1PO4-40 The orientation growth mechanism of the YBa₂Cu₃O_7-y films during vapor-liquid-solid

Yutaka Yoshida 1, Koichi Gotou 2, Yoshiaki Takai 2, Masato Hasegawa 3, and Izumi Hirabayashi 3. 1 Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japan. 2 Department of Energy Engineering and Science, Nagoya University, Nagoya 464-8603, Japan. 3 Superconducting Research Laboratory, International Superconductivity Technology Center, Nagoya 456-8587, Japan.

Presenting Author: Y. Yoshida

We have made the examination in detail on the Vapor-Liquid-Solid (VLS) growth of oxide superconducting films. The VLS growth was discussed in the growth mechanism of ice and Si-whisker. We confirmed that the VLS growth induced by surface liquid layer lying in the YBa₂Cu₃O_7-y (YBCO) films, over 750 ° C of the substrate temperature. In addition, it was confirmed that the growth rate was enhanced about 5 times by controlling the liquid layer. Furthermore, the application of VLS growth to formation of the superconducting wire is examined. In this report, we discussed about the VLS growth process of the YBCO films examined from the viewpoint of the crystalline orientation control and the growth rate in VLS growth.

We fabricated the YBCO thin films deposited by MOCVD method; firstly Ba-Cu-O liquid layer of 130Å, and next YBCO layer. We obtained the c-axis oriented and in-plane aligned YBCO. However, it is confirmed that a/b-axis oriented grains grow when the film thickness become over 1000Å, and also confirmed that the growth rate of the YBCO layer over the liquid layer rapidly lowers as this film thickness of about 1000Å. The details of the relationship between the growth rate and the a/b orientation layer will be discussed with the TEM observation.

1PO4-41 Thin HTSC Films by Polymer Metal Precursor Technique

Irene von Lampe, Frank Zygalsky, and Georg Hinrichsen, Technical University Berlin, Institute of Nonmetallic Materials, Englische Str. 20, D-10587, Germany

Presenting Author: I. von Lampe

There exists a large variety of techniques for the production of HTSC films: thermal and electron beam evaporation or co-evaporation, thermal or laser molecular beam epitaxy (MBE), on- and off-axis high pressure sputtering, on- and off-axis laser ablation, metal organic chemical vapor deposition (MOCVD), sol gel and plasma spray deposition. Our polymer precursor technique requires a simple equipment (spincoater, oven) and saves processing steps by photolithographic generation of microbridges in the precursor film. The measurements of the specific conductivity show that it is possible to improve the HTSC properties by the knowledge of precursor properties characterized by TGA/MS and IR spectroscopic measurements. We used polyacryolic acid, polymethacrylic acid or novolacs for the preparation of Bi-Sr-Ca-Cu- and Y-Ba-Cu-precursors. The transition temperature of our YBCO films is 90K, of the BSCCO films 90 - 115K, the critical current density 5. 10⁶ A/^cm2. Films on large-area substrates have the same quality.

1PO4-42 BSCCO Superlattice Thin Film Fabrication by Reactive Co-deposition Method

Qi Yang, Sakai Kazuo, Murakami Hironaru, and Ito Toshimichi, Dept. of Electrical Eng., Osaka Univ., 2-1 Yamada-oka, Suita-shi 565-0871, Osaka, Japan

Presenting Author: Q. Yang

SNS (SIS) junction with high-T_c superconductor is expected as some high frequency devices. Therefore, the superlattice thin film consisted of YBCO/PrBCO/YBCO trilayer (Y-system) has been rapidly developed because of the goodness of the thin film quality. On the other hand, the development of the high-T_c BSCCO superconductor is late for Y-system due to the its quality inferiority. However, it is well known to have the strong two-dimensionality and three types the stable phases with the different T_c. The BSCCO series are composed of the alternative stacking along c-axis with the insulating layer and the layer in which the supercurrent flows, therefore adjustment among layers superior to that of Y-system is anticipated. It is also of interest that even a single phase displays the intrinsic Josephson character.

Thus, we have fabricated high-T_c BSCCO superconducting thin films by means of a reactive con-deposition method [1]. The dependence of the composition, substrate temperature, ozone pressures and growth rated on BSCCO phase formations has been already investigated and the optimum fabrication conditions were found out [2]. The oriented along c-axis Bi2212 single phases and Bi2212/Bi2201/Bi2212 superlattice thin films have been fabricated on MgO(100) substrate. The junction with area about 30x30m m is fabricated by FIB. The physical properties of these thin films are under investigation.

[1] Y.Qi et al. Advances in Superconductivity, (1996)997

[2] Y.Qi et al. Proceeding in VASSOAA-1.

1PO4-43 Thermodynamics and Growth of BiSrCaCuO Thin Films by MOCVD

Milos Nevriva 1, Josef Stejskal 1, David Sedmidubsky 1, Premysl Beran 1, Ales Strejc 1, and Jindrich Leitner 2. 1 Department of Inorganic Chemistry,Institute of Chemical Technology,Technicka 5,166 28 Prague 6, Czech Republic. 2 Department of Solid State Engineering, Institute of Chemical Technology, Technicka 5, 166 28 Prague 6, Czech Republic.

Presenting Author: M. Nevriva

In spite of an extensive study of BiSrCaCuO superconductors preparation methods of the high quality single-phase thin films in this system still represents one of the most significant problem. Thin films of general composition Bi₂(Sr,Ca)_n+1Cu_nO_2n+4 (n=1,2,3 corressponding to Bi-2201, Bi-2212 and Bi-2223 phase, respectively) have been grown in a cold-wall rf heated quartz reactor, using the 2,2,6,6-tetramethyl-3,5-heptanedionates of Cu, Ca and Sr and triphenylbismuth as metal precursors. To propose feasible deposition conditions a thermodynamic analysis of the Bi-Sr-Ca-Cu-O-C-H-Ar system have been performed by the use of Gibbs energy minimization technique. Some simplifications have been adopted in this study as the thermodynamic data for complex oxides existing in the BSCCO system were unavailable. The obtained results show that for the temperature range of 600-900°C and the input metal ratio varying between 2212 and 2223 the deposition resulted in formation of the Bi-2212 phase as the major and the equilibrium amounts of accompanying phases were insignificant. Nevertheless, under above mentioned deposition conditions pure Bi-2201 and Bi-2212 phases were formed on both theYGa₅O₁₂ and the SrTiO₃ substrates. The Bi-2212 single phase was only formed at temperature 850°C in the case when the flow rate ratios of Sr, Ca and Cu to Bi were significantly higher than the ideal stoichiometry Bi:Sr:Ca:Cu=2:2:1:2. The Bi-2223 phase, not predicted by the calculation, was found in some deposited layers only in a mixture with Bi-2201 and 2212 phase. The discrepancies between thermodynamic model and experimental results were ascribed to both the assumption made in thermodynamic model and the complicated evaporation of b diketonate of Cu,Ca and Sr precursors. The prepared layers were characterized by X-ray diffraction, electron microprobe analysis and scanning electron microscopy. The critical temperature was determined by measuring the ac magnetic response.

This work was supported by the Ministry of Education of the Czech Republic through Grant No.VS 96070

1PO4-44 Domain Structure of YBa₂Cu₃O_7-x Epitaxial Thin Films

Igor K. Bdikin 1, Alexander D. Mashtakov 2, Peter B. Mozhaev 2, Ekaterina I. Raksha 2, and Gennady A. Ovsyannikov 2. 1 Institute of Solid State Physics, Chernogolovka, Moscow dist. 142432, Russia. 2 Institute of Radio Engineering and Electronics, Moscow, 103907, Russia.

Presenting Author: I.K. Bdikin

Using X-ray diffraction methods we have studied the structure of YBa₂Cu₃O_7-x thin films on (110) NdGaO3, (130) and (120) NdGaO₃ with CeO₂ buffer layer and (1102) Al₂O₃ with CeO₂ buffer layer.

These thin films layers were deposited using RF-magnetron sputtering technique at growth temperatures 620-770°C. The thickness of films are 1300Å and buffer layers is 300Å. The structural parameter (c=11.65Å) of YBa₂Cu₃O_7-x films is corresponded to superconducting YBa₂Cu₃O₇ phase.

Twinning structure of (001)YBa₂Cu₀O₇ / (110) NdGaO₃ films from analysis (103) and (113) reflections were investigated. Disorientation between films and NdGaO₃ substrates were found.

(001)YBa₂Cu₃O_7-x films on Al₂O₃ substrates with CeO₂ buffer layer have part twinning structure. Disorientation of CeO₂ and YBa₂Cu₃O_7-x grains about the direction in the surface of the substrate is 0.7° and 0.8° correspondingly. It shows a correlation in the disorientation of YBa₂Cu₃O_7-x and CeO₂ grains in this direction. The disorientation in the plan of the surface for CeO₂ is 1.2° and for YBa₂Cu₃O_7-x is 0.8°.

YBa₂Cu₃O_7-x thin films YBa₂Cu₃O₇ on (130) NdGaO₃ with CeO₂ buffer layer consist from two main domain orientations of YBa₂Cu₃O_7-x. Orientation characteristics of domain structure were determination. Orientations of main planes buffer layer and films are (111) CeO₂ ½ ½ (130) NdGaO₃, (001) YBa₂Cu₃O_7-x ½ ½ (100) CeO₂ ½ ½ (110) NdGaO₃, (001)YBa₂Cu₃O_7-x ½ ½ (110) CeO₂ ½ ½ (110) NdGaO₃ and (110)YBa₂Cu₃O_7-x ½ ½ (100) CeO₂.

1PO4-45 Anisotropic Transport Properties in Epitaxial YBCO Films Grown on Vicinal SrTiO₃ Surfaces

Qidu Jiang 1, Marie G. Medici 2, M. Dezaneti 3, and C.W. Chu 3. 1 Chemistry Department, University of Houston, Houston, TX 77204-5641. 2 Lab of Condensed Matter Physics UMR CNRS 6622, University of Nice Sophia Antipolis, France. 3 TcSUH, University of Houston, Houston, TX 77204-5932.

Presenting Author: M.G. Medici

YBCO epitaxial films were grown by RF magnetron sputtering onto vicinal (off-cut angle up to 10 degrees) SrTiO₃ (001) substrates which exhibit quasi-periodic and atomically flat terraces as were characterized with scanning tunneling and atomic force microscopy in ultrahigh vacuum. The morphology of YBCO films display similar self-organized structures as that of substrates. Measurements of resistivity and critical currents of the YBCO bridges parallel and perpendicular to the step edge in magnetic fields show strong anisotropy. In particularly, our results show strong correlation of the transport properties on the density of defects in the films induced by surface steps and different step heights on the substrates.

1PO4-46 Anisotropy in Current Distribution of YBCO Thin Films related to Substrate Misorientation.

Dimitris Vassiloyannis, School of Metallurgy & Materials, University of Birmingham, Edgbaston, Birmingham, B15 2TT, United Kingdom

Presenting Author: D. Vassiloyannis

Magneto-optical (MO) measurements have been performed on a series of YBa₂Cu₃O_7-d (YBCO) thin films deposited on misoriented SrTiO₃ (STO) substrates. We report a newly observed correlation between the substrate offcut and the anisotropic current densities measured in the resulting films: Lower critical current densities (J_c//) have been observed along the substrate step-terrace direction, while higher current values (J_c^ ) have been recorded for currents flowing in the perpendicular direction.

The degree of anisotropy in the current distribution along the substrate a and b planes has been found to increase by a factor of approximately 20% as the offcut angle is increased from 2° to 5° , while it remains stable for individual samples for temperatures up to 60 ° C. We compare these findings with a series of VSM average magnetisation measurements performed for the same samples in various temperatures and fields.

1PO4-47 Ultrathin Films of Y-Ba-Cu-O on YSZ Substrate with Eu-Cu-O buffer

C.Y. Yau, W.H. Tang, and J. Gao, Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong

Presenting Author: C.Y.B. Yau

Due to the large lattice mismatch between YBa₂Cu₃O_7-d (YBCO) (a=0.382nm, b=0.389nm) and Y-stabilized ZrO2 (YSZ) (a/O₂=0.365nm), the initial growth of YBCO on YSZ (100) substrate usually results in many defects. Besides, an intermediate layer between YBCO and YSZ is often formed. Thus, the buffer layer is strongly required for growing of thin and ultrathin YBCO films on YSZ substrates. Eu₂CuO₄ (ECO) has a stable crystal structure, and its lattice parameter (a=0.389nm) matches well with those of YBCO. In this work, ECO has been used as a buffer layer for growing of YBCO ultrathin films on YSZ substrates. The epitaxy, crystallinity and superconductivity of YBCO ultrathin films have been significantly improved. The intermediate layer between YBCO and YSZ has been effectively buffered by 10nm-thick ECO layer. High quality superconducting YBCO ultrathin films have been obtained.

1PO4-48 Growth of Eu-Cu-O Thin Films on STO and YSZ Substrates

C.Y. Yau, W.H. Tang, and J. Gao, Department of Physics, The University of Hong Kong, Pokfulam Road, Hong Kong

Presenting Author: C.Y. Yau

Compared with PrBa₂Cu₃O_7-d , Eu₂CuO₄ (ECO) shows higher stability in crystal structure and has no structural transition when its oxygen content changes. Besides, the lattice parameter of ECO (a=0.389nm) matches well with those of YBa₂Cu₃O_7-d (a=0.382nm, b=0.389nm). Thus, ECO could be used as a barrier for fabricating high-T_c Josephson junctions. In this work, ECO thin films have been grown on SrTiO₃ (STO) (100) and Y-stabilized ZrO₂ (YSZ) (100) substrates by magnetron rf sputtering method. The growth temperature on YSZ is about 30K higher than that on STO. Both kinds of films are highly c-axis oriented and epitaxial, as shown by x-ray diffraction, rocking curves and Phi-scanning. SEM and AFM indicate a very smooth surface for the ECO thin film of ~100nm in thickness. The changes in the c-axis length and the resistance behavior were studied on the films treated by post vacuum annealing. The post vacuum annealing removes the excess oxygen from the structure and reduces the c-axis length. As a result, the resistance of the films increases accordingly.

1PO4-49 Superconducting Properties of BSCCO (2223) Ultra Thin Films

Liliana Navarrete, Alvaro Marino, and Hernan Sanchez, Department of Physics, Universidad Nacional de Colombia, Bogotá, Colombia

Presenting Author: L. Navarrete

Ultra thin films of Bi₂Sr₂Ca₂Cu₃O_8+d (2223) were deposited by r.f. magnetron sputtering on MgO substrate. The temperature dependence of the resistivity of BSCCO films with thickness varying between 30 nm and 200 nm were studied systematically. A structural characterization of these was carried out and correlated with the electrical properties.