Control of Microstructure and Orientation in Solution-Deposited BaTiO3 and SrTiO3 Thin Films

Columnar and highly oriented (100) BaTiO₃ and SrTiO₃ thin films were prepared by a chelate-type chemical solution deposition (CSD) process by manipulation of film deposition conditions and seeded growth techniques. Randomly oriented columnar films were prepared on platinum-coated Si substrates by a multilayering process in which nucleation of the perovskite phase was restricted to the substrate or underlying layers by control of layer thickness. The columnar films displayed improvements in dielectric constant and dielectric loss compared to the fine-grain equiaxed films that typically result from CSD methods. Highly oriented BaTiO₃ and SrTiO₃ thin films were fabricated on LaAlO₃ by a seeded growth process that appeared to follow a standard "two-step" growth mechanism that has been previously reported. The film transformation process involved the bulk nucleation of BaTiO₃ throughout the film, followed by the consumption of this matrix by an epitaxial overgrowth process originating at the seed layer. Both BaTiO₃ and PbTiO₃ seed layers were effective in promoting the growth of highly oriented (100) BaTiO₃ films. Based on the various processing factors that can influence thin film microstructure, the decomposition pathway involving the formation of BaCO₃ and TiO₂ appeared to dictate thin film microstructural evolution.     

Microstructure and Dielectric Properties of YMnO3 Thin Films Prepared by Dip-Coating

YMnO₃ thin films with Y/Mn ratios from 1.00/1.05 to 1.00/0.90 were prepared by dip-coating from solution, in which starting materials were refluxed, and the effects of the Y/Mn ratio on the structure and dielectric properties of YMnO₃ thin films were investigated. XRD measurements indicated that the films with the Y/Mn ratios in this study were a single phase of polycrystalline YMnO₃. The lattice constants along the a -axis and c -axis lengthened with an increase in the Y/Mn ratio. FE-SEM micrographs of the films showed that the surface of the films became smoother and denser with an increase in the Y/Mn ratio. YMnO₃ thin films with good dielectric properties were obtained with an Y/Mn ratio of 1.00/0.90, which gave the smoothest and densest microstructure and the smallest leakage current.     

Grain-Boundary Migration and Dielectric Properties of Semiconducting SrTiO3 in the SrTiO3-BaTiO3-CaTiO3 System

Chemically induced grain-boundary migration and its effects on the interface and dielectric properties of semiconducting SrTiO₃ have been investigated. Strontium titanate specimens that had been doped with 0.2 mol% of Nb₂O₅ were sintered in 5H₂/95N₂. The sintered specimens were diffusion annealed at 1400°C in 5H₂/95N₂ with BaTiO₃ or 0.5BaTiO₃-0.5CaTiO₃ (mole fraction) packing powder. The grain boundaries of the annealed specimens were oxidized in air. In the case of BaTiO₃ packing, grain-boundary migration occurred with the diffusion of BaTiO₃ along the grain boundary. The effective dielectric constant of the specimen decreased gradually as the temperature increased but showed two peaks, possibly because of barium enrichment at the grain boundary and an oxidized Sr(Ba)TiO₃ layer. In the case of 0.5BaTiO₃-0.5CaTiO₃ packing, although barium and calcium were present at the grain boundary of the specimen, no boundary migration occurred, as in a previous investigation. With the diffusion of barium and calcium, the resistivity of the specimen increased and the variation of the effective dielectric constant with temperature was much reduced, in comparison to those without solute diffusion. These enhanced properties were attributed to the solute enrichment and the formation of a thin diffusional Sr(Ba,Ca)TiO₃ layer at the grain boundary.     

Microstructure of SrTiO3 Boundary-Layer Capacitor Material

SiTiO₃ capacitor material with indiffused Bi₂O₃ was studied using SEM, ESCA combined with Ar⁺ ion-etching, and TEM equipped with EDX. The apparent thickness of a second-phase layer observed with SEM was found to be influenced by in-depth effects. ESCA and TEM results show that only a 10- to 100-nm thick layer of second phase is present between the SrTiO₃ grains. In addition, it was found with TEM that the outer part of each grain contained Bi (at most 2 at.%), representing a diffusion layer. These results have implications for the boundary-layer model proposed to explain the dielectric properties .     

Effect of Stacking Layers on the Microwave Dielectric Properties of MgTiO3/CaTiO3 Multilayered Thin Films

Effect of stacking layers on the microwave dielectric properties of the MgTiO₃/CaTiO₃ (MTO/CTO) multilayered thin films prepared by the metalorganic solution deposition technique (MOSD) was investigated. As the thickness of CTO film in the MTO/CTO multilayered films increased, the dielectric constant ( K ) increased and temperature coefficient of dielectric constant ( TCK ) changed from positive to negative values by dielectric series mixing rule. Especially, MTO(100 nm)/CTO(200 nm) multilayered films exhibited a TCK of +10 ppm/°C, indicating temperature stability. The dielectric losses (tan δ) of MTO/CTO multilayered films increased with an increase of CTO layers. This result was attributed to the fact that the stresses induced by the higher thermal-expansion coefficient of CTO than that of MTO. Also, as compared with MTO(100 nm)/CTO(200 nm) film, the K and TCK of MTO(50 nm)/CTO(200 nm)/MTO(50 nm) film were not changed, but the dielectric losses increased. This result indicated that the dielectric loss was affected by the number of interfaces between CTO and MTO layers.     

Microstructure and Electrical Properties of Chemically Prepared Nb2O5-Doped SrTiO3 Ceramics

Chemically homogeneous SrTiO₃ powders of submicrometer size were obtained by alcohol dehydration and subsequent calcination of citrate/format solutions. Nb₂O₅-doped SrTiO₃ was prepared with various Sr:Ti ratios resulting in an anomalous increase in the dielectric constant ( K 'up to ∼8000) for donor-doped SrO-excess SrTiO₃. No semiconducting behavior was observed for donor-doped TiO₂-excess SrTiO₃ when fired in air. Therefore, a "brick-wall" type of microstructure was formed as a result of the excess SrO, giving rise to anomalously high dielectric constants.     

Reaction Rates of BaTiO3 and SrTiO3

The temperature dependence of the reaction rates in a 50(BaTiO₃)-50(SrTiO₃) ceramic mixture (wt%) was studied by high-temperature X-ray diffraction. The data were fitted to two theoretical models and times for complete reactions and activation energy were calculated. Use of the results in electronic material applications is discussed.     

Microstructure, Orientation, and Properties of Sol-Gel-Derived KTiOPO4 Thin Films

Dense and crack-free thin films of potassium titanyl phosphate (KTP) were synthesized using a sol-gel process. The decomposition of the organic precursors and the formation of the KTP were studied using thermal analysis, DRIFT, and XRD. Several processing parameters such as the nature of the precursors, the aging of the alkoxide solution, the firing atmosphere, time, and temperature were found to affect the final film microstructure. Thin films were deposited on single-crystal KTP, LaGaO₃, MgO, and sapphire substrates to promote crystallographic orientation. SEM and XRD studies reveal that films deposited on KTP and LaGaO₃ single crystals are highly oriented. The films deposited on MgO or sapphire were found to detach or chemically degrade. The orientation as well as the degradation mechanisms were examined in detail.     

Microstructure and Piezoelectric Properties of 0.95(Na0.5K0.5)NbO3–0.05SrTiO3 Ceramics

The 0.95(Na_0.5K_0.5)NbO₃–0.05SrTiO₃ (0.95NKN–0.05ST) ceramics formed in this study had a porous microstructure with small grains and low piezoelectric properties due to their low density. However, when a small amount of Na₂O was intentionally subtracted from the 0.95NKN–0.05ST ceramics, a liquid phase was formed, which led to increased density and grain size. Piezoelectric properties were also improved for the Na₂O-subtracted 0.95NKN–0.05ST ceramics. The increased density and grain size were responsible for the enhancement of the piezoelectric properties. In particular, the 0.95(Na_0.49K_0.5)NbO_2.995–0.05ST ceramics showed high piezoelectric properties of d ₃₃=220, k _p=0.4, Q _m=72, and ɛ₃^T/ɛ_o=1447, thereby demonstrating their promising potential as a candidate material for application to lead-free piezoelectric ceramics.     

Microstructure and Electrical Properties of MgAl2O4 Thin Films for Humidity Sensing

Active elements for humidity sensors based upon MgAl₂O₄ thin films or sintered pellets were investigated. Thin films were deposited on Si/SiO₂ substrates by radiofrequency (rf) sputtering. Sintered MgAl₂O₄ pellets were prepared by traditional ceramic processing. Scanning electron microscopy (SEM) analysis showed that the thin films were rather dense and homogeneous, made up of clustered particles of about 20–30 nm, while the pellets showed a wide pore-size distribution. X-ray photoelectron spectroscopy (XPS) demonstrated that the thin films have a stoichiometry close to that of MgAl₂O₄. Sintered MgAl₂O₄ is crystalline, while it is disordered in thin-film form. The presence of two different components of the Al 2 p peaks was correlated with the structural difference between pellets and thin films. The relationship between good film–substrate adhesive properties and the chemical composition at the interface was studied. The electrical properties of the sensing elements were studied at 40°C in environments at different relative humidity (RH) values between 2% and 95%, using ac impedance spectroscopy. MgAl₂O₄ thin films showed interesting characteristics in terms of their use in humidity-measurement devices. Resistance versus RH sensitivity values showed variations as high as 4 orders of magnitude in the RH range tested for thin films, and 5 orders of magnitude for pellets. The differences in the electrical behavior of MgAl₂O₄ pellets and thin films were correlated with their different microstructures.     

Preparation and Characterization of Alkoxy-Derived SrZrO3 and SrTiO3

High-purity strontium, zirconium, and titanium alkoxides were synthesized and characterized as precursors for complex oxides. Simultaneous hydrolytic decomposition either of strontium and zirconium alkoxides or of strontium and titanium alkoxides was used to obtain nearly stoichiometric, ideally mixed SrZrO₃ or SrTiO₃ powders of high surface activity. As-prepared helium-dried SrTiO₃ is crystalline before calcination. An ultraviolet radiation technique demonstrates the nucleation and growth of SrZrO₃ crystallites in the calcination temperature range to 350°C. The experimental results are supported by ir, TGA, and X-ray diffraction data. The high degree of control over purity, mixing uniformity, and crystallite size demonstrates the value of the alkoxide precursor approach for the solution of reproducibility problems encountered in the synthesis of electrical-quality ceramics.     

Microstructure and Electrical Properties of Sodium-Diffused and Potassium-Diffused SrTiO3 Barrier-Layer Capacitors Exhibiting Varistor Behavior

In sodium-diffused strontium titanate internal-boundary-layer capacitors exhibiting good varistor properties, high-resolution transmission electron microscopy and scanning transmission electron microscopy revealed sodium segregation at grain boundaries in the absence of intergranular phases. The segregation layer is narrow (≅10 nm), unlike much broader diffused boundary layers which have also been observed. The nonohmic behavior in these and in potassium-diffused specimens suggests that segregated acceptor alkali ions act as grain-boundary electron traps leading to varistor electrical barriers.     

Electrical Conductivity of Epitaxial SrTiO3 Thin Films as a Function of Oxygen Partial Pressure and Temperature

SrTiO₃ (100) epitaxial films with thicknesses of 3, 1 μm, and 250 nm were prepared on MgO (100) substrates by pulsed-laser deposition. The electrical conductivities of the thin films were systematically investigated as a function of temperature and ambient oxygen partial pressure. This was made possible by using a specially designed measurement setup, allowing the reliable determination of resistances of up to 25 GΩ in the temperature range of 600°–1000°C under continuously adjustable oxygen partial pressures ranging from 10⁻²⁰ to 1 bar. The capabilities of the measurement setup were tested thoroughly by measuring a SrTiO₃ single crystal. The well-known characteristics, e.g., the decline of the conductivity with a slope of –1/4 under reducing conditions and the opposite +1/4 behavior in oxidizing atmospheres, are found in the log(σ)–log( p O₂) profiles of the epitaxial films. However, the p -type conductivity decreases, and the n -type conductivity increases with decreasing film thickness. This phenomenon is attributed to the charge carrier redistribution in the surface space charge layers. Owing to the high surface-to-volume ratio, the space charge layers play an important role in thin films.     

Microstructure and Electromagnetic Properties of SrTiO3/Ni0.8Zn0.2Fe2O4 Composites by Hybrid Process

High permittivity and low-loss SrTiO₃/Ni_0.8Zn_0.2Fe₂O₄ (STO/NZO) composites with concrete-like morphology were prepared through hybrid processing route. The dielectric properties in the low-frequency range (100 Hz–1 MHz) follow the rule of Maxwell–Wagner interfacial polarization. The dielectric and magnetic properties in the high-frequency range (10 MHz–1 GHz) exhibit a well integration of dielectric and magnetic properties in the composites with low dielectric and magnetic losses. The STO/NZO composites show good dielectric properties and magnetic properties with low loss in high-frequency to microwave-frequency range. The results show that this kind of magnetic–dielectric composites can be used in high-frequency communications for the capacitor–inductor integrating devices such as electromagnetic interference filters and antennas.     

Oxidation Kinetics of Single-Crystal SrTiO3

The oxidation kinetics were determined for single-crystal SrTiO₃ by measuring the time and temperature dependence of the weight gain of reduced crystals. The oxidation can be described as a diffusion-controlled process. The calculated diffusion coefficients between 850° and 1460°C are represented by D = 0.33 exp (-22.5 ± 5.0 kcal/ RT ) cm²/sec. Directly measured oxygen ion diffusion coefficients in the same temperature interval reported earlier are interpreted as being extrinsic and can be represented by D = 5.2 × 10⁻⁶ exp (-26.1 ± 5.0 kcal/ RT ) cm²/sec, where the activation energy is for mobility only. Assuming that the calculated diffusion coefficients are for vacancy diffusion and the two activation energies are equivalent within experimental error, a vacancy concentration (fraction of vacant lattice sites), [O□], fixed by impurities in the fully oxidized crystal is calculated to be 1.6 × 10⁻⁵ by virtue of the relation between the oxygen self-diffusion coefficient, D_02-, and the oxygen vacancy diffusion coefficient, D_o□ ; D _o2-= [O□] D _o□ where the oxygen ion concentration [O^2-] is taken as unity.     

Microcontact Printed BaTiO3 and LaNiO3 Thin Films for Capacitors

There is an ongoing need to develop new technologies to enable further down-scaling of layer thicknesses in multilayer ceramic devices, for example, in multilayer capacitors (MLC). Microcontact printing of chemical solutions of both the dielectric and electrode layers was explored as an economical means of preparing patterned thin films for MLC without requiring photolithography. For this purpose, methanol/acetic acid-based BaTiO₃ solutions were spun onto polydimethylsiloxane stamps, printed onto substrates, pyrolyzed, and crystallized. LaNiO₃ was used as a prototype electrode that could also be microcontact printed. The line edge roughness produced this way was on the order of a tenth of a micrometer, which should enable very small margins. The printed layer thickness was also very uniform. Microcontact printed capacitors with a single dielectric layer were fabricated and found to have dielectric constants >800 with loss tangents <2%. Alignment between subsequent layers is readily achieved. Multilayer dielectric/electrode stacks could be fabricated without cracking or delaminations. Consequently, microcontact printing appears to be a viable potential means of preparing MLC with layer thicknesses in the range of ≤0.2 μm.     

Reverse Micellar Route to Nanocrystalline Titanates (SrTiO3, Sr2TiO4, and PbTiO3): Structural Aspects and Dielectric Properties

Nanoparticles of strontium titanates (SrTiO₃, Sr₂TiO₄) and lead titanate (PbTiO₃) have been obtained using reverse micelles as nanoreactors. Powder X-ray diffraction studies of the powders after calcining at 800°C show monophasic SrTiO₃, Sr₂TiO₄, and PbTiO₃. X-ray line broadening studies and transmission electron microscopic studies show spherical grains of 30–40 nm size for strontium titanates, while PbTiO₃ is obtained in the form of nanorods. The dielectric constant of SrTiO₃ and Sr₂TiO₄ is found to be 90 and 30, respectively, (at 100 kHz) for samples sintered at 1000°C. PbTiO₃ shows a dielectric constant of 160 (at 100 kHz) after sintering at 900°C. The dielectric constant of Sr₂TiO₄ (with temperature) is highly stable. The temperature variation studies of the dielectric constant of PbTiO₃ show a ferroelectric phase transition at 490°C (1 kHz). The T _c varies with frequency and is found to decrease to 470°C at 100 kHz.     

Conversion of SiO2 Diatom Frustules to BaTiO3 and SrTiO3

Diatom frustules were used as bio-templates to synthesize functional ceramics via solid–gas displacement reactions. Silica-based frustules were exposed to TiF₄ at 330°C to form TiOF₂, which was later converted to TiO₂ (anatase) by heat treatment in air at 600°C. The TiO₂ frustules were then exposed to molten Ba(OH)₂ or Sr(OH)₂ to form BaTiO₃ or SrTiO₃, respectively. In both cases, near-complete conversion was achieved while retaining the morphology of the original silica frustules. BaTiO₃ and SrTiO₃ frustules exhibit nearly phase pure, nanocrystalline perovskite structure.     

Dielectric Properties of Electrophoretically Deposited and Isothermally Pressed BaTiO3 Thick Films

Thick BaTiO₃ films were prepared on platinum metallic foils by the electrophoretic deposition (EPD) technique using BaTiO₃ nanoparticles. In order to increase the density of the thick film, the green film was pressed under an isostatic pressure of 200 MPa before high-temperature sintering. The microstructures of deposited films were examined using X-ray diffraction and scanning electron microscopy techniques. Dielectric properties of the thick films were investigated. As the films grow thicker, the dielectric constant increases gradually and the dielectric loss decreases slightly. The experimental results indicate that isostatic pressing is an effective method to process thick films with dense microstructure and better dielectric properties.     

Fabrication and Properties of Sol–Gel-Derived BiScO3–PbTiO3 Thin Films

BiScO₃–PbTiO₃ (BSPT) thin films near the morphotropic phase boundary were successfully fabricated on Pt(111)/Ti/SiO₂/Si substrates via an aqueous sol–gel method. The thin films exhibited good crystalline quality and dense, uniform microstructures with an average grain size of 50 nm. The dielectric, ferroelectric, and piezoelectric properties of the sol–gel-derived BSPT thin films were investigated. A remanent polarization of 74 μC/cm² and a coercive field of 177 kV/cm were obtained. The local effective piezoelectric coefficient d ^*₃₃ was 23 pC/N at 2 V, measured by a scanning probe microscopy system. The dielectric peak appeared at 435°C, which was 80°C higher than that of Pb(Ti, Zr)O₃ thin films.