Strontium Bismuth Tantalate Thin Films Research Articles

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.

Preparation and ferroelectric properties of barium-ion-doped strontium bismuth tantalate thin films

(Sr 0.5Ba 0.5) x Bi y Ta 2O z (SBBT) films were prepared on Pt/Ti/SiO 2/Si substrates by a metal-organic decomposition method. At low temperatures, the phase transformation of films from fluorite-type structure to SBBT structure was suppressed with increasing the amounts of bismuth ions. As temperature increased, the phase transformation was improved by excess bismuth ions. Adding excess bismuth ions also resulted in an increase in the grain size of the prepared films. The value of the remanent polarization of thin film was also increased by adding excess bismuth contents. It was found that the ferroelectric characteristics of these films were significantly affected by the composition in the films.

Behavior of the ferroelectric polarization as a function of temperature in sol-gel derived strontium bismuth tantalate thin films

Strontium bismuth tantalate (SBT) thin films of nominal composition Sr0.8Bi2.2Ta2O9 have been prepared on (111)Pt/TiO2/SiO2/(100)Si substrates by Chemical Solution Deposition (CSD) methods, using two different thermal treatments of crystallization. The evolution of the ferroelectric polarization as a function of temperature has been studied using hysteresis loops, thermo stimulated current (TSC) measurements and the evolution of the pyroelectric coefficient (γM) with temperature. The results of remnant polarization (Pr) obtained from hysteresis loops and γM as a function of temperature showed different trends, indicating the presence of polarizing bias field in the prepared thin films due to induced asymmetry of the electrical and mechanical conditions produced at the top and bottom film electrode interfaces. The competition between internal bias fields due to differences in contact potentials of bottom and top electrodes and flexoelectric bias fields induced by stress gradient, can be in the origin of the γM fluctuation with temperature. A critical temperature (T*) is found that can be explained as the crossover between the dominant effects. It can be related to a possible ferroelectric-ferroelectric phase transition previously reported. The results also indicate that the Pr is not stable for temperatures over ≈425 K.

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.

Optical and electric properties of strontium bismuth tantalate thin films

High quality SrBi 2Ta 2O 9 ferroelectric thin films were fabricated on platinized silicon using pulsed laser deposition. The optical properties of the thin films were studied by spectroscopic ellipsometry from the ultraviolet to the infrared region. Optical constants, n ∼ 0.25 in the infrared region and n ∼ 2.18 in the visible spectral region, were determined through multilayer analyses on their respective pseudodielectric functions. The band-gap energy is estimated to be 4.14 eV. It was found that the leakage current mechanism of the film was from bulk limited Poole–Frenkle emission to interface-controlled Schottky emission with applied field increasing, and that the breakdown field of the film had a negative linear variation with the logarithm of the electrode area.

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.

Dielectric and ferroelectric properties of c-axis oriented strontium bismuth tantalate thin films applied transverse electric fields

The in-plane dielectric and ferroelectric properties of c-axis oriented epitaxial strontium bismuth tantalate (SBT) thin films were investigated by interdigital surface electrode measurement. The four types of SBT thin films, which have various Sr∕Bi atomic ratios, were prepared on MgO substrates by pulsed laser deposition. The dielectric properties at kilohertz and megahertz frequencies were studied from 20to870K. Above room temperature, all the films show the phase transition. Sr0.99Bi1.61Ta2O9, Sr0.97Bi2.10Ta2O9, and Sr0.88Bi1.79Ta2O9 thin films show frequency dispersions near phase transition temperature. At low temperatures, dielectric and ferroelectric properties of Sr0.97Bi2.10Ta2O9 and Sr0.83Bi2.08Ta2O9 thin films were investigated. Both dielectric constant and remanent polarization show a tendency to decrease as temperature decreases, and the rapid change, which may indicate a phase transition, was observed in Sr0.97Bi2.10Ta2O9 near 80K.

Enhanced Crystallization of Strontium Bismuth Tantalate Thin Films by Irradiation of Elongated Pulses of KrF Excimer Laser

The pulse width of a KrF excimer laser was elongated from 40 to 60 ns by coupling dually split laser beams with different optical paths. By the irradiation of such elongated laser pulses up to 103 times, strontium bismuth tantalate thin films can be crystallized efficiently to perovskite at a substrate temperature of 650 °C, while the irradiation of nonelongated pulses fails to crystallize the films. The crystallized films have high remanent polarization and coercive field values. The presence of the energy threshold and the critical thickness for crystallization to perovskite is suggested.

Structural and electrical characterization of strontium bismuth tantalate (SBT) thin films

Ferroelectric strontium bismuth tantalate (SBT) thin films were deposited by thermal metalorganic chemical vapour deposition (MOCVD) onto a complex layered Pt/IrO 2 /Ir/Ti(Al)N substrate. A study of ultra-violet (UV)-assisted rapid thermal processing (RTP) annealing strategies of the SBT thin films was performed. The influence of UV irradiation temperature and annealing atmosphere on the crystallinity of the deposited films was evaluated using both microstructural and electrical analysis techniques. A UV-RTP strategy in an oxygen atmosphere above 400 °C, followed by a furnace treatment at 700 °C, provided an optimum remnant polarization figure of merit.

Atomic vapour deposition (AVD) of SrBi 2Ta 2O 9 using an all alkoxide precursor

A “single-source” Sr–Ta heterometal alkoxide precursor, Sr[Ta(OEt) 5(dmae)] 2 (dmae=OCH 2CH 2NMe 2), has been used for atomic vapour deposition (AVD) of SrBi 2Ta 2O 9 (SBT). This single-source precursor is designed to alleviate the mismatch between conventional Sr and Ta sources. Strontium tantalate thin films were deposited on silicon using the Sr[Ta(OEt) 5(dmae)] 2, and the optimum temperatures for deposition of strontium tantalate with a Sr:Ta ratio of 0.5 was found to be ∼510 °C. Deposition of Bi-oxide films using Bi(mmp) 3 (mmp=OCMe 2CH 2OMe) indicates similar decomposition behaviour to the Sr–Ta alkoxide precursor, demonstrating its suitability as a complementary source of Bi for SBT. The co-incorporation of Bi and Sr within the SBT films is promoted through the deposition of bismuth oxide/strontium tantalate super lattices. After post-growth annealing the super lattices are converted to strontium bismuth tantalate thin films.

Microstructural Studies on the Low-Temperature Crystallization Process of Strontium Bismuth Tantalate Thin Films

The preparation of strontium bismuth tantalate (SBT) thin films at low temperatures for electronic applications is at present the subject of intense study. However, the microstructural evolution of these films has not been extensively reported, despite its importance in the determination of the final properties of the film. In this work, we study SBT thin films with various nominal compositions obtained by the crystallization at 650°3C of spin-coated solutions on a silicon-based substrate. Both the formation of the perovskite phase and the evolution of the grains are analyzed, with special attention to the formation of platelet-like grains. A process of coalescence of initially round grains is proposed to explain the grain growth in this system. We also show that the use of a preannealing step inhibits the grain growth and, therefore, should be avoided. The role that the film thickness plays in the development of large grains is discussed.

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.

Enhanced Aurivillius phase formation kinetics in seeded SBT thin films

Strontium bismuth tantalate thin films having composition of Sr 0.7Bi 2.4Ta 2O 9 (SBT) were deposited on the pre-crystallized SBT seed layer with the same composition using sol–gel and spin coating methods. Unseeded and seeded SBT thin films were crystallized to fluorite by heating at 600°C for 1 h and they were further heated in the range of 710–740°C at 10°C intervals for different time periods to analyze fluorite-to-Aurivillius phase transformation characteristics. X-ray diffraction (XRD) results on the heated films indicate that the phase transformation kinetics is greatly enhanced in the seeded ones. Quantitative X-ray diffraction (Q-XRD) was performed and the results were used for Johnson–Mehl–Avrami (JMA) isothermal kinetic analyses. From JMA plots Avrami exponent values for unseeded and seeded films were determined as ∼1.5 and ∼1.1, respectively, which implies that each film has a different nucleation mechanism. Using Arrhenius plots the activation energy values for the phase transformation of unseeded and seeded films were determined as ∼263 and ∼183 kJ/mol, respectively. The nucleation mechanism difference and the resultant activation energy difference provide the origin of enhanced kinetics in seeded SBT thin films.

Processing effects on the microstructure and ferroelectric properties of strontium bismuth tantalate thin films

Ferroelectric thin films of strontium bismuth tantalate compositions with Bi/Sr ratios of 2.75 and 2.50 have been produced by deposition of solutions derived from sol-gels followed by crystallization using rapid thermal processing (RTP) at 650 °C. Single-step and two-step processes have been used for the RTP crystallization of the films. Both the composition and the heating process used affect the grain size, grain shape and compositional heterogeneity of the films, acting on their ferroelectric switching behaviour and dielectric properties. Larger and more elongated grains are obtained by crystallization using a single-step process. These films exhibit less slanted hysteresis loops. The combination of the study of the pure-ferroelectric-switching loop and the lambda curves through the application of a Curie–Weiss-like model gives a deeper insight into the influence of film heterogeneity on its ferroelectric properties. The observation of net polarisation without poling in all the films is attributed to the effects of 180° ferroelectric domains.

Comparison of the Electrical Characteristics of PZT and SBT Thin Films

The electrical properties of lead zirconate titanate (PZT) and strontium bismuth tantalate (SBT) thin films are compared for nonvolatile random access memory applications. PZT thin films have high switchable polarization with poor fatigue resistance and higher leakage current densities as compared to SBT thin films deposited on platinized silicon substrates. Characterization of these films in terms of their domain dynamics under application of sub-switchable and switchable electric fields give valuable insight about the observed differences in the electrical behavior. In the present work the dielectric behaviors of SBT and PZT thin films at sub-switchable electric field were analyzed in terms of Rayleigh law. The reversible and irreversible polarization component of switchable polarization was separated by the measurement of capacitance-voltage (C-V) and polarization hysteresis loops. The conduction mechanisms in these films were evaluated by measuring field dependence of leakage current densities at various temperatures. The observed differences in electrical properties are explained in terms of the intrinsic defects and defect-domain interaction of these two materials.

Crystallographic, Dielectric and Optical Properties of SrBi 2 Ta 2 O 9 Thin Films Prepared by the Polymeric Precursor Method

Strontium bismuth tantalate thin films were prepared on several substrates (platinized silicon (Pt/Ti/SiO 2 /Si), n -type (100)-oriented and p -type (111)-oriented silicon wafers, and fused silica) by the solution deposition method. The resin was obtained by the polymeric precursor method, based on the Pechini process, using strontium carbonate, bismuth oxide, and tantalum ethoxide as starting reagents. Characterizations by XRD and SEM were performed for structural and microstructural evaluations. The electrical measurements, carried on the MFM configuration, showed P r values of 6.24 w C/cm 2 and 31.5 kV/cm for the film annealed at 800 °C. The film deposited onto fused silica and treated at 700 °C presented around 80% of transmittance.

Crystallographic, Dielectric and Optical Properties of SrBi 2 Ta 2 O 9 Thin Films Prepared by the Polymeric Precursor Method

Strontium bismuth tantalate thin films were prepared on several substrates (platinized silicon (Pt/Ti/SiO 2 /Si), n -type (100)-oriented and p -type (111)-oriented silicon wafers, and fused silica) by the solution deposition method. The resin was obtained by the polymeric precursor method, based on the Pechini process, using strontium carbonate, bismuth oxide, and tantalum ethoxide as starting reagents. Characterizations by XRD and SEM were performed for structural and microstructural evaluations. The electrical measurements, carried on the MFM configuration, showed P r values of 6.24 w C/cm 2 and 31.5 kV/cm for the film annealed at 800 °C. The film deposited onto fused silica and treated at 700 °C presented around 80% of transmittance.

Top Electrode Induced Self-Polarization in CSD Processed SBT Thin Films

The CSD processed Strontium Bismuth Tantalate (SBT) ferroelectric films presents a certain degree of self-polarization when no further annealing after sputtered Pt top electrode deposition is performed. Using the measurement of the first pulsed hysteresis loop the percentage of switchable self-polarized polarization is quantitatively analyzed. Pyroelectric coefficient measurements assess the validity of the results obtained. The hysteresis loop measurements indicate that the polarization vector points to the bottom electrode. When the top electrode is a Hg drop instead of Pt, the degree of self-polarization obtained is different both in magnitude and sign. These results are explained as a competition between the self-polarization induced by the top and bottom platinum electrodes.

Tem Characterisation of Electrolytic Separated Specimens of Strontium Bismuth Tantalate Ferroelectric Thin Films

Preparation of specimens for Transmission Electron Microscopy usually involves elaborate and aggressive procedures that can produce artefacts in the observations. In this work we use an original preparation method for thin films that involves the electrolytic separation of the film from the substrate. Developed initially to study the characteristics of films isolated from the substrate, this method proves to be easier, less time consuming and in principle less intrusive than the traditional ones. The materials analysed are Strontium Bismuth Tantalate (SBT) ferroelectric films, processed by multiple deposition onto Pt/TiO 2 /SiO 2 substrates of sol-gel derived solutions and crystallisation by Rapid Thermal Processing (RTP) at 650°C. Final thickness range between 70 and 500 nm, which test the versatility of the method to obtain TEM specimens from films with varying thickness, and, at the same time, allows us to study the differences that the deposition of several layers may induce. Several issues concerning the optimisation of the SBT thin films processing need TEM analysis, and are also addressed in this work.

Factors controlling the a-axis orientation of strontium bismuth tantalate thin films fabricated by chemical solution deposition

The a-axis orientation of strontium bismuth tantalate (SBT) thin films on Pt/TiOx/SiO2/Si substrates was found to be controlled by the atomic composition of the precursor solution, heating rate for thermal annealing, and thickness of the single-annealed layer of chemical solution deposition. Under optimized conditions, an increase in the thickness of the total film caused the orientation to change from random to a-axis preferred. It was considered that anisotropic growth is available for preparing a-axis preferentially oriented films. The SBT thin film, whose relative intensity of the (200) peak [I(200)/I(115)] was 2.3, had a remanent polarization (2Pr) value of 30.4 μC/cm2.