Strontium Bismuth Tantalate Research Articles

Area-Scalable 109-Cycle-High-Endurance FeFET of Strontium Bismuth Tantalate Using a Dummy-Gate Process.

Strontium bismuth tantalate (SBT) ferroelectric-gate field-effect transistors (FeFETs) with channel lengths of 85 nm were fabricated by a replacement-gate process. They had metal/ferroelectric/insulator/semiconductor stacked-gate structures of Ir/SBT/HfO2/Si. In the fabrication process, we prepared dummy-gate transistor patterns and then replaced the dummy substances with an SBT precursor. After forming Ir gate electrodes on the SBT, the whole gate stacks were annealed for SBT crystallization. Nonvolatility was confirmed by long stable data retention measured for 105 s. High erase-and-program endurance of the FeFETs was demonstrated for up to 109 cycles. By the new process proposed in this work, SBT-FeFETs acquire good channel-area scalability in geometry along with lithography ability.

Effect of V2O5 B-site substitution on the microstructure, Raman spectrum, and dielectric properties of SrBi2Ta2O9 ceramics

Strontium bismuth tantalate vanadate [SrBi2(Ta2−xVx)O9, SBTV] ceramics, which are bismuth-layered perovskite ferroelectrics, were synthesized through the solid-state reaction method. The effects of different sintering temperatures and V2O5 contents on the structure of the microstructure, Raman spectrum, and dielectric properties of the SBTV ceramics were investigated. As sintered at high temperature (980–1040 °C) and different V2O5 contents (x = 0.1 − x = 0.4), only disk-like grains of the SBTV ceramics were observed in the scanning electron micrographs. Preferential orientation of the crystals of the SBTV ceramics was confirmed through X-ray diffraction studies. The higher dielectric constant and Curie temperature of the SBTV ceramics compared with those of strontium bismuth tantalite (SrBi2Ta2O9, SBT) ceramics are ascribe to the partial replace of Ta5+ ions by V5+ ions in the B sites. The Curie–Weiss law and the modified Curie–Weiss law were used to discuss the normal-type or relaxor-type ferroelectric characteristic of the SBTV ceramics. The Ta5+ ion replaced by V5+ ion site in SBT ceramics to form SBTV ceramics exerted a pronounced effect on the BO6 mode, as demonstrated by Raman spectrum results.

Investigating structural features of Ba and Zr co-substituted strontium bismuth tantalate thin films

Structural (crystal and microstructure), chemical and electronic states, and ferroelectric and electrical features of Ba and Zr co-substituted strontium bismuth tantalate (SBT) were probed in this study. Distinctly, Ba and Zr were substituted for Ta and Sr sites of \(\hbox {Sr}_{0.8}\hbox {Bi}_{2.2}\hbox {Ta}_{2}\hbox {O}_{9 }\) in the form of \(\hbox {Sr}_{0.8-x}\hbox {Ba}_{x}\hbox {Bi}_{2.2}\hbox {Ta}_{2-y}\hbox {Zr}_{y}\hbox {O}_{9}\). To investigate the impact of the co-substitution on the crystal structure, microstructure, ferroelectric and electrical properties, \(\hbox {Sr}_{0.8-x}\hbox {Ba}_{x}\hbox {Bi}_{2.2}\hbox {Ta}_{2-y}\hbox {Zr}_{y}\hbox {O}_{9}\) thin films were deposited on \(\hbox {Pt/Ti/}\hbox {SiO}_{2}/\hbox {Si}(100)\) wafers by sol–gel spin by coating method. Crystal structure, microstructure, chemical and electronic states, ferroelectric, capacitance and leakage current characteristics of the films were studied to investigate potential for one transistor type ferroelectric random access memories (1T-type FeRAMs). Successful substitutions up to 10 mol% lead to reduction of double remanent polarization \((2P_{\mathrm{r}})\) to \(10.26\,\upmu \hbox {C\,cm}^{-2}\), and dielectric constant \((\varepsilon _{\mathrm{r}})\) to 135. These values demonstrate that successful co-substitution of limited Ba and Zr in SBT with stable crystal structure has the ability to decrease \(P_{\mathrm{r}}\) and \(\varepsilon _{\mathrm{r}}\) values of the ferroelectric material which can be a candidate gate to be utilized in ferroelectric field-effect transistors (FeFETs) for 1T-type FeRAM applications.

Impurity Phases in Polycrystalline Films of Ferroelectric Oxides of the Perovskite-Type on the Basis of Bi2SrTa2O9 and Pb(Zr,Ti)O3

Processes of the formation of inclusions of impurity phases in lead zirconate titanate and strontium bismuth tantalate (SBT) films by changing the excess concentration of the most volatile cation component and temperature-time regimes of their formation are studied. The conditions in which the introduction of an excess Pb content does not lead to the formation of impurity phases are selected.

Barium silicate modified strontium bismuth tantalate ferroelectric thin films

Novel strontium bismuth tantalate (Sr0.8Bi2.2Ta2O9 (SBT)) modified with 3 and 5 mol% ratio barium silicate (Ba2SiO4) thin films were grown on Pt(100 nm)/Ti(50 nm)/SiO2/Si(100) substrates by spin coating technique. The influence of barium silicate doping in SBT was studied from the view point of changing dielectric and ferroelectric properties like dielectric constant (εr) and remnant polarization (Pr). Well crystallized thin films showed convenient ferroelectric properties with comparatively lower Pr in the range between 1.52 and 0.44 µC/cm2 and smaller εr value of 163. Thus, with such reduced values of Pr and εr barium silicate modified SBT offers a useful potential to be used in Ferroelectric Field Effect Transistor (FeFET) type (1T-type) Ferroelectric Random Access Memories (FeRAMs) upon improving insulation properties.

A silicon doped hafnium oxide ferroelectric p–n–p–n SOI tunneling field–effect transistor with steep subthreshold slope and high switching state current ratio

In this paper, a silicon–on–insulator (SOI) p–n–p–n tunneling field–effect transistor (TFET) with a silicon doped hafnium oxide (Si:HfO2) ferroelectric gate stack is proposed and investigated via 2D device simulation with a calibrated nonlocal band–to–band tunneling model. Utilization of Si:HfO2 instead of conventional perovskite ferroelectrics such as lead zirconium titanate (PbZrTiO3) and strontium bismuth tantalate (SrBi2Ta2O9) provides compatibility to the CMOS process as well as improved device scalability. By using Si:HfO2 ferroelectric gate stack, the applied gate voltage is effectively amplified that causes increased electric field at the tunneling junction and reduced tunneling barrier width. Compared with the conventional p–n–p–n SOI TFET, the on–state current and switching state current ratio are appreciably increased; and the average subthreshold slope (SS) is effectively reduced. The simulation results of Si:HfO2 ferroelectric p–n–p–n SOI TFET show significant improvement in transconductance (∼9.8X enhancement) at high overdrive voltage and average subthreshold slope (∼35% enhancement over nine decades of drain current) at room temperature, indicating that this device is a promising candidate to strengthen the performance of p–n–p–n and conventional TFET for a switching performance.

Influence of the annealing temperature on the ferroelectric properties of niobium-doped strontium–bismuth tantalate

Characteristics of ferroelectric thin films of niobium-doped strontium–bismuth tantalate (SBTN), which were deposited by magnetron sputtering on Pt/TiO2/SiO2/Si substrates, are investigated. To form the ferroelectric structure, deposited films were subjected to subsequent annealing at 700–800°C in an O2 atmosphere. The results of X-ray diffraction showed that the films immediately after the deposition have an amorphous structure. Annealing at 700–800°C results in the formation of the Aurivillius structure. The dependences of permittivity, residual polarization, and the coercitivity of SBTN films on the modes of subsequent annealing are established. Films with residual polarization 2Pr = 9.2 µC/cm2, coercitivity 2Ec = 157 kV/cm, and leakage current 10–6 A/cm2 are obtained at the annealing temperature of 800°C. The dielectric constant and loss tangent at frequency of 1.0 MHz were e = 152 and tan δ = 0.06. The ferroelectric characteristics allow us to use the SBTN films in the capacitor cell of high density ferroelectric random-access non-volatile memory (FeRAM).

Ferroelectric properties of niobium-doped strontium bismuth tantalate films

The characteristics of ferroelectric thin films of strontium bismuth tantalate (SBT) and niobium-doped strontium bismuth tantalate (SBTN) deposited by radio-frequency (RF) magnetron sputtering on Pt/TiO2/SiO2/Si substrates were investigated. For the formation of the structure of the ferroelectric material, the deposited films were subjected to a subsequent annealing at temperatures of 970–1070 K in an O2 atmosphere. The results of the X-ray diffraction analysis demonstrated that, in contrast to SBT films, in which the Aurivillius phase is formed only at annealing temperatures of 1050–1070 K, the formation of this phase in SBTN films is observed already at a temperature of 970 K. The dependences of the dielectric permittivity, remanent polarization, and coercive force of the SBT and SBTN films on the subsequent annealing conditions were determined. It was found that, upon doping of the SBT films with niobium, the remanent polarization increases by a factor of approximately three, the Curie temperature increases by 50 K, and the dielectric permittivity also increases. It was revealed that, in contrast to the SBT films, the polarization of the SBTN films is observed already at an annealing temperature of approximately 970 K. It was shown that the replacement of SBT films by SBTN films in the manufacture of high-density nonvolatile ferroelectric randomaccess memory (FeRAM) capacitor modules makes it possible to decrease the synthesis temperature from 1070 to 990–1000 K, which improves the compatibility with the planar technology of semiconductor devices. However, it turned out that an increase in the coercive field makes niobium-doped SBT films less attractive for the use in FeRAM.

Synthesis, Structural and Dielectric Properties of SrBi<sub>1.8</sub>Ce<sub>0.2</sub>Ta<sub>2</sub>O<sub>9</sub>

This Ce-doped strontium bismuth tantalate SrBi1.8Ce0.2Ta2O9 was prepared by solid-state reaction. X-ray diffraction was used to determine the crystal structure of the powders. The Raman spectrum of SrBi1.8Ce0.2Ta2O9 sample was measured to confirm X-ray diffraction result. The microstructure of ceramic was observed by Scanning Electron Microscope (SEM). The Temperature dependence of the dielectric properties of ceramic was investigated from the room temperature to 400°C.

Influence of samarium doping on structural and dielectric properties of strontium bismuth tantalate ceramics derived by molten salt synthesis route

Layered Sr(Bi1−xSmx)2Ta2O9 ceramics with x ranging from 0 to 0.10 (10 mol%) were fabricated by the low temperature molten salt synthesis route. X-ray powder diffraction studies revealed that the single phase orthorhombic layered perovskite structure is retained in all these compositions. Scanning electron microscopic studies on these ceramics confirmed the presence of well packed equiaxed plate shaped grains. The dielectric and electrical conductivity properties were studied in the 100 Hz–1 MHz frequency range at 300 K. Interestingly, the 10 mol% samarium doped SrBi2Ta2O9 ceramics exhibited high dielectric constant (er = 155) and low dielectric loss (0.00298) compared to those of other compositions. The electrical conductivity of undoped and samarium doped ceramics increased linearly with increase in frequency.

Effect of the Sintering Temperature on Nanocrystalline Non-stoichiometric Sr0.8Bi2.2Ta2O9 Ferroelectric Ceramics

In the present work, nanocrystalline non-stoichiometric strontium bismuth tantalate (SBT) of composition Sr0.8Bi2.2Ta2O9 was synthesized by mechanical activation technique using a high energy planetary ball mill. The synthesized powder was sintered at different sintering temperature i.e. 1050°C, 1100°C, 1150°C. X-ray diffractograms reveal the formation of single phase layered perovskite structure in all samples sintered at various temperatures. Scanning electron microscopy (SEM) and Transmission electron microscopy (TEM) have been used for the microstructural investigation. Detailed dielectric, ferroelectric and piezoelectric properties have been studied as a function of sintering temperature. Present study reveals that the electrical properties of the synthesized samples are sensitive to the sintering temperature variation.

Enhanced photoluminescence and structure of Dy3+-doped SrBi2Ta2O9-containing transparent glass-ceramics

Trivalent dysprosium (Dy3+)-doped precursor glass in the K2O–SiO2–SrO–Bi2O3–Ta2O5 (KSSBT) system have been prepared by melt-quench technique and strontium bismuth tantalate, SrBi2Ta2O9 (SBT) glass-ceramics has been synthesized by a controlled crystallization process of the precursor glass. With progression of heat-treatment it is observed that Dy3+:glass exhibit a blue emission at 486nm (4F9/2→6H15/2) and also a bright fluorescent yellow emission at 576nm (4F9/2→6H13/2) have been observed with λex=455nm (6H15/2→4I15/2). These spectra reveal that the Dy3+ ions are gradually entering into the SBT nanocrystals of the glass-ceramics. The photoluminescence characteristics originating from Dy3+-doping in nanocrystalline SBT reveals the dependence of the luminescent intensity on heat-treatment time. Their structural properties have also been evaluated by FTIR spectroscopic and microstructural studies. Such luminescent glass-ceramics are expected to find potential applications such as solid-state yellow lasers and optical display systems.

Synthesis and characterization of nanocrystalline ferroelectric Sr0.8Bi2.2Ta2O9 by conventional and microwave sintering: A comparative study

In recent years mechanical activation technique has been utilized to synthesize the nanocrystalline form of compounds resulting in enhancement in the properties. Also, microwave sintering is being preferred over conventional sintering due to rapid processing and uniform temperature distribution throughout the specimen. In the present work, nanocrystalline non-stoichiometric strontium bismuth tantalate (SBT) of the composition Sr0.8Bi2.2Ta2O9 ferroelectric ceramics were synthesized by microwave sintering process (with sintering temperatures of 1000°C and 1100°C) and conventional solid state reaction process (with sintering temperature of 1100°C) with an objective of comparing the properties of the synthesized specimens by the two processes. X-ray diffraction analysis shows the formation of single phase layered perovskite structure formation by both the processes. Scanning electron microscopy reveals the formation of a finer granular microstructure in the specimen synthesized by microwave sintering compared to that in the specimen prepared by conventional sintering. The specimen prepared by microwave sintering process exhibits improved electrical properties with higher dielectric constant, higher piezoelectric and pyroelectric coefficients and lower dielectric loss.

Effect of sintering temperature on the microstructural and electrical properties of non stoichiometric strontium bismuth tantalate ferroelectric ceramics

In recent times due to the increased environmental awareness, lead free bismuth layer structured ferroelectrics are being investigated as an alternative to the lead based ferroelectrics due to their high dielectric constant and remnant polarization. Strontium bismuth tantalate is one of the interesting lead free compounds being widely investigated. However, in these compounds it is difficult to maintain the stoichiometry as bismuth gets evaporated due to sintering at high temperature leaving behind the vacancies which affect the properties. In the present work, specimens with different strontium and bismuth content having compositions Sr1−xBi2+xTa2O9 (x = 0.0 and 0.2) were prepared. The preparation conditions have been optimized with an objective to get enhanced electrical properties. The structural characterizations have been done using X-ray diffraction and scanning electron microscopy. Dielectric studies as a function of temperature have been carried out. Dielectric constant is observed to be higher in non-stoichiometric composition and dielectric loss reduces significantly. A maximum 2Pr (~16 μC/cm2) is obtained in composition with the x = 0.2 when sintered at 1,150 °C. The observed behavior has been explained in terms of the microstructural features and inherent lattice defects. The present study reveals an optimum sintering temperature of 1,150 °C for the enhanced structural and electrical properties.

Impedance spectroscopy and conductivity studies in SrBi2(Ta1−xWx)2O9 ferroelectric ceramics

Abstract Complex impedance spectroscopy (CIS) technique has been utilized to investigate the intra- and intergranular contributions to the impedance in pristine and wolframium (tungsten, W) -substituted strontium bismuth tantalate [SrBi 2 (Ta 1− x W x ) 2 O 9 (SBTW); x =0.0, 0.025, 0.05, 0.075, 0.1 and 0.2] ceramics as a function of temperature and frequency. CIS studies reveal that the electrical relaxation process was temperature dependent and non-Debye type. The temperature dependence of the relaxation time was found to obey the Arrhenius law. DC conductivity of the studied samples obtained from the CIS data decreased for W content upto x =0.05, followed by a subsequent increase with x >0.05. Electrical conductivity data including the typical values of the activation energies at high temperature indicated that the conductivity in the studied ceramics was essentially due to the contribution of doubly ionized oxygen vacancies to the conduction process.

Synthesis of SrBi2Ta2O9 by solution combustion and its characterization

Solution combustion synthesis is an inexpensive technique to obtain high purity, homogeneous nanostructured materials. Today, ferroelectric materials are employed in electronic devices such as ferroelectric random access memories (FeRAM). This paper describes the preparation of strontium bismuth tantalate (SBT) by solution combustion synthesis (SCS), using strontium nitrate, bismuth nitrate pentahydrate, and tantalum pentachloride as oxidation reagents and urea as fuel. The influence of fuel on the electrical properties of the ferroelectric material was investigated by adding different amounts of fuel (stoichiometric (AS), 300% enriched (AS300) and 500% enriched (AS500). The AS500 powder presented a surface area of approximately 16.2m2/g and a crystallite size of about 38nm and its ferroelectric phase was obtained after calcination at 800°C for 2h. The electrical resistance increased substantially in response to an increase in the amount of fuel.

Nanocrystalline non-stoichiometric SBT: Effect of milling duration on structural and electrical characteristics

Abstract In the present work, nanocrystalline specimens of non-stoichiometric Strontium Bismuth Tantalate (SBT) ferroelectric ceramics were synthesized by mechanical activation process using a high energy planetary ball mill. The powders were milled for different milling durations (5, 10, 20 h) keeping the milling speed fixed at 300 rpm. Microstructural characterizations have been performed using X-ray diffraction, scanning electron microscopy, electron diffraction and transmission electron microscopy. Nanocrystallites with average grain size in the range of 30–50 nm are observed to be formed. Grain size is observed to decrease with increasing milling duration. Detailed dielectric study as a function of temperature has been carried out. It is observed that dielectric constant increases and dielectric loss decreases with increasing milling duration. The observed characteristics have been explained in terms of increased number of grain boundaries due to the reduction of granular size.

Structural and Dielectric Investigations of Bismuth Excess Strontium Bismuth Tantalate Ferroelectric Ceramics

In recent times due to the increased environmental awareness, lead free bismuth layer structured ferroelectrics are being investigated due to its high dielectric constant and remnant polarization. In these compounds it is difficult to maintain the stoichiometry as bismuth gets evaporated at high temperature leaving behind the vacancies which affects the properties. The bismuth content in strontium bismuth tantalate ferroelectric ceramics is very crucial for determining the properties of this compound. In the present work specimens of varying bismuth and strontium content with compositions Sr1-xBi2+xTa2O9 were prepared and the preparation conditions have been optimized with an objective to get enhanced dielectric properties. The structural characterization has been done using X-ray diffraction and scanning electron microscopy. Energy Dispersive Spectroscopic analysis has been carried out to determine the final composition of the synthesized specimens. Dielectric studies as a function of temperature have been carried out. The PE loops of the specimens have been recorded. The obtained data has been interpreted to explain the observed properties.

Characteristics of Strontium Bismuth Tantalate Film with ZrO2 Buffer Layer for Non-Volatile Memory Applications

We have investigated the interfacial and electrical properties of metal-ferroelectric-insulator-semiconductor capacitors with SrBi2Ta2O9 ferroelectric films grown on ZrO2 buffer layer coated Si. Interfacial and surface roughness parameters of heterostructures were extracted from the simulation of specular x-ray reflectivity data. For as-deposited film, no interfacial layer is found between SBT and ZrO2 from simulation data, however, an accurate fit can only be possible if a thin SiO2 layer is assumed at ZrO2/Si interface. On annealing, the formation of an inhomogeneous interfacial layer between SBT/ZrO2 interfaces having thickness 1.3 nm is observed.

X-ray photoelectron spectroscopy characterization of fluorite and perovskite phases in Sr1−xBi2+yTa2O9−z films

High-resolution x-ray photoelectron spectroscopy of strontium bismuth tantalate films revealed distinct chemical states of constituent atoms in fluorite and perovskite crystals. Reflecting the number of nearest-neighbor oxygen atoms coordinated with Bi3+ ions, the Bi 4f peak could be deconvoluted into Bi2O3 (sixfold coordination), Bi2O2 (fourfold coordination), and (Bi2O2)2+ (intermediate between sixfold and fourfold coordination) components. We found that amorphous and fluorite phases could be expressed as a mixture of Bi2O3 and Bi2O2, whereas the (Bi2O2)2+ component representing the bismuth oxide layer, being adjacent to the (SrTa2O7)2− block, predominated in the perovskite phase. Similarly, the Sr 3d peak could be deconvoluted into the components of Sr oxide (SrO) and Sr in (SrTa2O7)2−. The volume fraction of the (SrTa2O7)2− component increased at higher annealing temperatures. The Ta 4d peaks, in contrast, located at the same binding energy for all samples, indicated that an energetically stable TaO6 octahedron unit is preferentially created. The binding energies of the O 1s state corresponding to O–Bi, O–Ta, and O–Sr bonds were identified at 531, 530, and 528.9 eV, respectively.