SrBi2Ta2O9 Thin Films Research Articles

Growth and ferroelectric properties of SrBi2Ta2O9 thin films on highly oriented pyrolytic graphite substrates with atomic structure like graphene surface

Growth and ferroelectric properties of SrBi2Ta2O9 thin films on highly oriented pyrolytic graphite substrates with atomic structure like graphene surface

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.

Domain wall current, ferroelectric, and piezoelectric properties of flexible SrBi2Ta2O9 thin films affected by the distribution of a-oriented domains and c-oriented domains

We investigated the ferroelectric, piezoelectric, and domain wall current properties affected by the distribution of a-oriented domains and c-oriented domains in flexible SrBi2Ta2O9 (SBT) thin films on flexible Pt/mica substrates. On the flexible Pt/mica substrates, the SBT thin films with a mixed crystallinity of a-oriented and c-oriented crystallinities exhibited the domain structures mixed with a-oriented domains and c-oriented domains. The SBT thin films with excellent a-oriented crystallinity showed improved ferroelectric and piezoelectric properties due to the polar axis perpendicular to the c-axis. In particular, 90o domain walls composed of a-domain and c-domain have larger domain wall currents than 180o domain walls composed of c-domains, which increases the leakage current of the SBT thin films.

Metal (Pt)/ferroelectric (SrBi2Ta2O9)/insulator (La2O3)/semiconductor (Si), MFIS structures for nonvolatile memory applications

Extended data retention is a cardinal impediment for ferroelectric memories and serves a pivotal role for nonvolatile memory applications. Here, nonvolatile Metal–Ferroelectric–Insulator–Semiconductor (MFIS) structures are fabricated by using thin films of Strontium Bismuth Tantalum Oxide (SrBi2Ta2O9) as ferroelectric and high-κ lanthanum oxide (La2O3) as a buffer insulator on p-Si substrates via RF magnetron sputtering. The grazing incidence x-ray diffraction analysis confirms the dominant (111) and (115) ferroelectric perovskite phases of SrBi2Ta2O9 thin films. Albeit, atomic force microscopy surface micrographs revealed highly smooth La2O3 and SBT (SrBi2Ta2O9) thin films with a surface roughness of ∼0.22 ± 0.04 nm and ∼1.05 ± 0.03 nm, respectively. Capacitance–voltage (C–V), capacitance–time (C–T), and current–voltage (I–V) characteristics of Pt/SrBi2Ta2O9/La2O3/Si, MFIS structures, exhibited a high memory window of ∼1.1 V at ±5 V sweep voltage, data retention measured until ∼104 s even on the extrapolation up to 10 years, and a low leakage current density of ∼12.8 μA/cm2 at −1 V and 300 K. Far from it, the probed conduction mechanism is studied for Pt/SrBi2Ta2O9/La2O3/Si MFIS device structures. The optimum nonvolatile memory characteristics are attributed to the high-quality SBT ferroelectric and the buffer layer La2O3/Si interface of the investigated MFIS structure and also assert from the control Pt/SBT/Pt and Pt/La2O3/Si results. Thus, the proposed Pt/SrBi2Ta2O9/La2O3/Si structure is a potential candidate for a gate stack of one-transistor (1T) type Ferroelectric Field-Effect Transistors nonvolatile memory applications.

La3+-doped SrBi2Ta2O9 thin films for FRAM synthesized by sol-gel method

This paper discusses the possibility of synthesis of SBTL sol-gel films for use as active layers for non-volatile memory (FRAM). La-doped SrBi2Ta2O9 thin films were synthesized by sol-gel method on Pt/TiO2/BPSG/SiO2/Si substrates. The structural features of the surface (AFM), crystallization behavior (XRD) during the heating and ferroelectric properties of synthesized films were discussed. It was shown that an optimum surface structure and a high share of perovskite phase of SBTL-films were compared to SBT-films (Theating=800 °C). Achieved ferroelectric parameters suggested the possibility of using synthesized SBTL sol-gel films in non-volatile memory devices.

Variation in ferroelectric polarization direction of epitaxial (001) SrBi2Ta2O9 thin film induced by oxygen vacancy

We report the enhancement of c-axis ferroelectric properties in an epitaxial (001) SrBi2Ta2O9 (SBT) thin film originating from the oxygen vacancy. We controlled the oxygen vacancy in the SBT thin film by using the electrical stress process triggering the polarization fatigue. As a result of the fatigue test for the Pt/SBT/Nb:STO capacitor, we observed the gradual increase in the ferroelectric polarization up to 1012 fatigue cycles and then subsequently rapid decrease over 1012 cycles. Based on piezoresponse force microscopy (PFM) measurements, we demonstrated the increase in the polarization and PFM signal resulting from the creation of oxygen vacancy.

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.

Ferroelectric SrBi2Ta2O9 Thin Film Studied by Micro-Raman Scattering and Atomic Force Microscopy

Ferroelectric SrBi2Ta2O9 thin films were prepared by metal-organic decomposition technique on Si wafers. The thickness dependence of the films was studied by micro-Raman scattering and atomic force microscopy (AFM). From the AFM observation, it is found that films were constructed of c-axis-oriented grains. The low frequency component of the Raman spectra of the films decreases as the temperature increases. The spectral analysis of the low frequency component has been carried out by using a damped harmonic-oscillator model and a relaxational model. The relaxation time has the similar value of that reported in KDP. It is suggested the physical origin of the low frequency component may be polarization fluctuation.

Memory Properties of SrBi<sub>2</sub>Ta<sub>2</sub>O<sub>9</sub>/ Ba(Zr<sub>0.1</sub>Ti<sub>0.9</sub>)O<sub>3</sub> Thin Films Prepared on Si Substrate

In this study, ferroelectric thin films of SrBi2Ta2O9 (SBT) or bilayered SrBi2Ta2O9/ Ba(Zr0.1Ti0.9)O3 (SBT/BZT) are successfully deposited on Si substrate under the optimal RF magnetron sputtering conditions, and their electrical and ferroelectric characteristics are discussed. Ferroelectric thin films are deposited on Si substrate under the RF power of 80 W, chamber pressure of 10 mTorr, substrate temperature of 550oC, and different oxygen concentrations. The surface morphology of deposited thin films is observed from the FESEM images, and the memory windows and leakage current of the Al/SBT/BZT/Si (MFS) structure are measured by an impendence phase analyzer and a semiconductor parameter analyzer, respectively. The memory window, capacitance and leakage current density of MFS structures under different oxygen concentrations are also reported. We find that the memory window of bilayered SBT/BZT structure shows larger than one of single layer SBT structure.

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.

Memory Properties of SrBi2Ta2O9 Ferroelectric Thin Film Prepared on SiO2/Si Substrate

In this study, stoichiometry SrBi2Ta2O9 and SrBi2Ta2O9 + 4 wt% Bi2O3 was used as the compositions to fabricate the ceramic targets. The SrBi2Ta2O9 (SBT) thin films were deposited using the two different targets under the optimal depositing parameters with different oxygen concentration. After that, the metal-ferroelectric-insulator-semiconductor (MFIS) structure devices of Al/SrBi2Ta2O9 (SBT)/insulator (SiO2)/Silicon was fabricated and their electrical characteristics were investigated. The SBT thin films on SiO2/Si substrates show good capacitance-voltage characteristics and the threshold voltage shift of 23 V at applied voltage of ± 30 V bias. Finally, the memory properties of SrBi2Ta2O9 ferroelectric thin film prepared on SiO2/Si substrate were also discussed.

Temperature dependent leakage current behavior of pulsed laser ablated SrBi2Ta2O9 thin films

Polycrystalline SrBi2Ta2O9 (SBT) thin films were grown on Pt/Ti/SiO2/Si substrates by pulsed laser ablation technique. Phase analyses of the deposited films were studied by grazing incidence x-ray diffraction. Microstructural and interfaces of the SBT film were investigated using a field emission scanning electron microscope. The dc leakage current behavior was studied at different temperatures, and the current transport mechanism was investigated. The calculated activation energies from the Arrhenius plot were attributed to the shallow traps existing near the conduction band of the SBT thin films. The current-voltage plot could be clearly separated into three regions, i.e., Ohm’s law, trap-filled limited, and Child’s law. At a low electric field, the current density–voltage characteristics showed the Ohmic behavior. Lampert’s theory of space charge limited conduction was found to be suitable to explain the current conduction through SBT films. The trap-filled limited voltage increases with increasing temperature up to 100 °C and then decreases with temperature.

Interfacial and electrical properties of SrBi2Ta2O9/ZrO2/Si heterostructures for ferroelectric memory devices

We have investigated the interfacial and frequency dependent electrical properties of metal-ferroelectric-insulator-semiconductor capacitors with SrBi2Ta2O9 (SBT) ferroelectric films grown on ZrO2 buffer layer coated Si. Heterostructure SBT and ZrO2 thin films were deposited using rf magnetron sputtering. Interfacial and surface roughness parameters of heterostructures were extracted from the simulation of specular x-ray reflectivity data. The structure exhibited clockwise capacitance-voltage hysteresis with a maximum memory window of 2.0 V at a bias voltage of ±7 V. Frequency dependent (5 kHz–1 MHz) measurements at room temperature indicated that the clockwise hysteresis originates from the ferroelectric domain reversal. A minimum leakage current density of 4×10−8 A/cm2 of fabricated capacitors at an applied voltage of ±5 V revealed that the ZrO2 buffer layer prevents the interfacial diffusion between SBT thin film and the substrate, resulting in an improvement of interface quality. The charge retention time of the ferroelectric capacitor was studied as a function of buffer layer thickness.

Effects of acetoin as chelating agent on the preparation of SrBi2Ta2O9 thin films from non-hydrolyzing precursors

Ferroelectric thin films of strontium bismuth tantalate (SBT) have been fabricated by a chemical solution deposition technique using non-hydrolyzing precursors. Strontium acetate, bismuth nitrate and tantalum ethoxide were used as precursor materials, with methanol and glacial acetic as solvents. We investigate the effects of the precursor chemistry, by the selection of the chelating agent, on the elimination of residual organic compounds, thermal evolution of phase formation, and microstructure evolution of derived films. We found that the utilization of alkanolamines as chelating agent produce the segregation of metallic bismuth in as-prepared powders. On the other hand, acetoin, one of the hydroxyketones, showed the elimination of residual organics at low temperature, an earlier onset of crystallization, and no segregation of secondary phases during the whole crystallization process. A comparative investigation of the surface microstructure, grain size distribution, crystallinity, and degree of crystal orientation of films fabricated with the different chelating agents is presented. The dielectric and ferroelectric properties of films prepared with acetoin are investigated.

Enhanced domain wall speed in non-c-oriented SrBi2Ta2O9 thin film

A non-c-oriented SrBi2Ta2O9 thin film was fabricated by eclipse pulsed laser deposition. It showed improved ferroelectric properties as well as enhanced domain wall speed. The enhanced domain wall speed directly indicates that the non-c-oriented domain has a lower activation field in terms of Merz's law than the highly c-oriented domain. The non-c-oriented domains of the low activation field give easier domain switching with the fast domain wall speeds than the c-oriented domains.

Effect of Ta Nanobarrier in Magnetron Sputtering of Nd-Doped SrBi2Ta2O9 Thin Films

SrBi2Ta2O9 (SBT) is a bismuth layered perovskite with attractive ferroelectric properties for random access memory applications. Our previous studies showed that Nd-doped SBT (SNBT) thin films exhibited an improved remnant polarization and reduced coercivity. This paper concentrates on the effect of Ta nanobarrier in between the SNBT and the Pt layers. Without the nanobarrier, severe bismuth diffusion is revealed by the secondary ion mass spectroscopy. However, with a nano layer (up to 2 nm) of Ta metal, the interfacial diffusion is effectively suppressed even at 800 degrees C. Details of the composition profiling, film crystallinity and remnant polarization are discussed in view of the nanobarrier thickness.

Structural and electrical properties of metal–ferroelectric–insulator–semiconductor structure of Al/SrBi2Ta2O9/HfO2/Si using HfO2 as buffer layer

Ferroelectric SrBi2Ta2O9 (SBT) thin films have been deposited by the radio-frequency magnetron sputtering technique on bare p-Si as well as on HfO2 insulating buffer p-Si. XRD patterns revealed the formation of a well-crystallized SBT perovskite thin film on the HfO2 buffer layer. The electrical properties of the metal–ferroelectric–insulator–semiconductor (MFIS) structure were characterized by varying thicknesses of the HfO2 layer. The MFIS structure exhibits a maximum clockwise C–V memory window of 1.60 V when the thickness of the HfO2 layer was 12 nm with a lower leakage current density of 6.20 × 10−7 A cm−2 at a positive applied voltage of 7 V. However, the memory window reaches a maximum value of 0.7 V at a bias voltage of ±5 and then decreases due to charge injection in the case of the insulating buffer layer thickness of 3 nm. The density of oxide trapped charges at/near the buffer layer–ferroelectric interface is studied by the voltage stress method. Capacitance–voltage (C–V) and leakage current density (J–V) characteristics of the Al/SBT/HfO2/Si(1 0 0) capacitor indicate that the introduction of the HfO2 buffer layer prevents interfacial diffusion between the SBT thin film and the Si substrate effectively and improves the interface quality. Furthermore, the Al/SBT/HfO2/Si structures exhibit excellent retention characteristics, the high and low capacitance values clearly distinguishable for over 1 h and 30 min. This shows that the proposed Al/SrBi2Ta2O9/HfO2/Si structure is ideally suitable for high performance ferroelectric memories.

The Effects of Various Substrate Temperature of Bi2O3 Buffer Layer on SBT (SrBi2Ta2O9) Thin Films Deposited by R.F. Magnetron Sputtering

The SBT(SrBi2Ta2O9) thin films with Bi2O3 buffer layer were deposited on Pt/Ti/SiO2/Si substrate by R.F. magnetron sputtering method in order to improve the ferroelectric characteristics. In SBT thin films, the deficiency of bismuth due to its volatility during the process results in an obvious non stoichiometry of the films and the presence of secondary phases. Bi2O3 buffer layer was found to be effective to achieve lower temperature crystallization and improve ferroelectric properties of SBT thin films. Ferroelectric properties and crystallinities of SBT thin films with various substrate temperature of Bi2O3 buffer layer were observed, using X-Ray Diffraction (XRD), Precision LC (Radient Technologies. Inc.) and GDS (glow discharge spectrometer).

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.

Formation and stability of pyrochlore in sputtered SBT thin films

In magnetron sputtering of SrBi2Ta2O9 (SBT) thin films, bismuth deficiency tends to result in formation of an unwanted pyrochlore phase. In this paper, the dependence of deposition parameters on phase evolution in Sr deficient SBT thin films is studied. Only when bismuth is over stoichiometric (Sr0.74Bi2.2Ta2O9+x) can one obtain pure SBT structure. Decreasing the Bi content encourages formation of the Bi deficient pyrochlore phase. At 1.04 of Bi, only the pyrochlore phase forms. The pyrochlore phase is very stable even at 900°C.