SrBi2Ta2O9 Thin Films Research Articles

Holistic RBS–PIXE data reanalysis of SBT thin film samples

The growth of SrBi2Ta2O9 (SBT) films on top of Pt electrode substrates is an important issue for the fabrication of ferroelectric based memories. In a recent publication, SBT thin films grown using seeded and unseeded procedures were studied by PIXE and RBS. Difficulties and misfits found in the comparison of results from both techniques were, at that time, overcome by physical considerations. These, although not rendering interpretation impossible, left out the possibility of understanding the exact nature of the differences between the interface behavior in each case. In the present work it is shown that the reanalysis of the same data using the recently developed RBS–PIXE simultaneous and self-consistent calculation present in NDF leads to stronger conclusions on the solid state reaction occurring during the deposition stage for both types of samples. Allowing for the occurrence of solid state reactions between the deposited film and the Pt electrode even in the case of seeded samples leads to a full explanation of IBA data, this result showing that in complex systems such as this, the PIXE data are essential and must be integrated with the RBS data in an holistic analysis, in order to properly unfold the depth profile of the samples. Contrary to what is usually supposed PIXE data is therefore shown to contain hidden depth structure information.

Preparation of SrBi<sub>2</sub>Ta<sub>2</sub>O<sub>9</sub> Ferroelectric Thin Films by Liquid Source Misted Chemical Vapor Deposition Method Using Inorganic Salt Solution

A liquid source misted chemical vapor deposition (LSMCVD) system was used to deposit SrBi2Ta2O9 ferroelectric thin films on Pt/TiO2/SiO2/Si substrates using strontium nitrate, bismuth nitrate and fluorine tantalum acid as the starting ingredients. Citric acid, ethylene glycol, and ethylenediaminetetraacctic acid were employed as chelating agents to form homogeneous and stable sol. A density, homogenous, crack-free and c-axis oriented SrBi2Ta2O9 thin films were prepared successfully and the resultant thin film exhibits the ferroelectric properties of 2Pr of 3.8 C/cm2, 2Ec of 60kV/cm at ±3.5 V, respectively.

Interfacial Layer Formation during Crystallization of Sol-Gel Derived SrBi2Ta2O9 Thin Films on Silicon

Interfacial Layer Formation during Crystallization of Sol-Gel Derived SrBi2Ta2O9 Thin Films on Silicon

Strong thickness dependence of aurivillius phase formation in SrBi2Ta2O9 thin films

Sr0.7Bi2.4Ta2O9 (SBT) thin films were studied for the dependence of Aurivillius phase formation kinetics on their film thickness. SBT thin films were fabricated using a sol-gel process and spin coating, and their thickness was varied controlling the number of spin coating. The films were first heated at a low temperature for the complete crystallization of amorphous film to fluorite phase and then further heated at different elevated temperatures for the phase transformation to Aurivillius for 40 min. It was found that the phase transformation kinetics apparently increased with thickness up to ∼390 nm, and then it sharply decreased at higher values. The Aurivillius crystal size decreased and the density of crystals increased with the increase of film thickness up to ∼390 nm, implying increasing number of nuclei due to the reduced energy barrier for nucleation. Above the critical value both the size and density of crystals decreased. It is suggested that up to ∼390 nm the tensile strain energy in the films, which was stored by the shrinkage of thin films during the removal of remaining organic components from sol-gel chemistry, plays a major role for determining the phase transformation kinetics and above the critical value SBT films act as a free bulk material without substrate constraints.

IMPROVEMENT OF MEMORY RETENTION IN METAL-FERROELECTRIC-INSULATOR-SEMICONDUCTOR STRUCTURE BY SrBi2Ta2O9 SURFACE MODIFICATION INDUCED BY NITROGEN AND OXYGEN RADICAL IRRADIATION

ABSTRACT A Pt/SrBi2Ta2O9(SBT)/SiO2/n-Si metal–ferroelectric–insulator–semiconductor (MFIS) structure has been prepared with SrBi2Ta2O9 thin film whose surface was modified by irradiation of nitrogen or oxygen radical. The leakage current is reduced by irradiation treatment and C-V characteristic shows clear memory window and sharp slopes corresponding to good interface layer. It is estimated from X-ray Photoelectron Spectroscopy and Ultraviolet Photoyield Spectroscopy that the SBT thin films with nitrogen treatment have exhibited the higher threshold energy than that without the irradiation. Memory window of the MFIS structures are in the range of 1–2 V when the gate voltage is varied from 3–6 V. Retention time of ON and OFF states is 1 week in the structure irradiated with oxygen radical and 12 days in that with nitrogen radical although that without the irradiation is only 3 hours.

Parametric study of sputtered Sr-deficient SrBi2Ta2O9 thin films

Sr-deficient SrBi2Ta2O9 (SBT) thin films for nonvolatile ferroelectric random access memory applications were deposited by radio-frequency magnetron sputtering on Pt∕Ta∕SiO2∕Si substrates. The effect of deposition parameters on microstructures and chemical composition were studied using x-ray diffraction (XRD), field emission scanning electron microscopy, and x-ray photoelectron spectroscopy (XPS). The composition of the films was dependent on the sputtering conditions. The undesirable pyrochlore phase could be eliminated by adjusting process pressure, target power density, and target-to-substrate distance. The evolution of microstructures at various deposition conditions was attributed to changes in the Bi∕Ta and Sr∕Ta ratios. When Sr became deficient and Bi excessive (Sr0.74Bi2.2Ta2O9+x as determined by XPS), no pyrochlore phase was detected with XRD. Under an electric field of 240kV∕cm, the Sr-deficient SBT film demonstrated a remnant polarization (2Pr) of 11.6μC∕cm2 and a coercive field (2Ec) of 96kV∕cm.

STUDIES ON THE RELAX BEHAVIOR OF SrBi2Ta2O9 THIN FILMS

ABSTRACT Relax behavior of SrBi2Ta2O9 thin film is investigated. In very short-time scale (less than 10 μs), the relaxation curve follows the characteristics of the voltage change, due to the signal and polarization delay. After 100 μs's relax time, the curve tends to be linear in logarithmic time axes, suggesting that the decay of retained polarization is logarithmic damping. Applied voltage and probe polarity can influence the relaxation curve. Higher applied voltage causes higher decay rate. This can be attributed to the depolarization field, which aggravates the retained polarization's decay. Different probe polarities cause different curves, probably due to the asymmetry of the top electrode and the bottom electrode.

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.

Controlled crystal growth of layered-perovskite thin films as an approach to study their basic properties

This article describes the current progress in thin bismuth layer-structured ferroelectric films (BLSFs) including SrBi2Ta2O9 and (Bi,La)4Ti3O12, particularly those developed in the last ten years. BLSF thin films can be applied to ferroelectric random access memories because of their durable fatigue-free properties and lead-free composition. We will briefly introduce epitaxial thin films grown on a variety of substrates. Because of the difficulty in growing single crystals of sufficient size to characterize the ferroelectric behavior in specific crystal growth directions, we will characterize epitaxially grown thin films to obtain basic information about the anisotropic switching behavior, which is important for evaluating the performance of emerging materials. We will then discuss the fiber-textured growth on the (111)Pt-covered Si substrates of SrBi2Ta2O9 and Bi4Ti3O12 thin films. Because we expect that the spread crystal orientation will affect the bit-to-bit errors, we believe that the fiber-textured growth and the characterization technique for the deposited film orientation are interesting from a practical standpoint. Another specific challenge of thin film growth is the growth of a-axis-(polar axis)-oriented films. a-∕b-axis-oriented films are characterized both crystallographically and by electric hysteresis loop. The hysteresis performance was in accordance with the volume fraction of the a∕b domains; however, no evidence for 90° switching of the b domain by an external electric field was obtained. The control of film orientation also allows systematic studies on the effects of a structural modification and relation between spontaneous polarization and Curie temperature, examples of which are given in this paper. After a short description of the piezoelectric properties, we will conclude with a summary and the future prospects of BLSF thin films for research and applications.

Integration of SrBi2Ta2O9 thin films for high density ferroelectric random access memory

Ferroelectric random access memory (FeRAM) is an attractive candidate technology for embedded nonvolatile memory, especially in applications where low power and high program speed are important. Market introduction of high-density FeRAM is, however, lagging behind standard complementary metal-oxide semiconductor (CMOS) because of the difficult integration technology. This paper discusses the major integration issues for high-density FeRAM, based on SrBi2Ta2O9 (strontium bismuth tantalate or SBT), in relation to the fabrication of our stacked cell structure. We have worked in the previous years on the development of SBT-FeRAM integration technology, based on a so-called pseudo-three-dimensional (3D) cell, with a capacitor that can be scaled from quasi two-dimensional towards a true three-dimensional capacitor where the sidewalls will importantly contribute to the signal. In the first phase of our integration development, we integrated our FeRAM cell in a 0.35μm CMOS technology. In a second phase, then, possibility of scaling of our cell is demonstrated in 0.18μm technology. The excellent electrical and reliability properties of the small integrated ferroelectric capacitors prove the feasibility of the technology, while the verification of the potential 3D effect confirms the basic scaling potential of our concept beyond that of the single-mask capacitor. The paper outlines the different material and technological challenges, and working solutions are demonstrated. While some issues are specific to our own cell, many are applicable to different stacked FeRAM cell concepts, or will become more general concerns when more developments are moving into 3D structures.

90° switching of polarization in La3+-doped SrBi2Ta2O9 thin films

The crystal structure and polarization switching behavior of SrBi1.4La0.6Ta2O9 (SBLT) thin films have been studied by x-ray diffraction and piezoresponse force microscopy (PFM), respectively. Compared with SrBi2Ta2O9 (SBT), SBLT thin films show a reduced orthorhombic distortion. The polarization rotation of SBLT thin film, which is driven by negative and positive direct current (dc) biases, has been investigated by a combination of vertical and lateral PFM (VPFM and LPFM, respectively). After dc bias applications, the VPFM image is hardly changed, whereas the LPFM image experiences an obvious variation. It is believed that such difference is caused by 90° polarization switching. However, this kind of switching can be only realized by the exchange of a axis and b axis. By virtue of the reduced orthorhombic distortion, the a-b exchange in SBLT is easier than that in SBT. Unfortunately, stress is created due to the 90° polarization switching in SBLT thin films. The internal stress is found to increase with the repeated switching cycles, and so the polarization reorientation in SBLT is constrained. Thus, the fatigue resistance of SBLT thin films is not thought to be as good as that of SBT.

The XPS and GDS Study of SrBi<sub>2</sub>Ta<sub>2</sub>O<sub>9</sub>/ZrO<sub>2</sub>/Si (MFIS) Structure According to Heat Treatment Effects of ZrO<sub>2</sub> Buffer Layer

ZrO2 buffer layer and SrBi2Ta2O9 (SBT) thin films were deposited on the P-type Si(111) substrates by the R.F. magnetron-sputtering method. We studied the effect of the post-annealing of the ZrO2 buffer layer on the MFIS structure. We could conclude that the elements of Zr, Sr, Bi, and Ta etc. were diffused by the post-annealing, and according to the process with and without the post-annealing of the ZrO2 layer, the diffusion amount of Sr, Bi, Ta elements shows slight difference through the glow discharge spectrometer (GDS) analysis. From the results of the XPS analysis, we can confirm that a small amount of SiO2 and metallic Zr exist at the interface, and ZrO2 exists as the amorphous state with the weak binding energy before the post-annealing process. Contrarily, after the post-annealing of the ZrO2/Si structure, SiO2 and metallic Zr are observed at the wide range, but the bonding state of ZrO2 is strengthened.

High Energy Oxygen Ion Induced Modifications in Ferroelectric SrBi2Ta2O9 Thin Films

The effects of high energy oxygen ion irradiation on the ferroelectric properties of SrBi 2 Ta 2 O 9 (SBT) thin films are reported. The films were grown using pulsed laser ablation. Fluence dependant degradation in the ferroelectric properties was observed. The dielectric constant did not show any frequency dispersion in both the unirradiated and irradiated films. After irradiation, the films showed only a decrease in the capacitance value retaining their ferroelectric nature. The switching voltages increased with broader peaks and showed a partial recovery after annealing of the irradiated film. A slim hysteresis loop with decreased P s , P r and E c values was observed. The current density increased to a larger extent. The decrease in the dielectric constant, capacitance and polarization values was to a lesser extent as compared to those of PZT thin films under identical irradiation conditions. The results presented in this paper are illustrative of the fact that the layered perovskites have a stable structure and are less sensitive to extrinsic effects. The relatively high resistance of bismuth layered perovskites such as SBT to the high energy radiation along with the greater fatigue endurance as compared to PZT make them better candidates for applications in memory devices.

Dielectric properties of SrBi2Ta2O9 films in the low-temperature range

Ferroelectric materials, which show various interesting properties, such as high-dielectric constants, large spontaneous polarization, and optical nonlinearities, have attracted great attention for device application in nonvolatile memory, uncooled infrared detectors, and optical sensor protection [1–3]. In recent years, ferroelectric thin films are being used to develop a new class of tunable microwave devices. These devices are based on the large electric field dependent dielectric constant observed in ferroelectric materials, which are desirable to have a large capacitance change ratio [tunability(η) = (Cmax − Cmin)/Cmax] under a certain electric-field range accompanied by a small dielectric loss. The devices employing ferroelectric films have high-tuning speed, high-radiation resistance, low power consumption, and low cost [4–9]. In this letter, we report on the dielectric properties and the voltage tunability of SrBi2Ta2O9 thin films in low temperature range of 10–300 K. Origin for the temperature effect of the dielectric properties and the voltage tunability of SBT films, which may arise from switchable polarization that is frozen at lower temperature, is discussed. These results provide additional insight into their use for ferroelectric microwave device applications. The SBT thin films were fabricated on platinized silicon (Pt/Ti/SiO2/Si) using pulsed laser deposition (PLD) assisted by a dc glow discharge plasma. The laser used for SBT thin film deposition was an ArF (Lambda Physik LPX220icc, wavelength 193 nm) excimer laser with 5 Hz repetition frequency, 17 ns pulse duration, and an energy of 160 mJ/pulse. The output laser beam was focused onto a rotating SBT ceramic target at an angle of 45◦ by a UV lens with a focal length of 50 cm. The platinized silicon (Pt/Ti/SiO2/Si) substrates were mounted onto a heated substrate holder and placed parallel to the target at a distance of 4–5 cm. The stability of the incoming beam was monitored using an energy meter. In order to minimize the chemical reaction between the film and substrate and control stoichiometry and structure, a low oxygen pressure dc glow discharge was used during laser deposition. A copper ring was placed halfway between the target and substrate. The substrate and target were electrically grounded while the ring was held at +700 V.

“Greening” the synthesis of SrBi2Ta2O9 thin films

A new procedure based on an ethanolic solution of the appropriate cations to obtain SrBi2Ta2O9 (SBT) thin films is described, reducing the necessity of toxic organicals. According to X-ray diffraction analysis the desired SBT crystalline phase is obtained after heat treatment at 720 °C, in air, for 20 min. The ferroelectric properties of the obtained thin film include a remnant polarization value of 5.0 μC/cm2 and a coercive field of 50 kV/cm, which are rather good results for a stoichiometric SBT thin film.

Domain evolution in ferroelectric thin films during fatigue process

A combination of piezoresponse force microscopy and x-ray diffraction sin2Ψ stress measurement have been used to investigate domain evolutions in (111)-oriented Pb(Zr0.55Ti0.45)O3 (PZT) and (115)-oriented SrBi2Ta2O9 (SBT) thin films with the repeated switching cycles (fatigue process). In the PZT film, it has been found that 90° a-c domain walls form at the beginning of fatigue process and increase in amount with accumulation of switching cycles, accompanied by increase of stress. However, domain pattern and stress in the SBT film seem to be unchanged as the fatigue process is going on. The increase in stress with the repeated switching cycles in the PZT film can be attributed to the lattice distortion caused by polarization rotation. On the contrary, polarization switching in SBT involves little lattice distortion and lets it in a nearly stress-free status during the fatigue process. The difference in domain evolution during the fatigue process suggests the important role of stress in fatigue behaviors of PZT and SBT films.

Ferroelectric Property Dependence on the Texture of SrBi2Ta2O9 Thin Films

ABSTRACT SrBi2Ta2O9 (SBT) thin films with different textures were fabricated by pulsed laser deposition under different deposition conditions. The influence of the deposition condition on the texture of the SBT films can be explained by the interactions between the plume and the substrate. With the aid of x-ray diffractometry, piezoreponse scanning probe microscopy, and ferroelectric-property measurements, a correlation between the film texture, as well as the domain structure, and the ferroelectric properties was established. It was found that the ferroelectrics properties, such as the polarization and the coercive field, had a strong dependence on the texture of the SBT films. What's more, 90° domains were observed in the (115)-textured SBT films. These results are of interest for the fabrication of ferroelectric memories with fatigue free SBT thin films.

Crystallization and Structural Phase Transformation in Sub-100-nm-Thick SrBi2Ta2O9 Thin Film

Crystal phase and structural properties in sub-100-nm-thick SrBi2Ta2O9 (SBT) thin films deposited on silicon substrates have been quantitatively investigated by X-ray diffraction (XRD) and X-ray reflection (XRR) analyses. The simulation fitting of XRR showed that the density and the surface roughness of the SBT film increased and saturated as the annealing temperature increased, indicating the phase transformation from fluorite to Aurivillius and the grain growth were enhanced by the high-temperature annealing. The SBT film was composed of small crystals of fluorite-type and Bi-layered Aurivillius phase after crystallization at temperatures of 730–785°C for 1 h. When annealing temperature was higher than 800°C, the film crystallized into a single phase of Bi-layered Aurivillius. It was also shown that the average crystal size in the SBT film increased with annealing temperature and saturated to be ∼75 nm at annealing temperatures higher than 800°C. The annealing temperature dependence of the integrated intensity of the diffraction suggested that the activation energy of the crystallization into the Aurivillius phase was significantly changed at ∼800°C. The change in the activation energy had a close relationship with the structural phase transformation of fluorite to Aurivillius and the grain growth.

A superlattice approach to the synthesis of ferroelectric Strontium Bismuth Tantalate thin films using liquid-injection-MOCVD

AbstractThe synthesis of SrBi2Ta2O9 (SBT) thin films has been investigated using a superlattice approach. Thin films were deposited on silicon by independent injection of each source to produce Bi2O3/SrTa2O6 superlattices. The effects of post-deposition annealing have been investigated using high-resolution TEM and medium energy ion scattering (MEIS) to depth profile the superlattices. X-ray diffraction has also been used to characterize the conversion of the superlattices from distinct layers of Bi2O3 and SrTa2O6 into a polycrystalline layer of strontium bismuth tantalate.

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.