Properties Of SrBi2Ta2O9 Thin Films Research Articles

Enhanced ferroelectric and piezoelectric properties of SrBi2Ta2O9 thin films through crystalline orientation

This study investigates how variations in substrate temperatures and deposition rates during pulsed laser deposition on Nb-doped SrTiO3 substrates affect the crystallinity, and consequently, the ferroelectric and piezoelectric properties of SrBi2Ta2O9 (SBT) thin films. The SBT thin film developed at 850 °C with a deposition rate of 0.34 nm/pulse showed lower remanent polarization and piezoelectric coefficients, a result of its c-oriented crystallinity. In contrast, SBT thin films produced at 750 °C with deposition rates of 0.37 nm/pulse demonstrated enhanced ferroelectric and piezoelectric responses due to mixed a-oriented crystallinity. In particular, the SBT thin film, fabricated at 750 °C with a lower deposition rate of 0.04 nm/pulse, demonstrated the most enhanced ferroelectric and piezoelectric properties due to its highly refined a-oriented crystallinity. Our findings illuminate the relationship between the characteristics of the crystals and functional properties in SBT thin films, highlighting the integration of a-domains within c-domains as a key strategy for optimizing SBT film performance and opening avenues for designing specialized Bi-layered perovskite thin films.

Impact of forming gas annealing on the dielectric properties of SrBi2Ta2O9 thin films prepared by metalorganic decomposition

SrBi2Ta2O9 (SBT) thin films were prepared on Pt/TiO2/SiO2/Si substrates by metalorganic decomposition method. The dielectric properties of SBT films strongly depend on annealing conditions and annealing time. Compared with films not annealed in forming gas, the relative dielectric constant and the dissipation factor for SBT films annealed at 400 °C decrease by 23.4% and 30.6%, respectively. It implies that the dominant dielectric loss mechanism is related to the degradation of films. Forming gas ambient may have played an important role in the increase of oxygen vacancies in SBT thin films and the degradation of dielectric properties.

Seeding Effect by Off-Stoichiometric SrBi2Ta2xO9 Thin Buffer Layer on the Properties of SrBi2Ta2O9 Thin Films

The seeding effect on the crystallization behaviors of the strontium bismuth tantalite (SBT) phase on top of a thin Aurivillius buffer layer, , was studied. A uniform microstructure and a preferred polar -axis orientation within the buffered thin film were promoted to achieve a high remanent polarization value of about . Furthermore, the SBT/buffer heterostructure constructed by the two-step process showed a significantly improved leakage property compared with the nonbuffered SBT. The off-stoichiometric thin buffer layer demonstrates a simple method to improve the microstructure, polar -axis orientation, and electric characteristics and to introduce less defects in the superposed thin films.

Ferroelectric properties of SrBi2Ta2O9 thin films on Si (100) with a LaZrO x buffer layer

To obtain a metal–ferroelectric–insulator–semiconductor (MFIS) structure, we fabricated ferroelectric SrBi2Ta2O9 (SBT) film on a p-type Si (100) wafer with a LaZrOx (LZO) buffer layer by means of a sol–gel technique. The sol–gel deposited LZO film according to the different annealing temperatures had a good surface morphology even though the crystalline phase was not an amorphous phase. In particular, the root-mean-squared (RMS) surface roughness of the 750-°C-annealed LZO film was about 0.365 nm and its leakage current density was about 8.2 × 10−7 A/cm2 at 10 V. A Au/SBT/LZO/Si structure with different SBT film was fabricated. The C–V characteristics of the Au/SBT/LZO/Si structure showed a clockwise hysteresis loop. The memory window width increased as the SBT film thickness increased. The 600-nm-thick SBT film was crystallized in a polycrystalline phase with a highly preferred (115) orientation. The memory window width of the 600-nm-thick SBT film was about 1.94 V at the bias sweep voltage ±9 V and the leakage current density was about 6.48 × 10−8 A/cm2 at 10 V.

Effects of bottom electrode conditions on ferroelectric properties of SrBi2Ta2O9 thin films

The effect of adhesion layers under the bottom electrodes on ferroelectric characteristics of SrBi2Ta2O9 (SBT) thin films synthesized by sol-gel method was investigated. The remanent polarization (2Pr) of SBT films was found to be smaller for the tantalum oxide (TaOx) adhesion layer than for the titanium oxide (TiOx) adhesion layer, whereas the coercive voltage (2Vc) was larger for the TaOx adhesion layer than for the TiOx adhesion layer. The difference was due to a larger c-axis distribution of crystals in the SBT film on Pt with the TaOx layer than with the TiOx layer. Moreover, the imprint characteristics of SBT capacitors on the TaOx layer were better than those on the TiOx layer because the 2Vc in SBT capacitors was larger for the TaOx layer than for the TiOx layer, and the polarization stability against the hysteresis shift was better for the TaOx layer than for the TiOx layer.

Effect of Silicon Addition on Electrical Properties of SrBi2Ta2O9 Thin Films

Silicon-added SrBi2Ta2O7 (SBT) thin films were fabricated on a Pt/Ti/SiO2/Si substrate by metal-organic decomposition (MOD), and then a Pt top electrode was deposited on the SBT thin film by DC magnetron sputtering. The remanent polarization and the relative dielectric constant of the SBT thin film decreased as the amount of silicon addition increased. The SBT thin film had a randomly oriented structure. The X-ray diffraction pattern of the SBT thin film changed negligibly even if silicon was added. The silicon-added SBT thin film showed a comparatively excellent fatigue characteristic. The silicon-added SBT thin film is suitable for application in ferroelectric gate field-effect-transistor type memory because of its excellent fatigue characteristic, low remanent polarization and low relative dielectric constant.

Effects of LaNiO3 on the structures and properties of SrBi2Ta2O9 thin films

SrBi2Ta2O9(SBT)/LaNiO3(LNO)/Si and SBT/Pt/TiO2/SiO2/Si multilayers were fabricated by pulsed laser deposition. With Pt top electrodes, the measured remanent polarization (2Pr) of Pt/SBT/LNO/Si and Pt/SBT/Pt/TiO2/SiO2/Si capacitors was 6.5 μC/cm2 and 5.2 μC/cm2, respectively. Using LNO as both bottom electrodes and buffer layers, enhanced non-c-axis crystalline SBT films were induced, which resulted in a 2Pr greater than that of the Pt/SBT/Pt/TiO2/SiO2/Si capacitor. The hysteresis loop of the Pt/SBT/LNO/Si capacitor showed a great external-field-dependent horizontal shift. Using an electron-injection model, this dependence was addressed. The fatigue-free property of the Pt/SBT/LNO/Si capacitor was experimentally established, in that the non-volatile polarization decreased by less than 5% of the initial value after 1.44×109 switching cycles .

Impact of Sr content on dielectric and electrical properties of pulsed laser ablated SrBi2Ta2O9 thin films

The ferroelectric properties of SrBi2Ta2O9 thin films grown by laser ablation were investigated as a function of the Sr+2 content in the films. Different target compositions were used to obtain films with different Sr+2/Ta+5 ratios. The initial composition was according to the stoichiometric composition (1/2), and the Sr+2/Ta+5 ratio was varied to 0.7/2.0. It was seen that the remanent polarization showed a consistent increase, as the film became more deficient of “Sr+2” up to a certain extent. Similarly, a decrease in the dielectric constant and the leakage current with the decrease of Sr+2 in the film was observed. The dielectric transition temperature showed an increase with the reduction of Sr+2 content and was seen to approach the bulk value.

The Crystallinity and Electrical Properties of SrBi2Ta2O9 Thin Films Fabricated by New Low Temperature Annealing

We studied growth and characterization of <TEX>$SrBi_2Ta_2O_9$</TEX> (SBT) thin films fabricated by low temperature process under vacuum and/or oxygen ambient. A metal organic decomposition (MOD) method based on a spin-on technique and annealing process using a rapid thermal annealing (RTA) method was used to prepare the SBT films. The crystallinity of a ferroelectric phase of SBT thin films is related to the oxygen partial pressure during RTA process. Under an oxygen partial pressure higher than 30 Torr, the crystallization temperature inducing the ferroelectric SBT phase can be lowered to <TEX>$650^{\circ}C$</TEX>. Those films annealed at <TEX>$650^{\circ}C$</TEX> in vacuum and oxygen ambient showed good ferroelectric properties, that is, the memory window of 0.5~0.9 V at applied voltage of 3~7 V and the leakage current density of <TEX>1.80{\times}10^{-8}$</TEX> A/<TEX>$\textrm{cm}^2$</TEX> at an applied voltage of 5V. In comparison with the SBT thin films prepared at 80<TEX>$0^{\circ}C$</TEX> in <TEX>$O_2$</TEX> ambient by furnace annealing process, the SBT thin films prepared at <TEX>$650^{\circ}C$</TEX> in vacuum and oxygen ambient using the RTA process showed a good crystallization and electrical properties which would be able to apply to the virtul device fabrication precess.

Time effect on the ferroelectric properties of SrBi2Ta2O9 thin films in forming gas processing

Abstract The systematic measurements of hysteresis loops of SrBi 2 Ta 2 O 9 (SBT) thin films (440 nm) were carried out after they were isothermally processed in forming gas (FG) for different periods of time. It was found that the remnant polarization of SBT thin films dropped about 50% when they were annealed in forming gas at 400 °C for only 1 min. At the same time, the coercive voltage dropped about 20%. Then during anneal time, from 1 to 6.5 min, the remnant polarization and coercive voltage did not have obvious change. When the anneal time was up to 7.5 min, the normal hysteresis loops could not be obtained. The possible physical mechanism was discussed in this paper to give an explanation of this time effect on the degradation of ferroelectric properties of SBT thin films during forming gas processing.

Improvement in ferroelectric properties of SrBi2Ta2O9 thin films with Bi2O3 buffer layers by liquid-delivery metalorganic chemical-vapor deposition

Ferroelectric SrBi2Ta2O9 (SBT) thin films were deposited onto the Bi2O3 buffered Pt/Ti/SiO2/Si substrates using liquid-delivery metalorganic chemical-vapor deposition. The SBT films with a 5-nm-thick Bi2O3 buffer layer on a Pt bottom electrode showed (115) orientation stronger than those without the Bi2O3 buffer layer after annealing at 750 °C. The Bi2O3 buffer layer in Pt/Bi2O3/SBT/Bi2O3/Pt/Ti/SiO2/Si capacitors effectively prevents the evaporation of Bi through the Pt top electrode and diffusion of Bi into the Pt bottom electrode. The remanent polarization and leakage current densities of SBT films with Bi2O3 buffer layers were improved significantly in comparison with those for the films without the Bi2O3 buffer layer. The remanent polarization (2Pr) and coercive field (Ec) of SBT films without and with the Bi2O3 buffer layer annealed at 750 °C were 12 and 23 μC/cm2, 57 and 38 kV/cm at an applied voltage of 5 V, respectively.

Effects of anneal atmosphere on the structural and ferroelectric properties of SrBi2Ta2O9 thin films

SrBi2Ta2O9 (SBT) films were prepared on Pt/TiO2/SiO2/Si substrates at 750°C in oxygen by metalorganic decomposition method. SBT film capacitors were re-annealed in Ar (N2) at 350-750°C and then followed the O2 recovery at 750°C. Effects of anneal atmosphere on the structure, morphology and ferroelectric properties have been investigated deeply. After above 550°C 100% Ar or N2 reanneal, the remnant polarization decreases and coercive field increases significantly. The subsequent O2 recovery can hardly rejuvenated them. The result is different from that from forming gas processing (annealing in hydrogen atmosphere). The possible origin and mechanism is discussed and proposed.

The post annealing effects on ferroelectric properties of SrBi2Ta2O9 thin films prepared by R.F. magnetron sputtering

SrBi2Ta2O9(SBT) films were prepared on Pt/Ti/SiO2/Si substrates by conventional r.f. magnetron sputtering. The ferroelectric and dielectric properties of bismuth layer structured ferroelectrics are studied for the purpose of examining the effects of oxygen flow rate during annealing. Oxygen flow rate influenced the shape of hysteresis loop, polarization value and current voltage characteristics. The remanent polarization (2Pr) and the coercive field (2Ec) obtained from a 240 nm thick SBT films annealed at 760Torr were 4.4uC/cm2 and 72kV/cm at an applied voltage of 5 V, respectively. The films annealed at oxygen flowing condition showed leakage current density of ∼10−7 A/cm2 at 150kV/cm. The films showed fatigue-free characteristics independent of annealing condition up to 1011 cycles under 5 V bipolar square pulse.

Effects of bi2o3 buffer layer on ferroelectric properties of srbi2ta2o9 thin films by liquid-delivery mocvd

Ferroelectric SrBi2Ta2O9 thin films were deposited on the Bi2O3 buffered Pt/Ti/SiO2/Si substrates using liquid-delivery metalorganic chemical vapor deposition. SBT films with 5nm thick-Bi2O3 buffer layer on Pt bottom electrode showed stronger (115) orientation than those without Bi2O3 buffer layer after annealing at 750°C. The value of the remanent polarization of SBT films with Bi2O3 buffer layer were improved significantly in comparison with those for the films without Bi2O3 buffer layer. The remanent polarization(2Pr) and coercive field(Ec) of SBT films without and with Bi2O3buffer layer annealed at 750°C were 11.9, 20.8 μ C/cm2 and 57, 37.8kV/cm at an applied voltage of 5 V, respectively.

Influence of annealing temperature on ferroelectric properties of SrBi2Ta2O9 thin films prepared by off-axis radio frequency magnetron sputtering

Ferroelectric SrBi2Ta2O9 (SBT) thin films were deposited on Pt/TiOx/SiO2/Si substrates using the off-axis radio frequency magnetron sputtering technique. X-ray diffraction and atomic force microscopy experiments showed that the crystallization of SBT thin films at &amp;gt;700 °C correlated with the formation of rod-like grains. Cross-sectional field emission scanning electron microscopy images revealed that the apparent thickness of SBT decreased while the thickness of Pt increased as the annealing temperature was increased. The apparent decrease in the thickness of SBT was attributed to crystallization and densification in the film whereas the apparent increase in Pt thickness was due to diffusion of Ti and Bi into the Pt layer. This diffusion at high annealing temperatures (800 °C and above) alters the Pt purity and degrades the Pt as the bottom electrode for the ferroelectric capacitor. Good insulating properties were obtained when the SBT film was annealed at 700 and 750 °C whereas higher leakage currents were observed at annealing temperatures &amp;gt;800 °C. A remnant polarization (Pr) of 4.35 μC/cm2 and coercive field (Ec) of 31.5 kV/cm were obtained for the SBT thin film annealed at 750 °C with a leakage current density of &amp;lt;10−7 A/cm2.

Control of Crystal Orientation of Ferroelectric SrBi2Ta2O9 Thin Films with Multi-Seeding Layers

A Bi-Ta multi-seeding layer system is proposed to improve the ferroelectric properties of SrBi2Ta2O9 (SBT) thin films prepared by a sol-gel method for applications to nonvolatile memories. For seeding layers composed of different ratios of Bi to Ta, three different structures were observed after heat treatment at 800°C: γ-Bi2O3, Bi3TaO7 and Bi1.4Ta2O6.75. γ-Bi2O3 with a sillenite structure exhibited a plate-like surface morphology. Bi3TaO7 with a fluorite structure exhibited a flat surface morphology. Bi1.4Ta2O6.75 with a pyrochlore structure exhibited a rough surface morphology. Because a flat surface morphology with a fluorite structure is desirable for a seeding layer, the Bi–Ta seeding layers, with a Bi to Ta ratio equal to 3 to 1, were used as multi-seeding layers. When the seeding layers were sandwiched between SBT thin films, multi-seeding layers provided, after heat treatment at 800°C, a higher degree of crystal orientation in the a- or b-axis direction than a simple SBT thin film without seeding layers. Orientation in the a- or b-axis direction is desirable for SBT thin films to obtain improved ferroelectric properties, especially high remanent polarization values. The SBT thin film with Bi–Ta multi-seeding layers was found to have higher saturation characteristics than that without seeding layers, which suggests that the former type of SBT thin film has a good potential for low-voltage operation. The multi-seeding method is thus effective for improving ferroelectric properties of SBT thin films for nonvolatile memories.

Etching characteristics of SrBi2Ta2O9 film with Ar/CHF3 plasma

Among the ferroelectric thin films that have been widely investigated for ferroelectric random access memory (FRAM) applications, the SrBi2Ta2O9 (SBT) thin film is appropriate as a memory capacitor material due to its excellent fatigue endurance. However, very few studies on the etch properties of SBT thin films have been reported, even though dry etching is an area that demands a great deal of attention in the very-large-scale integration of ferroelectric thin-film capacitors for FRAM applications. In this study, SrBi2Ta2O9 thin films were etched using a magnetically enhanced inductively coupled Ar/CHF3 plasma. Etch properties, such as etch rate, selectivity, and profile, were measured for different gas mixing ratios of CHF3/(Ar+CHF3), while the other process conditions were fixed at rf power of 600 W, dc-bias voltage of −150 V, and chamber pressure of 5 mTorr. The maximum etch rate of SBT thin films was 1650 Å/min under CHF3/(Ar+CHF3) of 0.1. Selectivities of SBT to Pt and photoresist masks were 1.35 and 0.94, respectively. The chemical reaction and compositional change of the etched surfaces were investigated by x-ray photoelectron spectroscopy analysis. The Sr and Ta atoms of SBT films react with fluorine; Sr–F and Ta–F are then removed by physical sputtering by Ar ions. The surface of a SBT film etched with CHF3/(Ar+CHF3)=0.1 was analyzed using secondary ion mass spectrometry. Scanning electron microscopy was used to examine etched profiles of SBT thin films. The wall angle of a SBT film etched using CHF3/(Ar+CHF3)=0.1 was approximately 85°.

Role of bismuth precursor in crystallization of SrBi 2 Ta 2 O 9 thin films

Crystallization of SrBi2Ta2O9 (SBT) thin films was studied as a function of viscosity of bismuth precursor and baking temperature, in order to fabricate capacitors with improved ferroelectric properties. SBT thin films were deposited on to Pt substrates using a chemical solution deposition (CSD) technique. Post-deposition anneal at 750 °C for 1 h in oxygen atmosphere revealed a significant influence of baking temperature and the viscosity of bismuth precursor on the microstructure and the ferroelectric properties of SBT thin films. A high baking temperature (350 °C) and a low viscosity of bismuth precursor (8 cp) yielded larger amounts of Bi2O3 secondary phase, smaller SBT grains (104 nm), and lower remanent polarization (Pr=2.0 μc/cm2). Additionally, these films exhibited a very high rate of ageing (>45% reduction in Pr after 7 days). A modified CSD process is suggested, which could suppress the formation of Bi2O3 secondary phase. Films fabricated using modified CSD technique exhibited a much larger grain size of 165 nm, higher Pr of 7.2 μc/cm2, and significantly improved ageing characteristics (<1% reduction in Pr after 7 days). A qualitative model to describe the ageing in SBT-based capacitors is also suggested.

Characterization of metalorganic decomposition-derived SrBi2Ta2O9 thin films with different thicknesses

Ferroelectric capacitors having sputtered Pt bottom and top electrodes and metalorganic decomposition derived ferroelectric thin films of SrBi2Ta2O9 (SBT) were prepared. The thickness effects on the structure, surface morphology, and electrical properties of SBT thin films were studied and discussed. Structure and surface morphology were analyzed by x-ray diffraction and atomic force microscope. The electrical properties were characterized by the measurements of hysteresis, capacitance, and pulse switching. The crystalline structure, grain size, and electrical properties do not depend on film thickness when films are less than 440 nm thick. A larger grain size is observed for films thicker than 440 nm, which results in a larger remnant polarization. The thickness dependence of the coercive voltage and reciprocal capacitance can be explained by an interfacial layer model. The thickness-independent coercive field and dielectric constant, calculated from the interfacial layer model, are around 19 kV/cm and 343, respectively. Nonvolatile polarization (Pnv), obtained from pulse-switching measurements, increases with the applied electric field and finally saturates with a slight thickness effect. The relaxation of the remnant polarization that occurs at zero-voltage intervals between the pulses and the thickness-independent behavior of Pnv are explained based on depolarization fields within the ferroelectric due to nonswitching interfacial layers at the film/electrode interfaces.

Switching properties of SrBi2Ta2O9 thin films produced by metalorganic decomposition

The process of polarization reversal in a SrBi2Ta2O9 (SBT) thin film capacitor produced by a metalorganic decomposition method was investigated. The switching time (100–600 ns) and the reversible polarization have been measured at different voltages in the range of 0–5 V. It was found that by annealing the SBT sample in AR atmosphere at 400 °C, the switching times were significantly reduced. This indicates that oxygen vacancies in the film speed up nucleation and growth of new domains.