High Resolution Double Crystal Diffractometry of High T c Superconducting Epitaxial Gd-Ba-Cu-O Films
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HIGH RESOLUTION DOUBLE CRYSTAL DIFFRACTOMETRY OF HIGH T. SUPERCONDUCTING EPITAXIAL GCd-Ba-Cu-O FILMS D Y DAI, G S GREEN, B K TANNER, H C LI*, H R YI* AND R L WANG Department of Physics, University of Durham, South Road, Durham,DH1 3LE, UK *Instituteof Physics, Chinese Academy of Sciences, Beijing 100080, People'sRepublic of China ABSTRACT High-Tc superconducting epitaxial thin films of gadolinium barium copper oxide, grown in situ by dc-magnetron sputtering onto substrates of single crystal (001) orientated LaAIO 3 , SrTiO 3 and yttrium stabilised ZrO have been studied by high resolution double axis difffactometry, X-ray white beam-Laue topography and double crystal topography. Rocking curve halfwidths were found to vary from about 1000 to 3000 arc seconds, very good in the context of these complex materials. The topographs showed that the orientations of the small crystallites was randomly distributed across the sample. INTRODUCTION Since the discovery of high-Tc superconducting oxides, numerous investigations of superconducting thin films have been carried out for both scientific interest and possible technical applications [1]. For epitaxial growth of films, a close match between substrate and layer lattice parameters is necessary. Conventional single crystal X-ray characterisation of thin films in the symmetric geometry using theta - two theta scans provides lattice parameter information only for the direction perpendicular to the substrate surface. A complete analysis of film lattice parameters can be obtained by high resolution double crystal X-ray diffractometry using both surface symmetric and asymmetric reflections. With a fixed wavelength X-ray source the number of asymmetric reflections from the film is restricted, but by exploiting the tunability of synchrotron radiation data may be recorded in a wide range of asymmetric geometries, with incidence angles limited only by that for total external reflection. Such techniques are widely applied to the characterisation of semiconducting epitaxial structures [2], but their application to superconducting films has so far been rare. We report here the application o high resolution double crystal diffractometry and topography to the study of the perfection of thin films of GBCO on a range of dielectric substrates. EXPERIMENTAL PROCEDURE Details of the preparation and superconducting properties of the GdBa2 Cu2 O7 thin films have been presented elsewhere [3,4,5]. Briefly, the epitaxial, c-axis oriented films were grown in situ on (001) LaA10 3 (LAO), SrTi0 3 (STO) and yttrium-stabilized Zr02 (YSZ) single crystal substrates respectively, by dc-magnetron sputtering using stoichiometric targets prepared from oxide powders. The thickness of the films grown on the LAO substrates was 1000A, on the STO 2500A and on the YSZ 3000A respectively as determined by a DAKTAK IIA surface profile meter. The superconducting transition temperature of the samples was measured by AC susceptibility to be about 90K. The single crystalline nature of the films has been indicated by single
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