High resolution transmission electron microscopy study of interface structures and growth defects in epitaxial Bi 2 Sr 2
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High resolution transmission electron microscopy study of interface structures and growth defects in epitaxial Bi2 Sr2 Can21 Cun O412n1d films on SrTiO3 and LaAlO3 N. D. Zakharov and D. Hesse Max-Planck-Institut f¨ur Mikrostrukturphysik, Weinberg 2, D-06120 Halle/Saale, Germany
J. Auge, H. G. Roskos, and H. Kurz Institut f¨ur Halbleitertechnik II, RWTH Aachen, Templergraben 55, D-52056 Aachen, Germany
H. Hoffschulz, J. Dreßen, H. Stahl, and G. G¨untherodt 2. Physikalisches Institut, RWTH Aachen, Templergraben 55, D-52056 Aachen, Germany (Received 15 January 1996; accepted 15 April 1996)
The defect structure of epitaxial, c-oriented Bi2 Sr2 Can21 Cun O412n1d (BSCCO) thin films grown by dc-sputtering and layer-by-layer MBE on SrTiO3 and LaAlO3 single crystal substrates was investigated by high-resolution transmission electron microscopy (HRTEM). Particular emphasis was put on the structure of the film/substrate interface. The films grown by dc-sputtering show a rather perfect structure involving a regular stacking of the unit cells. In spite of this regularity, there are many defects, such as twins, chemical stacking faults, and precipitates, as well as interfacial dislocations accommodating the film/substrate lattice misfit. The MBE-grown films contain twins and interfacial dislocations, but most prominent are precipitates of various size and rather high number density. Composition and structure of the precipitates were analyzed. Interfacial dislocations were found to be located in the films at a distance of up to 3 nm from the film/substrate interface. The experiments showed that the quality of the film/substrate interface in MBE-grown films is considerably higher with respect to smoothness, sharpness, and regularity, if the layer-by-layer MBE process starts with a Sr–O layer instead of a Bi–O layer. This observation is in correspondence to the observed interface structure of the dc-sputtered films, where the first film layer was a Sr–O layer, not a Bi–O layer, in spite of the films being sputtered from a composite target. A structure model of the Bi2 Sr2 Can21 Cun O412n1dy(100)SrTiO3 interface is proposed. The prolonged MBE process was shown to imply a chemical interaction between the SrTiO3 substrate and the growing film, resulting in the formation of Sr-rich phases in the near-interface substrate regions.
I. INTRODUCTION
The properties of epitaxially grown high-Tc superconducting thin films depend on the composition and on compositional inhomogeneities and structure defects present within the film or at the film/substrate interface. All these characteristics in turn are determined by the growth conditions.1–4 The substrate surface, which during film growth may be subjected to structural and compositional modifications, can severely influence the growth process, especially at its beginning. Thus investigations of structure and composition of the film/substrate interface and of near-interface regions at the atomic resolution level can provide a deeper insight into
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