Strontium Bismuth Tantalate Films Research Articles

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.

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).

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.

Effect of Eu/Sr Ratios on Ferroelectric and Fluorescent Properties of Eu-Substituted Strontium Bismuth Tantalate Films

The ferroelectric and fluorescent properties of Eu-substituted strontium bismuth tantalate (Eu-SBT) films grown on Pt/Ti/SiO2/Si substrates by spin coating were investigated. The polycrystalline Eu-SBT films with Aurivillius phases were grown at Eu/Sr ratios between 0 and 0.25. Moreover, the lattice constants along the a- and c-axes at Eu/Sr=0.25 in comparison with Eu/Sr=0 decreased by approximately 0.36 and 0.19%, respectively. The remnant polarization values of the Eu-SBT films with Eu/Sr ratios of 0 and 0.25 were approximately 6.6 and 5.8 µC/cm2, respectively. The 80%-fatigue endurance of a Eu-SBT film with Eu/Sr=0.25 was approximately 3.1×109 cycles when a 10 kHz triangular wave with an amplitude of 8 V (approximately 530 kV/cm electric field) was used. The photoluminescence intensity of the Eu-SBT films was increased by Eu doping with negligible change of the remnant polarization. Emission peaks at approximately 600 nm in wavelength, which were associated with the 5D0–7F transitions of Eu3+, were observed in the Eu-SBT films.

Correlation Between Ferroelectric and Fluorescent Properties by Introducing Eu Atoms Into Strontium Bismuth Tantalate Films

AbstractElectrical and fluorescent properties of Sr-deficient strontium bismuth tantalate co-doped with excess-Bi and Eu atoms (BiEu-SBT) were investigated, in which the Eu/Bi ratio was changed between 0 and 0.1. BiEu-SBT were synthesized on Pt (200 nm)/Ti (50 nm)/SiO2/Si substrates by using Sr0.8Bi2.2Ta2O9 and Sr0.8(Bi2, Eu0.2)Ta2O9 mixed precursor solutions with a concentration of 0.33 mol/kg, in which Sr concentration was fixed at 0.8. From X-ray diffraction analysis, the diffraction peaks from Aurivillius phases were observed in the synthesized BiEu-SBT as well as in pure SBT. In BiEu-SBT, the lattice parameters along a- and c-axes at the Eu/Bi ratio of 0.1 in comparison with those of 0 decreased approximately 0.27 and 0.19 %, respectively. The remnant polarization (2Pr) values of the synthesized BiEu-SBT with Eu/Bi ratio of 0 and 0.1 were approximately 8.4 and 6.8 μC/cm2, respectively. From photoluminescence (PL) measurement at a wavelength of 615 nm as a function of Eu/Bi ratio, the PL intensity of the synthesized BiEu-SBT was proportional to the Eu/Bi ratio, whereas, the 2Pr values were slightly decreased by Eu-doping.

Ferroelectric and Luminescent Properties of Eu-Doped SrBi2Ta2O9 Films

Sr-deficient and Bi-excess strontium bismuth tantalate (SBT) films doped with Eu atoms with concentrations from 1.25 to 5 mol % (SBT:Eu) were grown on Pt/Ti/SiO2/Si substrates by spin coating. The crystallinity of the SBT:Eu films was characterized by X-ray diffraction analysis. The ferroelectric and luminescent properties of the SBT:Eu films were measured at room temperature. Polycrystalline SBT:Eu films with the Aurivillius phase were grown, in which the Eu concentration was less than 5 mol %. The remnant polarization values of the 260-nm-thick SBT:Eu films with Eu concentrations of 1.25 and 5 mol % were approximately 4.0 and 0.86 µC/cm2, respectively. The PL intensity of the SBT:Eu films was significantly dependent on both annealing temperature and Eu concentrations. The maximum PL intensity was obtained in a 5 mol %-Eu-doped SBT film at an annealing temperature of 850 °C.

Influence of Ta content on the physical properties of SrBi2Ta2O9 ferroelectric thin films

We have investigated the effect that the Ta content has on the ferroelectric properties of strontium bismuth tantalate (SBT) thin films synthesized using metalorganic decomposition (MOD) and spin coating techniques. The physical properties of these SBT samples were strongly dependent upon the Ta ratio. Polarization measurements revealed that Ta-deficient SBT exhibited a relatively low coercive field (2Ec ∼ 87 kV/cm) and a high remanent polarization (2Pr ∼ 15 μC/cm2). The value of 2Pr decreased as the Ta ratio in SBT increased. The improved ferroelectric properties of the Ta-deficient SBT samples may have resulted from the uniformly well-grown bismuth-layered-structured (BLS) phases of the films and their highly preferential orientation along the a and b axes. We suggest that the incorporation of Ta vacancies plays an important role in enhancing the crystallinities and microstructures of Ta-deficient SBT films.

TRIBUTYLBISMUTH AS A LIQUID BISMUTH PRECURSOR IN AN AVD SYSTEM FOR STRONTIUM BISMUTH TANTALATE THIN FILM DEPOSITION

ABSTRACT In this study, tributylbismuth has been tested as a bismuth source for strontium bismuth tantalate (SBT) deposition using atomic vapor deposition (AVD) system. The effects of annealing temperature on the structural and electrical properties of deposited films were investigated. SBT films annealed at 750°C were crystallized in ferroelectric perovskite phase. Ferroelectric hysteresis loops of the SBT films have been measured, from which a remnant polarization of 3.5 μC/cm2 and a coercive voltage of 3 V of the crystallized SBT layer have been evaluated. SBT thin films annealed at 750°C also showed good fatigue properties. Fatigue was found to be about 4% after 109 cycles.

Oxygen ion implantation in Strontium Bismuth Tantalate thin films

Strontium Bismuth Tantalate (SBT) ferroelectric thin films have attracted considerable attention for the development of non-volatile ferroelectric random access memories (NV-FRAMs). These films, however, have a critical problem: a high processing temperature ( > 700oC) is required for the crystallization of the perovskite phase. The thermal evolution of the SBT films prepared by Chemical Solution Deposition (CSD) techniques showed the formation of an intermediate oxygen-deficient fluorite phase at ~ 550oC. The SBT perovskite phase crystallizes at higher temperatures. To favor an earlier perovskite crystallization, SBT thin films were implanted with oxygen ions pulses produced by a Plasma Focus (1 kJ). The samples were annealed at different temperatures in oxygen atmosphere and characterized by GI-XRD and Atomic Force Microscopy techniques. It was found that oxygen addition into the SBT structure promotes a better crystallization of the perovskite phase.

Liquid‐Delivery MOCVD of Strontium Bismuth Tantalate Thin Films Using Sr[Ta(OC2H5)5(OCH2CH2OCH3)]2 and Liquid Bi(CH2CH=CH2)3 as Precursors

AbstractThin films of BiOX, SrXTaYOZ, and strontium bismuth tantalate (SBT) were deposited by metal‐organic (MO)CVD on 150 mm silicon (100) wafers. Some of the wafers were pre‐deposited with Pt electrodes. The substrate temperature and the deposition pressure were varied from 300 °C to 600 °C and from 0.35 mbar to 7 mbar, respectively. Bi(CH2CH=CH2)3 (triallylbismuth) and Sr[Ta(OEt)5(OC2H4OMe)]2 (strontium‐bis[tantalum(pentaethoxide)(2‐methoxyethoxide)]) were used as Bi precursor and as Sr‐Ta precursor, respectively. A liquid delivery system was used to supply and to vaporize the precursors into the reactor. X‐ray photoelectron spectroscopy (XPS) and ellipsometry were carried out to characterize the film properties. The growth rate of the MOCVD of BiOX and SrXTaYOZ was compared to the growth rate of SBT to obtain information about mutual interaction between the precursors. The deposition rate of bismuth oxide thin films was low (∼10 nm h–1 at 0.35 mbar) and virtually independent of the temperature. On the contrary, the growth rate of strontium tantalate films depended strongly on the temperature. The deposition rate of the SBT films was similar to the bismuth oxide film deposition, which slightly increased with increasing substrate temperature. However, the deposition rate of SBT was always lower than deposition rate of the single precursors. The growth rate significantly depended on pressure. The decrease of the deposition pressure in the reactor chamber reduced the deposition rate of BiOX, SrXTaYOZ, and SBT but on the other hand, it improved the uniformity of the film thickness. XPS measurements showed a deficit of bismuth in the SBT films even though the concentration of the Bi precursor had been several times higher than the concentration of Sr‐Ta precursor. The XPS depth‐profiling by Ar+ ion sputtering indicated more metallic bond characteristics of Ti, Sr, and Bi after ion‐beam bombardment.

Interfacial Phases and Electrical Characteristics of Ferreoelectric Strontium Bismuth Tantalate Films on Pt/TiO2 and Ti/Pt/Ti Heterostructure Electrodes

Strontium bismuth tantalate (SBT) thin films containing Sr defect and Bi excess have been prepared by chemical solution deposition onto spontaneously oxidized Si(100) substrates with two different heterostructure electrodes: Pt/TiO2 and Ti/Pt/Ti. Layered perovskite SBT films were crystallized in oxygen by rapid thermal processing at 973 K for 1 h with an intermediate treatment at 823 K for 2 h. Crystalline phases of the films were monitored by X-ray diffraction analysis with Bragg−Brentano geometry and with grazing incidence. Interfaces developed between the SBT layer and the bottom electrode during thermal treatment were analyzed by X-ray photoelectron and Rutherford backscattering spectroscopies. Chemical composition of the interfaces was dependent on the heterostructure electrode. Interfaces of the SBT films on Pt/TiO2 contain Pt and Bi, whereas those of the films on Ti/Pt/Ti contain Pt, Bi, Ti, and O. Electrical characteristics of these interfaces were deduced from their leakage current behavior. Leakages of SBT films onto Pt/TiO2 are governed by a Schottky emission model at all temperatures, with a barrier height of ∼0.89 eV. Leakages of SBT films onto Ti/Pt/Ti can only be fit to a Schottky model at high temperatures. Despite the so different leakage behavior of the films, their ferroelectric properties and fatigue characteristics are similar, with values of remanent polarization of Pr ≈ 5 μC/cm2 and of the coercive field of Ec ≈ 65 KV/cm, and with a ∼20% of reduction of Pr after ∼1010 cycles. These properties are appropriate for their use in nonvolatile memories.

Crystallization, Heterostructure, Microstructure, and Properties of Ferroelectric Strontium Bismuth Tantalate Films Derived from Tantalum Glycolate Solutions.

AbstractFor Abstract see ChemInform Abstract in Full Text.

MOCVD kinetics of strontium bismuth tantalate thin film growth

The thin films of BiOx, SrxTayOz , and strontium bismuth tantalate (SBT) were deposited by metalorganic chemical vapour deposition (MOCVD) on 150 mm silicon (100) wafers. Some of the wafers were pre-deposited with Pt electrodes. The substrate temperature and the deposition pressure were varied from 300 to 600 ◦C and from 0.35 to 7 mbar, respectively. Bi(/\//)3 (triallyl bismuth) and Sr[Ta(OEt)5(OC2H4OMe)]2 (strontium bis[tantalum(pentaethoxy)(2-methoxyethoxide)] were used as a Bi precursor and as a Sr–Ta precursor, respectively. A liquid delivery system was used to supply and to evaporate the precursor into the reactor. X-ray photoelectron spectroscopy (XPS) and ellipsometry were carried out to characterize the film properties. The growth rate of the MOCVD of BiOx and SrxTayOz was compared to the growth rate of SBT to obtain information about mutual interaction between precursors. The deposition rate of bismuth oxide thin films was low (∼10 nm/h at 0.35 mbar) and did virtually not depend on the temperature. On the contrary, the growth rate of strontium tantalate films depended strongly on the temperature. The deposition rate of the SBT films was similar to the bismuth oxide film deposition, which slightly increased with increasing substrate temperature. However, the deposition rate of SBT was always lower than the deposition rate of the single precursors. The growth rate significantly depends on pressure. The decrease of the deposition pressure in the reactor chamber reduces the deposition rate of BiOx, SrxTayOz, and SBT, but on the other hand, it improves the uniformity of the film thickness. The XPS measurements showed a deficit of bismuth in the SBT films even though the concentration of the Bi precursor was several times higher. The XPS depth-profiling by Ar ion sputtering indicated different bond characteristics of Ti, Sr, and Bi before and after ion beam bombardment.

Crystallization, Heterostructure, Microstructure, and Properties of Ferroelectric Strontium Bismuth Tantalate Films Derived from Tantalum Glycolate Solutions

Strontium bismuth tantalate (SBT) films were prepared by chemical solution deposition (CSD) onto Pt/TiO2/SiO2/(100)Si substrates. Nominal chemical compositions of the precursor solutions of SrBi2Ta2O9, SrBi2.2Ta2O9, Sr0.8Bi2Ta2O9, and Sr0.8Bi2.2Ta2O9 were tested for the deposition of the films. Crystallization of the films was carried out by a two-step process (TS) or a single-step process (SS). The TS treatment consisted of thermal treatment of the films in oxygen atmosphere at 550 °C for 7200 s, with a heating rate of 8 °C/s, followed by a rapid thermal processing (RTP) in oxygen at a temperature of 650 °C for 3600 s, using a heating rate of 200 °C/s. The SS process was just the RTP treatment of the films. The X-ray diffraction (XRD) patterns of the crystalline films showed the formation of the layered perovskite together with a second phase. Analysis of the films by means of Rutherford backscattering spectroscopy (RBS) showed that this second phase was placed at the interface and it was formed by the r...

Photoelectron spectroscopic studies of bismuth-excess strontium bismuth tantalate thin films and their high-pressure-O2-annealing effects

The electronic structure of strontium bismuth tantalate (SBT) thin films has been studied by x-ray photoelectron spectroscopy and ultraviolet-ray photoyield spectroscopy (UV-PYS). The SBT films were prepared by metalorganic decomposition (MOD) method using a chemical solution of Sr:Bi:Ta=0.8:2.3:2. The XPS spectra imply that pyrochlore-dominant MOD-SBT films include bismuth oxides other than oxidized bismuth [(Bi2O2)2+] layers after high-pressure-O2 annealing. The UV-PYS spectra exhibited that high-pressure-O2-annealed SBT thin films with dominant pyrochlore structures have electron excitation energy of about 6.0 eV, which is excitation from the Fermi level to the vacuum level, while the as-crystallized SBT thin films with dominant perovskite structures exhibit about 5.5 eV. These results indicate that the high-pressure-O2 annealing oxidizes Bi atoms more in the pyrochlore-dominant SBT thin films than in the perovskite-dominant ones. The high-pressure-O2 annealing tends to suppress current through the pyrochlore-dominant MOD-SBT thin films, which is considered to be partly because bismuth oxides other than (Bi2O2)2+ layers have been produced at the surface by the high-pressure-O2 annealing.

Nanoscale properties of ferroelectric ultrathin SBT films

Strontium bismuth tantalate (SBT) ultrathin films with thickness about 40 nm have been prepared by a chemical solution deposition (CSD) method based on the synthesis of a Ta-glycolate derivative. Layers of the precursor solution diluted with 2-ethyl-1-hexanol were spin-coated onto Pt/TiO 2/SiO 2/(100)Si substrates. Crystallisation was carried out in oxygen at 650 °C by Rapid Thermal Processing (RTP). Topography of the films was observed by Scanning Force Microscopy (SFM). The piezoresponse mode of the SFM equipment was used to perform the nanoscale observation of the ferroelectric domain structure of the SBT ultrathin films and to measure piezoelectric hysteresis loops. The macroscopic ferroelectric response was measured on capacitors prepared after the deposition on the film surfaces of top platinum electrodes of ∼0.05 mm 2 of area.

Effects of scaling the film thickness on the ferroelectric properties of SrBi2Ta2O9 ultra thin films

We have investigated the effect of reducing the thickness of strontium bismuth tantalate film to as low as 25 nm on its ferroelectric characteristics. A degradation of ferroelectric properties such as significant reduction in remanent polarization is generally observed with reduction in film thickness, in particular below 100 nm. This has been overcome by using a modified deposition process sequence and a crystallization technique based completely on the rapid thermal annealing process. The resulting ultrathin films show good remanent polarization, low-voltage saturation, low leakage current, high breakdown strength, and good endurance. These films demonstrate the potential for scaling and are excellent candidates for several generations of ferroelectric random access memory applications.

Ultrathin ferroelectric strontium bismuth tantalate films

Continuous strontium bismuth tantalate films with an average thickness of ∼36 nm have been prepared by spin-coating deposition of solutions onto Pt/TiO2/SiO2/(100)Si substrates and by rapid thermal processing. The films microstructure was formed by rod-shaped grains and fine equiaxed grains. This densely packed microstructure makes the growth of a continuous ultrathin film possible without shorts in the capacitor. The films exhibited values of coercive field of Ec∼226±40 kV/cm and of remanent polarization of Pr∼7.4±1.5 μC/cm2. They retain their Pr up to ∼105 s and do not fatigue up to ∼1010 cycles.

Non-destructive characterization of strontium bismuth tantalate films

Optical, compositional, and structural properties of strontium bismuth tantalate (SBT) films deposited in a high throughput low-pressure chemical vapor deposition reactor using liquid metal-organic precursors were characterized using three non-destructive techniques, spectroscopic ellipsometry (SE), Rutherford backscattering spectrometry (RBS), and X-ray diffraction (XRD). The thicknesses and the refractive indices of the SBT layers were calculated with SE using different parametric dielectric function models. The samples were characterized with RBS using different tilt angles and probe ions to enhance the depth resolution and the mass separation. Comparison with SE measurements supports the results of Bahng et al. revealing an increasing refractive index ( n) with increasing Bi/Sr ratio. The decreasing grain size measured by XRD was reflected as a decrease of n in the SE measurement. We show that RBS, XRD, and SE supply a wide range of information about the SBT layers, which can be used for qualification as well as for feedback to layer production. The results suggest that by SE, being used as in situ or in line characterization tool, the control of even complex MOCVD deposition looks feasible.