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Ion beam studies of Tl-based superconducting films prepared from fluorides

The elemental composition and depth profiles of Ba-Ca-Cu-(O, F) precursor and Tl-Ba-Ca-Cu-O (TBCCO) superconducting films were studied by ion beam analysis, such as Rutherford backscattering spectrometry (RBS) and resonant nuclear reaction (RNR). A series of 150-200 nm thick BCC(O, F) 212 and 223 precursor films was prepared by sequential thermal evaporation of BaF2, Cu and CaF2 components onto single-crystalline MgO and/or CeO2 buffered sapphire substrates. The BaF2 and CaF2 components are chemically more stable under atmospheric conditions than those of BaO and CaO oxides. Fluorine was partially removed from some of the as-evaporated BCC(O, F) precursor films by means of an ex situ annealing in vacuum and dry oxygen at 720 °C. RBS and RNR measurements were performed on these films. Both types of precursor, i.e. the as-evaporated as well as the defluorinated ones, were then thallinated in a single-zone reaction chamber at 860 °C for 30 minutes using a crude TBCCO pellet as a source of thallous oxide Tl2O. The synthesized superconducting TBCCO films were characterized by resistance versus temperature measurements, x-ray diffraction, RBS and also RNR analysis using the resonance at a proton energy of 340 keV of the 19F(p,αγ)16O nuclear reaction. Mainly the Tl-2212 phase was observed in the superconducting TBCCO films, with Tc values up to 106 K. From the RBS investigations we found that films with higher Tc have a slight excess of Tl, Ca and Cu components. While the RNR study revealed the presence of fluorine in both types of precursor film, no fluorine was observed in the superconducting ones (within the limit of sensitivity of our method).