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Defects in Bi 2 Sr 2 Ca n -iCu,,O}, superconducting thin films annealed in an oxygen atmosphere are examined by high-resolution electron microscopy (HREM). In addition to the majority 2212 (n = 2) phase, subsequent slabs of other homologous phases with n values up to n = 10 are found intergrown in the films. Large-angle tilt grain boundaries and various secondary phases such as CuO, S r - C u - 0 oxides are formed in the films. The occurrence of these defects is attributed to an inhomogeneous Sr, Ca, and Cu distribution induced by the post-annealing. Superconducting transition temperature (Tc) is increased by the annealing under suitable conditions.

I. INTRODUCTION The structures of B i - S r - C a - C u - 0 superconductors with high Tc have been studied extensively by means of transmission electron microscopy (TEM).1"7 The composition of these compounds can be generally written as Bi 2 Sr 2 Ca n _ 1 Cu,,O J where the subscript n denotes the number of CuO2 planes sandwiched between (BiO)2 layers. The phases with n = 1 and 2 (referred to as 2201 and 2212 phases) were found more frequently in spite of intergrown lamella of n = 3 (2223) phase being observed, (e.g., Ref. 5) Phases with higher n values, i.e., n = 4, 5, 6, and 7, have been observed, too, in bulk B i - S r - C a - C u - 0 compounds by Eibl 89 and Losch et al.10 when Pb was added to the compounds. The larger the n value, the larger the c constant. Table I lists the oparameters of these phases. Tc are about 20 K, 85 K, and 110 K for the 2201, 2212, and 2223 phases, respectively. No higher Tc has been achieved in phases with higher n values (n > 3). Producing high-r c superconducting thin films is more attractive from an application point of view. B i S r - C a - C u - O thin films have been produced successfully by many methods, e.g., pulsed-laser deposition."~16 The microstructures of as-deposited Bi 2 Sr 2 CaCu 2 0 } epitaxial thin films have been investigated by TEM from both cross-section and plane-view directions.17"21 The as-deposited films are found to grow epitaxially from the very first layer with the c-axis perpendicular to the SrTiC>3 (001) surface plane. Slabs of the intergrown 2223 phase are sometimes present, but they have only a single slab along the c-direction. Neither phases with n values beyond 3 nor any secondary phases or precipitates have been detected in as-deposited films on the (001) SrTiO3 substrates.1718 The first deposition layers of the 2212 thin films are a BiO monolayer, BiO double layers, or SrO layer.19 Tc values of the as1872 http://journals.cambridge.org

J. Mater. Res., Vol. 10, No. 8, Aug 1995 Downloaded: 15 Mar 2015

grown 2212 thin films we have previously studied17 20 are generally lower than those of corresponding bulk materials. It has been reported by many groups that post-annealing of bulk B i - S r - C a - C u - 0 materials can influence Tc.22~25 Optimum Tc can be achieved by an appropriate annealing e.g., Refs. 23, 25-28; therefore we performed post-annealing of the 2212 thin films using an oxygen pressure high