Crystallization of YBa 2 Cu 3 O 7-y films on SrTiO 3 (100) by postannealing of precursors prepared by dipping-pyrolysis
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The effects of annealing temperature and oxygen partial pressure [p(O2)] were investigated on the crystallization and orientation of superconducting YBa2Cu307_y (YBCO) films on SrTiO3(100) prepared by the dipping-pyrolysis process. Annealing conditions for precursor films consisting of a Y 2 O 3 -BaCO3-CuO mixture were varied in the temperature range 700-850 °C and initial p(O2) range 2 X 10^-10" 2 atm followed by pure O 2 treatment. Three different types of crystallization behavior were observed. The c-axis-oriented epitaxial YBCO films were obtained on the lower p(O2) and higher temperature side in the p(O2)-T diagram. The c/a-axis-oriented growth was observed in films heat-treated at 750-800 °C under p(O2) of 10"3 atm. In addition, mixtures of YBCO and YBa 2 Cu 4 0 8 resulted when the films were annealed at 700 °C.
I. INTRODUCTION Recently, epitaxial YBazCuaOy^ (YBCO) films with excellent superconducting properties have been fabricated by ex situ postannealing processes including co-evaporation1"5 and dipping-pyrolysis (D-P) or the metal-organic deposition (MOD) process,6"8 besides by a variety of in situ techniques. Dipping-pyrolysis process is a wet chemical method starting with metal-organic solutions such as metal-organic acid salts through precursors containing BaF 2 6 ' 7 or BaCO 3 . 8 The effects of parameters in the postannealing, i.e., temperature and oxygen partial pressure p(O2), on the preferred orientation of the final films have been extensively investigated for BaF2-containing precursor films1"4 and for metal-based precursor films.5 These studies revealed that c-axis-oriented films grow at higher temperatures and lower-p(O2) values. Therefore, precise control of these annealing parameters is essential for producing high quality YBCO films. However, when the kinds of precursors and substrate materials are varied, orientation behavior and optimal annealing conditions of YBCO films differ appreciably, because reaction mechanism from each precursor to final film is different. From the precursor containing BaF 2 and BaCO 3 , YBCO films are formed through hydrolysis of BaF 2 and through BaCO 3 decomposition, respectively, and from metal-based precursor films through O 2 diffusion. In this paper, the effects of annealing temperature and initial p(O2) were investigated on crystallization and orientation of superconducting YBCO films from BaCO3-containing precursor, and its behavior was compared with that from other precursor films, as an extension of our previous work,8"12 since only little knowledge 858 http://journals.cambridge.org
J. Mater. Res., Vol. 9, No. 4, Apr 1994 Downloaded: 13 Mar 2015
has so far been obtained for YBCO films starting with BaCO3-containing precursors, especially about their inplane alignment. Heat-treatment conditions were set in the temperature range 700-850 °C and initial p(O2) range 2 X lO^-lO" 2 atm, i.e., near the CuO-Cu 2 O-O 2 equilibrium and YBCO-stability lines in the p(O2)-T diagram.13 Crystallinity and in-plane alignment of the product films were evaluated using x-
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