K. Tsukada

Preparation of Epitaxial YBCO Films by a Novel Excimer-Laser-Assisted MOD

Epitaxial (YBCO) films were prepared on cerium oxide-buffered (40 nm)-cut single-crystal sapphire substrates by a novel excimer-laser-assisted metalorganic deposition (ELAMOD). Laser irradiation after application of a fluorine-free ingredient solution onto the substrates revealed to have the advantages of shorter heating (prefiring and heat-treatment) time and higher superconducting current density . The in excess of 5 was measured by an inductive method at 77.3 K. The laser irradiation is considered to produce a desirable precursor, in which three metal components are well-mixed in short time. The X-ray diffraction measurement of the irradiated area clearly showed a complete -axis oriented YBCO and no impurity phase, whereas -axis oriented YBCO was also observed in the area without irradiation. The ratio of the -axis component decreased with an increase of the laser irradiation.

Preparation of CeO/sub 2/-buffer layers for large-area MOD-YBCO films (10/spl times/30 cm/sup 2/) with high-J/sub c/

Epitaxial cerium oxide (CeO/sub 2/; thickness: 40 nm) buffer layers were deposited on large area (maximum size 10/spl times/30 cm/sup 2/) R-cut single-crystal sapphire (/spl alpha/-Al/sub 2/O/sub 3/) substrates by double electron-beam guns. Substrate heater-and-holder system was modified to maintain the substrate temperature uniform over the large area during deposition. Oxygen gas of a pressure 4/spl times/10/sup -2/ Pa was introduced to keep radio frequency plasma. When the substrate temperature was controlled in the range 640/spl deg/-700/spl deg/C, the CeO/sub 2/ buffer showed a complete [001] orientation and very smooth surfaces by x-ray diffraction analysis and atomic force microscopic observations, respectively, in the whole area of the large size substrates. These buffer layers without post-deposition annealing were quite suitable for preparing YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) films by a metal-organic deposition (MOD) using a metal acetylacetonate-based solution. The average critical current density of the 210-nm-thick MOD-YBCO film was in excess of 2.6 MA/cm/sup 2/ measured by an inductive method at 77.3 K.

Structural aspect of high-Jc MOD-YBCO films prepared on large area CeO2-buffered YSZ substrates

YBa2Cu3O7 (YBCO) film was prepared by fluorine-free metalorganic deposition (MOD) on a 2-inch-diameter CeO2-buffered yttria-stabilized zirconia (CbYSZ) substrate. The MOD-YBCO films on a flat CbYSZ prepared using a metal acetylacetonate-based coating solution demonstrated high critical current density (Jc), the average being higher than 4 MA/cm2 at 77.3 K and in a self field using an inductive method. The Jc distribution was uniform in the whole area of the MOD-YBCO films on the large size substrates. Cross sectional transmission electron microscopic images of the MOD-YBCO film exhibited a very smooth CeO2/ YSZ and CeO2 buffer layer/ YBCO film interfaces. Both the buffer and the YBCO layers were dense; however, a small amount of (100)-oriented YBCO occurred inside the matrix of (001)-oriented YBCO layers. Also, some distinct dislocations were observed at the CeO2/ YBCO interface. From around the stepped surface of the CeO2 buffer layer there seemed to grow a slightly distorted (001)-oriented YBCO domain, however, the domain restored its (001)-oriented periodicity in the region at some distance above the steps. Such growth mode is considered to correlate the high Jc of the MOD-YBCO film with very small average roughness of CeO2 buffer layer.

Microstructure of Epitaxial Y123 Films on

Epitaxial YBa2Cu3O7 (Y123) films have been prepared by fluorine-free metalorganic deposition (MOD) using a metal acetylacetonate-based coating solution on cerium oxide (CeO2: 40 nm)-buffered yttria-stabilized zirconia (CbYSZ) substrates. The 210-nm-thick Y123 film demonstrated a high critical current density (Jc), the average Jc being 4.5 MA/cm2 at 77.3 K using an inductive method. However, with increasing thickness the Jc values decreased; the average Jc was 3.4 and 2.2 MA/cm2 for 400-nm- and 550-nm-thick films, respectively. In low-magnification cross sectional transmission electron microscopy (XTEM) images for the films, the stripe-like contrast due to the c-axis orientation of Y123 was clearly seen from the interface with CeO2 to the surface. The whole c-axis-oriented Y123 layers were very dense even in the 550-nm-thick film. Full width at half maximum (FWHM) of Deltaomega for the 308/038 reflections in XRD omega - 2thetas scanning decreased with increasing film thickness. High resolution XTEM images of the 550-nm-thick film showed wavy c-planes of Y123 due to the short stacking faults near the interface with CeO2 buffer and near the surface whereas the c-planes were very flat but contained long stacking faults in the middle part of the film. It is suggested that the absence of wavy stacking faults, which probably work as pinning centers, decreased Jc in the thicker films. Further decrease in Jc in the 550-nm-thick film is presumably caused by the roughness in the outer layer near the surface.

{\rm CeO}_{2}

Distribution of J/sub c/ in the two-dimensional large-size YBa/sub 2/Cu/sub 3/O/sub 7/ (YBCO) film was investigated by an inductive method for the application to fault current limiter elements. The YBCO films were prepared by metal-organic deposition (MOD) using a fluorine-free acetylacetonate-based complex solution on R-plane sapphire with an evaporated-CeO/sub 2/ buffer layer. The average J/sub c/ values of 1-3 MA/cm/sup 2/ at 77 K were obtained for 1 cm /spl times/ 12 cm-, 3 cm /spl times/ 12 cm-, and 10 cm /spl times/ 30 cm-rectangular YBCO films as well as on 2-inch-diameter films. In contrast with the TFA-MOD method via BaF/sub 2/ precursor, the fluctuation of J/sub c/ in the present films was fairly small; no significant size effects or shape effects were recognized on the fluctuation of J/sub c/. Instead, fluctuation of J/sub c/ depended on the average J/sub c/ values; higher-J/sub c/ films tended to exhibit more uniform J/sub c/ distribution.

-Buffered YSZ Prepared by Fluorine-Free MOD

Large-size, two-dimensional Y Ba2Cu3O7 (YBCO) films were successfully prepared by a chemical solution-based coating pyrolysis (CP) process on 30 × 10 cm2 R-plane sapphire substrates with a vacuum-deposited CeO2 buffer layer. The CeO2 layer was very smooth and uniform with an average roughness (Ra) of 0.48 nm by atomic force microscopic observations and highly (001)-oriented by large-area x-ray diffraction θ–2θ scans. The c-axis-oriented YBCO films were prepared by a CP process using a fluorine-free, metal acetylacetonate-based coating solution. A 30 × 10 cm2-rectangular c-axis-oriented YBCO film annealed in a large-diameter tube furnace demonstrated high inductive-Jc values, for which the distribution was fairly uniform, with an average 1.0 MA cm−2 at 77 K.

Distribution of inductive J/sub c/ in two-dimensional large-size YBCO films prepared by fluorine-free MOD on CeO/sub 2/-buffered sapphire

It has been difficult to prepare Y123 films with a thick ness greater than 400 nm by using a single coating-preflring procedure in metal organic deposition (MOD), since shrinkage of the film causes cracks in the pyrolysis process. In the present study, we developed a novel coating solution for crack-free films with thicknesses greater than 500 nm. We used a Y-, Ba-, and Cu-acetylacetonate-based solution (metal content: ~2mol metal/L) with various additives for the coating solution. We prepared prefired films on CeO2-buffered YSZ (CbYSZ) or CeO2-buffered SrTiO3 (CbSTO) substrates using the single coating-preflring process and this solution. When using some additives, we could obtain crack-free preflred films with thicknesses corresponding to 350-800 nm after crystallization to Y123. These prefired films were heat-treated at high-temperatures, resulted in epitaxial Y123 films. The Jc and Ic of the film on CbSTO was ~2.3 MA/cm2 and 143 A/cm, respectively.

Two-dimensional large-size Y Ba2Cu3O7 films (30 × 10 cm2) on CeO2-buffered sapphire by a coating pyrolysis process

Abstract Thin films of Bi 2 VO 5.5 (BVO) were prepared on polycrystalline yttria-stabilized zirconia (YSZ) and single-crystal SrTiO 3 [STO] (100) and (110) substrates by a coating-pyrolysis process. X-ray diffraction (XRD) θ–2θ scans showed that the crystallinity of the films increased with increasing heat-treatment temperature. The crystallinity of the BVO (001) films was so high that the XRD peak intensity of the 002 reflection was comparable with that of the STO 100 reflection. XRD pole-figure analysis and reciprocal-space mapping revealed that the films on YSZ are uniaxially c -axis oriented, while epitaxial BVO films with the (001) orientation have grown on STO (100). On the other hand, the occurrence of epitaxial (114) orientation in the BVO films was discovered by the present study and is similar to the case of (116)-oriented SrBi 2 Ta 2 O 9 films on STO (110).

Preparation of Y123 Thick Films by Fluorine-Free MOD Using a Novel Solution

Abstract Epitaxial thin films of Bi 2 VO 5.5 (BVO) were prepared on La-doped SrTiO 3 (LSTO) by a coating-pyrolysis process. The BVO films were (001)- and (114)-oriented on LSTO(100) and (110), respectively. Atomic force microscopic observations showed that very dense surfaces were obtained in the BVO films annealed at 600 °C and p O 2 =10 −4 atm. The step heights of the BVO(001) film were in the range of 2–12 nm, each approximately corresponding to a multiple of the c -axis parameter of BVO. The low- p O 2 -processed and O 2 -postannealed BVO films resulted in reproducible P – E hysteresis loops for both orientations, although somewhat leaky characteristics were observed at higher applied voltages. Anisotropic behavior based on the orientation difference was clearly observed.

Preparation of (001)- and (114)-oriented epitaxial thin films of Bi2VO5.5 by a coating pyrolysis process

We investigated microfabrication using the metal–organic deposition (MOD) process with an electron beam. Metal naphthenates in the irradiated area remained after development, which indicates negative exposure characteristics. The optimized electron dose was estimated to be 3400 µC/cm2. Drawn patterns shrink after heat treatment because of the vaporation of organic components. A fine line with a width of 90 nm was successfully obtained after heat treatment. Numerous voids were observed on the surface of the large patterned film, compared with unirradiated films. Electron beam irradiation decomposed metal naphthenates, and cohesion was promoted by the annealing process. The optimization of the annealing profile is required to prepare uniform micropatterns.