STRUCTURAL AND ELECTRICAL PROPERTIES OF (BaxSr1-x)TiO3 THIN FILMS PREPARED BY A SOL-GEL METHOD

ABSTRACT

Ba_xSr_1-xTiO₃(BST) thin films with x = 0.5, 0.6, 0.7 and 0.8 were fabricated on p-type Si(100) substrates by a sol-gel spin coating method. All samples show the perovskite polycrystalline structures without the pyrochlore phase and have dense surface texture and crack-free uniform microstructures. The C-V characteristics of the BST thin film/p-type Si(100) capacitors exhibit clockwise hysteresis loops and also clearly show regions of accumulation, depletion and inversion corresponding to those of metal-ferroelectric-semiconductor structures. Furthermore, a constant memory window has been observed through the entire frequency ranges from 1 kHz to 10 MHz.     

High quality BaxSr1-xTiO3 films grown by mocld and novel ferroelectric/ferrite structures for dual-tuning microwave devices

Abstract

Excellent single crystal Ba_xSr_1-xTiO₃ (BST) films were grown on LaAlO₃ substrates using the metal-organic chemical liquid deposition (MOCLD) method. Very low losses (tanδ ∽0.002-0.008) were measured from these films at 400 KHz. Biaxially oriented BST films were successfully grown on polycrystalline YIG substrates using both MOCLD and pulsed laser deposition methods with biaxially oriented MgO and YSZ buffer layers. The dielectric losses of the films range from 0.005 to 0.015 while 25% of dielectric constant change was observed with 40V bias voltage up to 10 MHz. Both the dissipation and dielectric constant of the films remained nearly constants over a wide temperature range (77 K to 380 K). A dual-tuning microwave coplanar phase shifter using a BST film grown on a MgO buffered polycrystalline YIG substrate was fabricated. A significant phase shift was observed in GHz frequency range when an electric bias or a magnetic field was applied to the device.     

YBa2Cu3O7-x/SrTiO3//LaAlO3 heterostructures obtained by injection MOCVD

Abstract

YBa₂Cu₃O_7-x /Ba_xSr_1-xTiO₃ /LaAlO₃ heterostructures can be used as a basis for devices with voltage control in microwave circuits.

Ba_xSr_1-xTiO₃ (x=0–0.1) (BST) thin films have been epitaxially grown on LaAlO₃ substrates using injection MOCVD. The excellent crystalline quality of the obtained BST films can be proven by a FWHM of <0,2° for the rocking curve of the (002) BST reflection. An AFM study revealed flat surfaces, showing a surface roughness R_s as low as 1nm. YBa₂Cu₃O_7-x/Ba_xSr_1-x TiO₃//LaAlO₃ heterostructures were than optimised. The YBa₂Cu₃O_7-x (YBCO) layers obtained on Ba_xSr_1-xTiO₃ films are epitaxial with a FWHM of 0.45° for the (005) YBCO rocking curve and display very promising superconducting properties of T_c=92K.

Finally the microwave properties of the superconducting films were studied. For YBa₂Cu₃O_7-x layers directly deposited on LaAlO₃, surface resistance values of 0,32mΩ were obtained, while for YBa₂Cu₃O_7-x /SrTiO₃//LaAlO₃ heterostructures, higher values of 1mΩ at 8.5GHz were measured.     

The electrical properties of Ba1-x SrxTiO3 thin films grown by sputter deposition

Abstract

Ba_1-x Sr_xTiO₃ (BST) thin films were deposited by reactive rf-magnetron sputtering onto Si substrates. The influence of the deposition parameters such as temperature and oxygen ambient on the dielectric constant of the films is presented. BST films deposited at 450°C and optimum conditions exhibited a dielectric constant of approximately 200 at frequencies as high as 1GHz. In addition, the films were found to have leakage current densities of <0.1μAmp/cm² at fields of 5×10⁵V/cm. An extrapolated lifetime greater than 10 years was obtained from stress tests at elevated temperatures and fields. These films compared favorably with published data.     

Effect of Ba2+ – Sr2+ co-substitution on the structural and dielectric properties of Lead Titanate

Growing interest in developing new materials for device applications led to study of ferroelectric oxides in a wide range and variety of compositions. In the present work, polycrystalline samples of lead barium strontium titanate (Pb_1-xBa_0.5xSr_0.5xTiO₃) solid solution system have been synthesized. Phase formation studies and crystal structure analysis were carried out by X ray diffractometry at room temperature, which suggested formation of single phase compound with tetragonal structure up to x?=?0.8 and cubic structure for x?=?1.0. The XRD pattern has been analyzed by employing Rietveld method. The phase transition in the system was confirmed by Differential Scanning Calorimetry (DSC). Samples with 0.0????x????0.8 are in ferroelectric state whereas with x?=?1.0 is found to be in paraelectric state at room temperature. Co-substitution of Ba²⁺ and Sr²⁺ into lead titanate shows reduction in anisotropy as well as porosity. The dielectric studies of the system as a function of temperature and frequency were carried out in the range 323?K to 773?K and 100?Hz to 1?MHz respectively. Variation of dielectric constant and loss tangent with temperature shows peaking effect near Curie temperature. Frequency dependant dielectric studies clearly show that the dielectric constant and loss tangent decrease exponentially with increased frequency.     

Advanced model of metal-organic chemical vapor Deposition of BaxSr1-xTiO3 oxides

Abstract

An advanced model of Ba_xSr_1-xTiO₃ Chemical Vapor Deposition is proposed. The model implies that metal-organic precursor decomposition occurs primarily at the reactor surfaces. Deposition of various parasitic solid phases (TiO₂, BaO, SrO, etc.) is taken into account. The model predicts deposition rate, solid phase composition and fraction of parasitic phases in the grown film as a function of growth parameters.     

Dielectric and Mechanical Losses in (Ba,Sr)TiO3 Systems

In the application of tuneable microwave devices of ferroelectric (BaSr)TiO₃ systems the two critical parameters needed for optimal device performance are high tunability and low dielectric loss. The dielectric loss of the materials is strongly dependent on microstructure. This paper is concerned with an investigation of the variation in the dielectric and mechanical losses in Ba _x Sr_{1 – x} TiO₃ systems (x = 0.5, 0.6, 0.7 and 1.0) with microstructure (grain sizes from 1 m to 50 m). The magnitude of the loss peak and sharpness of the anomaly in the dielectric constant/elastic modulus observed for the phase transitions in Ba _x Sr_{1 – x} TiO₃, depend not only on the composition and but also on the grain size. A relaxation peak has been observed in large grain material, which is indication of interactions between different configurations of domain walls and the diffusion of oxygen vacancies in the domains.     

Dielectric properties of (Ba1-2x Sr x Ca x )TiO3 ferroelectric ceramics

Ferroelectric ceramics of (Ba_1?2x Sr _x Ca _x )TiO₃ (0?≤?x?≤?0.30) were prepared by a routine solid-state reaction technique. Co-substitution of Sr²⁺ and Ca²⁺ for Ba²⁺ with equal mole in BaTiO₃ restrain the maximal dielectric constant K _m strongly when 0.2?≥?x?>?0. However, composition (Ba_0.4Sr_0.3Ca_0.3)TiO₃ shows the higher maximal dielectric constant. With increasing Sr and Ca content x, temperature T _m of the maximal dielectric constant K _m shifts to low temperature and all selected compositions for study exhibit a broad phase transition temperature range. The composition (Ba_0.4Sr_0.3Ca_0.3)TiO₃ presents characteristics of ferroelectric relaxor, value of K _m decreases and temperature T _m increases with increasing frequency.     

Coplanar Waveguide Using Ferroelectric Thin Oxide Film: Dielectric Constant

Coplanar waveguide (CPW) transmission lines were fabricated on thin ferroelectric Ba_{1 – x}Sr_xTiO₃ films for tunable microwave applications. The growth of the ferroelectric oxide films was accomplished by a pulsed laser deposition with a partial oxygen background. Microwave properties of the CPW phase shifter were measured using a HP 8510C vector network analyzer from 0.045–20 GHz with –40–40 V of dc bias. A large phase shift angle of 120 at 10 GHz was observed from the CPW (gap = 4m, length = 3 mm) with a 40 V of dc bias change. The dielectric constant of the thin ferroelectric film was extracted from the dimension of the CPW (gap, width, length) and the measured S-parameter by a modified conformal mapping. However, the dielectric constant of the ferroelectric thin film exhibits a gap dependency; dielectric constant (990–830) decreases with increasing gap size (4–19 m, respectively). By adjusting the filling factors of the film, a constant dielectric constant of BST film is found to be 810 ± 5.     

Ferroelectric effect in spin coated Ba0.9Sr0.1TiO3 thin films

Abstract

The achievement of excellent growth of lead zirconium titanate (PZT) films by various techniques for use as ferroelectric memories has generated an extensive research interest in the synthesis of various other perovskite and layered oxides. Ba_xSr_1?xTiO₃ thin films have also been deposited by various methods to study their dielectric behavior. We report the synthesis of Ba_xSr_1?xTiO₃ (where x ' 0.9, 0.1) by a solution method using hydroxides, acetates, and nitrate salts as precursors for barium and strontium, and titanium isopropoxide for titanium. The films deposited by spin coating on ITO coated glass substrates showed ferroelectric behavior.     

Deposition and characterization of low-loss epitaxial non-linear dielectric thin films for microwave devices

Abstract

Oxide ferroelectric thin films for frequency-tunable microwave devices, in which the dielectric constant of the non-linear dielectric is varied by application of electric fields, have been deposited using PLD. We have fabricated single phase epitaxial Ba_0.6Sr_0.4TiO₃ and KTaO₃ thin film capacitors for applications at 300K and 77K, respectively. Single phase KTaO₃ films were obtained only with excess potassium source in the target along with KTaO₃ perovskite phase. The films have been characterized for structure and morphology by X-ray diffraction and AFM. The dielectric properties were measured in the low frequency range from 100 kHz to 10 MHz, using interdigitated capacitors. Low loss tangents (0.002 at 300K) were observed for highly oriented Ba_0.6Sr_0.4TiO₃ films. The importance of low losses for various devices is discussed and the dielectric constants, loss tangents and tunability of these films are reported in this paper.     

Evaluating voltage-tunable materials for RF phase shifter technology

Abstract

Today's phased-array antennas use hundreds of radiating elements that use relatively high-loss phase shifters that operate over a limited bandwidth. The number of elements and the phase shifter losses affect the overall cost of the antenna system. Ferroelectric RF phase shifters have the potential to meet the low-loss, low-cost requirements driving many phased-array applications. Some of the issues affecting the development of ferroelectric phase shifters include ferroelectric tunability, dielectric losses, conductor losses, and impedance mismatch. We used the measured tunability (250 kHz, room temperature), dielectric constant, and loss tangent (10 GHz, room temperature) of Ba_1-xSr_x/TiO₃ (0.4x 0.6) with various amounts of MgO additive, 0 to 60 wt.%, to estimate the device performance of microstrip phase shifters. The electromagnetic model of the microstrip (which uses a standard 3-mil-wide 1-oz. copper line, 3-mil-thick BST/MgO composite and the bias criteria of 2 V/μm) has produced performance benchmarks for a number of composites providing 360° of phase shift. While the accuracy of the electromagnetic model used to evaluate these materials has limitations, the results do provide some insight as to which materials may be better suited for 10-GHz phase shift devices.     

Thin film BaxSr1-xTiO3 ku- and k-band phase shifters grown on MgO substrates

Abstract

We report measurements of gold circuits fabricated on four Ba_xSr_1-xTiO₃ ferroelectric films doped with 1% Mn grown on MgO substrates by laser ablation. Low frequency (1 MHz) measurements of σ_T and tanδ on interdigital capacitors are compared with high frequency measurements of phase shift and insertion loss on coupled microstrip phase shifters patterned onto the same films. The variation in temperature of both high and low frequency device parameters is compared. Annealed with amorphous buffer layer and unannealed films are compared. Room temperature figures of merit of phase shift per insertion loss of up to 58.4°/dB at 18 GHz and 400 V dc bias were measured.     

Frequency and Voltage Dependent Dielectric Properties of Ni-doped Ba0.6Sr0.4TiO3 Thin Films

Highly (100) preferred undoped and 1–5% Ni-doped Ba_1–xSr_xTiO₃ (BST) thin films were deposited onto MgO (100) single crystal substrate at 750°C using pulsed laser deposition. BST thin film-based interdigital capacitors (IDC) were prepared by standard photolithography process. The microwave properties of BST films were measured at 10 GHz. Ni-doped BST films showed better dielectric properties by exhibiting improved dielectric Q while retaining an appropriate capacitance tuning compared to undoped BST films. 1% Ni-doped BST film showed the maximum figure of merit of 2896.1. It is suggested that 1 mol% Ni doped BST film is an effective candidate for high performance tunable device applications.     

Phase transition,microstructure and properties of (Na<Subscript>0.5</Subscript>Bi<Subscript>0.5</Subscript>)<Subscript>1-x</Subscript>Ba<Subscript>x</Subscript>TiO<Subscript>3</Subscript> lead-free piezoelectric ceramics

In this article, (Na_0.5Bi_0.5)_1-xBa_xTiO₃ lead-free piezoelectric ceramics were prepared by solid-state reaction. The influence of Ba contents on phase structures, compositional distribution and electrical properties of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ ceramics were systematically investigated to further understand the nature of phase transition. It was found that the phase structure of (Na_0.5Bi_0.5)_1-xBa_xTiO₃ transforms from rhombohedral to tetragonal symmetry at x = 0.06 ~ 0.07 and Ba²⁺ segregation forms the coexistence of Ba-rich tetragonal and Ba-deficient rhombohedral phases close to MPB. The electrical properties of prepared samples regularly changed with Ba content, which is closely related to the distribution of rhombohedral and tetragonal phases. The prepared sample near MPB exhibited the largest dielectric constant and the excellent piezoelectric properties (the maximal measuring field reached 78 kV/cm and the piezoelectric constant d ₃₃ = 151pC/N).     

Diffuse phase transition behavior of dysprosium doped barium titanate ceramic

The effect of dysprosium (Dy) doping on the structural and dielectric properties of barium titanate (BaTiO₃) ceramic has been studied. Dysprosium-doped barium titanate (BT) with general formula Ba_1-xDy_2x/3TiO₃ (x?=?0.0–0.1) is prepared through solid state reaction route. The XRD pattern reports the single phase tetragonal structure up to x?=?0.025 with space group of P4mm and a secondary phase is observed for higher concentration of Dy. The Raman study of Ba_1-xDy_2x/3TiO₃ (x?=?0.00, 0.01, 0.025) also reveals that the ceramic is showing tetragonal symmetry and with an increase in Dy doping the tetragonal phase is moving toward more symmetric phase i.e. cubic pm3m. The surface morphology of the sintered pellets shows a compact and homogeneous grain distribution with a decrease in grain size as the Dy content increases. The temperature and frequency dependency dielectric study of the ceramic compositions (x?=?0.00, 0.01, 0.025) are studied to show the effect of Dy ion on the A site of BaTiO₃ perovskite ceramic. The dielectric constant decreases with increase in Dy content. The dielectric diffusivity increases with increase in doping concentration. The P-E hysteresis loop confirms the ferroelectric behavior of the sample with decrease in remanent polarization and increase in coercive field.     

Microstructure and dielectric properties of ferroelectric barium strontium titanate ceramics prepared by hydrothermal method

Ba _x Sr_1-x TiO₃, nanoparticles with different Ba compositions were synthesized by a hydrothermal method. The mechanism of hydrothermal reactions was discussed based on DTA/TG, XRD and TEM characterizations. The result showed that perovskite structure was developed through the mutual diffusion between the intermediate phases and TiO₂ phase. The grain size of the Ba_0.77Sr_0.23TiO₃ (BST77) powders was about 20–40 nm. BST ceramics were made from the hydrothermal-derived BST powders and the dielectric properties of the BST ceramics were measured. Due to the small grain size and active surface energy of the BST powders prepared by hydrothermal method, the BST ceramics showed low sintering temperature. It was found that the BST77 ceramics sintered at 1280 °C showed dielectric constant peak dispersion which was believed to be caused by dimension domino effect.     

Synthesis of (Ba0.5Sr0.5)(Ti1 − x Zr x )O3 ceramics: Effect of Zr content on room temperature electrical properties

The phase formation behavior and room temperature dielectric properties of bulk perovskite solid solution composition (Ba_0.5Sr_0.5)(Ti_1 − x Zr _x )O₃ have been investigated. The samples with different Zr-content were prepared through solid state reaction. The XRD investigation showed that Zr⁺⁴ is systematically dissolved in Ba_0.5Sr_0.5TiO₃ lattice up to about 60 atm.% substitution, having cubic Pm3m structure. Eighty atom percent Zr substituted composition showed to contain a cubic phase similar to that of x = 0.6 composition and a tetragonal (I4/mcm) phase. That is the solid solution breaks around at 80 atm.% Zr substitutions. Ba_0.5Sr_0.5ZrO₃ was having orthorhombic Imma structure. Decrease in grain sizes were observed with increase in Zr content. The permittivity of the ceramics decreased with the increase in Zr substitution. The frequency dependency of dielectric loss in the frequency range 10 Hz to 10 MHz, were improved with Zr substitution in the ceramics. The room temperature ac and dc conductivity also decreased significantly with the increase in Zr-content.     

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

Epitaxial (111) oriented ferroelectric (Ba_{1 ? x}Sr_x)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.     

Substrate effect on in-plane ferroelectric and dielectric properties of Ba0.7Sr0.3TiO3 thin films

Heteroepitaxial Ba_0.7Sr_0.3TiO₃ thin films were grown on (LaAlO₃)_0.3(Sr₂AlTaO₆)_0.35 (001) (LSAT) and SrTiO₃ (001) (STO) single crystal substrates using pulsed laser deposition (PLD). X-ray diffraction characterization revealed a good crystallinity and a pure perovskite structure for films grown on both LSAT and STO substrates. The in-plane ferroelectric and dielectric properties of the films were studied using interdigital electrodes (IDE). The film grown on LSAT substrate exhibited an enhanced in-plane ferroelectricity, including a well-defined P-E hysteresis loop with the remnant polarization P _r = 10.5 μC/cm² and a butterfly-shaped C-V curve. Nevertheless, only a slim hysteresis loop was observed in the film grown on STO substrate. Curie temperature T _c of the film grown on LSAT substrate was found to be ∼105^∘C, which is nearly 70^∘C higher than that of the bulk Ba_0.7Sr_0.3TiO₃ ceramics. T _c of the film grown on STO substrate has almost no change compared to the bulk Ba_0.7Sr_0.3TiO₃ ceramics. The dielectric tunabilities were found to be 64% and 52% at 1 MHz for the films grown on LSAT and STO substrates, respectively.