Vibrational spectroscopic studies on Ba0.8Sr0.2TiO3 thin films prepared by RF sputtering technique

Thin films of Ba_0.8Sr_0.2TiO₃ have been deposited on p-type Si substrate by radio frequency magnetron sputtering. Polycrystalline bulk Ba_0.8Sr_0.2TiO₃ sample has also been studied for comparison. X-ray diffraction patterns reveal that both the bulk sample and thin films are polycrystalline without any preferential orientation and belong to paraelectric cubic phase. We have compared the room temperature Raman and IR spectra of powder and thin films (both annealed and as-deposited) of Ba_0.8Sr_0.2TiO₃. The extra feature in the Raman spectrum for the annealed film has been explained as due to the presence of intergrain stresses from the submicron size grains in it.     

The Field Effect in a Metal–Ferroelectric–Semiconductor System of Multilayer Ferroelectric Films with Various Structure Types

The ability to grow the interfacial defect-poor Sr_0.5Ba_0.5Nb₂O₆ + Ba_0.2Sr_0.8TiO₃ and Ba_0.8Sr_0.2TiO₃ + Ba_0.4Sr_0.6TiO₃ ferroelectric films onto the doped silicon substrates is discussed. A study of piezo-response via the quasi-static method (using the electrode area of 0.07 mm²) reveals that heterostructures possess an initial polarized ferroelectric state with a spontaneous polarization vector perpendicular to the substrate at any type of Si conductivity. The polarized state is established to refer to two-dimension stresses in the ferroelectric, which is tunable through a preprepared Ba_xSr_{1 – x}TiO₃ onto a sublayer substrate as well as to a thickness of this sublayer. Polarization switching in Sr_0.5Ba_0.5Nb₂O₆/Si and Ba_0.8Sr_0.2TiO₃/Si heterostructures under the external field arises at only using the barium–strontium titanate sublayer predeposited onto silicon. A 15% decrease in switching polarization in Ba_0.8Sr_0.2TiO₃/Ba_0.4Sr_0.6TiO₃/Si structures is observed after 500 h.     

Polarization of thin barium-strontium titanate films by an external electric field

The behavior of polarization of a Ba_0.8Sr_0.2TiO₃ thin film in an external electric field for a planar geometry of electrodes is described using a simple phenomenological model. The “van der Waals field” E _z formed by the substrate along the normal to the film surface is introduced for fitting to experimental data. This field is estimated as E _z = 1.1 × 10⁸ V/m. The phase diagram of the Ba_0.8Sr_0.2TiO₃ film in the E _z (misfit strain) at room temperature is analyzed.     

溶胶-凝胶法制备MgO/(Ba0.8Sr0.2)TiO3多层薄膜及其介电和漏电特性研究

采用改进的溶胶-凝胶方法在LaNiO₃/Si(100)衬底上制备了MgO/(Ba_{0.8Sr0.2)TiO3多层薄膜.实验结果表明，MgO层的引入改变了(Ba0.8}Sr_0.2)TiO₃的介电特性和漏电流行为，使薄膜的漏电 流降低了3个数量级，但介电常数也有相应降低.漏电流的显著降低是由MgO子层的高阻特性 以及微量Mg向(Ba_0.8关键词： 0.8Sr_0.2)TiO₃多层薄膜')" href="#">MgO/(Ba_0.8Sr_0.2)TiO₃多层薄膜 漏电流 介电常 数     

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.     

溶胶-凝胶法制备的Ba0.8Sr0.2TiO3薄膜的结构及铁电性质研究

采用0.05mol/L的前驱体溶液,利用溶胶-凝胶法成功制备了室温下具有优良铁电性质的Ba_0.8Sr_0.2TiO₃ (BST)薄膜.X射线衍射分析表明,制得的BST薄膜室温下呈四方相,场发射扫描电子显微观测显示BST薄膜表面平整、致密、无裂纹出现,薄膜晶粒呈柱状结构、尺寸在150nm左右.电学测量表明制备的BST薄膜室温下具有优良的铁电性能.薄膜的剩余极化P_r约为35μC/cm²,矫顽电场E关键词：     

Sol-gel-derived pyroelectric barium strontium titanate thin films for infrared detector applications

Quality pyroelectric Ba_0.8Sr_0.2TiO₃ films have been successfully fabricated by a sol-gel processing using a highly diluted precursor solution. The remnant polarization of the films decreases with the temperature increasing, which results in a large pyroelectric coefficient at room temperature. Infrared response measured using a 500-K chopped blackbody at room temperature exhibits a typical pyroelectric response waveform. Frequency dependence of the infrared response measurement for a pixel with area of 2.5×10^-3 cm² showed that the maximum response output voltage of 3.2 mV was obtained at 6 Hz. Better infrared response can be expected by the improvement in thermal isolation of the pixels and electrode materials. Received: 26 April 2000 / Accepted: 9 May 2000 / Published online: 9 August 2000     

Characteristics of Ba xSr 1–xTiO 3 thin films grown by pulsed laser ablation of rotating split targets of BaTiO 3 and SrTiO 3

We have grown heterostructures of Ba_xSr_1-xTiO₃(BST)/La_0.7Sr_0.3MnO₃(LSMO) on LaAlO₃(LAO) substrates by the pulsed laser deposition method. BST films with x=0.2,0.5,0.7 and 0.8 have been prepared using a novel rotating split-target arrangement. The lattice constant of the BST films is found to vary linearly with Ba/Sr ratio. An excellent cube-on-cube epitaxial relationship of (100)_BST||(100)_LSMO||(100)_LAO has also been obtained. Scanning electron microscopy studies have revealed smooth and crack-free BST films with a uniform grain size of about 100 nm. Dielectric measurements, made with patterned Au top electrodes and conducting LSMO bottom layers, have shown a maximum permittivity of approximately 280 at 5 kHz in BST films with x=0.7. P–E loop analyses of the Ba_0.7Sr_0.3TiO₃ and Ba_0.8Sr_0.2TiO₃ films have yielded remnant polarization values of 1.66 and 1.81C/cm², respectively. PACS 81.15Fg; 68.55Jk; 77.55+f     

Preparation and characterisation of compositionally graded BaxSr1-xTiO3 thin films

Ba_xSr_1-xTiO₃ thin films with a compositional gradient of x=0.3 to 1 (in 0.1 mole fraction increments) were fabricated on Pt/Ti/SiO₂/Si substrates using a modified sol–gel technique. The graded film crystallised in a perovskite structure and consists of a uniform microstructure with comparatively larger grains. The room-temperature relative dielectric constant (ε_r) and dielectric loss (cosδ) at 100 kHz were found to be 305 and 0.03 respectively. Dielectric peaks were not observed in the temperature range from -20 °C to 120 °C. The dielectric constant and dielectric loss were almost independent of temperature. Polarisation–electric field measurements at room temperature revealed a saturated but slim hysteresis loop with a remanent polarisation (P_r) of 0.6 μC/cm² and a coercive field (E_c) of 2.4 kV/mm. The graded film behaves as a stack of Ba_xSr_1-xTiO₃ capacitors connected in series and hence the dielectric Curie peaks are removed. Received: 4 October 2001 / Accepted: 17 October 2001 / Published online: 23 January 2002     

Relaxation of polarized states in thin films of BST

The possibility of the local polarization of epitaxial Ba_0.8Sr_0.2TiO₃ thin film is studied by means of piezoresponse force microscopy. The existence of a local polarized state depends on the duration of the applied pulse. The kinetics of polarized states explaining the experiment is considered in the context of phenomenological theory.     

High-resolution transmission electron microscopy of heteroepitaxial barium strontium titanate films grown on MgO substrates

Nanoscale (5–100 nm) ferroelectric heteroepitaxial Ba_0.8Sr_0.2TiO₃ films on MgO(001) substrates were studied by methods of high-resolution transmission electron microscopy using Tecnai G² and FEI Titan 80–300 microscopes. Misfit dislocations resulting from stress relaxation in films and insignificant local misorientation of film regions (block structure) were detected. The heterostructure was numerically simulated, which allowed the detection of the formation of oxygen (substrate)-titanium (film) bonds or the oxygen (substrate)-barium (film) bond at the film-substrate interface.     

Dielectric properties and magnetoelectric effect of (x)NiFe2O4+(1−x)Ba0.8Sr0.2TiO3 composites

Magnetoelectric composites of NiFe₂O₄ and Ba_0.8Sr_0.2TiO₃ were prepared using conventional double-sintering ceramic method. The phase formation of magnetoelectric composites was confirmed by XRD technique. Variation of dielectric constant and loss tangent at room temperature with frequency in the range 100 Hz-1 MHz has been studied. Also the variation of dielectric constant and loss tangent with temperature and composition at fixed frequencies of 1 kHz, 10 kHz, 100 kHz and 1 MHz is reported. The static value of the magnetoelectric conversion factor was measured as a function of intensity of the magnetic field. The ME voltage coefficient of about 430 μV/cm Oe was observed for 15% NiFe₂O₄+85% Ba_0.8Sr_0.2TiO₃ composite. All the samples show linear variation of magnetoelectric conversion in the presence of static magnetic field.     

Effect of growth mechanisms on the deformation of a unit cell and polarization reversal in barium–strontium titanate heterostructures on magnesium oxide

The effect of a growth mechanism on the unit cell strain and the related change in the properties of single-crystal Ba_0.8Sr_0.2TiO₃ films grown on MgO substrates according to the Frank–van der Merwe and Volmer–Weber growth mechanisms is studied. The unit cell strain is shown to depend substantially on the film thickness and the growth mechanism. It is found that the same film–substrate pair can be used to vary stresses in the film from two-dimensional tensile to compressive stresses due to a change in the growth mechanism and the film thickness.     

Effect of internal strain fields on the controllability of nanodimensional ferroelectric films in a plane capacitor

Nanodimensional ferroelectric heteroepitaxial Ba_0.8Sr_0.2TiO₃ films grown by the layer-by-layer mechanism on MgO(100) substrates are examined by the X-ray diffraction and transmission electron microscopy methods. It is established that, when the thickness of the film changes, the stress relaxation proceeds via generation of misfit dislocations at the film-substrate interface. There exists a critical thickness (≈40 nm) of the film below and above which the film possesses tensile and compression stresses, respectively. Examples of how the stresses influence the insulating properties of the films are given.     

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.     

Physical properties of Ba<Subscript>0.8</Subscript>Sr<Subscript>0.2</Subscript>TiO<Subscript>3</Subscript> thin films

The behavior of material constants in ferroelectric Ba_0.8Sr_0.2TiO₃ thin films is studied depending on the misfit strain at room temperature in the context of nonlinear thermodynamic potential of the phenomenological theory. Some constants are found to undergo drastic changes with the alternating strain at the interfaces. The gathered results allow one to evaluate the material constants for a specific film and to outline the direction in searching the ways to synthesize films with the needed properties.     

Optical properties of the Ba0.8Sr0.2TiO3/(Bi0.82, Nd0.02)FeO3 superlattice

A Ba_0.8Sr_0.2TiO₃/(Bi_0.82, Nd_0.02)FeO₃ multilayer heteroepitaxial structure is prepared by successively depositing 20 Ba_0.8Sr_0.2TiO₃ and (Bi_0.82, Nd_0.02)FeO₃ layers, each 4 nm thick, using the method of rf sputtering. The layers grow on a single-crystal MgO substrate following the layer-by-layer mechanism. The structural perfection of the layers is examined by X-ray diffraction. The optical transmission is studied in the wavelength interval 200–1100 nm. The spectra are processed using a dispersion formula for the permittivity represented as a superposition of two oscillators with regard to relaxation. Such an approach allows us to separate direct and indirect transitions. The absorption edge of the superlattice for direct transitions is 80 nm wide and is estimated to be 3.78 eV.     

Improvement of dielectric tunability and loss tangent of (Ba,Sr)TiO3 thin films with K doping

Ba_0.6Sr_0.4TiO₃ thin films doped with K were deposited on Pt/Ti/SiO₂/Si substrates by chemical solution deposition method. The structure, surface morphology, and dielectric and tunable properties of Ba_0.6Sr_0.4TiO₃ thin films have been studied in detail. The K content in Ba_0.6Sr_0.4TiO₃ thin films has a strong influence on the material's properties including surface morphology, dielectric and tunable properties. It is found that the Curie temperature of K-doped Ba_0.6Sr_0.4TiO₃ films shifts to higher values compared with that of undoped Ba_0.6Sr_0.4TiO₃ thin films, which leads to a dielectric enhancement of K-doped Ba_0.6Sr_0.4TiO₃ films at room temperature. At the optimized content of 0.02 mol, the dielectric loss tangent is reduced significantly from 0.057 to 0.020. Meanwhile, the tunability is enhanced obviously from 26% to 48% at the measured frequency of 1 MHz and the maximum value of the figure of merit is 23.8. This suggests that such films have potential applications for tunable devices.     

Influence of thickness on the domain structure of barium-strontium-titanate films on MgO substrates

Ba_0.8Sr_0.2TiO₃/MgO(001) heterostructures are studied using atomic-force spectroscopy in several modes (the contact and semicontact modes and that of piezoelectric response) to obtain information on the relief and the distribution of the electric potential on the surface. Based on data obtained for films with different thicknesses, the correlation between the domain sizes and orientations and the film thickness and the surface relief is established. It is shown that the films with thicknesses less than 36 nm contain only aa domains whose dimensions decrease with increasing film thickness. The films with thicknesses greater than 36 nm contain only c domains whose dimensions increase with increasing film thickness.     

Improvement of electrical properties of Sr-modified (Pb,Ca)TiO3 thin films grown by soft chemistry route

Strontium and calcium-modified lead titanate (Pb_0.70Ca_0.15Sr_0.15)TiO₃ soft chemistry-derived thin films were prepared on platinum-coated silicon substrate by spin-coating method. Investigations were made on the structure, surface morphology and electrical properties of the film. The results by XRD and FE-SEM showed that the film exhibits a pure tetragonal perovskite phase and an average grain size of about 50-60 nm, respectively. Electrical measurements of a metal-ferroelectric-metal type capacitor exhibited a stable and switchable electrical polarization in the film. The structure of the Au/PCST/Pt capacitor showed well-saturated hysteresis loops at an applied voltage of 300 kV/cm with remanent polarization and coercive field values of 22 μC/cm² and 100 kV/cm, respectively. At 100 kHz, the dielectric constant and the dielectric loss of the (Pb_0.70Ca_0.15Sr_0.15)TiO₃ thin film with thickness 240 nm were 528 and 0.05, respectively.