Growth process of Ba-poor YBCO film fabricated by TFA-MOD process K. Tada a , J. Yoshida a , N. Mori a, * , K. Yamada a , R. Teranishi a , M. Mukaida a , T. Kiss a , M. Inoue a , Y. Shiohara b , T. Izumi b , J. Matsuda b , K. Nakaoka b a Department of Materials Science and Engineering, Faculty of Engineering, Kyushu University, Motooka 744, Nishi-ku, Fukuoka-city, Fukuoka 819-0395, Japan b Superconductivity Research Laboratory, ISTEC, 1-10-13, Shinonome, Koto-ku, Tokyo 135-0062, Japan article info Article history: Available online 2 June 2008 PACS: 74.78.Bz 74.72.Bk Keywords: TFA-MOD TEM YBa 2 Cu 3 O 7d abstract Metal organic deposition process using trifluoroacetates (TFA-MOD) is one of the most promising pro- cesses to fabricate YBCO film. It has been reported that YBCO films grown by the starting solution with Ba-poor (cation ratio as Y:Ba:Cu = 1:1.5:3) have higher J C value and has smaller and less pores than those of the YBCO film with stoichiometric composition. It is important to investigate the growth mechanism of YBCO crystals to obtain a high J C film by controlling the crystal structures. In this study, YBCO films were fabricated under various Ba concentrations in the TFA starting solution, and the influences of Ba compo- sition on the growth process and microstructures were investigated. As a result, the Ba-poor YBCO film with Ba/Y = 1.5 and high J C had less a-axis oriented Y123 in comparison with the film with Ba/Y = 2. Fur- thermore, pores in the Ba-poor film were less than that in the stoichiometric composition film. This decrease of pores in the Ba-poor film was considered to be caused by the smaller size of non-reacted phases especially such as Ba–F rich particles entrapped by growing Y123 in the growing Y123 layer. It is considered that both the reductions of a-axis oriented Y123 and pores were the reasons of improving J C values in Ba-poor film. Ó 2008 Elsevier B.V. All rights reserved. 1. Introduction Metal organic deposition process using trifluoroacetates (TFA- MOD) is one of the most promising processes for fabricating YBCO superconducting thin films because this process can produce high- quality YBCO films by a low cost procedure [1–4]. In the TFA-MOD method, YBCO is crystallized through the simple way of coating the solution and two-step heat treatments. The overall reaction in this process is proposed as [5,6]; 1 2 Y 2 Cu 2 O 5 þ 2BaF 2 þ 2CuO þ 2H 2 O ! YBa 2 Cu 3 O 7d þ 4HF In the previous researches, it has been reported that J C value of YBCO films by this process were improved by reducing the Ba con- centration in the starting materials, and one of the reason of the J C improvement was the decrease of the size and number of pores in the YBCO films [7]. However, there are still few reports about the influence of the Ba reduction of the starting solution on the micro- structure of YBCO in the TFA-MOD process in detail. In this study, we study the effects of Ba reduction in starting solutions on J C of YBCO by examining the microstructures of YBCO films in the TFA- MOD process. In order to consider the differences of the YBCO films in growth process, the quenched films were prepared, and the microstructures of these films were investigated. 2. Experimental The starting solutions were prepared by dissolving Y, Ba-TFA salts and Cu–Naphtanate salts (F-free) in solvent, and their cation ratios were adjusted as Y:Ba:Cu = 1:x:3 (x = 1, 1.5, 2, 3). They were coated on (1 0 0) oriented LaAlO 3 (LAO) single crystal substrates by a spin coating at 4000 rpm for 120 s. After coating, the gel films were treated through three heating profiles. Firstly, the coated films calcined at heated at a heating rate of 2 °C/min to a temper- ature of 400 °C in a humid O 2 atmosphere with water vapor partial pressure, P H 2 O , of 2 vol.% to get precursor films of quasi-amorphous containing oxides and fluorides. Then, the precursor films were heated at a heating rate of 25 °C/min to a temperature (T max ) of 760 °C in a humid Ar–0.1%O 2 atmosphere with P H 2 O of 10 vol.% for 120 min to get crystallized YBCO films. Finally, the crystallized films were annealed at the temperature of 500 °C for 120 min in a dry O 2 gas to obtain the YBCO superconducting films. In order to thicken the YBCO film, starting solutions were coated on the pre- cursor films after the calcination again (multi-coating). These coat- ings and calcinations were repeated for three times in this study. The thicknesses of the YBCO films were about 200 nm by single coating, and that of three times coated films were about 600 nm. 0921-4534/$ - see front matter Ó 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.physc.2008.05.222 * Corresponding author. Tel.: +81 92 802 2979; fax: +81 92 802 2978. E-mail address: mori-ny@zaiko.kyushu-u.ac.jp (N. Mori). Physica C 468 (2008) 1554–1558 Contents lists available at ScienceDirect Physica C journal homepage: www.elsevier.com/locate/physc