Sr TiO Research Articles

In situ investigation of the early stage of TiO2 epitaxy on (001) SrTiO3

We report on a systematic study of the growth of epitaxial TiO(2) films deposited by pulsed laser deposition on Ti-terminated SrTiO(3) (001) single crystals. By using in situ reflection high energy electron diffraction, low energy electron diffraction, x-ray photoemission spectroscopy, and scanning probe microscopy, we show that the stabilization of the anatase (001) phase is preceded by the growth of a few nanometers thick pseudomorphic Sr(x)TiO(2+y) (x, y < 1) intermediate layer. The data demonstrate that the formation of this intermediate phase is related to the activation of a long range Sr migration from the SrTiO(3) substrate into the film. Our results enrich the phase diagram of the Sr-Ti-O system under epitaxial strain opening a route for the study of the electronic and dielectric properties of the reported Sr-deficient SrTiO(3) phase.

Phase Transitions in Ferroelectric Thin Films and Superlattices: Thermophysical Approach

It is shown that the information about thermophysical parameters of the nanostructured ferroelectrics is a useful supplement to the traditional x-ray and dielectric data. The results of the study of the temperature dependence of heat capacity of BaTiO 3 /MgO epitaxial thin films by dynamical 3ω method for the thickness range 50–450 nm are presented. The blurred specific heat anomalies corresponding to the phase transitions were observed, and the temperature of the transitions increases with decreasing of film thickness compared with T c in a bulk sample. The causes of the severe smearing of specific heat anomalies as well as pronounced nonlinearity of the dependence of the transition temperature on the film thickness are discussed. Specific heat data for ferroelectric superlattices (BaTiO 3 ) n /(Sr TiO 3 ) m /Sr TiO 3 were obtained for the first time for the films with the average thicknesses near 100 nm.

Si field emitter arrays coated with thin ferroelectric films

This paper demonstrates novel approach on Si field emitter arrays (FEAs) coated with thin ferroelectric films for vacuum microelectronic applications, which exhibit enhanced electron emission behaviors. The films were deposited using sol–gel and sputtering process, respectively. In sol–gel approach, the emission behavior is highly correlated to the crystallinity of (Ba,Sr)TiO 3 (BST) layer. The interfacial reaction between Si and BST film would deteriorate the crystallinity of the films, and in turn impede the electron emission from silicon tips. The film thickness and the dopants also affect the emission behaviors significantly. In sputtering process, the nitrogen-incorporated SrTiO 3 (STO) films are deposited with eliminated interfacial due to relatively lower processing temperature. The enhanced emission characteristics are highly correlated with nitrogen-incorporation and film thickness. These encouraging results have offered great promise for the application of ferroelectric films in field emission devices.

High-spin polarized Co-doped (La,Sr)TiO 3 thin films on high-mobility SrTiO 3 substrates

We report on the magnetotransport properties and spin polarization of Co-doped (La,Sr)TiO 3 (Co-LSTO) thin films in which the host oxide is a strongly correlated metal with a high density of carriers. In order to probe the spin polarization, we have performed tunneling magnetoresistance (TMR) measurements on magnetic tunnel junctions associating Co-LSTO and Co electrodes. A large spin polarization of around −80% has been determined at low temperatures, indicating that the carriers in this system are highly spin-polarized. We also report on the planar magnetotransport experiments on Co-LSTO films grown on SrTiO 3 substrates. The electrical properties of these samples are strongly dependent on the growth conditions and large electronic mobilities as high as 10 4 cm 2/V s at T<10 K are observed for specimens grown at the lowest oxygen pressure. We show that this high-mobility state is three-dimensional and we relate it to the doping of the SrTiO 3 substrate with oxygen vacancies.

In situ photoemission study of the contact formation of (Ba,Sr)TiO 3 with Cu and Au

The contact formation of (Ba,Sr)TiO 3 thin films and SrTiO 3 single crystals with Cu and Au have been studied using X-ray and ultraviolet photoelectron spectroscopy with in situ sample preparation. During metal deposition a partial reduction of the substrate occurs. The Fermi level at the interface is found to be close to the conduction band minimum, indicating small Schottky barrier heights (< 0.2 eV).

Nonaqueous and Halide‐Free Route to Crystalline BaTiO3, SrTiO3, and (Ba,Sr)TiO3 Nanoparticles via a Mechanism Involving C—C Bond Formation.

AbstractFor Abstract see ChemInform Abstract in Full Text.

Nonaqueous and halide-free route to crystalline BaTiO3, SrTiO3, and (Ba,Sr)TiO3 nanoparticles via a mechanism involving C-C bond formation.

A novel nonaqueous route for the preparation of nanocrystalline BaTiO(3), SrTiO(3), and (Ba,Sr)TiO(3) has been developed. In a simple one-pot reaction process, the elemental alkaline earth metals are directly dissolved in benzyl alcohol at slightly elevated temperatures. After the addition of Ti(O(i)Pr)(4), the reaction mixture is heated to 200 degrees C, resulting in the formation of a white precipitate. XRD measurements prove the exclusive presence of the perovskite phase without any other crystalline byproducts such as BaCO(3) or TiO(2). TEM investigations reveal that the BaTiO(3) nanoparticles are nearly spherical in shape with diameters ranging from 4 to 5 nm. The SrTiO(3) particles display less uniform particle shapes, and the size varies between 5 and 10 nm. Lattice fringes observed in HRTEM measurements further prove the high crystallinity of the nanoparticles. Surprisingly, GC-MS analysis of the reaction solution after hydrothermal treatment shows that hardly any ether formation occurs during the BaTiO(3) synthesis. Instead, the presence of 4-phenyl-2-butanol in stoichiometric amounts gives evidence that the formation mechanism proceeds mainly via a novel pathway involving C-C bond formation between benzyl alcohol and the isopropanolate ligand.

Integration of&amp;gt;tex&amp;lt;$hbox Ba_xhbox Sr_1-xhbox TiO_3$&amp;gt;/tex&amp;lt;Thin Films With AlGaN/GaN HEMT Circuits

Ba Sr TiO (BST) thin films have large dielectric constants that can be varied by as much as a factor of 3 with an applied field, making them attractive for radio frequency (RF) cir- cuits as small-area ac bypass/dc blocking capacitors, or high-power varactors. However, BST must be deposited at relatively high tem- peratures in an oxidizing environment, presenting significant in- tegration challenges for MMIC applications. This letter describes the successful integration of BST films on AlGaN/GaN high elec- tron-mobility transistor (HEMT) monolithic microwave integrated circuits on sapphire substrates. A sacrificial SiO buffer layer is used to protect the underlying AlGaN during the RF magnetron sputtering of the BST film at an elevated temperature, with a care- fully controlled heater ramp rate to avoid degradation of the ohmic contacts on the HEMT.

Preparation of high quality strontium titanate based thin films by ECR plasma sputtering

Strontium titanate based thin films were prepared using a mirror-confinement-type electron-cyclotron-resonance plasma sputtering method. Typically films were deposited Ar/O 2 gas mixture with a thickness of approximately 100 nm on Si wafers and Pt/Ti/SiO 2/Si multilayer substrates at floating potential and at a temperature below 473 K. Films on Si substrates were found to be amorphous-like both in the preparation of SrTiO 3 (STO) films and (Ba, Sr)TiO 3 (BST) ones. While on Pt-multilayer substrates STO films could be crystallized well even at as-deposited condition, but BST films were poorly crystallized. As for the post-annealing of these films, 28 GHz millimeter-wave radiation could drastically lower their crystallization temperature. STO films on Si substrates were crystallized at 673 K and BST at 923 K by the conventional method, while by the millimeter-wave method the crystallization temperature of STO films decreased to 573 K and that of BST to 673 K. The millimeter-wave irradiation to STO on Pt-multilayer substrates did not give remarkable contribution to further crystallization, since they were already crystallized well at as-deposited condition. While for BST on Pt-multilayer substrates the temperature again decreased to 673 K by the millimeter-wave irradiation. The dielectric constant of as-deposited STO films on the Pt-multilayer substrates was approximately 70, and it increased drastically to a value of approximately 260, which was comparable with the bulk value, by the millimeter-wave annealing at 573 K.

XRF and SIMS/SNMS analyses of BaxSr1-xTiO3 dielectrics.

The development of analytical tools and procedures for process control is a prerequisite for the integration of high permittivity and/or ferroelectric materials in CMOS devices. The thickness and composition of perovskite oxide films were determined by wavelength dispersive X-ray fluorescence analysis (XRF) with special emphasis on the ratio of the group-II elements to the Ti content, and a precision of 0.5% was achieved for a typical film thickness of 20-30 nm. Secondary ion mass spectrometry (SIMS) and sputtered neutrals mass spectrometry (SNMS) was used for depth profiling to determine film homogeneity and elemental interdiffusion at hetero-interfaces. Examples are given for Ba(x)Sr(1-x)TiO(3) and SrTiO(x) thin films which were grown in a prototype MOCVD production tool. No interdiffusion was observed for films grown at 600 degrees C on Pt electrodes in contrast to films grown directly on Si.

Study of the Ferroelectric Domain Structure and Phase Transitions by Confocal Scanning Optical Microscopy

Confocal scanning optical microscopy (CSOM) has been shown to be a promising method to investigate different properties of ferroelectric domain structures. It combines diffraction-limited spatial resolution with non-invasiveness and sensitivity of optical techniques. We have obtained optical and electrooptic images in ac modulation mode using model ferroelectrics systems, including both single crystals and thin films. The origin of CSOM contrast, its dependence on mutual orientation of the electric field, ferroelectric polarization, and light polarization vector have been established. The direction of ferroelectric polarization in a given point can be determined accurately, within a few degrees. The phase transition in solid solution (Ba,Sr)TiO 3 thin films with different compositions has been studied. It has been shown that paraelectric and ferroelectric areas coexist on both sides of dielectric permittivity maximum. The phase transition was observed to be locally sharp, contrary to wide diffused peak in macroscopic characteristic. Local ferroelectricity was also observed in pure SrTiO 3 thin films, a quantum paraelectric in bulk form.

PLD growth of ZnO film free from deep level emission using (La,Sr)TiO 3 substrate

This paper describes the pulsed laser deposition (PLD) technology of making ZnO thin films free from deep level emission (a problem of conventionaly grown ZnO films). A visible broad peak (deep level emission) is often seen in the ZnO photoluminescence (PL) spectrum and also a sharp exciton peak. ZnO films PLD-grown on La-doped (0.5%) SrTiO 3 substrates were compared with those grown on SrTiO 3 substrates. When grown on La-doped SrTiO 3 substrate, the deep level emission disappeared almost completely. Even when (LaSr)TiO 3 film was inserted between the ZnO film and SrTiO 3 substrate, deep level emission was suppressed to a large extent. Using the present result, we fabricated a NiO/ZnO/La x Sr 1− x TiO 3 structure and characterized it.

Growth of (Ba,Sr)TiO 3 Thin Films by MOCVD: Stoichiometry Effects

(Ba x Sr 1 m x )TiO 3 thin films were deposited in a planetary multi-wafer MOCVD reactor combined with a liquid delivery system using 0.35 molar solutions of Ba(thd) 2 and Sr(thd) 2 and a 0.4 molar solution of Ti(O-i-Pr) 2 (thd) 2 . The film growth on Pt-(111) was investigated within a wide parameter field, e.g., the deposition temperature was varied between 560C and 650C, which yields films with microstructures ranging from randomly oriented polycrystalline to perfectly (100)-textured columnar structures. Special emphasis is given to film stoichiometry: starting with (Ba 0.7 Sr 0.3 )TiO 3 the Group-II/Ti content was varied from 0.9 to 1.1 and the Ba content was reduced to the limit of pure SrTiO 3 films. The electrical properties of MIM structures were investigated after deposition of Pt top electrodes. The nominal thickness of the films was varied between 5 and 100 nm and permittivity and leakage current both are shown to depend strongly on the film thickness. These dependencies on the film thickness are analyzed within the phenomenological dead layer model. The dependence of the electrical properties on stoichiometry are discussed in detail.

Factors affecting the photoelectrochemical fixation of carbon dioxide with semiconductor colloids

Carbon dioxide was reduced photocatalytically using aqueous CdS or ZnS colloids containing tetramethylammonium chloride to give the dimeric and tetrameric products namely, oxalate, glyoxylate, glycolate and tartrate. A model is presented to explain the role of the tetramethylammonium ions. Studies were also performed using ZnO, SiC, BaTiO 3 and Sr TiO 3, which in the absence of tetramethylammonium ions produced formate and formaldehyde. The relative quantum efficiencies of the six semiconductors were related to their band gaps and conduction band potentials. The role and effectiveness of several `hole acceptor' (electron donor) compounds in this process is shown to be related to their redox potentials.

Structural Control of Epitaxially Grown Sputtered Perovskite Thin Films

AbstractThin films of perovskite Pb–Ti–O3 families were heteroepitaxially grown by sputtering on (0001)sapphire and/or (001)SrTiO (ST). These epitaxial films contained microstructures, although X–ray diffraction analysis suggested formation of single crystal phase with three dimentional crystal orientation. Their microstructures were studied by the electron microscopy, atomic force microscopy, and spectroscopic ellipsometry so as to find factors which influence the formation of the microstructure. It was found that the orientation of the substrate surface and the chemical composition of adatoms during initial film growth strongly affected the formation of the microstructures. Sputtered PbTiO3 (PT) thin films under a stoichiometric condition on a miscut(001) ST(miscut 1.7 degree) realized the growth of continuous single crystal thin films of 10–100nm thick with extremely smooth surface with surface roughness less than 3nm. Deposition on a miscut substrate under a stoichiometric condition is essential to make continuous thin films of perovskite of single crystal phase.

Minor phases in the Earth's mantle: Evidence from trace- and minor-element patterns in primitive alkaline magmas

A review of trace- and minor-element distributions in primitive alkaline rocks (alkali basalts to melilitites with Ni = 200–500 ppm and Mg' = Mg ( Mg + 0.9 Fe) atomic = 0.66–0.75 ) shows element patterns consistent with the presence of several residual minor phases in the source regions for some small percentage melts. TiO 2 concentrations separate the primitive magmas into two distinct groups apparently reflecting fundamental differences in their source compositions. RbSr data for both the high- and low-TiO 2 magma groups display an arrow-shaped distribution of points with the highest Rb concentrations at intermediate (∼ 1000 ppm) Sr values. Modelling shows that the disappearance of phlogopite as a residual phase, in generating intermediate- to large-percentage melts (< 1100 ppm Sr) can explain the data patterns. BaSr data are consistent with this conclusion but also hint at residual K-richterite in the source for some low-TiO 2 magmas. Modelling of the constant P 2 O 5 Ce ratio and positive correlation between P 2O 5 and Ce concentrations in magmas shows that the correlation need not indicate that apatite is never a residual phase. However, the simplest explanation for the positive correlation between both SrP 2O 5 and CeP 2O 5 data is that apatite is not normally a residual phase. Two low-TiO 2 samples plotting off the CeP 2O 5 correlation line (and with high SiO 2 concentrations) may reflect rare instances of residual apatite. SrTiO 2 data for high-TiO 2 samples define an arrow-shaped pattern consistent with the disappearance of a Ti-bearing phase. Consideration of TiO 2ZrNb data indicates that phlogopite is most likely the residual mineral though a role for ilmenite cannot be eliminated. Low Nb concentrations (compared with Zr concentrations) in low-TiO 2 samples suggest that rutile is a residual mineral in the source regions for these magmas. Examination of the Sr concentrations at which inflections occur on Sr vs. Rb, and TiO 2 plots indicates that phlogopite and, if present, ilmenite disappear at about the same percentage of melting in the high-TiO 2 magmas. Similarly, phlogopite (and/or K-richterite) disappear during the transition from nephelinites to alkali basalts in the low-TiO 2 magmas but rutile appears capable of persisting to higher percentages of melting.

Preparation and properties of (Ba, Sr)TiO 3 single crystals

High quality solid solution single crystals of (Ba xSr1 -x)TiO 3 (BST) with any value of x can be pulled from a melt containing about 36 mol-% BaO/SrO and 64 mol-% TiO 2. The low frequency dielectric behaviour resembles that of the respective ceramics. The character of phase transition changes from 1st to 2nd order when the SrTiO 3-content exceeds around 50 mol-%. Accordingly the dielectric nonlinearity constant of the free crystal reverses its sign at that composition.

Doping of ferroelectric solutions of (Ba, Sr Ti O 3) with In 2 O 3 to improve the RC time constant

The principle of conversion of solar energy into electrical energy using ferroelectric materials has been discussed by Hoh [1]. A capacitor made of ferroelectric material is subjected to heat cycling above the Curie temperature. During heating the permittivity across the ferroelectric material decreases and hence the voltage across a charged capacitor increases. The heat energy supplied to the capacitor is converted into electrical energy and could be obtained by discharging the capacitor. It is known that the Curie temperature of Ba Ti O 3 can be decreased by doping it with Sr TiO 3. It has been observed by us that each 1 per cent by weight of Sr Ti O 3 lowers the Curie temperature of Ba Ti O 3 by 3·5 deg. It has been pointed out by Hoh [1] that Ba Ti O 3 is not suitable for ferroelectric energy conversion devices because of semiconduction. Solid solutions of various percentages of Sr Ti O 3 in Ba Ti O 3 are also not suitable for such devices because of semiconduction. The efficiency of the device depends upon the RC time constant of the capacitor. The larger the RC time constant, the more efficient is the device. The product RC = ϱε, where ϱ is the resistivity in Ω-m and ε is the permittivity in F/m. Since changes of ε in orders of magnitude are not conceivable, it is essential that the resistivity determines the RC time constant. In this work a method for increasing the resistivity of (Ba, Sr) Ti O 3 solid solutions was developed by utilizing the principles of electrical compensation. It was found that (Ba 0.77, Sr 0.23) Ti O 3 solid solution with a Curie temperature of nearly 30°C showed an N-type of conduction. To neutralize the excess free electrons, an appropriate amount of indium was added in the form of In 2 O 3. The resistivity was observed to increase by two orders of magnitude with optimum doping. Excess doping impaired the compensation.

Ein modulator für das synchrozyklotron bonn mit ferroelektrischer keramik

The frequency modulation system for the university of Bonn 190 cm synchrocyclotron is described. The unusual feature of this system is the use of BaSrTiO 3 sinterceramic capacitors for the variable capacity. 4.3% frequency modulation is obtained at 11 Mc/s with 24 capacitors (0.6 mm thick and 11 mm dia, mounted in xylene) each controlled by a low frequency modulating voltage up to 1400 volts. Design problems, concerning heat transfer and breakdown are discussed. The main advantage of this system is that many modulating wave-shapes and repition rates are possible.