the elements of the film diffuse little at the interface between the SrBi2Ta2O9 thin film and SiO2. The energy distributions of the density of interface states are below 1012 cm-2eV-1 in SiO2/Si at 550°C and below 3×1011 cm-2eV-1 at 400°C. A good capacitance-voltage hysteresis curve is obtained in the metal-ferroelectric-oxide-semiconductor structure using the SrBi2Ta2O9 film on SiO2/Si at T s of 550°C.

Fatigue Characteristics of SrBi2Ta2O9 Thin Films by RF Magnetron Sputtering Method

Ferroelectric SrBi2Ta2O9 thin films were synthesized on various substrates, such as MgO(100), glass, Pd/MgO(100) and Pd/glass by the rf magnetron sputtering method. Single-phase thin films were prepared on these substrates using a target with the starting chemical composition of SrCO3:Bi2O3:Ta2O5=0.7:1.2:1. At the higher substrate temperature of 1073 K, (h 0 l) orientation was found. The hysteresis loop of the thin film on Pd(100)/MgO(100) substrate did not change up to 109 switching cycles. These properties are very attractive for nonvolatile memory application.

Electrical properties of SrBi2Ta2O9 thin films deposited by RF magnetron sputtering

Ferroelectric bismuth layer oxide SrBi2Ta2O9(SBT) thin films were deposited on Pt/Ti/SiO2/Si substrates by rf magnetron sputtering at 500°C and then, were annealed at 850°C for 1 hr in oxygen ambient. The remanent polarization(2Pr) and the coercive field(2Ec) of a 300nm thick SBT film deposited in 10mtorr were 18.5 μC/cm2 and 150kV/cm at an applied voltage of 5V, respectively. The SBT films showed a fatigue-free characteristics up to 1010 cycles under 5V bipolar pulse. The leakage current density of the SBT films were about 8.6 × 10−8 A/cm2 at 100kV/cm.

Electrode dependence of hydrogen-induced degradation in ferroelectric Pb(Zr,Ti)O3 and SrBi2Ta2O9 thin films

Forming gas annealing causes changes in the remanent polarization (Pr), coercive field (Ec), and leakage current (I) in both PZT [Pb(Zr,Ti)O3] and SBT (SrBi2Ta2O9) samples with a variety of top electrode materials (Pt, Au, Ag, Cu, Ni, and In2O3), and the degree of degradation depends strongly on the top electrode material. These results may be explained by a model that is based on the catalytic activities of the top electrode to dissociate hydrogen molecules into hydrogen atoms, with the latter subsequently migrating into PZT or SBT films to cause oxygen deficiency and its associated property degradation. This model can be expanded to explain the recovery phenomenon resulting from oxygen annealing, which also depends on the catalytic activity of the top electrode to produce atomic oxygen from molecular oxygen.

Characterization of SrBi2Ta2O9 ferroelectric thin films deposited at low temperatures by plasma-enhanced metalorganic chemical vapor deposition

Ferroelectric bismuth-layer SrBi2Ta2O9(SBT) thin films were prepared on Pt/Ti/SiO2/Si substrate by plasma-enhanced metalorganic chemical vapor deposition. The films were crystallized at temperatures between 500 and 600 °C. The dielectric constant and dissipation factor of SBT films were 320 and 0.04 at an applied frequency of 1 MHz, respectively. The remanent polarization (Pr) and the coercive field (Ec) obtained for a 200 nm thick Sr0.9Bi2.3Ta2.0O9 films deposited at 550 °C were 15 μC/cm2 and 50 kV/cm at an applied voltage of 3 V, respectively. The leakage current density was about 5.0×10−8 A/cm2 at 300 kV/cm. The films showed fatigue-free characteristics up to 1.0×1011 switching cycles under 6 V bipolar pulse.

Properties of SrBi2Ta2O9 ferroelectric thin films prepared by a modified metalorganic solution deposition technique

Polycrystalline SrBi2Ta2O9 thin films having a layered-perovskite structure were fabricated by a modified metalorganic solution deposition technique using room temperature processed alkoxidecarboxylate precursor solution. It was possible to obtain a complete perovskite phase at an annealing temperature of 650 °C and no pyrochlore phase was observed even up to 600 °C. In addition, the SrBi2Ta2O9 thin films annealed at 750 °C exhibited better structural, dielectric, and ferroelectric properties than those reported by previous techniques. The effects of postdeposition annealing on the structural, dielectric, and ferroelectric properties were analyzed. The electrical measurements were conducted on Pt/SrBi2Ta2O9/Pt capacitors. The typical measured small signal dielectric constant and dissipation factor at 100 kHz were 330 and 0.023 and the remanent polarization and the coercive field were 8.6 μC/cm2 and 23 kV/cm, respectively, for 0.25-μm-thick films annealed at 750 °C. The leakage current density was lower than 10−8 A/cm2 at an applied electric field of 150 kV/cm. The films showed good switching endurance under bipolar stressing at least up to 1010 switching cycles.

SrBi2Ta2O9(SBT) thin films prepared by electrostatic spray

Ferroelectric SrBi2Ta2O9 (SBT) thin films were deposited on Pt/SiO2/Si substrates by electrostatic spray, and their structural and electrical properties were investigated. The spray-coated films crystallized upon heating at 680°C-760°C. The XRD patterns showed distinct peaks of (105), (110) and (200) orientations. P-E hysteresis loops were observed with a typical coercive field around 55kV/cm. The remanent polarization, 2P r , was above 12μC/cm2 for a 160nm film. The fatigue endurance was good, as the reduction of P r during 1012 switching cycles was less than 10% of its initial value. The electrostatic spray technique is capable of injecting submicron, stoichiometrically correct, metal-organic SBT droplets onto a rotating substrate, and has the potential to achieve a better step coverage than the spin-on technique while keeping its original advantages.