SBT Films Research Articles

Dual interfacial modification via anodizing method for achieving enhanced breakdown strength in multi-layer anodized alumina/Sr0.85Bi0.1TiO3 films

Prepared with two layers of anodized alumina (Al2O3), a novel structure of multi-layer anodized alumina/Sr0.85Bi0.1TiO3 (AO/SBT) dielectric capacitor has been designed and fabricated to realize significant improvement in energy density. The first Al2O3 layer (AO1) was artificially introduced through anodic oxidation of aluminum layer in electrolyte, while the second Al2O3 layer (AO2) was automatically generated during the I–V testing process. With ultrahigh breakdown strength, the embedded Al2O3 layers could be taken as the patch to repair the micro-holes located at the interface. In addition, they could be used as barrier layers to dramatically reduce the leakage current. More importantly, it was found that the different location of the AO1 layer has enormous impact on the ultimate breakdown strength. Based on finite element analysis, the simulation results showed the electric field was mainly concentrated on the Al2O3 layer, which could effectively reduce the breakdown probability of SBT films. By dual interfacial modification, the multi-layer thin films with an optimal structure show a remarkable enhancement in breakdown strength from 134.09 to 534.72 MV m−1. As a result, the maximum energy density of 29.71 J cm−3 has been achieved, which represents a ∼741% increase in comparison to the samples without dual interfacial modification. The unique dual interfacial modification technology employed in this work offers a new approach to increase the energy storage density of dielectric capacitors from the perspective of structural design.

Enhanced ferroelectric properties of multilayer SBT-BTN thin films for NVRAM applications

ABSTRACTFerroelectric SrBi2Ta2O9 – (Bi4Ti3)1-xNbxO12 (SBT-BTN) multilayer thin films with various stacking periodicity have been synthesized on Ir/Ti/SiO2/Si substrates by metal organic chemical vapor deposition technique (MOCVD). Tributylbismuth [Bi(C4H9)3], strontium-bis[tantal(pentane-ethoxy)(2-methoxyethoxide)] [Sr[Ta(OEt)5(OC2H4OMe)]2], titanium bis(isopropoxy)bis(1-methoxy-2-methyl-2-propoxide) [Ti(OiPr)2(mmp)2] and niobium-ethoxide [Nb(OC2H5)5] were selected as precursors. X-ray diffraction patterns show that the multilayer films annealed at 800°C consisted of a fully formed perovskite phase with polycrystalline structure. The remanent polarization (2·Pr) and coercive field strength (Ec) were 16.2 μC/cm2 and 230 kV/cm, respectively, values which are much higher compared to pure SBT film (2·Pr = 6.4 μC/cm2, Ec = 154 kV/cm).

Enhanced Ferroelectric Properties of Multilayer SBT-BTN Thin Films for NVRAM Applications

Ferroelectric SrBi2Ta2O9-(Bi4Ti3)1-xNbxO12 (x = 0.02) (SBT-BTN) multilayer thin films with various stacking periodicity have been synthesized on Ir/Ti/SiO2/Si substrate by metal organic chemical vapor deposition technique (MOCVD). Tributylbismuth [Bi(C4H9)3], Strontium-bis[Tantal(pentanethoxy)(2-methoxyethoxid)] [Sr[Ta(OEt)5(OC2H4OMe)]2], Titanium Bis(isopropoxy)bis(1-methoxy-2-methyl-2-propoxide) [Ti(OiPr)2(mmp)2] and Niob-ethoxide [Nb(OC2H5)5] were selected as precursors. X-ray diffraction patterns show that the multilayer films annealed at 800oC consist of fully formed perovskite phase with polycrystalline structure and plate-like grains with no crack. The remanent polarization () and coercive field (Ec) are 16.2 μC/cm2 and 230 kV/cm, respectively, which is much higher, compared to pure SBT film ( = 6.4 μC/cm2, Ec = 154 kV/cm). In the films prepared above 700oC, postannealing increased the capacitor shorting rate; this was attributed to oxidizing of the top iridium layer. In this paper, the dependence of composition variation around stoichiometric on ferroelectric properties in SBT-BTN multilayer films is studied.

Fabrication and Characteristics of Ferroelectric/Fluorescent Oxide Structures for Ferroelectrically Controlled Emission Devices

ABSTRACTThe crystallinity, electrical, and optical properties of the ferroelectric/fluorescent oxide structures using sol-gel-derived (Ba0.6Sr0.4)TiO3 (BST) and (Sr0.8Eu0.2)Bi2.2Ta2O9 (Eu-SBT) grown on STO(110) single crystal substrates were introduced for the first time. In the present structures, the SBT films partly included a (116)-oriented Eu-SBT crystallite. The polarization vs. voltage characteristics of the BST/Eu-SBT structures showed the hysteresis loop caused by spontaneous polarization reversal, and then several emission peaks from Eu3+ ion were observed in a photoluminescence spectrum of a present BST/Eu-SBT structure.

Electrical properties of metal-ferroelectric-insulator-semiconductor structure with Lanthanum Dysprosium Oxide and SrBi2Ta2O9

layer was prepared. The LDO thin film and the SBT film were deposited by using the sol-gel method. The sol-gel-derived LDO thin films on Si had very flat and smooth surface morphologies. The equivalent oxide thickness (EOT) values were about 10.1 nm, 12.5 nm, and 12.8 nm for 750 C, 800 C, and 850 C, respectively. Also, the relative dielectric constants of the 750 C-annealed, 800 C-annealed and 850 C-annealed LDO films were about 15.4, 12.5, and 12.2, respectively. On the whole, the leakage current densities showed good characteristics regardless of the annealing temperature. Especially, the leakage current property of the 850 C-annealed LDO film was visibly superior to those of the others at voltages over 6 V. The C-V characteristics of the Au/SBT/LDO/Si structure showed clockwise hysteresis loops, and the memory window width increased as the bias

Halogen-Light-Enhanced Metallorganic Decomposition of SrBi2Ta2O9 Ferroelectric Thin Films

We investigated the influence of the halogen-light exposure on the metallorganic decomposition of strontium bismuth tantalate [ (SBT)] thin films deposited by chemical-solution deposition (CSD) on platinum (Pt) capacitors. The UV absorption spectrum obtained using Fourier transform infrared showed that the removal of the residual organic species from the SBT solution layer was enhanced by the halogen light during drying at . The halogen-light exposure improved ferroelectric properties and the formation of a (200)-preferred film structure, a superior ferroelectric property. In addition, the decomposition of oxidated bismuth (Bi–O bonds) in SBT as a result of the halogen-light exposure was also observed. Some reduction in metallic Bi from the oxidated state that occurred during the high temperature rapid thermal annealing was enhanced by the halogen-light exposure. The high ratio in the SBT film samples exposed to the halogen light was responsible for their high leakage current. At , the annealing in a ambient was effective in oxidizing Bi and in reducing the leakage current densities by about 2 orders of magnitude. Although the specific interaction between the halogen light and the CSD film remains unclear, the cost-effective and safe halogen lamp may be an appropriate choice for photoassisted thermal processes for the production of ferroelectrics.

Improved electrical properties of Sr0.8Bi2.2Ta2O9 films by ZrSiO4 doping for low voltage operations of metal-ferroelectric-insulator-Si devices

Sr 0.8 Bi 2.2 Ta 2 O 9 (SBT) films were prepared by chemical solution deposition in which ZrSiO4 (ZSO) with concentration ranging from 0 to 10 wt % was incorporated for improving dielectric and leakage current characteristics of the films. X-ray diffraction analysis revealed that no secondary phase and clear degradation of crystallization can be found in ZSO-doped SBT films. Smaller grain size and reduced surface roughness were found for the samples with higher ZSO doping concentration as observed by atomic force microscopy and scanning electron microscopy. The dielectric constant was observed to be much reduced for ZSO-doped SBT films as well as the remnant polarization and coercive field. Films preannealed at 400 °C have a much smaller dielectric constant when compared with that of the films preannealed at 750 °C. Furthermore, a clear reduction in the leakage current and improved fatigue characteristics were observed for ZSO-doped SBT films preannealed at 400 °C. Such improved electrical properties as reduced dielectric constant, leakage current, and coercive field for the ZSO-doped SBT films will be very beneficial for the low voltage operations in metal-ferroelectric-insulator-Si devices.

Improved ferroelectric polarization of KSrBi 2Ta 3O 12 thin films

KSrBi 2Ta 3O 12 (KSBT, in which there should be three TaO 6 octahedra between neighboring (Bi 2O 2) 2+ layers), was designed to improve ferroelectric polarization of SrBi 2Ta 2O 9 (SBT). Thin films of SBT and KSBT were fabricated on Pt/TiO 2/SiO 2/Si substrates by pulsed laser deposition for comparison study. No x-ray diffraction peaks of KTaO 3 could be detected in the XRD patterns of the KSBT films, indicating a single phase of KSBT was obtained. The measured remnant polarization ( 2 P r ) values of SBT and KSBT films were 3.6 μC/cm 2 and 15.6 μC/cm 2, respectively. The possible microstructural background responsible for the 4-times improved polarization was discussed. Furthermore, excellent ferroelectric fatigue-free property, retention property, and dielectric property of the KSBT films were exhibited experimentally. These results indicate that other than conventional site engineering such as substitution, adjusting the octahedra number between neighboring (Bi 2O 2) 2+ layers is an alternative way to improve polarization of SBT while keeping its fatigue-free characteristics.

Catalytic Studies of RuO2 Films Deposited on Ferroelectrics Films by Spin Coating Process

Films of a catalytic compound (RuO 2 ) were deposited by spin-coating process on ferroelectric films mainly constituted of SrBi 2 Ta 2 O 9 (SBT) and Ba 2 NaNb 5 O 15 (BNN) phases. SBT films were deposited by MOCVD method, and BNN films were deposited by sputtering. After thermal treatment under air, these ferroelectric-catalytic systems were characterized by X-ray diffraction and scanning electron microscopy (SEM). SEM images showed that RuO 2 film morphology depended on substrate nature. A study of CH 4 conversion into CO 2 and H 2 O was carried out using these catalytic-ferroelectric multilayers: the conversion was analyzed from Fourier Transform Infrared (FT-IR) spectroscopy, at various temperatures. Improved catalytic properties were observed for RuO 2 films deposited on BNN oxide layer.

Metal–ferroelectric–insulator–Si devices using HfTaO buffer layers

Metal–ferroelectric–insulator–Si (MFIS) diodes and transistors using Pt/Sr0.8Bi2.2Ta2O9 (SBT)/HfTaO/Si gate structures were fabricated. HfTaO films were deposited at room temperature by electron beam evaporation under a background vacuum of ∼1.5 × 10−9 Torr. By process optimization of post-deposition annealing, a small effective oxide thickness of 1.6 nm, a small leakage current of 2.4 × 10−4 A cm−2 at a voltage shifted from the flat band voltage by 1 V and a negligible hysteresis loop were obtained for Al/HfTaO (4 nm)/Si diodes. The MFIS diodes were fabricated by the deposition of SBT films on the HfTaO (4 nm)/Si substrate using chemical solution deposition. A memory window of 0.62 V was obtained for Pt/SBT (300 nm)/HfTaO (4 nm)/Si diodes for a voltage sweep between +4 V and −4 V. P-channel ferroelectric-gate transistors were fabricated using the same gate structure, which showed a memory window of 0.6 V and good long-term retention characteristics. A drain current ON/OFF ratio as high as 103 was attained at a fixed reading voltage of −0.7 V even after over 104 s has elapsed.

Transmission Electron Microscopy Observations on the Interfacial Structures of the Pt/SrBi2Ta2O9/Pt Thin-Film Capacitors Prepared by Metallo-Organic Decomposition

Interfacial structures of the Pt/SBT/Pt capacitors prepared by metallo-organic decomposition were examined by transmission electron microscopy (TEM) and high-resolution TEM (HRTEM) as a function of the postannealed time at 750°C to investigate the interfacial evolution developed in the Pt/SBT/Pt capacitors. The results show that after postannealing at 750°C for a short time (e.g., 300 s), a thin amorphous layer was observed at the interfaces of the SBT/Pt-bottom and the SBT/Pt-top electrodes in the Pt/SBT/Pt capacitors, whereas after postannealing for a moderate time (e.g., 20 min), the interfaces of the SBT/Pt-bottom and the SBT/Pt-top electrodes became quite smooth and uniform, and almost free of the interfacial phase. However, after postannealing for a longer time (e.g., 60 min), an interfacial phase consisting of oval-shaped nanocrystallites was developed at both the SBT/Pt-bottom and the SBT/Pt-top electrode interfaces, which resulted in weakly bonded interfaces between the SBT films and Pt electrodes. This is the reason why a Pt electrode peeling phenomenon (similar to that reported in the Pt/SBT/Pt/capacitors annealed in the forming gas atmosphere) is frequently observed in the Pt/SBT/Pt capacitors postannealed for 60 min during the fabrication of cross-sectional TEM samples. Based on two-dimensional lattice fringes of the HRTEM images from the small nanocrystallites and the corresponding fast Fourier transform patterns, the oval-shaped nanocrystallites precipitated at both the SBT/Pt-bottom and the SBT/Pt-top electrode interfaces were determined to be a PtBi2 phase with a cubic structure. The formation of the Pt–Bi-based interfaces in the Pt/SBT/Pt capacitors is due to the reaction of bismuth (out-diffused from the SBT films) with Pt electrodes during the postannealing process. The elliptical morphology of the interfacial PtBi2 phase can be ascribed to its different growth rates in the parallel and vertical directions of the interface, and also the limited space for grain growth perpendicular to the interface.

IMPROVED FERROELECTRIC PROPERTIES OF SrBi4−0.1Pr0.1Ti4O15 THIN FILMS

ABSTRACT Thin films with the composition of SrBi4Ti4O15(SBTi) and SrBi4−0.1Pr0.1Ti4O15 (SBPTi) have been prepared on Pt/TiO2/SiO2/Si substrates by sol-gel method structure characterization of the SBT and SBPT thin films was revealed by x-ray diffraction (XRD). Using Pt/SBPT/Pt/TiO2/SiO2/Si configuration, ferroelectric properties of the film were well established with the hysteresis loop. A notable enlargement of remnant polarization (2Pr) and a decrease of the coercive field (Ec) are observed. The 2Pr of SBPTi reaches a value as large as34.76 μC/cm2, nearly twice greater than that of SrBi4Ti4O15 without Pr-doping. The SBPTi film shows little change of Pns and −Pns up to 1010 switching cycles, suggesting an excellent fatigue-endurance characteristic.

Electric and ferroelectric properties of PZT/SBT multilayer films prepared by photochemical metal-organic deposition

Abstract The electric and ferroelectric properties of lead zirconate titanate and strontium bismuth tantalate multilayer films prepared using photosensitive starting precursors were characterized. The electric and ferroelectric properties were investigated by characterization of the effect of stacking order of four ferroelectric layers of PZT or SBT in the multilayer films of 4-PZT, PZT/2-SBT/PZT, SBT/2-PZT/SBT, and 4-SBT. The Pr value of the 4-SBT multilayer film was relatively small (6 μC/cm2) and a two times higher value (12 μC/cm2) was obtained with the SBT/2-PZT/SBT multilayer film. The films with SBT layers at the top and bottom showed improved leakage current and fatigue resistance compared to the films with PZT layers at the top and bottom. It was revealed that the defect dipole was reduced at the SBT/Pt interface due to a self-regulation layer such as (Bi2O2)2+ in the SBT film. Also, the bottom layer on the Pt substrate showed a significant influence on the growth orientation of the entire ferroelectric films.

Mechanical constraint effect on the crystallinity and the ferroelectric properties of SrBi 2Ta 2O 9 thin films

We applied tensile and compressive constraints during annealing processes to investigate the effect of stress on the crystallinity and ferroelectric properties of SrBi 2Ta 2O 9 thin films. Both the tensile and compressive stresses exerted pronounced effects on the volume fraction of the pyrochlore phase and the grain growth mechanisms of the SBT films. The effects of the stress correlated with the change of crystallization temperatures. Because the stress applied to an SBT sample during its crystallization led to the creation of distinct microstructures and constituted phases of the film, our SBT samples exhibited distinct ferroelectric behavior under various stressing conditions.

Ferroelectric properties of bilayer structured Pb(Zr 0.52Ti 0.48)O 3/SrBi 2Ta 2O 9 (PZT/SBT) thin films on Pt/TiO 2/SiO 2/Si substrates

Pb(Zr 0.52Ti 0.48)O 3 (PZT) thin films with large remanent polarization and SrBi 2Ta 2O 9 (SBT) thin films with excellent fatigue-resisting characteristic have been widely studied for non-volatile random access memories, respectively. To combine these two advantages , bilayered Pb(Zr 0.52Ti 0.48)O 3/SrBi 2Ta 2O 9 (PZT/SBT) thin films were fabricated on Pt/TiO 2/SiO 2/Si substrates by chemical solution deposition method. X-ray diffraction patterns revealed that the diffraction peaks of PZT/SBT thin films were completely composed of PZT and SBT, and no other secondary phase was observed. The electrical properties of the bilayered structure PZT/SBT films have been investigated in comparison with pure PZT and SBT films. PZT/SBT bilayered thin films showed larger remanent polarization (2 P r) of 18.37 μC/cm 2 than pure SBT and less polarization fatigue up to 1 × 10 9 switching cycles than pure PZT. These results indicated that this bilayered structure of PZT/SBT is a promising material combination for ferroelectric memory applications.

Study on the Electrical Properties of Direct-Patternable SBT Films Formed by Photochemical Metal-Organic Deposition

The ferroelectric properties of UV irradiated and non-irradiated SBT thin films using photosensitive starting precursors were investigated. The observation of surface microstructure showed that UV irradiation and increase in anneal temperature induced the grain growth of SBT. The measured remnant polarization values of UV irradiated and non-irradiated SBT films after anneal at 700oC were 5.8 and 4.7 )C/cm2 and after anneal at 750oC, the values were 10.8 and 9.3 )C/cm2, respectively.

Polarization inscription in ferroelectric (111) PZT and (100) SBT films

Abstract Ferroelectric thin films form an equilibrium domain structure compatible with their respective crystallographic symmetry. In tetragonal (111) PZT, 90° domains prevail; in (pseudo-tetragonal) (100) SBT both 90° and 180° domains are present. The size of 90° domains has been measured for e.g., PZT as slabs of ∼15 nm width. Domain size is a result of stress minimization in the film during the paraelectric (PE) → ferroelectric (FE) transition. A precise and regular domain pattern for (111) PZT and (100) SBT films has been investigated in detail by TMSFM. Single domains can be addressed mechanically with the tip of an AFM. Such single domain switching corresponds to a data storage density of ∼200 Gbit/inch 2 . Applications of ferroelectric and high- ɛ paraelectric materials for e.g., non-volatile data storage replacing DRAM devices or as sensors in infrared cameras are increasingly becoming popular.

Electrical and ferroelectric properties of SBT thin films formed by photochemical metal-organic deposition

The electrical and ferroelectric properties of SBT thin films prepared by photochemical metal-organic deposition using photosensitive starting precursors were characterized. Fourier transform infra-red spectroscopic observation showed that a complete removal of organic groups was possible by UV exposure of spin-coated SBT precursor film at room temperature. The values of measured remnant polarization and leakage current density at 400 kV/cm were 5.8 μC/cm 2 and 3.48 × 10 −8 A/cm 2 after anneal treatment at 700 °C of the UV-exposed SBT precursor film and 10.8 μC/cm 2 and 6.94 × 10 −8 A/cm 2 after anneal treatment at 750 °C. The SBT films annealed at 700 and 750 °C remained fatigue-free up to 10 10 switching cycles.

Relevance of the pulsed capacitance–voltage measurement technique for the optimization of SrBi 2Ta 2O 9/high-k stack combination to be used in FeFET devices

The ferroelectric field effect transistor (FeFET) with metal–ferroelectric–insulator–semiconductor (MFIS) structure has attracted a lot of interest recently as a nonvolatile memory device. For best performances of FeFET with ferroelectric/high-k gate stacks, the deposition techniques as well as the materials for both the ferroelectric layer and the high-k buffer layer must be carefully selected. In this work, 120 nm SrBi 2Ta 2O 9 (SBT) films were deposited on Al 2O 3 or HfO 2 buffer layers by metal organic chemical vapor deposition (MOCVD) or metal organic decomposition (MOD) techniques. The different SBT/high-k stack combinations were optimized considering not only the remnant polarization of the SBT film, extracted from high-frequency capacitance–voltage (CV) measurements, but also considering the electron trapping–detrapping delay accessed by pulsed CV measurements, which gives insight into the electrical response to very fast program and read pulses. Based on these criteria, the MOCVD SBT/HfO 2 gate stack was found to be the best choice, resulting in a memory window of 0.45 V measured for a gate voltage scan range of only ±5 V. The memory switching occurs at rather low voltage due to the use of thinner SBT than typically reported. This optimized stack combination also exhibited low electron trap density and low leakage.

Enhanced ferroelectric properties of multilayer SrBi 2Ta 2O 9/SrBi 2Nb 2O 9 thin films for NVRAM applications

Ferroelectric SrBi 2Ta 2O 9/SrBi 2Nb 2O 9 (SBT/SBN) multilayer thin films with various stacking periodicity were deposited on Pt/TiO 2/SiO 2/Si substrate by pulsed laser deposition technique. The X-ray diffraction patterns indicated that the perovskite phase was fully formed with polycrystalline structure in all the films. The Raman spectra showed the frequency of the O–Ta–O stretching mode for multilayer and single layer SrBi 2(Ta 0.5Nb 0.5) 2O 9 (SBNT) samples was ∼827–829 cm −1, which was in between the stretching mode frequency in SBT (∼813 cm −1) and SBN (∼834 cm −1) thin films. The dielectric constant was increased from 300 (SBT) to ∼373 at 100 kHz in the double layer SBT/SBN sample with thickness of each layer being ∼200 nm. The remanent polarization (2 P r) for this film was obtained ∼41.7 μC/cm 2, which is much higher, compared to pure SBT film (∼19.2 μC/cm 2). The coercive field of this double layer film (67 kV/cm) was found to be lower than SBN film (98 kV/cm).