Superconducting Multilayers: Microstructural Properties Studied by X-Ray Diffraction
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A. VAILIONIS, A. BRAZDEIKIS AND A.S. FLODSTROM Dept. of Physics, Materials Physics, Royal Institute of Technology, S-100 44 Stockholm, Sweden
ABSTRACT
Structural properties of molecular beam epitaxy-grown Bi 2Sr2Cu,Oy/Bi 2Sr2Ca,Cu 2Oy multilayers have been studied by x-ray diffraction. A one-dimensional kinematic x-ray diffraction model has been used to describe the structural quality of the multilayers. Interface roughness, stacking defects and unit cell disorder are obtained by an iterative fitting of the calculated diffraction profile to the experimental spectra. The type and amount of disorder in the films was qualitatively determined. Results demonstrate that structural imperfections are present in the multilayers and have to be considered when transport properties are studied.
INTRODUCTION
Multilayers of conventional superconductors have been extensively studied both for fundamental research and device applications.1 Superlattices of high temperature superconductors (HTS) offer further possibilities of studying a variety of phenomena such as the proximity effect,2 Kosterlitz-Thouless transition,3 and electric field effect. 4 Although advanced techniques such as molecular beam epitaxy (MBE),5 7 provide the opportunity to produce high quality HTS multilayers, the complexity of the composition and structure of the high-Tc cuprates makes reproducible preparation of epitaxial films difficult. Many of the physical properties of8 superconducting multilayers are sensitive to the structural qualities such as interface roughness, unit cell disorder,9 and stacking faults'.' Structural analysis, therefore is important before the transport properties are studied. Only a few techniques are commonly used for the analysis of thin film structure. High-resolution transmission electron microscopy reveals only a local structural arrangement of the atoms. Due to a relatively small probing volume it does not give average information about the interfaces and unit cell structure of the entire film. Reflection high-energy electron diffraction is sensitive to a growing surface morphology, however due to a short coherence length it is impossible to study the structure of the whole layer or interface. X-ray diffraction (XRD) is the most common and nondestructive method for structural analysis. It gives average structural information on the atomic scale. In this study the structure of Bi 2Sr2CujO/Bi 2Sr2CajCu 2Oy (2201/2212) multilayers has been quantitatively analyzed. The results show that interface roughness as well as stacking faults are the most common type of defects in the multilayered HTS thin films. These defects play an important role and, therefore have to be considered in the interpretation of transport properties. 217 Mat. Res. Soc. Symp. Proc. Vol. 382 @1995 Materials Research Society
EXPERIMENTAL
HTS multilayers were grown by molecular beam epitaxy using a layer-by-layer technique. The experimental details are given elsewhere." The multilayers were designed by assembling half unit cell thick 2212 and 2201 molecular
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