Superconducting Ba 0.6 K 0.4 BiO 3 : Thin film preparation by RF magnetron sputtering
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Claude Hilbert and Harry Kroger Computer Physics Laboratory, Microelectronics and Computer Technology Corporation, 12100 Technology Boulevard, Austin, Texas 78727 (Received 23 April 1992; accepted 16 April 1993)
Superconducting Bao.6Ka4Bi03 thin films with transition temperatures up to 25 K have been successfully grown on SrTiO3 substrates using RF magnetron sputtering and postgrowth oxygen annealing. Systematic variation of the sputtering process parameters showed that optimum films can be grown on substrates heated to 400 °C in a 10 mTorr atmosphere containing 96% Ar and 4% O 2 and using a target containing Ba, K, and Bi in ratios of 0.6:1.2:1.4. High transition temperature superconductivity was observed only in highly oriented, crystalline films having the ideal stoichiometry, Bao.6Ko.4Bi03.
I. INTRODUCTION Superconductivity in the B a - K - B i - 0 (BKBO) system was first reported in 1988 by Mattheiss et al} 2
Wignacourt et al. established the perovskite structure and the anisotropic vibration ellipsoid parameters of the oxygen ions in the nonsuperconducting phase Bao.87Ko.i3Bi03; Schneemeyer et al? isolated the superconducting phase, Ba0.6Ko.4Bi03, and determined that it also has the cubic perovskite structure. Hinks and collaborators 4 " 6 have developed a two-step process for the synthesis of bulk BKBO compounds having various potassium stoichiometries. A low oxygen atmosphere is used to facilitate K substitution for Ba during reaction at 725-800 °C; the compound is then cooled to 475-500 °C and annealed in oxygen to fill the oxygen vacancies. Thin film work on BKBO compounds has been reported by several investigators7"16; a variety of film growth techniques, such as RF magnetron sputtering, molecular beam epitaxy, and laser deposition, have been used with varying degrees of success. Of these techniques, RF magnetron sputtering is best suited for commercial exploitation. Although BKBO has a superconducting transition temperature (30 K) that is much lower than that of YBa 2 Cu 3 0 7 (YBCO), (90 K), it does have at least two distinct advantages over YBCO for the preparation of useful junction devices. First, BKBO is cubic, with isotropic properties. Second, its 4 nm coherence length is significantly larger than that of YBCO, which is 1.5 nm in the a-b plane and 0.5 nm parallel to the c axis.10 Both these characteristics should facilitate the formation of reproducible junction devices. This paper presents the results of a study of the use of RF magnetron sputtering 1798
J. Mater. Res., Vol. 8, No. 8, Aug 1993
for the fabrication of superconducting thin films of BKBO and of S-I-N junctions made from these films. II. EXPERIMENTAL PROCEDURES The RF magnetron sputtering system used for these experiments features two sputtering guns mounted at 90° to each other, one of which was normally used for growth of the superconducting films and the other for the growth of insulating barriers necessary for the formation of tunneling junctions. Each gun was shuttered to allow for sputter cleaning of the targets prior to f
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