Twinning of YBa2Cu3O7 thin films on different substrates

The twinning of YBa₂Cu₃O₇ (YBCO) thin films with c-axis orientation on (001) MgO, (001) SrTiO₃, (012) LaAlO₃, (110) NdGaO₃ and (001) NdGaO₃ substrates, prepared by laser ablation, has been examined using a combination of and θ/2θ scans at a four-circle diffractometer. On all substrates, except for (001) NdGaO₃, the tetragonal to orthorhombal phase transition results in four different orientations of YBCO twins relating to the substrate. On (001) NdGaO₃ only two different twin orientations, accompanied by a slight lattice monoclinization, has been observed.     

Preparation and characterization of YBa2Cu3O7−x high Tc superconducting thin films deposited by S-gun sputtering

We have prepared YBa₂Cu₃O_7−x high T_c superconducting (HTS) thin films on (100) yttria-stabilized zirconia (YSZ) and LaAlO₃ (LAO) substrates, using a 2 kW S-gun in an off-axis mode. By varying the temperature of the substrates, films with a axis and c axis orientations were readily obtained. The X-ray diffraction pattern and Laue pattern confirmed that films with a axis orientation exhibited a single-crystal texture. All films had a good mirror-like surface. For films grown on YSZ substrates, scanning electron microscopy (SEM) revealed a clear distinction between the surfaces of the films grown at various temperatures (520–780°C). Films grown on LAO substrates exhibited even smoother and flatter surfaces. The SEM changes will be discussed in correlation with J_c. The best HTS thin films were obtained on LAO substrates at a temperature of 820°C, with T_c=89 K and J_c=1×10⁶ A cm^-2 (77 K).     

Dissolution of Y2BaCuO5 through the peritectic reaction rim formed at the front of growing YBa2Cu3O7−y crystal

Microstructure related to the peritectic reaction was investigated in melt-textured Y-Ba-Cu-O. It was found that Y₂BaCuO₅ particles dissolved in the peritectic reaction rim formed at the interface between YBa₂Cu₃O_7−y, crystal and Ba-Cu-O liquid phase. Y₂BaCuO₅ morphology was a blocky shape in the liquid phase while it was changed to a round shape in the textured YBa₂Cu₃O_7−y crystal through an irregular shape due to inhomogeneous dissolution of Y₂BaCuO₅ particles. By way of dissolution, one Y₂BaCuO₅ particle was divided into several particles. The refinement process of Y₂BaCuO₅ is discussed in terms of the dissolution process of Y₂BaCuO₅ and its morphology.     

Evaporated YBa2Cu3O7 thin films and device technology

Epitaxial thin films of YBa₂Cu₃O₇ were grown by evaporation of the metals from electron-guns in the presence of atomic oxygen. The films were characterized structurally by X-ray diffraction and electrically by measurements of d.c. magnetization and flux creep. The progress in device technology is discussed. Results are presented on Josephson junctions, and a superconducting microwave filter with the ground plane grown on the back.     

Total dose radiation effects of Au/PbZr0.52Ti0.48O3/YBa2Cu3O7-δ capacitors

PbZr_0.52Ti_0.48O₃/YBa₂Cu₃O₇₋δ (PZT/YBCO) thin films have been fabricated on Y₂O₃ stabilized zirconate (YSZ) substrates by a pulsed excimer laser deposition (PLD) method. In order to investigate total dose radiation effects on the Au/PZT/YBCO ferroelectric capacitor, the capacitance–voltage (C–V) curves and the retained polarization property of the capacitor have been measured before and after γ-ray irradiation. The results showed that, with an increased total dose, the retained polarization and the dielectric constant decreased, but the coercive field drifted towards positive voltage direction. This is caused by charges trapped by defects in the PZT capacitor during irradiation.     

Ferroelectric/superconductor heterostructures

This review covers the fabrication and characterization of ferroelectric/superconductor heterostructures such as Pb(Zr_xTi_1−x)O₃/YBa₂Cu₃O_7−δ (YBCO), BaTiO₃/YBCO and Ba_xSr_1−xTiO₃/YBCO etc. on various single crystal substrates. Pulsed laser deposition, laser molecular beam epitaxy, and magnetron-sputtering methods are compared. This report shows that pulsed laser deposition equipped with in situ reflection high-energy electron diffraction is a good method to control the growth mode of YBCO thin films. Furthermore, laser molecular beam epitaxy is a superb method for research of complex oxide films and their superlattices. Atomic force microscopy and transmission electron microscopy showed the ferroelectric films grown on the rough surface of the YBCO films produced high-density planar defects in the film and is detrimental to the ferroelectric/dielectric properties of the heterostructures. Therefore, for device usage, it is more advantageous to use SrRuO₃ than YBCO as the bottom electrode material. For growing atomically smooth surface films step-flow mode is highly recommended. Prospects of microwave device application of the ferroelectric/superconductor heterostructures are discussed, and proposed the BSTO films as the best candidate for passive microwave components.     

Molecular beam epitaxial growth of SmBa2Cu3O7−δ thin films and effect of substrate temperature on the surface roughness

Stoichiometrically optimized, epitaxial SmBa₂Cu₃O_7-δ thin films with high T_{c, R = 0} and high critical current densities j_c have been prepared for the first time in a tightly controlled molecular beam epitaxy process in non-reactive molecular oxygen, followed by an in situ loading process with molecular oxygen. The surface roughness (on a submicrometre scale) of single-crystal films with their c axes perpendicular to the surface depends markedly on the surface temperature of the substrate during the deposition of the epitaxial films, within a range of only a few degrees centigrade. The calibrated optimal temperature for the preparation of epitaxial films 200 nm thick of this single orientation is found to be 680 ± 5 °C. In scanning tunnelling microscopy investigations, they show a surface roughness of less than 6 nm (five SmBa₂Cu₃O_7−δ unit cells) on a 2 μm × 2 μm scale. At deposition temperatures below this optimal deposition temperature, the well-known a-axis growth increases rapidly, whereas higher temperatures give a significantly higher surface roughness, which can be observed by scanning electron microscopy.     

Investigation on the cross sensitivity of NO2 sensors based on In2O3 thin films prepared by sol-gel and vacuum thermal evaporation

In₂O₃ thin films have been prepared from commercially available pure In₂O₃ powders by high vacuum thermal evaporation (HVTE) and from indium iso-propoxide solutions by sol-gel techniques (SG). The films have been deposited on sapphire substrates provided with platinum interdigital sputtered electrodes. The as-deposited HVTE and SG films have been annealed at 500°C for 24 and 1 h, respectively. The film morphology, crystalline phase and chemical composition have been characterised by SEM, glancing angle XRD and XPS techniques. After annealing at 500°C the films’ microstructure turns from amorphous to crystalline with the development of highly crystalline cubic In₂O_3−x (JCPDS card 6-0416). XPS characterisation has revealed the formation of stoichiometric In₂O₃ (HVTE) and nearly stoichiometric In₂O_3−x (SG) after annealing. SEM characterisation has highlighted substantial morphological differences between the SG (highly porous microstructure) and HVTE (denser) films. All the films show the highest sensitivity to NO₂ gas (0.7–7 ppm concentration range), at 250°C working temperature. At this temperature and 0.7 ppm NO₂ the calculated sensitivities (S=R_g/R_a) yield S=10 and S=7 for SG and HVTE, respectively. No cross sensitivity have been found by exposing the In₂O₃ films to CO and CH₄. Negligible H₂O cross has resulted in the 40–80% relative humidity range, as well as to 1 ppm Cl₂ and 10 ppm NO. Only 1000 ppm C₂H₅OH has resulted to have a significant cross to the NO₂ response.     

Magnetic properties and critical currents of epitaxial YBa2Cu3O7-x films

The authors report results of magnetic and transport measurements on thin epitaxial films of YBa₂Cu₃O_7-x which show critical current densities of 10⁷ A/cm² at 4.2 K. They exhibit well-formed symmetrical hysteresis loops and flux-trapping effects and linear susceptibilities at low fields. Magnetic and transport critical currents are in good agreement at low temperatures. The above properties are attributed to strong pinning from point defects which are suggested to be more numerous in films than in bulk single crystals. Diamagnetic shielding effects can be very large and are proportional to the critical current at zero field; however, there is a large penetration of H_a at all field values. Field-cooled magnetization is always very small, being only a few percent of the diamagnetic shielding. This small value is attributed to a balance between trapped flux and expelled flux in the cooling process. The strong pinning in attributed to a high density of defects in the film     

Normal metal, ohmic contacts to high-temperature superconducting YBa2Cu3O7-δ thin films suitable for multichip modules

Normal metal, ohmic contacts to high-temperature superconductor (HTSC) materials will be used to form via structures between HTSC interconnect levels, and also, substrate bonding pads in a superconducting multichip module (SMCM). Specific contact resistivities below 10⁻⁸ Ω cm² will be required for such contacts to control signal attenuation and local contact heating of the LN₂cooled SMCM. Previous work on normal metal/superconducting contacts has not focused on metallization schemes which will be stable during subsequent high-temperature processing. Metal contacts of gold, silver, and palladium were formed on superconducting thin films of YBa₂Cu₃O_7-δ via evaporation and sputtering through a shadow mask followed by annealing in various ambients and at several temperatures. Palladium contacts oxidized readily during anneal, and sputtered gold contacts required additional processing and exhibited higher specific contact resistivities. The best contacts were obtained by a controlled-cooling oxygen anneal of evaporated gold or silver, as indicated by normal-state specific contact resistivities of 3 × 10⁻⁵ Ω cm² and 4 × 10⁻⁵ Ω cm², respectively. This work differs from previously published results by describing contacts which required no extensive preparation of the HTSC surface and were stable to 700 °C, indicating these contacts would be compatible with subsequent high-temperature processing of the additional HTSC layers required in a multi-level SMCM.     

YNixMn1−xO3 thin films by pulsed laser deposition: Structure and magnetic properties

Highly oriented YNi_xMn_1−xO₃ thin films on SrTiO₃ (100) substrates were achieved by using pulsed laser deposition for x = 0.33 and x = 0.50. We used a combination of X-ray diffraction, scanning electron microscopy, atomic force microscopy, and magnetic-property measurements. The magnetic transition temperatures (T_c) of the as-grown films are higher than the corresponding bulk values (typically 85 K instead of 80 K, for x = 0.5, and 60 K instead of 50 K, for x = 0.33). Our magnetic measurements also suggest a spin-glass characteristic in the x = 0.33 films, while a cluster glasslike behavior is observed for the films with x = 0.5, which is quite different from that of the bulk samples. Finally, the influence of post-deposition heat treatment on the magnetic properties of the as-grown films is discussed.     

Electronic structure of the ladder-chain compound Sr14−xCaxCu24O41

The electronic structure of the ladder-chain compound Sr_14−xCa_xCu₂₄O₄₁ is studied by ab initio calculations within the local density approximation. The effects of Ca substitution and structure modulation on electronic structure are discussed. It is found that 0.05 holes per copper atom are on the ladder layers for fully substituted compound, Ca₁₄Cu₂₄O₄₁.     

Pyroelectric properties of BST/PLT composite thick films with addition of xPbO–(1 − x)B2O3 glass

The Ba_xSr_1−xTiO₃ (BST)/Pb_1−xLa_xTiO₃ (PLT) composite thick films (20 μm) with 12 mol% amount of xPbO–(1 − x)B₂O₃ glass additives (x = 0.2, 0.35, 0.5, 0.65 and 0.8) have been prepared by screen-printing the paste onto the alumina substrates with silver bottom electrode. X-ray diffraction (XRD), scanning electron microscope (SEM) and an impedance analyzer and an electrometer were used to analyze the phase structures, morphologies and dielectric and pyroelectric properties of the composite thick films, respectively. The wetting and infiltration of the liquid phase on the particles results in the densification of the composite thick films sintered at 750 °C. Nice porous structure formed in the composite thick films with xPbO–(1 − x)B₂O₃ glass as the PbO content (x) is 0.5 ≥ x ≥ 0.35, while dense structure formed in these thick films as the PbO content (x) is 0.8 ≥ x ≥ 0.65. The volatilization of the PbO in PLT and the interdiffusion between the PLT and the glass lead to the reduction of the c-axis of the PLT phase. The operating temperature range of our composite thick films is 0–200 °C. At room temperature (20 °C), the BST/PLT composite thick films with 0.35PbO–0.65B₂O₃ glass additives provided low heat capacity and good pyroelectric figure-of-merit because of their porous structure. The pyroelectric coefficient and figure-of-merit F_D are 364 μC/(m² K) and 14.3 μPa^−1/2, respectively. These good pyroelectric properties as well as being able to produce low-cost devices make this kind of thick films a promising candidate for high-performance pyroelectric applications.     

Deposition of CeO2 films including areas with the different orientation and sharp border between them

Epitaxial films from one material, with sharp borders between contacting regions having different film orientation are grown on one surface of the substrate for the first time. The main reason for the deposition of thin ceria layers with mixed (001) and (111) orientations on a (1 02) sapphire substrate is determined. We suggest that this is related to the availability of surface defects which, in thin near-surface layers, deviate from stoichiometric composition. This in turn is connected with the loss of oxygen.

A technique for influencing CeO₂ film orientation is demonstrated. This involves specific preliminary processing of the substrate, and the selection of oxygen partial pressure during the deposition process.

High quality thin (30–50 nm) “protective” (001) CeO₂ epitaxial layers are prepared on (1 02) Al₂O₃. Structures comprising two epitaxial protective CeO₂ layers, orientations (001) and (111), are made on the base of (0001) and (1 02) sapphire substrates. The interface between the epitaxial layers is <1 000 nm.

Preliminary results using this method are described, and the possibility of creating a “bi-epitaxial” transition in thin YBa₂Cu₃O_7−x layers is explored.     

Influence of rare-earth oxides on structure and crystallization properties of Bi2O3–B2O3 glass

Bi₂O₃·B₂O₃ glasses doped with rare-earth oxides (RE₂O₃) (RE³⁺ = La³⁺, Pr³⁺, Sm³⁺, Gd³⁺, Er³⁺ and Yb³⁺) were prepared by the melting–quenching method. The relationships between composition and properties were demonstrated by IR, DSC, XRD and SEM analysis. The results show that the network structure resembles that of undoped Bi₂O₃·B₂O₃ glass, composing of [BO₃], [BO₄] and [BiO₆] units. RE₂O₃ stabilizes the glass structure as a modifier. Transition temperature (T_g) increases linearly with cationic field strength (CFS) of RE³⁺. La₂O₃, Pr₂O₃, Sm₂O₃ and Gd₂O₃ are benefit to promote the formation of BiBO₃ crystal. When Er₂O₃ and Yb₂O₃ are introduced, respectively, the main crystal phase changes to Bi₆B₁₀O₂₄. Transparent surface crystallized samples are obtained by reheating at 460–540 °C for 5 h. In this case, needle like BiBO₃ crystal or rare-earth-doped BiBO₃ crystal (Pr_xBi_1−xBO₃ and Gd_xBi_1−xBO₃) are observed, which is promising for non-linear optical application.     

Infrared optical properties of Bi2Ti2O7 thin films by spectroscopic ellipsometry

Bi₂Ti₂O₇ thin films have been grown directly on n-type GaAs (1 0 0) by the chemical solution decomposition technique. X-ray diffraction analysis shows that the Bi₂Ti₂O₇ thin films are polycrystalline. The optical properties of the thin films are investigated using infrared spectroscopic ellipsometry (3.0–12.5 μm). By fitting the measured ellipsometric parameter (Ψ and Δ) data with a three-phase model (air/Bi₂Ti₂O₇/GaAs), and Lorentz–Drude dispersion relation, the optical constants and thickness of the thin films have been obtained simultaneously. The refractive index and extinction coefficient increase with increasing wavelength. The fitted plasma frequency ω_p is 1.64×10¹⁴ Hz, and the electron collision frequency γ is 1.05×10¹⁴ Hz, and it states that the electron average scattering time is 0.95×10⁻¹⁴ s. The absorption coefficient variation with respect to increasing wavelength has been obtained.     

Optical and electrical properties of p-type AgInSnxS2−x (x = 0–0.04) thin films prepared by spray pyrolysis

AgInSn_xS_2−x (x = 0–0.2) polycrystalline thin films were prepared by the spray pyrolysis technique. The samples were deposited on glass substrates at temperatures of 375 and 400 °C from alcoholic solutions comprising silver acetate, indium chloride, thiourea and tin chloride. All deposited films crystallized in the chalcopyrite structure of AgInS₂. A p-type conductivity was detected in the Sn-doped samples deposited at 375 °C, otherwise they are n-type. The optical properties of AgInSn_xS_2−x (x < 0.2) resemble those of chalcopyrite AgInS₂. Low-temperature PL measurements revealed that Sn occupying an S-site could be the responsible defect for the p-type conductivity observed in AgInSn_xS_2−x (x < 2) thin films.     

Fabrication and characterization of (1−x)SrBi2Ta2O9–xBi3TaTiO9 layered structure solid solution thin films for ferroelectric random access memory (FRAM) applications

We report on the properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ solid solution thin films for ferroelectric non-volatile memory applications. The solid solution thin films fabricated by modified metalorganic solution deposition technique showed much improved properties compared to SrBi₂Ta₂O₉. A pyrochlore free crystalline phase was obtained at a low annealing temperature of 600°C and grain size was found to be considerably increased for the solid solution compositions. The film properties were found to be strongly dependent on the composition and annealing temperatures. The measured dielectric constant of the solid solution thin films was in the range 180–225 for films with 10–50% of Bi₃TaTiO₉ content in the solid solution. Ferroelectric properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ thin films were significantly improved compared to SrBi₂Ta₂O₉. For example, the observed remanent polarization (2P_r) and coercive field (E_c) values for films with 0.7SrBi₂Ta₂O₉–0.3Bi₃TaTiO₉ composition, annealed at 650°C, were 12.4 μC/cm² and 80 kV/cm, respectively. The solid solution thin films showed less than 5% decay of the polarization charge after 10¹⁰ switching cycles and good memory retention characteristics after about 10⁶ s of memory retention. The improved microstructural and ferroelectric properties of (1−x)SrBi₂Ta₂O₉–xBi₃TaTiO₉ thin films compared to SrBi₂Ta₂O₉, especially at lower annealing temperatures, suggest their suitability for high density FRAM applications.     

Preparation and photoelectrochemical properties of Ti1−xVxO2 solid solution thin film photoelectrodes with gradient bandgap

A design of a gradient bandgap Ti_1−xV_xO₂ thin film electrode for wet-type solar cells is provided. The gradient bandgap film electrodes were prepared by heating stacked layers of varying V/Ti ratios using the sol-gel method. A composition gradient was observed for some of the samples by X-ray photoelectron spectroscopy although it was not very large. For the Ti_1−xV_xO₂ film electrodes, conspicuous visible light photoresponse and photoelectrochemical stability were observed. The photocurrent increased with increasing bias potential. However, the photocurrent onset potentials of the Ti_1−xV_xO₂ film electrodes were more positive than those of TiO₂ film electrodes, probably owing to the high surface state density introduced by the diffusion of vanadium ions.     

Nanocrystallization of SrBi2Nb2O9 from glasses in the system Li2B4O7---SrO---Bi2O3---Nb2O5

Transparent glasses in the system (100−x)Li₂B₄O₇–x(SrO---Bi₂O₃---Nb₂O₅) (10≤x≤60) (in molar ratio) were fabricated by a conventional melt-quenching technique. Amorphous and glassy characteristics of the as-quenched samples were established via X-ray powder diffraction (XRD) and differential thermal analyses (DTA) respectively. Glass–ceramics embedded with strontium bismuth niobate, SrBi₂Nb₂O₉ (SBN) nanocrystals were produced by heat-treating the as-quenched glasses at temperatures higher than 500 °C. Perovskite SBN phase formation through an intermediate fluorite phase in the glass matrix was confirmed by XRD and transmission electron microscopy (TEM). Infrared and Raman spectroscopic studies corroborate the observation of fluorite phase formation. The dielectric constant (_r) and the loss factor (D) for the lithium borate, Li₂B₄O₇ (LBO) glass comprising randomly oriented SBN nanocrystals were determined and compared with those predicted based on the various dielectric mixture rule formalism. The dielectric constant was found to increase with increasing SBN content in LBO glass matrix.