Microstructures and dielectric properties of compositionally graded (Ba1-xSrx) TiO3 thin films prepared by pulsed laser deposition

Compositionally graded (Ba_1-xSr_x)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. Received: 10 December 2001 / Accepted: 12 March 2002 / Published online: 19 July 2002 RID="*" ID="*"Corresponding author. Fax: 86-25/359-5535, E-mail: xhzhu@public1.ptt.js.cn     

Nonlinear dielectric response of epitaxial Ba<Subscript>0.6</Subscript>Sr<Subscript>0.4</Subscript> TiO<Subscript> 3</Subscript> thin films

Based on Landau-Devonshire (LD)-type phenomenological thermodynamic theory, the electric field dependence of the dielectric properties of tetragonal single-domain barium strontium titanate(Ba_1-xSr_xTiO₃) films on cubic substrates is theoretically investigated by taking into account the high order terms of the polarization. At room temperature, the nonlinear dielectric responses of epitaxial Ba_0.6Sr_0.4TiO₃ films are provided by adjusting the film thickness and growth temperature. The strong nonlinearity of relative dielectric constant and pyroelectric coefficient are attained around critical film thickness on MgO (69 nm) and LaAlO₃ (132 nm) substrates or critical growth temperature on MgO (337 °C) substrate with respect to epitaxy-induced lattice misfit and thermal stresses during deposition. This can be explained that small compressive stresses are effective to support high nonlinearity of dielectric constant and pyroelectric coefficient for Ba_0.6Sr_0.4TiO₃ films irrespective of whether they are on compressive substrate or tensile substrate. It is also predicted that a large tunability may be achieved by altering processing conditions, such as the film thickness and growth temperature for different substrates. Our theoretical results are in good agreement with the experimental data reported in literature.     

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 (Ba_1-xSr_x)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 (Ba_0.75Sr_0.25)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. PACS 81.15.-z; 77.55.+f; 68.37.Lp; 61.14.-x     

Fabrication and electrical properties of sol-gel-derived Ba0.8Sr0.2TiO3 ferroelectric films from a 0.05-M spin-on solution

Barium strontium titanate (Ba_0.8Sr_0.2TiO₃) films with good ferroelectricity have been obtained by a developed sol-gel processing, using a 0.05-M spin-on solution. X-ray diffraction and Raman spectroscopy investigations showed that the Ba_0.8Sr_0.2TiO₃ film exhibited a tetragonal structure at room temperature. Field-emission scanning electron microscopy measurements revealed that large columnar grains with the size of 100 to 200 nm in the film were formed from the highly dilute spin-on solution with layer-by-layer homoepitaxy. Electrical measurements for the prepared Ba_0.8Sr_0.2TiO₃ film showed a remnant polarization of 3.5 μC/cm², a coercive field of 53 kV/cm, two distinctive phase transitions, lower dissipation factor, and good insulating properties. These results indicate the sol-gel-derived Ba_0.8Sr_0.2TiO₃ film from a 0.05-M solution is suitable for uncooled infrared detector applications. Received: 19 August 1999 / Accepted: 11 October 1999 / Published online: 1 March 2000     

Critical concentrations in Ba-doped incipient ferroelectric SrTiO3

Temperature-induced variations of light refraction and dielectric permittivity in single-crystal Sr_1−x Ba_xTiO₃ (x=0.02, 0.05, 0.07, and 0.14), Sr_1−x Ca_xTiO₃ (x=0.014), and in nominally pure strontium titanate have been studied within the 17–300 K temperature range. The spontaneous polar contribution to the refractive index has been isolated. It was used to calculate the temperature and concentration dependences of the polarization autocorrelation function 〈P _s ^2〉 in the Sr_1−x Ba_xTiO₃ system. For x⩽0.07, the polarization P _s=〈P _s ² 〉^1/2 varies proportional to (x−x _g)^1/2, where x _g=0.0027 is the new critical concentration in Sr_1−x Ba_xTiO₃, below which short-range polar order vanishes. Fiz. Tverd. Tela (St. Petersburg) 39, 704–710 (April 1997)     

Polarization characteristics of graded thick Ba<Subscript>1 − <Emphasis Type="Italic">x</Emphasis></Subscript>Sr<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>TiO<Subscript>3</Subscript> films

The barium strontium titanate ceramics Ba_{1 − x} Sr _x TiO₃ with a spatially variable composition has been prepared according to the thick film technology (tape casting). The strontium content over the film thickness is varied from 0 to 30 mol %. The structure and polarization characteristics of the samples prepared have been investigated. It has been found that the polarization characteristics of multilayer structures are determined by the ratio between the thicknesses of layers with different compositions and by their properties. No shift of the hysteresis loops in the graded thick Ba_{1 − x} Sr _x TiO₃ ( _x = 0–0.3) films has been revealed. The results obtained have been analyzed in the framework of modern theoretical approaches.     

Growth of Ba2−x Sr x NaNb5O15 thin film on La0.05Sr0.95TiO3 substrate by pulsed laser deposition

Ba_2−x Sr _x NaNb₅O₁₅ thin films were prepared on La_0.05Sr_0.95TiO₃ substrates by pulsed laser deposition. The structural and ferroelectric properties of the thin films depended on substrate temperature (T _sub) and Sr concentration. When T _sub was fixed at 700 °C, the Ba_2−x Sr _x NaNb₅O₁₅ (x = 0, 0.6, 1.0, and 1.4) thin films exhibited a high c-axis orientation. The thin films consisted of well-developed grains and exhibited a smooth surface. The c-axis-oriented Ba_0.6Sr_1.4NaNb₅O₁₅ thin film with the lowest Curie temperature also exhibited a high c-axis orientation and a P-E hysteresis loop with a high ferroelectricity at T _sub 650 °C. Thus, its remanent polarization (P _r) and coercive field (E _c) were 2P _r 24.9 μC/cm² and 2E _c 107 kV/cm, respectively. These values indicate that Ba_2−x Sr _x NaNb₅O₁₅ has ferroelectricity in the thin film form.     

Size dependence of dielectric properties and structural metastability in ferroelectrics

The size effect of the dielectric properties and the barrier height was investigated in the ferroelectric solid solution Ba_xSr_1-xTiO₃ system. The decrease of the grain size causes the suppression of the ferroelectricity, and the increase of the relaxation frequency. Barrier heights increase with increasing grain size. The result is analogous to magnetic phase transitions in nanocrystals and other solid-solid phase transitions in nanocrystals. It suggests a general rule that may be of use in the discovery of new metastable phases. An explanation of this phenomenon was given by an electric potential model that agrees well with the experimental results. For Ba_xSr_1-xTiO₃ system, the decrease of xcauses the decrease of the barrier height. Received 3 August 1998 and Received in final form 22 November 1998     

Light-induced charge transport processes in photorefractive crystals II: Materials

3 , LiTaO₃, BaTiO₃, Ba_1-xSr_xTiO₃ (, BST), Ba_1-xCa_xTiO₃ (, BCT), KNbO₃, KTa_1-xNb_xO₃ (, KTN), Sr_1-xBa_xNb₂O₆ (, SBN) and Bi₁₂(Si,Ti,Ge)O₂₀ (BSO, BTO, BGO) are discussed. Utilizing the knowledge on the charge transport processes, consequences for applications are deduced; improved techniques for nondestructive readout of holograms with light of the recording wavelength are described. Received: 14. October 1996     

Ferroelectric properties of compositionally graded BST ceramics

Homogeneous and graded Ba_{1? x} Sr _x TiO₃ ceramics were produced by thick-film technique (tape casting). In the graded materials, the content of strontium was changed from 0 to 30 mol% in the direction normal to the surface. The microstructure, Ba/Sr spatial distribution, polarization characteristics, and dielectric properties of the samples were investigated. No hysteresis loop offset (neither along E- nor P-axis) in the graded Ba_{1? x} Sr _x TiO₃ structures (x = 0–0.3) was observed. This fact was analyzed on the basis of phenomenological models for graded ferroelectrics. The materials with spatially changing composition were characterized by high-temperature stability of dielectric properties and low temperature coefficient of capacitance.     

Enhanced spontaneous polarization in Sr and Ca co-doped BaTiO3 ceramics

The dielectric and ferroelectric properties of (Ba_xSr_1−x)_0.77Ca_0.23TiO₃ ceramics with x=1 to 0.7 were studied and compared with those of Ba_xSr_1−xTiO₃ and Ba_0.77Ca_0.23TiO₃ ceramics. It has been found that Sr doping of the Ba_0.77Ca_0.23TiO₃ ceramics causes a drastic decrease of the Curie temperature, just like Sr doping of pure BaTiO₃ ceramics, demonstrating a cell volume effect. However, the (Ba_xSr_1−x)_0.77Ca_0.23TiO₃ ceramics with x=0.9 and 0.8 have larger spontaneous polarization than those of the corresponding Ba_xSr_1−xTiO₃ and Ba_0.77Ca_0.23TiO₃ ceramics, along with sufficient insulating properties. The enhancement of their polarization was explained by the increase of the lattice parameter c/a ratio due to the lattice distortion and strain developed in the ceramics.     

Preparation of epitaxial compositionally graded (Ba1-xSrx)TiO3 thin films with enhanced dielectric properties

Epitaxial compositionally graded (Ba_1-xSr_x)TiO₃ (BST) (0.0x0.25) thin films were deposited on (100) LaAlO₃ substrates by pulsed laser ablation, the substrates having bottom electrodes made of 100-nm-thick conductive La_0.5Sr_0.5CoO₃ (LSCO). Extensive X-ray diffraction, rocking-curve, and -scan studies indicate that the graded films are (100)-oriented and exhibit good in-plane relationships of [010]_BST//[010]_LAO and [001]_BST//[001]_LAO. For the up-graded films with barium concentration (1-x) increasing across the film thickness in the direction from the film/substrate interface to the film surface, the full width at half maximum of the BST film (200) rocking curve and the surface roughness, examined by atomic force microscopy, were larger than those of the down-graded films with barium concentration decreasing from the film/substrate interface to the film surface. The dielectric properties of the graded films, measured using vertical structures, show that at room temperature, the dielectric constant (_r) and dissipation factor (cos) at 100 kHz were 380 and 0.013 for the up-graded films, and 650 and 0.010 for the down-graded films, respectively. The dielectric behavior was enhanced in the down-graded films, which was attributed to the fact that the pure BaTiO₃ layer in the down-graded BST films not only serves as a bottom layer but also acts as an excellent seeding layer for enhancing subsequent film growth, leading to better film crystallinity and larger grain sizes in the down-graded films. The graded BST films undergo a diffuse phase transition, giving a broad, flat capacitance-versus-temperature profile. With such a graded structure, it is possible to build a dielectric thin-film capacitor having a capacitance which has a low temperature dependence over a broad temperature regime. PACS 77.55.+f; 68.55.Jk; 81.15.Fg     

Dielectric tunability and conduction mechanisms of?nanostructured (Pb1?xSrx)TiO3 thin films

The effect of nanosize grains to enhance dielectric tunability in chemically prepared (Pb_1−x Sr _x )TiO₃ (PST) (x=0.1 to 0.5) thin films has been observed. The grain size is evaluated from X-ray diffraction patterns and atomic force microscopy. The average grain size lies in the range of 80–23 nm with varying Sr content. The nanosize grains in the PST films control the dielectric behavior up to the higher frequency region and exhibit large tunability with low loss factor at room temperature. The current–voltage characteristics show a large tunability as electron transport takes place within a highly resistive Fermi gap.     

Structure, ferroelectric and piezoelectric properties of?(Bi0.98?xLa0.02Na1?x)0.5BaxTiO3 lead-free ceramics

Lead-free (Bi_0.98−x La_0.02Na_1−x )_0.5Ba _x TiO₃ ceramics have been prepared by an ordinary sintering technique and their structure, ferroelectric and piezoelectric properties have been studied. The results of X-ray diffraction show that La²⁺ and Ba²⁺ diffuse into the Bi_0.5Na_0.5TiO₃ lattices to form a new solid solution with a pure perovskite structure, and a morphotropic phase boundary (MPB) exists at 0.04<x<0.10. Compared with pure Bi_0.5Na_0.5TiO₃ ceramics, the (Bi_0.98−x La_0.02Na_1−x )_0.5Ba _x TiO₃ ceramics possess much smaller coercive field E _c and larger remanent polarization P _r. Because of the low E _c (3.38 kV/mm), large P _r (46.2 μC/cm²) and the formation of the MPB of rhombohedral and tetragonal phases, the piezoelectric properties of the ceramics are significantly enhanced at x=0.06: d ₃₃=181 pC/N and k _p=36.3%. The depolarization temperature T _d reaches a minimum value near the MPB. The ceramics exhibit relaxor characteristic, which is probably a result from the cation disordering in the 12-fold coordination sites. The temperature dependences of the ferroelectric and dielectric properties suggest that the ceramics may contain both polar and non-polar regions at the temperatures above T _d.     

Gradient-stress on polarization in Ba1-xSrxTiO3 films

A group of position-thickness-dependent stresses are used to modified Landau-Devonshire theory to investigate the second-order phase transition in Ba_1−x Sr _x TiO₃ films. The result shows that the short-range interaction between the unit cells of the film and the substrate induces the phase transition dispersion and the rise of the transition temperature in the films. The dependence of the effective dielectric constant on the temperature and the average spontaneous polarization on the film thickness are computed, which qualitatively agree with the experiments.      

Ba1-xSrxTiO3及Ba0.6-xPbxSr0.4TiO3陶瓷的制备和介温特性研究

用固相反应法制备了Ba_1-xSr_xTiO₃(BST)及Ba_0.6-xPb_xSr_0.4TiO₃(BPST)陶瓷,通过XRD,FESEM和拉曼谱分析了Pb掺杂对Ba_0.6Sr_0.4TiO₃样品的晶格、相变及表面形貌的影响.测试了BST及B 关键词： BST BPST 弥散相 介温特性     

Effect of strontium additive on thermoluminescence properties of (Ba1-xSrxSO4)99.8%: Eu0.2% nanophosphor

Nanophosphors of barium strontium sulfate complex (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% (0 ≤ x ≤ 1) were prepared through the chemical co-precipitation method at room temperature. Precipitated samples were characterized using X-ray diffraction (XRD), dynamic light scattering (DLS) and high resolution transmission electron microscope (HRTEM) techniques. The obtained XRD patterns from the prepared nanophosphate series (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% exhibit an orthorhombic structure with semispherical particle shape. The lattice parameters of (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% solid crystals change and the cell volume decreases with the increase of x value of strontium. The thermoluminescence (TL) glow curves induced by gamma rays of (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% series were recorded and compared. The substitution of Ba²⁺ by Sr²⁺ cations shift the trap centers in the host of (Ba_1-xSr_xSO₄)_99.8%:Eu_0.2% material to the higher temperature side. The TL glow curve (GC) of sample with x = 0.12, with grain size ranging between 13–31 nm, reveals that it has deep trap centers, and higher TL sensitivity. The different heating rates effect of the glow peaks of samples with x = 0, 0.12 and 1 showed that they follow the first-order kinetics. These samples have been studied and analyzed with the help of both T_stop experimental method, and the computerized glow curve deconvolution (CGCD) program. T_m–T_stop experiment indicates that there are three trapping levels in both (BaSO₄)_99.8%:Eu_0.2% and (SrSO₄)_99.8%:Eu_0.2% sulfate samples at 452, 489, 543 K and 487, 513, 530 K respectively, while five peaks at 458, 486, 499, 544 and 556 K in the complex GC of (Ba_0.88Sr_0.12SO₄)_99.8%:Eu_0.2%. These values are used as input for CGCD. The figure of merit (FOM) during fitting procedures is determined.     

Effect of annealing temperature on microstructures and optical properties of Ba<Subscript>0.9</Subscript>Sr<Subscript>0.1</Subscript>TiO<Subscript>3</Subscript> films

Evolution of microstructure and optical property with annealing temperature has been examined for Ba_0.9Sr_0.1TiO₃ films derived from one single precursor solution containing polyethylene glycol polymer. The films sintered below 750°C exhibit a uniform phase structure across the cross-sections and an ordinary optical thin film feature, while the Ba_0.9Sr_0.1TiO₃ films crystallized at 750°C or higher temperature render a lamellar texture consisting of dense and porous Ba_0.9Sr_0.1TiO₃ layers and a good performance as a one-dimensional photonic crystal. The discrepancy in cross-sectional morphology and reflectance property observed in these Ba_0.9Sr_0.1TiO₃ films has been preliminarily explained.     

The ac conductivity and complex impedance of CuO-doped (Ba0.5Sr0.5)TiO3 ceramic

The AC conductivity and complex impedance spectroscopy of CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramic were investigated. X-ray diffraction analysis showed that CuO-doped (Ba_0.5Sr_0.5)TiO₃ has a perovskite structure without any pyrochlore phase. Frequency dependent dielectric permittivity was discussed at a different temperature range. The activation energy was calculated and discussed through the Arrehnius equation from the ac conductivity with different frequency plots. CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramics have a negative temperature coefficient of resistivity. Dependence of impedance spectroscopy on frequency and temperature showed that the conduction process in the CuO-doped (Ba_0.5Sr_0.5)TiO₃ ceramic follows the thermally activated conduction mechanism.     

Evaluation of the relative permittivity of Ba<Subscript>x</Subscript>Sr<Subscript>1-x</Subscript>TiO<Subscript>3</Subscript> ceramics at microwave frequencies using microstrip ring resonators

The microstrip-ring-resonance technique has been applied to determine the dielectric properties of high-permittivity barium strontium titanate (Ba_xSr_1-xTiO₃) ceramics at microwave frequencies. A microstrip ring resonator of diameter 5 mm has been fabricated on the ceramics by the standard photolithography process. The transmission coefficient S₂₁ spectra for the resonators have been measured using a vector network analyzer and simulated using a commercial electromagnetic simulation package, IE3D. By fitting the observed spectra to the simulated spectra, and tan of the Ba_xSr_1-xTiO₃ ceramics have been determined. The results obtained by this technique are in agreement with those determined by other conventional methods. This technique is relatively simple, especially for high-permittivity materials, since there are less rigorous requirements on the sample and the test conditions. PACS 77.22.-d; 77.84.Dy; 84.40.Az