BaxSr1-xTiO3 Research Articles

Improved Performance in BST-Based Tunable Circuits Employing Low-Loss Nontunable Dielectric

BaxSr1 − xTiO3 (BST) materials are being developed for tunable microwave applications. A figure of merit “K” defined as K = is frequently used to evaluate tunable dielectrics. Based on a simple equivalent-circuit model, multilayer structures consisting of BST films and low-loss non-tunable dielectric should have a higher K than BST alone. The maximum improvement achievable by this approach is predicted to increase with tunability of the BST material. For a BST film with 80% tunability and maximum loss tangent of 0.07, such as those typically deposited by pulsed laser deposition, the maximum predicted improvement is close to a factor of five for K. A four-electrode chip capacitor was fabricated for experimental validation of the modeling predictions. A 25% increase in K was demonstrated for a multilayer four-electrode capacitor using low tuning voltages. An alternative figure of merit the Commutation Quality Factor (CQF) was also examined, it is reported to be invariable with the respect to series and parallel secondary low loss inclusions and is therefore potentially more universal quality parameter for characterization of tunable dielectrics than the widely-used K. We present experimental results for a multilayer structure and evaluate its impact on the two alternative figures of merits.

Design of Tunable Balanced Amplifier Using Ferroelectric Materials

The non-linear electric field dependence of ferroelectric thin films can be used to design frequency and phase agile components. It is well known from the literature that ferroelectric-based tunable microwave component can easily be integrated into conventional microstrip circuits. These components are particularly attractive for broad-band and multi-frequency applications. However, most efforts have focused on the careful characterization of tunable passive devices while active components have been occasionally reported. In this work, simulated results from a full wave electromagnetic simulator are obtained to show the tunability of a quadrature hybrid made of Ba x Sr1 − x TiO3 (BSTO) ferroelectric thin film on MgO. The S-parameters of this passive structure are estimated for different bias conditions for the BSTO and used in the simulation of tunable balanced amplifier. Simulations of this type of amplifier are shown with and without matching networks.

Closed Loop Controlled BaxSr1–xTio3 Thin Films In Spray Flames

Control logic was developed for automated thin film deposition in the combustion chemical vapour deposition (CCVD) process. In situ film thickness and deposition temperature measurements were used as a feedback to control film thickness and optical properties through real-time adjustment of the deposition parameters. The closed loop controller was optimised for the deposition of barium–strontium–titanate (BaxSr1–xTiO3, BST) films on sapphire but it can be used for other coating/substrate combinations. Besides enabling repeatable depositions of BST films with optimal thickness and optical properties for electronic applications, this control process proved that unique features of the Nanomiser atomiser can be implemented to tune the film deposition rate. Comparison of scanning electron microscopy cross-sections of epitaxial BST films on sapphire substrate with the predicted film thickness suggests that the closed loop controller can be used to control the thickness of the deposited film to within ± 2%

Microwave Dielectric Properties of (111) Oriented Ferroelectric (Ba1 − xSrx)TiO3 Thin Films

Epitaxial (111) oriented ferroelectric (Ba1 − xSrx)TiO3 (BST) films were deposited on MgO (111) single crystals using pulsed laser deposition. Structural properties of BST films were investigated using X-ray diffractometer. The dielectric properties of BST films were investigated under the dc bias field of 0–40 V using interdigital capacitors (IDT) fabricated by photolithography and etching process. The small signal dielectric properties of BST films were calculated by modified conformal mapping both the measured data using an impedance gain/phase analyzer and the reflection coefficient data measured using a HP 8510C vector network analyzer in 0.05–10 GHz at room temperature. The IDT capacitor based on (111) oriented BST film exhibits about 40% of capacitance change with an dc bias of 40 V which value is somewhat smaller than that of the IDT device based on (001) oriented BST film. But the dielectric quality factor value is about twice that of the device based on (001) oriented BST film.

Dielectric Properties and Leakage Current Characteristics of Sol-Gel Derived (Ba0.5Sr0.5)TiO3:MgTiO3 Thin Film Composites

(Ba1-xSrx)TiO3 (BST) is a suitable material for microelectronic device applications due to its high response of the dielectric permittivity to an applied electric field. However, the large dielectric constants found in this system limit its usefulness at microwave frequencies. In our recent studies sol-gel prepared Ba0.5Sr0.5TiO3:MgO heterostructured thin films have shown an increase in the figure of merit (tunability/loss) compared to pure Ba0.5Sr0.5TiO3 films, but the leakage characteristics of these films did not improve significantly. Moreover, due to the hygroscopic nature of MgO, the BST:MgO heterostructured films may suffer with longer period of time instability. In this paper we used the low loss and non-hygroscopic MgTiO3 for the fabrication of the Ba0.5Sr0.5TiO3:MgTiO3 (BST50:MT) heterostructured thin films by sol-gel technique and studied the effects of MgTiO3 layers on the structural, physical, dielectric properties, and leakage current behavior of the BST:MT films. The X-ray studies indicated that both MT and Ba0.5Sr0.5TiO3 are highly oriented and remain as two distinct individual entities in the composite films and a considerable reduction in the dielectric loss and leakage currents has been observed. High frequency phase shifter studies (at low voltage) suggest that these films are suitable for the tunable microwave device applications.

Layered Cu-based electrode for high-dielectric constant oxide thin film-based devices

Ti–Al/Cu/Ta multilayered electrodes were fabricated on SiO2/Si substrates by ion beam sputtering deposition, to overcome the problems of Cu diffusion and oxidation encountered during the high dielectric constant (κ) materials integration. The Cu and Ta layers remained intact through the annealing in oxygen environment up to 600 °C. The thin oxide layer, formed on the Ti–Al surface, effectively prevented the oxygen penetration toward underneath layers. Complex oxide (BaxSr1−x)TiO3 (BST) thin films were grown on the layered Ti–Al/Cu/Ta electrodes using rf magnetron sputtering. The deposited BST films exhibited relatively high permittivity (150), low dielectric loss (0.007) at zero bias, and low leakage current <2×10−8 A/cm2 at 100 kV/cm.

Microstructures and dielectric properties of compositionally graded (Ba 1-x Sr x ) TiO 3 thin films prepared by pulsed laser deposition

Compositionally graded (Ba1-xSrx)TiO3 (BST) (x:0.0∼0.25) thin films were grown on Pt (111)/TiO2/SiO2/Si (100) substrates using layer-by-layer pulsed laser deposition in the temperature range 550–650 °C. Both downgraded (Ba/Sr ratio varying from 100/0 at the bottom surface to 75/25 at the top surface) and upgraded (Ba/Sr ratio varying from 75/25 at the bottom surface to 100/0 at the top surface) BST films were prepared. Their microstructures were systematically studied by X-ray diffractometry and scanning electron microscopy. A grain morphology transition from large ‘rosettes’ (>0.30 μm) to small compact grains (70–110 nm) was observed in the downgraded BST films as the deposition temperature was increased from 550 to 650 °C. No such grain morphology transition was detected in the upgraded BST films. Dielectric measurements with metal electrodes revealed an enhanced dielectric behavior in the downgraded films. This enhancement is mainly attributed to the large compressive stress field built up near the interface between the downgraded film and substrate. Furthermore, the BaTiO3 layer in the downgraded BST films not only serves as a bottom layer but also as an excellent seeding layer for enhancing the crystallization of the subsequent film layers in the downgraded films.

Low Temperature Dielectric Properties of BST/ZrO2 Multilayer Films

ABSTRACTThin films of Ba1-x SrxTiO3 (BST) are being actively investigated for applications in dynamic random access memories (DRAM) because of their properties such as high dielectric constant, low leakage current, and low fatigue. Several approaches have been used to improve the properties of thin films such as doping with aliovalent dopants, graded compositions, and layered structures. We have found that interposing layers of an electronic insulator such as ZrO2 in between BST layers results in a significant reduction in the leakage current. In this paper the low temperature electrical properties of these multilayer structures are reported. The structures consist of alternate layers of Ba0.8Sr0.2TiO3 and ZrO2 deposited by a sol-gel process on platinized Si substrates. The thickness and the number of layers are varied while keeping the total thickness of the film constant. Multiple peaks in the dielectric constant vs temperature plots at all frequencies are observed in the multilayered films. The properties of the multilayer films are a complex function of the number of layers and their thicknesses. A structure with several thin layers of ZrO2 interposed between the BST layers produces smoother plots than a single layer of ZrO2 of same total thickness. This is attributed to more uniform distribution, as determined by XPS, of ZrO2 in the multilayer structure due to smaller diffusion distances.

Preparation and Characterization of Ba1-xSrxTiO3 Based Thin Films for Pyroelectric Applications

ABSTRACTThe use of pyroelectric thin films in uncooled IR detectors has many advantages over the present IR detector technology, which requires extensive cooling for operation. These advantages include reduced weight, reduced footprint, reduced complexity, increased reliability, and decreased maintenance. Ba1-xSrxTiO3 (BST), based thin films are ideal candidates for use in these devices due to their tailorable materials properties. These properties include a high dielectric constant, low dielectric loss, high electrical resistivity, as well as a high pyroelectric constant. BST thin films were doped with Mg in varying amounts, from 0 to 20mol%. The thin films were deposited via metalorganic solution deposition (MOSD) on Pt/Ti/SiO2/Si substrates. Annealing temperatures ranged from 500 to 750 °C. The films were characterized for structural, microstructural, compositional, surface morphological, dielectric and insulating properties. Glancing angle x-ray diffraction (GAXRD) was used to determine crystallinity, phase formation and film orientation. Field emission scanning electron microscopy (FESEM) and cross sectional transmission electron microscopy (TEM) were employed to access surface morphology and microstructure. The Materials Detectivity Figure of Merit (FOM), D*, [D*=pi/(CV(ε0εrtanδ)1/2] was used to evaluate the film's detectivity response. Our results show that undoped Ba60Sr40TiO3, with a value D*= 0.08 (cm3/J)1/2, appears to out perform Pb based pyroelectric thin films, thus making it a viable candidate for IR pyroelectric detector applications.

Low Temperature Deposition of Ba0.96Ca0.04Ti0.84Zr0.16O3 Thin Films on Pt Electrodes by RF Magnetron Sputtering

ABSTRACTBa0.96Ca0.04Ti0.84Zr0.16O3 (BCTZ) films acceptor-doped with 0.8 at. % Sc were deposited on Pt/TiO2/SiO2/Si substrates using rf magnetron sputtering. Substrate temperatures throughout the fabrication process remained at or below 450°C, which allows this process to be compatible with many materials commonly used in IC manufacturing. In addition, this process made no use of oxygen in the sputter gas or in annealing atmospheres and thus it remains compatible with easily oxidized materials. A relative dielectric constant of 166 was achieved along with a loss tangent of 0.006 to 0.17 at 10 kHz. The tunability of the dielectric constant was greater than 50 %. Leakage current densities of 1.6×10-8 A/cm2 were observed at 300K with 300 kV/cm of applied electric field. In comparison, Ba1-xSrxTiO3 (BST) films prepared under similar conditions show much greater leakage.

Origin of antiphase domain boundaries and their effect on the dielectric constant of Ba0.5Sr0.5TiO3 films grown on MgO substrates

Epitaxial Ba1−xSrxTiO3 (BST) with x=0.5 films were grown on MgO substrates using pulsed-laser deposition. We have observed a high density of antiphase domain boundaries (ADB) in these BST films. We attribute the formation of the ADBs to the different crystal symmetry of the film and the substrate. Adjacent domains have an in plane phase shift of 12[110], or 12[11̄0] thus creating a phase shift of the in plane lattice planes of 12[010] or 12[100] across the boundary. We have used first-principles calculations to obtain the effect of the ADBs on the dielectric constant of SrTiO3 and found that they lower the effective in plane dielectric constant in the direction normal to the ADB. Upon annealing, the density of ADBs decreases and the dielectric properties improve.

Characterization of Mn-Doped Ba1-xSrxTiO3 ThinFilms Prepared by the Sol-Gel Method

Undoped and Mn-doped Ba1-xSrxTiO3 (BST) thin films havebeen fabricated on Pt/Ti/SiO2/Si by an aqueous acetatesol-gel method. The BST stock solution can be easily mixed with anaqueous metal ion solution and is stable at room temperature. Theannealing temperature of the doped and undoped films is between650-750°C. The x-ray photoelectron spectraresults show that the Mn 2p3/2 valence state in the BST is the same asthat of the original Mn(II) dopant. The dielectric constant of the BST thinfilms can be increased to 800, and the loss tangent can be decreased to0.01 due to the Mn(II) doping. The leakage current of the BST films canalso be greatly reduced.

Evaluation of Ta2O5 as a buffer layer film for integration of microwave tunable Ba1−xSrxTiO3 based thin films with silicon substrates

Successful integration of Ba1−xSrxTiO3 (BST) based thin films with affordable Si substrates has a potential significant commercial impact as the demand for high-frequency tunable devices intensifies. This work evaluates the material properties of postdeposition annealed Ta2O5 thin films fabricated via the metalorganic solution technique for use as a passive buffer layer between BST and Si. The microstructure, surface morphology, and interfacial properties of the Ta2O5 film deposited on n+-Si and PtSi substrates was evaluated via field-emission scanning electron microscopy, Rutherford backscattering spectrometry, Auger electron spectroscopy, and atomic force microscopy. The presence of a transition interaction zone between the Ta2O5 film and Si substrate was detected after annealing at temperatures >600 °C. The width of the interaction zone increased with increasing annealing temperature reaching a maximum of 29 nm at 750 °C. The width of the interaction zone was also found to be dependent on the Ta2O5 film microstructure. The Ta2O5 film was typified by a smooth; fine grain, crack/pinhole free surface morphology. The presence of the interfacial layer had no appreciable effect on the microstructure of the film or surface morphology. The narrowness of the interaction zone with respect to the overall film thickness combined with the fact that the interfacial layer did not seriously deteriorate the dielectric properties of the Ta2O5 film suggests Ta2O5 to be a favorable candidate buffer layer enabling the integration of BST with Si substrates.

Thin Pt layer insertion into the Ru bottom electrode: effects on the surface morphology of a (Ba, Sr)TiO3 dielectric film and on the performance of the TiN barrier in the pt/Ru/TiN/p-Si/Si heterostructure

We have investigated the effects of inserting a thin Pt layer into the bottom electrode heterostructure and of post-treatment by rapid thermal oxidation (RTO) on the surface morphology of Ba1−xSrxTiO3 (BST) films and on the thermal stability of the TiN barrier under oxygen exposure in a Pt/Ru/TiN/Si structure. In the case of the Ru/TiN/p-Si/Si structure without a Pt thin layer, the surface morphology of the Ru layer was rough due to heterogeneous grain growth formation, which resulted in the degradation of the BST/Ru interface. When a thin Pt layer was inserted on top of the Ru layer, the number of faceted protrusions observed for the Ru layer was either largely decreased for the Pt/Ru/TiN/p-Si/Si structure or unobserved for the BST/Pt/Ru/TiN/p-Si/Si structure. For the BST/Pt/Ru/TiN/p-Si/Si structure, the oxygen in-diffusion can be reduced by the BST/Pt layers during RTO in the two-step annealing process, resulting in the observed smooth surface morphology of the BST/Pt layer. The thermal stability of the TiN barrier for the Pt/Ru/TiN/poly-Si/Si layer structure was found to be up to 650 °C. This was attributed to the role of the thin oxidized RuOx layer formed at the surface of the Ru film and the thin Pt layer. Consequently, the surface morphology of the BST film and the performance of the TiN barrier can be simultaneously improved by the introduction of a thin Pt layer and by post-treatment for a Ru/TiN/p-Si/Si structure.

Epitaxial growth and dielectric properties of functionally graded (Ba1−xSrx)TiO3 thin films with stoichimetric variation

Functionally graded (Ba1−xSrx)TiO3 (BST) thin films with stoichimetric variation (x:0.0–0.25) were layer-by-layer grown epitaxially on MgO (100) single-crystal substrates with 100-nm-thick conductive La0.5Sr0.5CoO3 (LSCO) as the bottom electrode by pulsed-laser deposition. X-ray diffraction, rocking curve, and φ scans showed that the graded films are epitaxial grown with an orientation of [001](100)BST//[001](100)MgO. The surface roughness of the up-graded film was larger than that of the down-graded films, and the full width at half maximum of the BST (200) rocking curve of up-graded films was wider than that of the down-graded films. The compositional gradations along the depth in the films were confirmed by Rutherfold backscattering spectroscopy. Dielectric properties measured by vertical structures using LSCO as the bottom electrodes showed that the dielectric constant and dielectric loss at 10 kHz were 532 and 0.010 for the up-graded films, and 715 and 0.0103 for the down-graded films, respectively. An enhanced dielectric behavior was observed in the down-graded films. The graded BST films also exhibit a broad and flat profile of the dielectric constant versus the temperature. Such behavior of the dielectric response is attributed to the presence of the compositional and/or residual strain gradients in the epitaxial graded films. The possible origins of the enhanced dielectric behavior observed in the down-graded films are also briefly discussed.

Fabrication and Characterization of BaxSr1-xTiO3/YBa2Cu3Ox/SrTiO3 Structure

BaxSr1-xTiO3 (BST)/YBa2Cu3Ox (YBCO)/SrTiO3 (STO) structures were deposited, and the microstructure, orientation and electrical characteristics were investigated. (00l) oriented YBCO thin films were deposited on STO substrates using pulsed laser deposition, and (h00) oriented BST thin films were deposited on YBCO/STO substrates using electron cyclotron resonance (ECR) plasma assisted metal organic chemical vapor deposition (MOCVD). A new phase was formed at the interface between YBCO and BST films and was speculated to be (BaxY1-x)(TiyY1-y)O3. Ba-rich BST films showed a higher dielectric loss than Sr-rich BST films did, which indicates that Sr-rich BST films are more suited for application to microwave devices. The dielectric loss of the films was reduced as temperature decreased, which may be due to the conductivity change of YBCO film and the formation of a conduction path rather than a dielectric property change of the BST film itself. The capacitance vs voltage and the dielectric loss vs bias voltage curves of BST film showed a polarity dependence, which may be speculated to be because of the barrier height difference between the interfaces.

Simple chemical route for the quantitative precipitation of barium–strontium titanyl oxalate precursor leading to Ba1−xSrxTiO3 powders

Barium–strontium titanyl oxalate (BSTO) [Ba1−xSrxTiO(C2O4)2·4H2O with x=0.25 precursor powders] were prepared successfully by using the simple chemical exchange reaction between oxalotitanic acid (HTO) [H2TiO(C2O4)2·2H2O], barium hydroxide and strontium nitrate at room temperature. Initially, oxalotitanic acid was prepared by using the reaction between 0.1 M titanium tetrabutoxide and 0.2 M oxalic acid in isopropanol (IPA). Barium hydroxide and strontium nitrate were added directly to this solution in stoichiometric amounts. The required solubility achieved only after addition of distilled water dropwise leads to the controlled precipitation of BSTO molecular precursor. To maintain the equilibrium (pH conditions), more and more Ba(OH)2 and Sr(NO3)2 solids get dissolved and reaction continues. The kinetics of exchange reaction is governed by the rate of addition of water. The pyrolysis of BSTO at 750 °C/4 h in air produced the Ba1−xSrxTiO3 (BST) powders. The characterization studies were carried on the as-dried and calcined powders by various physicochemical techniques viz. microanalysis, chemical analysis, DTA/TGA, XRD, IR, scanning electron microscopy (SEM), etc. It revealed that the BST powders are cubic, stoichiometric, highly pure with yield>99% and agglomerated nature (size: 0.5–2 μm).

Characterization of quality of BaxSr1−xTiO3 thin film by the commutation quality factor measured at microwaves

The loss factor (tan δ) of ferroelectric material at microwaves and the tunability (n) defined as a ratio of the dielectric permittivity at zero dc field to the dielectric permittivity at the given field are strongly connected with the softness of so-called ferroelectric soft mode. The loss factor and the tunability are generalized by the commutation quality factor (CQF). The dielectric response of a ferroelectric material allows one to determine the CQF and to compare it to the theoretical CQF obtained for a perfect ferroelectric sample. In order to compare the theoretical prediction and experimental data, the influence of deposition parameters on microwave properties of BaxSr1−xTiO3 (BSTO) films deposited on sapphire and alumina substrate has been investigated in a wide frequency range (1–30 GHz). The highest CQF at room temperature was obtained for the 30% Ba content target and the deposition temperature 905 °C. The best BSTO films exhibited CQF≅1.6×104 with n=2.1 and tan δ=0.012 at 1 GHz and zero bias. This result is among the best reported for sputtered BSTO films both on sapphire and alumina substrates. Nevertheless the obtained CQF is 3–5 times lower than the upper theoretical limit.

Preparation and characterization of Ba1−xSrxTiO3 thin films for uncooled infrared focal plane arrays

(Ba1−xSrx)TiO3 (BST) thin films with different compositions have been prepared on Pt/Ti/SiO2/Si substrates by an improved sol–gel technique. DTA–TGA, XRD and SEM are employed to analyze the thermolysis process of BST gel. The dielectric and ferroelectric properties of BST thin films were measured. The results show that the films exhibit pure perovskite phase through rapid thermal anneal (RTA) above 650 °C and their grain size is about 50–100 nm. The dielectric constants of BT, BST5, BST10 and BST15 are 320, 375, 400 and 425, respectively, at 25 °C and 10 kHz. Their loss tangents are 0.035, 0.041, 0.024 and 0.01, respectively. The remnant polarization, Pr, and coercive field, Ec, of BST10 thin film are 2.5 μC cm−2 and 50 kV cm−1, respectively, and the maximal pyroelectric coefficient reaches 12×10−8 C cm−2 K−1 at 35 °C. BST15 thin film is the advanced candidate material for uncooled infrared focal plane arrays (UFPAs) applied at near ambient temperature.

Structural features and phase transition temperature of BaxSr1-xTiO3 films grownon various substrates

Ferroelectric BaxSr1−xTiO3 (BSTO) films were grown on various substrates (α-Al2O3, LaAlO3,MgO and polycrystalline alumina) by RF sputtering of a BSTO targetwith Ba/Sr ratio x in the rangefrom 0.50 to 0.65. The results of comprehensive structuraldiagnostics of the films are correlated with the temperatureTmcorresponding to the maximum capacitance of a planar BSTO varactor, measuredat a frequency of 1 MHz. BSTO films of the same cationic composition,grown on various single-crystal substrates, have markedly differentTmcorrelated with the structural ordering of ferroelectric films. Highlyoriented BaSrTiO3 films approximately 300 nm thick, grown on MgO andLaAlO3 substrates, are characterized by low tan δ ≤ 8 × 10−4and Tm = 140− 230 K. However, textured andpolycrystalline BSTO films of the same composition, grown on sapphireand alumina substrates, are less strained, having tanδ ≤ 10−3, andTm ≥250K.