Ion beam studies of Tl-based superconducting films prepared fromfluorides

Abstract

The elemental composition and depth profiles ofBa-Ca-Cu-(O, F) precursor and Tl-Ba-Ca-Cu-O (TBCCO)superconducting films were studied by ion beam analysis, such asRutherford backscattering spectrometry (RBS) and resonantnuclear reaction (RNR). A series of 150-200 nm thick BCC(O, F)212 and 223 precursor films was prepared by sequential thermalevaporation of BaF2, Cu and CaF2 componentsonto single-crystalline MgO and/or CeO2 bufferedsapphire substrates. The BaF2 and CaF2components are chemically more stable under atmosphericconditions than those of BaO and CaO oxides. Fluorine waspartially removed from some of the as-evaporated BCC(O, F)precursor films by means of an ex situ annealing invacuum and dry oxygen at 720 °C. RBS and RNRmeasurements were performed on these films. Both types ofprecursor, i.e. the as-evaporated as well as the defluorinatedones, were then thallinated in a single-zone reaction chamber at860 °C for 30 minutes using a crude TBCCO pellet as asource of thallous oxide Tl2O. The synthesizedsuperconducting TBCCO films were characterized by resistanceversus temperature measurements, x-ray diffraction, RBS and alsoRNR analysis using the resonance at a proton energy of 340 keVof the 19F(p,αγ)16O nuclearreaction. Mainly the Tl-2212 phase was observed in the superconductingTBCCO films, with Tc values up to 106 K. From the RBSinvestigations we found that films with higher Tc have aslight excess of Tl, Ca and Cu components. While the RNR studyrevealed the presence of fluorine in both types of precursorfilm, no fluorine was observed in the superconducting ones(within the limit of sensitivity of our method).