BaxSr1-xTiO3 Research Articles

Studying The Dielectric and Structural Properties of Baxsr1-Xtio3 (BST) Ferroelectric System Prepared by Using Oxalic Acid Route

Studying The Dielectric and Structural Properties of Baxsr1-Xtio3 (BST) Ferroelectric System Prepared by Using Oxalic Acid Route

Ba0.60Sr0.40TiO3 THIN FILMS FOR MICROWAVE PHASE SHIFTER DEVICES: THE INFLUENCE OF CRYSTALLIZATION TEMPERATURE ON THE ELECTRIC FIELD DEPENDENT PHASE SHIFT RESPONSE

ABSTRACT We present the correlation between film fabrication conditions (crystallization temperatures), microstructure, and dielectric phase shift of Ba1−xSrxTiO3 (BST) thin films synthesized by metal organic solution deposition (MOSD) on sapphire substrates. The structure, microstructure, surface morphology, and composition of the films were assessed by glancing angle X-ray diffraction (GAXRD), atomic force microscopy (AFM), and Rutherford backscattering spectroscopy (RBS). The dielectric phase shift measurements were carried out using coplanar waveguide (CPW) test circuits over a frequency range of 2–18 GHz. Our results indicate that BST processed at 950°C achieved large relative phase shift response with low attenuation of the microwave signal.

BST FILMS GROWN BY METAL ORGANIC CHEMICAL VAPOR DEPOSITION INCORPORATING REAL-TIME CONTROL OF STOICHIOMETRY

ABSTRACT We present preliminary results of metal organic chemical vapor deposition (MOCVD) growth of BaxSr1-xTiO3 (BST) thin films. Films were deposited on MgO, sapphire, Si, and Si/Pt/Ti substrates. Real-time monitoring of the precursor flux by UV absorption spectroscopy was used to control the composition of the films over the range 30 ≤ x ≤ 80. The films were characterized for a number of attributes including composition, crystalline structure, and morphology.

Tunable dual-mode filter using varactor and variable ring resonator on integrated BST/TiO2/Si substrate

A RF tunable dual-mode filter employing a BST (BaxSr1−xTiO3)-based ring resonator and a capacitor is proposed. BST thin film was deposited on TiO2/Si substrate and a TiO2 buffer layer grown by ALD (atomic layer deposition). The device with a BST varactor capacitance of 0.5 pF and 40% of tunability applied voltage of 40 V, and a tunable ring resonator frequency of 1.53–2.02 GHz with a bias of up to 40 V, has been realised. The fabricated bandpass filter has a centre frequency of 1.53 GHz and shows a tunability of approximately 20% with an applied voltage of 40 V. It also has a bandwidth of 74 MHz and shows a tunability of approximately 27% with an applied voltage of 40 V.

Synthesis and temperature dependence of permittivity of Ba1-xSrxTiO3 and Ba0.6-xPbxSr0.4TiO3 ceramics

Ba1-xSrxTiO3 (BST) and Ba0.6-xPbxSr0.4TiO3 (BPST) ceramics were prepared by conventional solid-state reaction method. The analysis of XRD, FESEM and Raman spectroscopy showed the effects of Pb-doping on the crystal lattice,phase transition and surface morphology of Ba0.6Sr0.4TiO3, respectively. And the temperature dependence of permittivity (ε-T properties)for all ceramics were measured at a frequency of 10 kHz. The mechanism that Curie temperature and the peak value of permittivity are modulated by Sr and Pb content in BST and BPST perovskite has been revealed. In addition, all samples exhibit a diffused phase transition (DPT), especially for the ceramic Ba0.5Pb0.4Sr0.1TiO3, which can improve its temperature stability of tunability as phase shifter than others. Some ferroelectric parameters are calculated to interpret the DPT by adopting Smolenski’s theory and Curie-Wise law. For example, the values of diffuseness exponent α,Curie constant and transition region have changed from 1.29 to 1.73, from 1.25×105 to 2.87×105 K, and from 13.2 ℃ to 22.3 ℃, respectively.

Novel Low Melting Point Barium and Strontium Precursors for the MOCVD Growth of Barium‐Strontium‐Titanate Films

AbstractNovel barium and strontium precursors, belonging to the family of volatile complexes of the alkaline earth element (AEE) fluorinated β‐diketonates with polyglymes, [M(dfhd)2(tetraglyme)] [Hdfnd = 1,1,1,2,2,6,6,7,7,7‐dekafluoro‐heptane‐3,5‐dione, M = Ba (I), Sr (II)], are synthesized and tested with an aim to use them for metal‐organic (MO)CVD of BaxSr1‐xTiO3 (BST) thin films. The thermal behavior and composition of vapor phases for compounds I and II are studied by thermal analysis and mass spectrometry (MS). Considerable reduction of the melting point (down to 80–90 °C) for both compounds I and II, as compared to known volatile complexes, is achieved. By means of the developed precursors, epitaxial BST films (x = 0.5) are grown on SrTiO3 substrates by using the modified MOCVD facility. The quality of the grown films is examined by high‐resolution (HR) X‐ray diffraction (XRD) and high‐resolution (HR) scanning electron microscopy (SEM).

Mechanical relaxation in tetragonal and orthorhombic phases of (Ba,Sr)TiO3 ceramics

AbstractTwo broad relaxation peaks in the tetragonal and orthorhombic phases, respectively, of BaxSr1–xTiO3 (x = 0.8 and 1) ceramics were investigated via internal friction and modulus measurements. The activation energy calculated from the Arrhenius relation shows that the relaxation process depends on the thermal history. A possible explanation is proposed that the relaxation during heating is associated with the motion of domain walls, the migration of oxygen vacancies and the interaction between oxygen vacancies and domain walls. In addition, the activation energies of Ba0.8Sr0.2TiO3 are smaller than those of BaTiO3, possibly resulting from the larger interspaces of the two neighbouring Ba2+ and Sr2+ along the c‐axis.

Mechanosynthesis and reactive sintering of Ba1–xSrxTiO3 ceramics

We report on the synthesis of Ba1–xSrxTiO3 (0≤x≤1) ceramics by solid state reaction using high energy ball milling from mixtures of carbonates of barium and strontium with titanium oxide in stoichiometric ratios and pressureless reactive sintering. Ba1–xSrxTiO3 ceramics were obtained by milling at room temperature to form precursors, compacting by uniaxial pressing and direct sintering of compacted powders. The tetragonal to cubic phase transformation was successfully tuned from 87·15 to 399·15 K by varying the ratio of Sr/Ba according to Raman scattering spectra and differential scanning calorimetry results. The orthorhombic–tetragonal phase transition for the pure BaTiO3 sample shifted from the nominal value in the range from 273·15 to 285·15 K towards 300·15 K. Orthorhombic phase of BaTiO3 ceramics was achieved at room temperature.

Polarization behavior in diffuse phase transition of BaxSr1−xTiO3 ceramics

Ultrawide band dielectric spectroscopy analysis of barium strontium titanate [BaxSr1−xTiO3 (BST)] ceramics at different temperatures disclosed a contribution of the dipole and the ionic polarizations to their diffuse phase transition (DPT). In the BST ceramics, thermal behavior of the ionic polarization governs the DPT. The dielectric maximum temperature (Tm) is in agreement with the maximum temperature of the permittivity determined by the ionic polarization. The maximum of the permittivity determined by the ionic polarization at the Tm is caused from the decrement of the vibration frequency of the soft mode. The dipole polarization gives small contribution to the DPT. The gradual increase in the permittivity determined by the dipole polarization with decreasing temperature could be explained by the increase in the size of the polar nanoregions.

Dielectric dispersion of BaxSr1−xTiO3 thin film with parallel-plate and coplanar interdigital electrodes

Ferroelectric BaxSr1−xTiO3 (BST) thin films with x = 0.25 and 0.5 were grown by pulsed laser deposition on single crystal LaAlO3 and Pt/Ti/SiO2/Si substrates, respectively. Capacitors were then fabricated from the BST thin films based on coplanar interdigital electrodes (CIEs) and parallel-plate electrodes (PPEs). The dielectric properties of the BST film with CIE and PPE were investigated and compared over a wide frequency range from 100 Hz to 10 GHz. The dielectric dispersion in PPE configuration, caused by the interfacial polarization in film/electrode interfaces, exhibited a strong dependence on frequency. However, the permittivity εCIE in CIE configuration shows a gentle variation with the frequency indicating interfacial polarization substantially suppressed. The influence upon the dielectric properties of the columnar BST grains due to the use of different forms of electrodes was discussed.

Interfacial characteristic of (Ba,Sr)TiO3 thin films deposited on different bottom electrodes

Barium strontium titanate (Ba1−x Sr x )TiO3 (BST) thin films were deposited on Pt, Ru, RuO2, and Pt/RuO2 electrodes by radio frequency magnetron sputtering. The interfacial structure characteristic of the BST films deposited on various electrodes was investigated. X-ray photoelectron spectroscopy investigations showed that the interfacial diffusion layer in BST/Pt and BST/Ru are approximately 6 and 10 nm, respectively. The BST films are short of Ba and O elements comparing with the stoichiometry Ba0.65Sr0.35TiO3 in the interface region. Dielectric measurement of the BST films with thickness ranging from 70 to 400 nm revealed that the BST films deposited on Pt and Pt/RuO2 bottom electrodes have similar dielectric property, the BST films deposited on Ru have the highest bulk dielectric constant, and the thickness dependence of dielectric constant on the BST film deposited on RuO2 electrode can be neglected. The interfacial layer dielectric constant of BST films deposited on Pt and Ru electrodes are estimated to be about 34.5 and 157.1, respectively. The effect of interfacial dead-layer on the dielectric constant could be eliminated through selecting appropriate bottom electrodes.

Phase transitions and the temperature dependence of the dielectric properties in tetragonally strained barium strontium titanate films

The dielectric properties of sputter-deposited Ba1−xSrxTiO3 (BST) thin films on (001) MgO substrates with in-plane or out-of-plane tetragonal lattice structure distortions were characterized as a function of temperature. A temperature-dependent interpolation calibration technique was developed for increased efficiency of the microwave measurements. The BST films showed significant differences in the ferroelectric phase transition due to lattice distortions with a strong temperature dependence of the in-plane dielectric behavior for films under tensile strain and a weak temperature dependence for films under compressive strain. The experimental data agreed well with theoretical modeling of the BST film strain effect based on Devonshire’s theory.

Ferroelectric BaxSr1−xTiO3 thin-film varactors with parallel plate and interdigital electrodes for microwave applications

Columnar BaxSr1−xTiO3 (BST) thin films with x=0.25 and 0.5 were grown by pulsed laser deposition on single crystal LaAlO3 and Pt/Ti/SiO2/Si substrates, respectively. Ferroelectric varactors were then fabricated from the BST thin films by photolithographic techniques based on parallel-plate capacitors (PPCs) and coplanar interdigital capacitors (IDCs). The microstructure, dielectric constant ε, loss tan δ, tunability τ, and temperature dependent characters of the BST film with IDC and PPC electrodes were investigated and compared in a wide frequency range from 1 kHz to 10 GHz. The Ba0.5Sr0.5TiO3 films exhibited larger permittivity, tunability, and loss tan δ at the level of IDC and PPC structures compared to Ba0.25Sr0.75TiO3 films. As measured by IDC and PPC electrodes, their in-plane dielectric properties also exhibited a larger permittivity, lower loss tan δ, and relative lower tunability than that of out-of-plane dielectric properties. Finally, the influence on the dielectric properties of the columnar BST grains due to the use of different types of electrodes was discussed.

BaTiO3-RELATED FERROELECTRIC THIN FILMS BY POLYMER ASSISTED DEPOSITION

ABSTRACT Epitaxial films of Ba1 - xSrxTiO3 (BST) and BaTi1 - xZrxO3 (BZT) have been grown using polymer assisted deposition. Lattice parameters of the BST films with different Ba/Sr ratio showed bulk like behavior, indicating that the films were relaxed. Dielectric properties of the BST films are comparable to the films grown by pulsed laser deposition. Highest dielectric tunability of 69% (at room temperature) has been obtained for Ba0.7Sr0.3TiO3 film, which is the phase boundary of the tetragonal and cubic phases. The dielectric properties of BaTi0.5Zr0.5O3 film showed relaxor like behavior.

ENHANCED DIELECTRIC RESPONSE AND TEMPERATURE INSENSITIVITY OF COMPOSITIONALLY STRATIFIED BA1 −xSRxTIO3 THIN FILMS

ABSTRACT In this work we have developed and fabricated a compositionally stratified multilayered Ba1 −xSrxTiO3(BST) thin film heterostructure with a systematically varied stiochoimetric composition from Ba0.60Sr0.40TiO3 to Ba0.90Sr0.10TiO3. The optimized material design was characterized for structural, microstructural, dielectric (100 kHz and 0.5–10 GHz), and temperature stability properties. Experimental results demonstrated that the compositionally stratified thin film heterostructure possessed low loss (tan δ = 0.012), and high tunability (65.5% at 444 kV/cm). In addition, the BST heterostructure exhibited minimal dielectric dispersion in the temperature range of 90 to −10°C, showing a 6.4% decrease in permittivity (corresponding to a temperature coefficient of capacitance TCC20−90 = −0.921) as the temperature was elevated from 20 to 90°C and only a 2.1 increase in permittivity (TCC20 − (− 10) = −0.716) as the temperature was lowered from 20 to −10°C. Our results suggest that the compositionally stratified BST heterostructure, with its enhanced tunability and excellent temperature stability, is an excellent candidate for the next generation of tunable devices operating in harsh, i.e., temperature extreme, environments.

Development of Pt/MgO(100) Buffer Layers for Orientation Control of Perovskite Oxide Thin Films

(100)-oriented MgO thin films can be effectively grown at a substrate temperature of room temperature (RT) on Fe seed layers by the facing-targets sputtering (FTS) method. Pt(100) orientation was observed when the Pt layer was deposited on MgO(100)/Fe buffer layers at a substrate temperature above 350 °C. Pt thin films can be grown effectively with (100) orientation and with smaller strain when they are prepared at a substrate temperature above 500 °C. Also, Ba1-xSrxTiO3 (BST) thin films with (100) preferential orientation can be grown on Pt(100)/MgO(100)/Fe buffer layers when Pt and BST films are deposited at a substrate temperature of 500 °C. Pt(100)/MgO(100)/Fe buffer layers are very effective for attaining BST(100) orientation.

Dielectric characterization of a ferroelectric film in the sub-GHz region

A new method is proposed for the measurement of the dielectric properties of BaxSr1−xTiO3 (BST) thin films in the frequency range 106–1.8 × 109 Hz. A co-planar waveguide (CPW) transmission line was employed for impedance measurements via on-wafer probing. The measured impedance of the CPW line was found as a function of the probing position on the line. A simplified model of the CPW line under test allows the intrinsic dielectric properties of BST thin films to be extracted from the measured position-dependent impedance. Experimental results obtained by this new method agree well with those obtained by two-port S-parameters measurement at microwave frequencies using the same CPW structure.

X-Ray Absorption Spectroscopy Study of Ba1-xSrxTiO3 (x = 0.0-0.2) at the Ti-K and Ba-LIII Edges

Barium titanate, BaTiO3 is a significant material for electronic components. It was known that Sr2+ substitution on Ba ion can strongly modify ferroelectric properties of BaTiO3 but the structural change has not been fully understood. An X-ray absorption spectroscopy at the Ti K-edge (4966 eV) and the Ba LIII-edge (5247 eV) was employed to investigate the structure of Ba1-xSrxTiO3 (BSTO) with 0<x<0.20. Results suggested that Ba2+ did not change its position with Sr2+ substitution. On the other hand, Ti4+ atoms gradually displaced as Sr2+ is increased.

Deposition of BaxSr1–xTiO3 thin Films by Double RF Hollow Cathode Plasma Jet System

AbstractLow‐pressure plasma jet system with two hollow cathodes was used for deposition of Bax Sr1–x TiO3 (BSTO) ferroelectric thin films. Two RF hollow cathode plasma jet guns were pointed to the substrate where BSTO thin films were deposited. High density RF hollow cathode discharge was generated in ceramic nozzles made of SrTiO3 (STO) and BaTiO3 (BTO). Pulse modulated RF power was applied on water‐cooled copper block insulated from the plasma by ceramic and grounded stainless steel shield. In the experiment, the composition of BSTO films was controlled by magnitude of average RF power applied on particular plasma jet guns with STO and BTO nozzles. By this method, BSTO films were deposited on single crystal silicon substrate and on Pt top of Pt/TiO2/SiO2/Si multi‐layer system. X‐ray diffraction proved that BSTO thin films were polycrystalline with perovskite structure and measurement of dielectric properties showed ferroelectric hysteresis loop at room temperature. Time resolved Langmuir probe system and plasma impedance monitor were used for in situ plasma diagnostics and control of deposition conditions. Furthermore, plasma jets were analyzed during the deposition process by spatially resolved optical emission spectroscopy. (© 2008 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Microstructure, optical and dielectric properties of compositional graded (Ba,Sr)TiO3 thin films derived by RF magnetron sputtering

Thin films of compositional graded Ba1−x Sr x TiO3 (BST) (x decreasing from 0.3 to 0) were prepared on fused quartz and Pt/Ti/SiO2/Si substrates by RF magnetron sputtering. The microstructure of the graded BST thin films was characterized by X-ray diffraction (XRD). It indicates that the films were crystallized with peroveskite structure and (100) + (111) preferred orientation. The refractive index and the band gap were determined at room temperature in the wavelength 200–1100 nm from spectrophotometric measurements of the transmittance. The average value of the refractive index is found to be 2.17 for the graded BST films in the wavelength 400–1000 nm. The optical band gap of the graded BST film was 3.77 eV. The dielectric measurement showed that the dielectric constant and loss factor of the graded BST film was 318.04 and 0.028 at 100 KHz and room temperature.