Soft X-Ray Studies of Y-Ba-Cu-O Thin Films Prepared by Laser Ablation
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SOFT X-RAY STUDIES OF Y-Ba-Cu-O THIN FILMS PREPARED BY LASER ABLATION
A. KROL*, C.J. SHER*, D.R. STORCH*, L.W. SONG*, Y.H. KAO*, S. WITANACHI*, Y.Z. ZHU*, S. PATEL*, D.T. SHAW* AND G.C. SMITH** *New York State Institute on Superconductivity and State University of New York at Buffalo, Buffalo, NY 14260 **Brookhaven National Laboratory, Instrumentation Division, Upton, NY 11973
ABSTRACT Angular variation of x-ray fluorescence due to oxygen atoms in high-T Y-Ba-Cu-O thin films is measured for the first time by using a new paralley plate avalanche chamber. This technique allows the possibility of nondestructive probing of the depth-profile of oxygen atoms in the superconducting materials. Our preliminary results indicate that the near surface region of the Y-Ba-Cu-O film may contain an oxygen-depleted layer of thickness around 20 nm.
X-ray fluorescence spectroscopy is a useful nondestructive method for probing microstructures in superconducting thin films. The grazing incidence angular profile of fluorescence emission at fixed energy of incoming radiation provides us with a convenient tool to investigate the depth distribution of selected atomic species and to examine the quality of buried interfaces. This element selectivity is realized by choosing appropriate energy of incident x-ray photons just above the strong absorption edges of atoms of interest, thus taking advantage of resonant photoabsorption processes followed by radiative decay of created core-hole. It is well known that oxygen is a key element of high-T . S c superconductors and its distribution in the material is of special interest. Unfortunately, due to its low Z value, the probability of fluorescence emission for this element is three orders of magnitude smaller than Auger transition, which results in an extremely small flux of characteristic oxygen K fluorescence photons. This is why up till now the techniques of oxygen-probing were all based on electron emission. The inherent drawback of those techniques is their very shallow information depth which does not exceed 10 nm [1]. However, the advent of a low pressure parallel plate avalanche chamber (PPAC) detector [2], (which works as a single photon counter) and synchrotron radiation made it practical to probe bulk properties by using soft x-ray fluorescence spectroscopy. The primary goal of this work was to obtain information on oxygen depth distribution in superconducting thin films using the depth-probing fluorescence method (with information depth of the order of a few hundred nanometers). In this contribution we will present preliminary results of an experiment performed at the U15 beamline at NSLS. The experimental set-up consists of monochromatic x-ray source (Torroidal Grating Monochromator), radiation intensity monitor, collimator, goniometer and detectors of emitted fluorescence photons (PPAC) and electrons (photocurrent amplifier). The PPAC detector was placed perpendicular to the plane of incidence. The experiment was carried out in TE geometry. Thin films of YBa 2Cu.30 7-6. about 200 n
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