Interactive Growth Effects of Rare‐Earth Nanoparticles on Nanorod Formation in YBa2Cu3Ox Thin Films

The controlled growth of self‐assembled second‐phase nanostructures has been shown to be an essential tool for enhancing properties of several composite oxide thin film systems. Here, the role of Y₂O₃ nanoparticles on the growth of BaZrO₃ (BZO) nanorods is investigated in order to understand the mechanisms governing their self‐assembly in YBa₂Cu₃O_7–x (YBCO) thin films and to more fully control the resulting defect landscape. By examining the microstructure and current‐carrying capacity of BZO‐doped YBCO films, it is shown that the nanorod growth dynamics are significantly enhanced when compared to films double‐doped with BZO and Y₂O₃ nanoparticles. The average nanorod length and associated critical current densities are found to increase at a significantly higher rate in the absence of Y₂O₃ nanoparticles when the growth temperature is increased. Using microstructural data from transmission electron microscopy studies and the response in critical current density, the interactive effects of multiple dopants that must be considered to fully control the defect landscape in oxide thin films are shown.     

Microwave properties of epitaxial large-area Ca-doped YBa2Cu3O7−δ thin films on r-plane sapphire

Ca doping of YBa₂Cu₃O_7−δ (YBCO) is well known to enhance the critical current density in large-angle grain boundaries for example of bicrystals. However, up to now no data are available on microwave properties of epitaxial Ca-doped YBa₂Cu₃O_7−δ thin films on r-plane sapphire with CeO₂ buffer layer.Therefore, first results are presented for large-area pulsed laser deposition (PLD) grown Ca_xY_1−xBa₂Cu₃O_7−δ films on 3-in. diameter sapphire wafers. The PLD process is optimised for undoped YBCO thin films and shows high reproducibility for YBCO. The microwave surface resistance R_s at 8.5 GHz of Ca-doped YBCO (x=0.1) thin films shows clear reduction (up to 20%) with respect to that of YBCO for temperatures from about 20–50 K. In addition, microwave surface resistance R_s of Ca-doped YBCO is lower than that of YBCO even for enhanced microwave surface magnetic field up to about 20 mT for temperatures 20 and 40 K.     

Superconducting Properties of (M x /YBa2Cu3O7−δy )N Multilayer Films with Variable Layer Thickness x

The superconducting properties of (M _x /YBa₂Cu₃O_7−δy )_N multilayer films were studied for varying layer thickness x. Different M phases were examined including green-phase Y₂BaCuO₅ (211), Y₂O₃, BaZrO₃, CeO₂, SmBa₂Cu₃O_7−δ (Sm123), brown-phase La₂BaCuO₅ (La211), and MgO. Multilayer (M _x /YBa₂ Cu₃O_7−δy )_N structures were grown by pulsed laser deposition onto SrTiO₃ or LaAlO₃ single-crystal substrates by alternate ablation of separate YBa₂Cu₃O_7−δ (123) and M targets, at temperatures of 750°C to 790°C. The x layer thickness was varied from 0.1 nm to 4.5 nm, and the y 123 layer thickness was kept constant within a given range of 10 to 25 nm. Different M phase and x layer thicknesses caused large variations of the microstructural and superconducting properties, including superconducting transition (T _c), critical current density as a function of applied magnetic field J _c(H), self-field J _c(77 K), and nanoparticle layer coverage. Strong flux-pinning enhancement up to 1 to 3x was observed to occur for M additions of 211 and BaZrO₃ at 65 to 77 K, Y₂O₃ at 65 K, and CeO₂ for H < 0.5 T. BaZrO₃ had a noticeably different epitaxy forming smaller size nanoparticles ∼8 nm with 3 to 4x higher areal surface particle densities than other M phases, reaching 5 × 10¹¹ nanoparticles cm⁻². To optimize flux pinning and J _c (65 to 77 K, H = 2 to 3 T), the M layer thickness had to be reduced below a critical value that correlated with a nanoparticle surface coverage <15% by area. Unusual effects were observed for poor pinning materials including Sm123 and La211, where properties such as self-field J _c unexpectedly increased with increasing x layer thickness.     

An Easily Sintered,Chemically Stable,Barium Zirconate‐Based Proton Conductor for High‐Performance Proton‐Conducting Solid Oxide Fuel Cells

Yttrium and indium co‐doped barium zirconate is investigated to develop a chemically stable and sintering active proton conductor for solid oxide fuel cells (SOFCs). BaZr_0.8Y_0.2‐xIn_xO_{3‐ δ} possesses a pure cubic perovskite structure. The sintering activity of BaZr_0.8Y_0.2‐xIn_xO_{3‐ δ} increases significantly with In concentration. BaZr_0.8Y_0.15In_0.05O_{3‐ δ} (BZYI5) exhibits the highest total electrical conductivity among the sintered oxides. BZYI5 also retains high chemical stability against CO₂, vapor, and reduction of H₂. The good sintering activity, high conductivity, and chemical stability of BZYI5 facilitate the fabrication of durable SOFCs based on a highly conductive BZYI5 electrolyte film by cost‐effective ceramic processes. Fully dense BZYI5 electrolyte film is successfully prepared on the anode substrate by a facile drop‐coating technique followed by co‐firing at 1400 °C for 5 h in air. The BZYI5 film exhibits one of the highest conductivity among the BaZrO₃‐based electrolyte films with various sintering aids. BZYI5‐based single cells output very encouraging and by far the highest peak power density for BaZrO₃‐based proton‐conducting SOFCs, reaching as high as 379 mW cm^?2 at 700 °C. The results demonstrate that Y and In co‐doping is an effective strategy for exploring sintering active and chemically stable BaZrO₃‐based proton conductors for high performance proton‐conducting SOFCs.     

Structural and magnetic properties of silver doped melt-textured YBCO prepared in a solar furnace

Magnetic measurements and structural investigation have been performed on melt-textured YBCO and AgYBCO HTS. The “sun” technique produces very dense YBCO (ρ=5.86 g cm⁻³) and AgYBCO [ρ=6.36 g cm⁻³; 10% b/w (by weight) silver. This technique renders samples with a large volume fraction of the Y₂BaCuO₅ (211) phase. The material is characterized by very high J_c values, as compared with bulk polycrystalline YBCO prepared by other methods. This feature is attributed to the enhanced amount of 211 particles which serve as pinning centers. Additional significant densification of the structure due to silver incorporation is obtained, and a reduction of the size of 211 inclusions is also observed. Silver doped samples show “butterfly”-like hysteresis loops at relatively high temperatures (T≥60K). This feature is probably associated with oxygen deficiency which arises from the slower oxygen diffusion into silver doped samples. J_c values enhancement was obtained in silver doped “sun” samples at high temperatures (T≥60K) and fields of 20–30 kOe. The temperature dependence of effective activation energy of pinning, U_eff, was measured for YBCO and AgYBCO materials. U_eff is higher in silver doped samples in the high temperature region T≥60K.     

Highly (1 0 0)-oriented Pb(Zr0.20Ti0.80)O3/(Pb1−xLax)Ti1−x/4O3 multilayered thin films prepared by RF magnetron sputtering

The Pb(Zr_0.20Ti_0.80)O₃/(Pb_1−xLa_x)Ti_1−x/4O₃ (x = 0, 0.10, 0.15, 0.20) (PZT/PLT_x) multilayered thin films were in situ deposited on the Pt(1 1 1)/Ti/SiO₂/Si(1 0 0) substrates by RF magnetron sputtering technique with a PbO_x buffer layer. With this method, all PZT/PLT_x multilayered thin films possess highly (1 0 0) orientation. The PbO_x buffer layer leads to the (1 0 0) orientation of the multilayered thin films. The effect of the La content in PLT_x layers on the dielectric and ferroelectric properties of the PZT multilayered thin films was systematically investigated. The enhanced dielectric and ferroelectric properties are observed in the PZT/PLT_x (x = 0.15) multilayered thin films. The dielectric constant reaches maximum value of 365 at 1 KHz for x = 0.15 with a low loss tangent of 0.0301. Along with enhanced dielectric properties, the multilayered thin films also exhibit large remnant polarization value of 2P_r = 76.5 μC/cm², and low coercive field of 2E_c = 238 KV/cm.     

Sputtered oxides used for passivation layers of amorphous InGaZnO thin film transistors

Four sputtered oxide films (SiO₂, Al₂O₃, Y₂O₃ and TiO₂) along with their passivating amorphous InGaZnO thin film transistors (a-IGZO TFTs) were comparatively studied in this paper. The device passivated by an Al₂O₃ thin film showed both satisfactory performance (μ_FE=5.3 cm²/V s, I_on/I_off>10⁷) and stability, as was probably related to smooth surface of Al₂O₃ thin films. Although the performance of the a-IGZO TFTs with a TiO₂ passivation layer was also good enough (μ_FE=3.5 cm²/V s, I_on/I_off>10⁷), apparent V_th shift occurred in positive bias-stress tests due to the abnormal interface state between IGZO and TiO₂ thin films. Sputtered Y₂O₃ was proved no potential for passivation layers of a-IGZO TFTs in this study. Despite unsatisfactory performance of the corresponding a-IGZO TFT devices, sputtered SiO₂ passivation layer might still be preferred for its high deposition rate and excellent transparency which benefit the mass production of flat panel displays, especially active-matrix liquid crystal displays.     

Synthesis and characterization of Pb(Zr0.54Ti0.46)O3 thin films on (100)Si using textured YBa2Cu3O7−δ and yttria-stabilized zirconia buffer layers by laser physical vapor deposition technique

We have fabricated high-quality <001> textured Pb(Zr_0.54Ti_0.46)O₃ (PZT) thin films on (00l)Si with interposing <001> textured YBa₂Cu₃O_7−δ (YBCO) and yttria-stabilized zirconia (YSZ) buffer layers using pulsed laser deposition (KrF excimer laser, λ, = 248 nm, τ = 20 nanosecs). The YBCO layer provides a seed for PZT growth and can also act as an electrode for the PZT films, whereas YSZ provides a diffusion barrier as well as a seed for the growth of YBCO films on (001)Si. These heterostructures were characterized using x-ray diffraction, high-resolution transmission electron microscopy, and Rutherford backscattering techniques. The YSZ films were deposited in oxygen ambient (∼9 × 10⁻⁴ Torr) at 775°C on (001)Si substrate having <001>YSZ // <001>Si texture. The YBCO thin films were deposited in-situ in oxygen ambient (200 mTorr) at 650°C. The temperature and oxygen ambient for the PZT deposition were optimized to be 530°C and 0.4-0.6 Torr, respectively. The laser fluence to deposit this multilayer structure was 2.5-5.0 J/cm². The <001> textured perovskite PZT films showed a dielectric constant of 800-1000, a saturation polarization of 37.81 μC/cm², remnant polarization of 24.38 μC/cm² and a coercive field of 125 kV/cm. The effects of processing parameters on microstructure and ferroelectric properties of PZT films and device implications of these structures are discussed.     

Study of high-quality epitaxial YBCO thin films grown directly on Y-Cut LiNbO3

Pulsed laser deposition was used to deposit high-quality YBa₂Cu₃O_7-δ (YBCO) thin films directly on y-cut LiNbO₃ substrates. The as-deposited YBCO films had a high degree of in-plane orientation and showed superconducting transition temperature (T_co) at 91K with a transition width of less than IK. Transport critical current densities were found to be ∼10⁶ A/cm² at 77K and zero field. An ion beam minimum channeling yield of 16% was obtained for YBCO films, indicating high crystallinity. High-resolution transmission electron microscopy studies showed that the interface between the film and the substrate was quite smooth and free from interfacial interdiffusion. The defects in thin films are also identified. The work showed that high-quality high T_c thin films can be deposited directly on LiNbO₃. Novel devices based on the properties of both YBCO and LiNbO₃ could be realized based on these results.     

MISFET structures with barium titanate as a dielectric layer for application in memory cells

This work shows investigations of La₂O₃ containing BaTiO₃ thin films deposited on Si substrates by Radio Frequency Plasma Sputtering (RF PS) of sintered BaTiO₃ + La₂O₃ (2 wt.%) target. Round, aluminum (Al) electrodes were evaporated on top of the deposited layers. Thus, metal–insulator–semiconductor (MIS) structures were created with barium titanate thin films playing the role of an insulator. The MIS structures enabled a subsequent electrical characterization of the studied film by means of current–voltage (I–V) and capacitance–voltage (C–V) measurements. Several electronic parameters, i.e., ε_ri, ρ, V_FB, ΔV_H were extracted from the obtained characteristics. Moreover, the paper describes technology process of MISFETs fabrication and possibility of their application as memory cells. The influence of voltage stress on transfer and output I–V characteristics of the transistors are presented and discussed.     

Engineered flux-pinning centers in Bi2Sr2CaCu2Ox and TIBa2Ca2Cu3Ox superconductors

Three methods were used to introduce flux-pinning centers into Bi₂Sr₂CaCu₂O_x (Bi-2212) and TlBa₂Ca₂Cu₃O_x (Tl-1223) samples. It was found that carbon induced local decomposition, that nanosized Al₂O₃ additions created stable reaction products, and that second phases could be isolated in Tl-1223 during synthesis. Each of these defects enhanced flux pinning and was of most benefit at temperatures ≤ 35K.     

Effect of substitution of Ti by Zr in BaTiO3 thin films grown by MOCVD

Thin films of the solid solution BaTiO₃–BaZrO₃ were studied. Such lead-free, environmental friendly materials are known, from dielectric measurements, to exhibit relaxor behaviour in bulk materials with increasing the Zr content. Ba[Ti_(1−x)Zr_x]O₃ thin films with various x values were prepared by liquid injection MOCVD by varying the corresponding x′ value in solution (from 0.00 to 0.80). The films were studied by X-ray diffraction, Raman spectroscopy and microprobe analysis. A single perovskite phase with a linear evolution of the out-of-plane lattice parameter was identified by X-ray diffraction up to x′=0.25. For higher Zr contents a secondary ZrO₂ phase was detected. Raman spectroscopy was used to follow the subtle evolution of the crystal structure as a function of the chemical composition. The dielectric properties of single-phase layers were investigated in the range 20–600 K and 0.02–100 kHz. For some composition, the measured dielectric constant displayed a frequency-dependent behaviour.     

Magnetic shielding effects in Ag-doped and pure YBa2Cu3O7 − x thin films probed by non-resonant microwave absorption

A non-resonant microwave absorption technique is used to study the magnetic shielding effects in Ag-doped and pure YBa₂Cu₃O_{7 − x}(YBCO) thin films for small modulating fields. While Ag-doped YBCO thin films show good shielding for small modulating fields (≈ 1 Oe) until 60 K, the pure YBCO films show significant shielding effect only up to 20 K. We interpret these results in terms of better grain alignment in Ag-doped YBCO films as compared to pure YBCO films, which can lead to better shielding effects.     

Structural,chemical, and electrical properties of Y2O3 thin films grown by atomic layer deposition with an (iPrCp)2Y(iPr-amd) precursor

Y₂O₃ thin films were grown by atomic layer deposition (ALD) through a heteroleptic liquid (iPrCp)₂Y(iPr-amd) precursor at 350 °C. The structural and chemical properties of both as-deposited and annealed Y₂O₃ films at 500 °C and 700 °C are analyzed by atomic force microscopy for variation in surface roughness, X-ray diffraction for crystalline structure, and X-ray photoelectron spectroscopy for chemical states. The as-deposited Y₂O₃ film shows the same crystalline orientation along the plane (222), a stoichiometric state, and minimal hydroxylate formation up to 700 °C. Being the dielectric layer in the metal-oxide-semiconductor capacitor, the as-deposited ALD-Y₂O₃ films with liquid (iPrCp)₂Y(iPr-amd) precursor without any post-deposition annealing show the much lower leakage density than ALD-Y₂O₃ with solid Y(MeCp)₃.     

Comparison of the surface morphology and microstructure of in situ and ex situ derived YBa2Cu3O7−x thin films

The surface morphology and microstructure of in situ and ex situ derived YBa₂Cu₃O_7−x (YBCO) thin films have been investigated. In situ films were deposited by single-target off-axis sputtering and three-target co-sputtering. Ex situ films were derived by metalorganic deposition (MOD) of trifluoroacetate precursors. Surface defects resulting from mixed a-axis and c-axis orientation as well as secondary phases have been identified in these films. Despite these defects, films with excellent electrical properties have been formed. However, defects interfere with film patterning and the fabrication of multi-layered structures. Several secondary phase precipitates have been identified, including CuO, Y₂O₃, Cu-Ba-O, and Y₂Cu₂O₅. Secondary phases resulting from a lack of stoichiometry can be eliminated by direct composition control in the MOD and three-target sputtering techniques, and by composition control through the application of an externally applied magnetic field in single-target off-axis sputtering. Secondary phases caused by contamination were also identified: Cr-Ba-O in off-axis sputtering, resulting from contamination by the oxidized heater block; and BaSO₄ in MOD, resulting from gas phase impurities. These results suggest that cation composition control is not sufficient to prevent the formation of secondary phases and that small levels of contamination may prevent phasepure material from being formed.     

Ferroelectric and Ferromagnetic Behavior of Pb(Zr0.52Ti0.48)O3–Co0.9Zn0.1Fe2O4 Multilayered Thin Films Prepared via Solution Processing

Multilayered multiferroic nanocomposite films of Pb(Zr_0.52Ti_0.48)O₃ (PZT) and Co_0.9Zn_0.1Fe₂O₄ (CZFO) are prepared on general Pt/Ti/SiO₂/Si substrates via a simple solution‐processing method. Structural characterization by X‐ray diffraction and electron microscopy techniques reveals good surface and cross‐sectional morphologies of these multilayered thin films. In particular, at room temperature strong ferroelectric and ferromagnetic responses are simultaneously observed in the multilayered thin films, depending on the deposited sequences and volume fractions of ferroelectric PZT phase and magnetic CZFO phase.     

An experimental investigation of the off-diagonal Seebeck effect on textured YBCO high-Tc superconductor

The off-diagonal Seebeck effect was investigated on a melt-textured YBa₂Cu₃O_7−δ (YBCO) high-T_c superconductor. It was found that the transverse voltage V_x was proportional to the longitudinal temperature difference Δ T_z, measured directly with a pair of differential thermocouples, for textured samples with their c-axes tilted with respect to the surface’s normal. This supported the idea that the off-diagonal thermoelectric effect accounts for the anomalously high laser-induced transverse voltage on the oriented YBCO superconducting thin films. The variation of the V_x against the sample’s thickness d, at a given Δ T_z, deviated from the inversely proportional relationship when the sample was too thin. The deviation was discussed qualitatively in terms of deteriorated surface layers. The data of V_x(d) was fitted to obtain a reasonably reliable ∣ S_c−S_ab∣ value of 12 μ V K⁻¹.     

The Difference Between Thermo- and Pyroelectric Co-Based Rare-Earth (Nd,Y, Gd,Ce) Oxide Composites Measured Using a High Temperature Gradient

The Seebeck voltage U _S of cobalt-based oxide composites containing rare-earth elements such as Nd, Y, Gd, and Ce was measured under high temperature gradients of up to ΔT = 700 K, as well as its time dependence U _S(t). The closed-circuit electric current as a function of Seebeck voltage I _S(U _S) or time I _S(t) was used for further characterization. While Nd₂O₃ + CoO and Y₂O₃ + CoO showed linear n-type U _S(t) behavior, as it is usual for thermoelectrics, Gd₂O₃ + CoO exhibited large hysteresis. For both Gd₂O₃ + CoO and Ce₂O₃ + CoO, a significant time dependence of the electric current I(t) was measured, indicating pyroelectric material behavior with large capacitance. Both anomalies became smaller when Fe₂O₃ was added, but became more apparent in Nd₂O₃ + CoO and Y₂O₃ + CoO when Al₂O₃ was added. This behavior can be explained by electron pull-out into the interfacial space-charge region, which is considered as a material phenomenon with great potential for further development.     

Oxide thin-film electroluminescent devices and materials

This report reviews newly developed oxide phosphors shown to be promising as the emitting layer of thin-film electroluminescent (TFEL) devices. Since the first report of a high-luminance TFEL device using a Mn-activated Zn₂SiO₄ (Zn₂SiO₄:Mn) phosphor, high-luminance multicolor-emitting TFEL devices have been fabricated using various oxide phosphors activated with Eu or Mn. In addition, many oxides that consist of binary and ternary compounds and multicomponent oxides have been developed and shown to be promising as a host material for TFEL phosphors. This report focuses on Mn-activated Y₂O₃-based oxide phosphors: a binary compound and various ternary compounds and multicomponent oxides, composed of Y₂O₃ in combination with another binary compound such as Ga₂O₃ or GeO₂. TFEL devices of which every constituent was an oxide material were fabricated using oxide phosphor thin films deposited by r.f. magnetron sputtering, pulsed laser deposition or a sol–gel process. High luminances and luminous efficiencies comparable to those of TFEL devices using ZnS:Mn sulfide phosphor were realized using Mn-activated Y₂O₃-based oxide phosphors. Luminances above 7000 cd/m² (1 kHz-driving voltage) and luminous efficiencies of approximately 10 lm/W (60 Hz driving voltage) were obtained in yellow emitting TFEL devices fabricated using a Y₂O₃:Mn, a ((Y₂O₃)_0.6–(GeO₂)_0.4):Mn or a ((Y₂O₃)_0.5–(Ga₂O₃)_0.5):Mn thin film and driven by an ac sinusoidal wave voltage. Also, a high luminance above 1000 cd/m² for green emission was obtained in a ((Y₂O₃)_0.3–(Ga₂O₃)_0.7):Mn TFEL device driven at 1 kHz.