BaxSr1-xTiO3 Research Articles

Specific heat of the Ba0.7Sr0.3Ti1−yZryO (y = 0, 0.03, 0.05, 0.1) ferroelectric ceramics obtained by the temperature relaxation method

In the last few years great interest has developed around Ba1–xSrxTiO3 (BST) or BST-based materials due to their great potential for technological applications such as integrated capacitors, ferroelectric memory and many others [1–3]. Thermal properties of materials used in the design and construction of electronic devices must be taken into account since heat will be unavoidably generated in a working electronic circuit. The behavior of the devices as their temperature change and their ability to store and dissipate heat will affect their performance and lifetime therefore their thermal properties cannot be neglected. Despite the extended use of the BST ceramic system, knowledge of the thermal properties is still scarce

DEPOSITION OF NANOCRYSTALLINE AND MICROCRYSTALLINE BaXSr1-XTiO3 BY MEANS OF PULSE MODULATED LOW PRESSURE PLASMA JET SYSTEM

ABSTRACT RF plasma jet system was used for deposition of BaxSr1-xTiO3 (BSTO) and SrTiO3 (STO) thin films. Optical emission spectroscopy was used for control of concentration of particles sputtered from the hollow cathode. Relation between ratio of spectral intensity of Ba, Ba+, Sr and Sr+ lines and ratio of Ba and Sr concentration in the deposited film was found. Deposited thin films were analyzed by XRD, which confirmed presence of BSTO perovskite phase in the films. All prepared films with high enough Ba concentration (0.5 ≤ x ≤ 0.9) revealed good shaped dielectric hysteresis loops typical for ferroelectric phase.

VOLTAGE-CONTROLLED OSCILLATOR DESIGN USING FERROELECTRIC VARACTORS

ABSTRACT Ferroelectric capacitors made from Ba1−xSrxTiO3 (BST) are applied as varactors in tunable, high frequency circuit applications. In this context, a voltage-controlled oscillator (VCO) is studied. VCOs are integral parts of phase locked loops, clock recovery circuits and frequency synthesizers and thus are widely used in both discrete and on-chip applications. Ferroelectric varactors are capable of out-performing solid-state varactors at microwave frequencies, due to their lower parasitic series resistance that leads to a higher Q-factor. In VCO implementations, this higher Q leads in turn to lower phase noise, a critical quality for these circuits. A VCO design is presented and the performance of ferroelectric varactors is compared with solid-state varactors.

Preparation and Electrical Properties of (Ca, Mn)-codoped Ba1xSrxTiO3 Solid Solution

(Ca, Mn)-codoped Ba1xSrxTiO3 (BST) solid solution, i.e. Ba0.4Sr0.5Ca0.08Mn0.02TiO3, was successfully synthesized via sol-gel process. The product was characterized by TG-DTA thermal analyses, XRD, TEM. The effects of both curing temperature and applied voltage on the resistance, etc. were investigated. It showed that the structure of BST was unable to be changed even though it was codoped by Ca2+ and Mn2+. TEM photographs demonstrated the influence of curing temperature on the morphologies of the tested samples. The dependence of resistance on the curing temperature illustrated that (Ca, Mn)-codoped BST was capable of exhibiting the positive temperature coefficient (PTC) effect at low frequency; nevertheless, it might display the negative temperature coefficient (NTC) effect at other frequencies. The relationships between the resistance and applied voltage revealed that the resistance property of (Ca, Mn)-codoped BST appeared at low frequencies, but its property of capacitive reactance might present at high frequency. These findings mean that the electrical property of BST can be adjusted by codoping method. These phenomena can be explained by the grain boundary resistance effect, Heywang model and ion polarization theory.

Structural and optical properties of BaxSr1−xTiO3 thin films on indium tin oxide/quartz substrates prepared by radio-frequency magnetron sputtering

Ba x Sr 1 − x Ti O 3 (x=0.6 and 0.8) thin films have been prepared on indium-doped tin oxide (ITO) coated quartz substrates using radio-frequency magnetron sputtering. Their structural properties and surface morphologies were examined by x-ray diffraction and atomic force microscopy, respectively. The BaxSr1−xTiO3 (BST) thin films with x=0.6 and 0.8 annealed at 650°C for 20min exhibit good surface morphology and well-crystallized perovskite structure. High quality BST ferroelectric thin films were further investigated by electrical measurements, showing the remnant polarization (Pr) of 6.75μC∕cm2 and the coercive field (Ec) of 43.2kV∕cm. Optical transmittance measurement indicated that the Ba concentration has an effect on the band gap energy (Eg) structure of the BaxSr1−xTiO3 thin films. The Eg decreases linearly with the increase of the Ba content. The refractive index (n) and extinction coefficient (k) of the BST films with x=0.6 and 0.8 were obtained by fitting the spectroscopic ellipsometric data using a parametric dielectric function model. The refractive index (n) and extinction coefficient (k) increase with increasing wavelength. All of the results show that the BST/ITO/quartz heterostructure is promising for applications in integrated optical waveguide devices.

Low Pressure RF Plasma Jet Sputtering Technique Applied to Ferroelectric Films: Ba<sub>1-x</sub> Sr<sub>x</sub>TiO<sub>3</sub>

Technology aspects and characterization of BaxSr1-xTiO3 (BST) films fabricated with low pressure plasma jet technique are presented. BST films were deposited on silicon coated with Pt/TiO2/SiO2 and on bare Si substrates. The nozzles-type RF hollow cathode has been fabricated from hot pressed BaTiO3, SrTiO3, and BST ceramics. Controlling of RF voltage, RF current and substrate temperature allowed us to deposit reproducible films with controlled grain size. Hysteresis loops, ellipsometric and micro-Raman investigation results are presented and discussed.

Optical properties of amorphous Ba0.7Sr0.3TiO3 thin films obtained by metal organic decomposition technique

Barium strontium titanate (BaxSr1−x)TiO3 (BST) thin film deposition techniques are the subjects of many research studies mainly due to their influence on the optical and electrical properties of this material, which are of increasing interest for the processing of optoelectronic integrated circuits. In this study, the amorphous Ba0.7Sr0.3TiO3 (BST0.7) thin films were grown onto fused quartz and silicon substrates at low temperature by using a metal organic decomposition (MOD)-spin-coating procedure from barium 2-caprylate Ba(C8H15O2)2 and 3-methylbutyl acetate CH3COOC2H4CH(CH3)2-based special precursors. The optical constants of amorphous BST0.7 thin films including refractive index, extinction coefficient and optical band gap energies were presented. The calculated extinction coefficient of 214-nm-thick amorphous BST0.7 thin films in visible and near-infrared region was in the order of 10−3, which is much lower than that of polycrystalline BST thin films. The optical band gap energy and refractive index n are estimated to be about 4.27 eV and n=1.94, respectively. The optical propagation loss of the groove optical waveguide with 10 μm width and 185-nm-thick amorphous BST0.7 films was 4.11 dB/cm at a wavelength of 632.8 nm.

Structural and luminescence properties of Eu3+ doped BaxSr1−xTiO3 (BST) nanocrystalline powders prepared by different methods

Abstract Eu3+ doped BaxSr1−xTiO3 (BST) nanocrystalline powders have been prepared using molten salts, sol–gel and hydrothermal techniques in order to compare their structural and luminescence properties. The sizes of the particles are around 50 nm for the BST powders obtained by the sol–gel and hydrothermal methods while they are around 400 nm for the samples prepared by the molten salts method. The Eu3+ doped BST samples show strong emission in the visible range. The luminescence spectra of the Eu3+ ions for the investigated samples depend strongly upon the preparation method. The luminescence bands are affected by a notable amount of inhomogeneous broadening. The broadening of the emission bands and the non-exponential behavior of the emission decay curves point to the presence of significant structural disorder.

Chemical Solution Deposition of (Ba1-xSrx) TiO3 Thin Films and Characterization

A simple system has been developed for the preparation of stable precursor solutions for dip coating (Ba1−xSrx)TiO3 thin films. From such stable solutions highly crystalline, uniform and crack-free thin films are obtained by dip coating, followed by annealing at temperatures as low as 600°C. The pyrolysis behavior, crystallization mechanism, and thin film morphology have been studied using thermogravimetric and differential thermal analysis, X-ray diffraction, and scanning electron microscopy. The dielectric tunability characteristics have been determined for thin films of composition (Ba1−xSrx)TiO3, x=0.30, to study their suitability for tunable device applications.

Electrical Characteristics of BaxSr1-xTiO3(BST) Capacitors Implemented with Ti-Al Electrodes

AbstractTi-Al was used as the electrode for RF magnetron sputtered BST film at a substrate temperature of 450°C for decoupling capacitor applications. X-ray diffraction analysis shows that the deposited film BST film is amorphous. Electrical characterizations of the devices performed by capacitance versus voltage measurements show a dielectric constant of about 55. With increase in annealing temperature above 450°C, the capacitance was found to decrease significantly.

A comparative microstructural study of compositionally up- and down-graded (Ba,Sr)TiO3 thin films epitaxially grown on (La,Sr)CoO3-covered MgO(100) substrates by pulsed laser deposition

We report a comparative study of the microstructure of compositionally graded (Ba1-xSrx)TiO3 (BST) films with two compositionally graded directions, up and down, with respect to the substrate, which were deposited epitaxially on (La,Sr)CoO3 (LSCO)-covered MgO(100) single-crystal substrates by pulsed laser deposition. Cross-sectional transmission electron microscopy (TEM) images and electron diffraction show that the graded films grow epitaxially with their (100) plane parallel to the (100) surface of the MgO single-crystal substrate, and with an in-plane orientation relationship of 〈001〉BST//〈001〉LSCO//〈001〉MgO. The crystalline quality and surface morphology of the graded films are closely related to the direction of the compositional gradient built into the graded films. Down-graded films (starting with a BaTiO3 layer at the film/substrate interface) have a much better crystalline quality and a smoother surface than the up-graded films (starting with a (Ba0.75Sr0.25)TiO3 layer at the film/substrate interface). Obviously, the BaTiO3 bottom layer in the down-graded film acts not only as a part of the graded film but also as an excellent seeding layer to enhance the crystallization of the subsequent film layers, resulting in a high crystalline quality of the down-graded film and an enhanced dielectric behavior. Planar (high-resolution) TEM images also demonstrate that down-graded films have a larger, and more uniform, grain size than up-graded films, and that the latter contain voids.

Stress-induced domain structure formation in nanodimensional barium strontium titanate films

Thin heteroepitaxial films of BaxSr1−x TiO3 (BST) with x = 0.80 possessing high structural perfection have been grown by RF magnetron sputtering on MgO(001) substrates. The microstructure and crystallographic characteristics of the BST films of various thicknesses have been studied using electron microscopy and X-ray diffraction. It is established that there is a threshold thickness at which the BST crystal lattice parameter in the direction perpendicular to the substrate exhibits a sharp change.

MICROWAVE PROPERTIES OF BST AND BST/BMT THIN FILMS GROWN ON SAPPHIRE SUBSTRATE BY EVANESCENT MICROWAVE PROBE

ABSTRACT Thin films of BaxSr1−xTiO3 (BST) serial materials have the advantages of adjustable tunability and are good candidates for the application in DRAM and microwave devices. However, these films usually have loss tangent higher than the order of 0.01 at microwave frequencies. To improve the crystal structures and suppress the microwave losses, an interlayer material with good microwave properties can be used. In this present work, a low loss Ba(Mg1/3Ta2/3)O3 (BMT) thin buffer layer with varying thickness and its effect on the microwave properties of Ba0.4Sr0.6TiO3 thin films is investigated. Moreover, to overcome the spatial limit in the traditional microwave measurement, a novel technique, evanescent microwave probe (EMP) method, is used to directly probe the microwave dielectric properties of the films. This technique also provides the capability to study the dielectric mechanism in micro-scale region. Pulsed laser deposition technique was used to synthesize thin films. The films shows (111) preferably oriented growth of BST films with the introduction of BMT layer for the films grown on sapphire substrates. As the thickness of BMT increases, this behavior is more obvious. The microwave dielectric constants (ϵ) and dielectric losses (tan δ) of the films grown on sapphire substrates have been measured by EMP. The dielectric constants of BST thin films decrease monotonously with the increase of BMT thickness. In contrast, the tan δ shows a discontinuity variation when the BMT buffer layer is deposited for 10–20 minutes.

A LARGE SCALE PRODUCTION TEST OF THIN FILM Ba x Sr 1−x TiO 3 MICROWAVE PHASE SHIFTERS FABRICATED ON LaAlO3 SUBSTRATES

ABSTRACT Many individual tunable ferroelectric-based microwave components have demonstrated excellent performance. However data on the production of many identical devices has been lacking. This paper will present data from NASA Glenn Research Center's fabrication of 822 phase shifters using commercially obtained pulsed laser deposited BaxSr1-xTiO3 (BST) thin films. These phase shifters of identical design were fabricated on 31 BST-coated LaAlO3 wafers. Issues of phase shifter performance, repeatability, yield, and long term stability will be addressed. Phase shifter tunability and insertion losses will be compared with BST microstructural film characteristics and also with manufacturing induced physical variations in the electrodes.

Dielectric Properties of the Compositionally Graded (Ba,Sr)TiO3 Thin Film

The compositionally graded Ba1–xSrxTiO3 films with a fine compositional gradient from BaTiO3 to Ba0.70Sr0.30TiO3 were fabricated on Pt/Ti/SiO2/Si substrates by a sol-gel deposition method. The graded films crystallized into a pure perovskite structure after post-deposition annealing. Dielectric constant peaks, common to a ferroelectric transition, were not observed in the temperature range from -35°C to 190°C, within which the dielectric constant and dielectric loss showed negligible temperature dependence. The improved temperature stability from this type of compositionally graded material is beneficial to applications requiring a wide range of operating temperatures, thereby eliminating the need for environmental controls.

Metal-organic chemical-vapor deposition of (Ba,Sr)TiO3: Nucleation and growth on Pt-(111)

Thin films of the (BaxSr1−x)TiO3 (BST) solid solution series were grown by metal-organic chemical-vapor deposition (MOCVD) on platinized silicon wafers with emphasis to (Ba0.7Sr0.3)TiO3 and SrTiO3. The nucleation behavior and the size of the stable nuclei were investigated by different scanning probe microscope techniques including local conductivity scanning. The characteristic differences were observed for different deposition temperatures, i.e., a homogeneous nucleation of small BST grains on the larger Pt grains at 565 °C and a dominating grain-boundary nucleation at 655 °C. X-ray photoelectron spectroscopy indicates a stoichiometric composition from the beginning. The microstructural evolution was investigated by high-resolution transmission electron microscopy and revealed randomly oriented grains (typical in-plane size 10–20 nm) with a high density of twins at 565 °C and (100)-oriented defect-free grains of slightly increased size at 655 °C. This remarkably stable (100) texture seems specific for MOCVD as it is not observed for other deposition methods. The grain structure and surface morphology and their dependence on film stoichiometry (group-II/Ti ratio) are investigated. The relation between the structural and the electrical properties, capacitance, and leakage current, is finally discussed.

Ferroelectric properties of barium strontium titanate thin films grown by RF co‐sputtering

In this work, we present the variation of the ferroelectric properties of Ba1–xSrxTiO3 films deposited on Pt/TiO2/SiO2/Si substrates by RF co-sputtering with 0 ≤ x ≤ 1. The co-sputtering was done using a single magnetron with BaTiO3/SrTiO3 targets in a pie mosaics configuration. Smooth and uniform films were obtained using the same conditions of growth and annealing temperature. The X-ray diffraction and EDS results show that the processes were managed to obtain crystalline materials with x from 0 to 1. The behaviour of P-E loops suggests that the ferroelectric properties of the films were tuned by changing the concentration of the cation. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Direct experimental evidence of hybridization of Pb states with O2p states in ferroelectric perovskite oxides

This work presents the OK- and TiL3,2-edge x-ray absorption near-edge structure (XANES) spectra of PbxSr1−xTiO3 (PxSTO) and BaxSr1−xTiO3 (BxSTO) compounds with various Pb and Ba concentrations. The result provides direct evidence that the Pb–O bonding strongly affects O2p–Ti3d hybridization in the TiO6 octahedron of PxSTO. In contrast, the Ba–O bonding does not substantially affect O2p–Ti3d hybridization in BxSTO. The TiL3-edge XANES spectra show the splitting of the eg band for PxSTO with x≳0.5, which provides an evidence of Pb-induced tetragonal distortion in the TiO6 octahedron. In contrast, eg band splitting is absent in BxSTO.

Microstructure of compositionally-graded (Ba1−xSrx)TiO3 thin films epitaxially grown on La0.5Sr0.5CoO3-covered (100) LaAlO3 substrates by pulsed laser deposition

Compositionally-graded (Ba1−xSrx)TiO3 (BST) epitaxial thin films (with x decreasing from 0.25 to 0.0) were deposited by pulsed laser deposition on (100)LaAlO3 (LAO) single-crystal substrates covered with a conductive La0.5Sr0.5CoO3 (LSCO) layer as a bottom electrode. X-ray and electron diffraction patterns demonstrate that the entire graded film has a single-crystal cubic structure. The epitaxial relationship between BST, LSCO, and LAO can be described as (100)BST‖(100)LSCO‖(100)LAO; [001]BST‖[001]LSCO‖[001]LAO. Cross-sectional transmission electron microscopy (TEM) images reveal that both the BST films and the LSCO bottom electrodes have sharp interfaces and overall uniform thickness across the entire specimen, and that they grow with a columnar structure. Planar TEM images show that the graded films exhibit granular and/or polyhedral morphologies with an average grain size of 50nm. High-resolution TEM images reveal aligned rectangular-shaped voids in the graded BST film, with length size of 12–17nm, and width of 5–8nm along the ⟨001⟩ direction in the (100) plane.

Influence of ambient oxygen pressure on the preferred orientation, microstructures, and dielectric properties of (Ba1-xSrx)TiO3 thin films with compositionally graded structures

Compositionally graded (Ba1-xSrx)TiO3 (BST) thin films, with x decreasing from 0.25 to 0.0, were deposited on Pt(111)/Ti/SiO2/Si(100) substrates by pulsed-laser ablation at 600 °C and under ambient oxygen pressures ranging from 50 to 400 mTorr. The influence of the ambient gas pressure on the preferred orientation, microstructures, and dielectric properties of compositionally graded BST films was investigated by X-ray diffraction, scanning electron microscopy, and dielectric frequency spectra, respectively. As the ambient oxygen pressure was increased, the preferred orientation evolved in the order: (100)+(110)→(110)+(111)→ random orientation, and the surface roughness of the graded BST films also increased. The graded BST films deposited at high ambient oxygen pressures (300∼400 mTorr) exhibited a grainy structure with polycrystalline grains throughout the film thickness, whereas the graded films deposited at low ambient oxygen pressures (50∼200 mTorr) possessed a columnar structure. The evolution of the microstructure was ascribed to the different physical and chemical properties of the species that were incident onto the substrates at the various oxygen pressures. The dielectric properties of the graded BST films were dependent upon the ambient oxygen pressures. The graded BST films deposited at 200 mTorr exhibited the highest dielectric constant.