Development of microstructure in Tl-2212 thin films and possible influence on microwave surface resistance values
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Z.U. Rek Stanford Synchrotron Radiation Laboratory, Stanford, California 94025
J.C. Bilello Department of Materials Science and Engineering, University of Michigan, Ann Arbor, Michigan 48109
C.R.M. Grovenor Department of Materials, University of Oxford, Oxford OX1 3PH, United Kingdom (Received 25 July 2005; accepted 14 December 2005)
The microstructures of Tl2Ba2Ca1Cu2O8 (Tl-2212) films are very strongly influenced by the processing parameters used to synthesize the superconducting phase and also control the microwave surface resistance values that are of key importance in the application of these materials in high-frequency devices. We report here on detailed studies of how the mesotexture of Tl-2212 films develops during synthesis at 820 and 855 °C. Our key observation is that the microstructure, and hence the superconducting properties, are controlled by the mechanism by which stress is relieved in the films and that apparently perfectly epitaxial films do not have the best microwave performance because in these samples the stress is relieved by macroscopic defects rather than local, low-angle grain misorientations.
I. INTRODUCTION
The deposition of large-area, high-temperature superconducting thin films for use in complex passive microwave devices has been an active area of study for many years,1–7 but most of the work published on the direct correlation of microstructure with microwave properties has concentrated on YBa2Cu3O7−␦ (YBCO) thin films. Many of these papers have taken the approach of measuring macroscopic surface resistance values and seeking correlation between these values and average areal concentrations of crystalline defects like a-axis grains,8 inplane-misoriented c-axis grains (often at 45°),8–11 nonsuperconducting particles and cracks,10 surface roughness,12 or the degree of mosaicity (measured from the peak width of rocking curves or phi scans).8,10,13 From these detailed studies of YBCO, it is possible to make the following general observations: (i) In-plane misoriented c-axis grains have the strongest effect on degrading surface resistance (Rs)
a)
Address all correspondence to this author. e-mail: [email protected] b) Present Address: School of Engineering, Coventry University, Coventry, United Kingdom DOI: 10.1557/JMR.2006.0208
values8,10,11 (as long as the films contain none of the microcracks often found in YBCO films on CeO2 when the thickness of either the film or the buffer layer exceeds a critical value10). (ii) The a-axis grains have a stronger influence on reducing critical current density (Jc) values than they have on increasing the surface resistance.8,10 (iii) Increasing width of a c-axis rocking curve (increasing mosaicity) results in a significant increase in Rs values, and more severe nonlinear characteristics are observed at higher microwave power levels in films with greater mosaic spread.10 These observations all suggest that to fabricate thin films with the lowest Rs values, one should avoid microstructural defects on a scale greater than a few
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