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

The elemental composition and depth profiles of Ba-Ca-Cu-(O, F) precursor a nd Tl-Ba-Ca-Cu-O (TBCCO) superconducting films were studied by ion beam ana lysis, such as Rutherford backscattering spectrometry (RBS) and resonant nu clear reaction (RNR). A series of 150-200 nm thick BCC(O, F) 212 and 223 pr ecursor films was prepared by sequential thermal evaporation of BaF2, Cu an d 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 parti ally removed from some of the as-evaporated BCC(O, F) precursor films by me ans of an ex situ annealing in vacuum and dry oxygen at 720 degreesC, RES a nd RNR measurements were performed on these films. Both types of precursor, i.e. the as-evaporated as well as the defluorinated ones, were then thalli nated in a single-zone reaction chamber at 860 degreesC for 30 minutes usin g a crude TBCCO pellet as a source of thallous oxide Tl2O. The synthesized superconducting TBCCO films were characterized by resistance versus tempera ture measurements, x-ray diffraction, RES and also RNR analysis using the r esonance at a proton energy of 340 keV of the F-19(p, alpha gamma)O-16 nucl ear reaction. Mainly the Tl-2212 phase was observed in the superconducting TBCCO films, w ith T-c values up to 106 K. From the RES investigations we found that films with higher T-c have a slight excess of Tl, Ca and Cu components. While th e RNR study revealed the presence of fluorine in both types of precursor fi lm, no fluorine was observed in the superconducting ones (within the limit of sensitivity of our method).