Study of oxygen content and of disorder in YBaCuO thin films with enlarged c-axis lattice parameter

Abstract

It was recently shown that YBaCuO thin films prepared at low oxygen partial pressure P O2 present enlarged c-axis lattice parameter. In this paper, we report on further results concerning the c-axis expansion effect, but on films produced at particular conditions: they were deposited at relatively high p O2 = 0.25 mbar and then cooled to RT at the same oxygen partial pressure. A question arises: are these films fully oxidized or are they oxygen deficient films, according to the relationship between the c-axis parameter and the oxygen content? YBaCuO thin films were deposited, on MgO single crystal heated to about 750°C, by inverted cylindrical magnetron sputtering of a stoichiometric Y 1Ba 2Cu 3O 7 target, at an oxygen partial pressure p O2 = 0.25 mbar. A series of samples was cooled down to room temperature at the same pressure (0.25 mbar) and a second one, at higher pressure (1 atm). As grown samples were characterized by ρ(T) resistivity measurements (T c, ΔT c). Their composition and structure were studied by Rutherford Backscattering Spectrometry (RBS) in random and channeling geometries, Nuclear Reaction Analysis (NRA), X-ray Diffraction (XRD) and Raman Spectroscopy. The oxygen contents, measured by RBS and NRA, were compared to those deduced from XRD (c-axis parameter). A good agreement was found for the YBaCuO films cooled at 1 atm, but significantly higher results were obtained by RBS and NRA for the films cooled at 0.25 mbar. These results suggest that the cooling at low oxygen partial pressure leads to the formation of films with enlarged c-axis parameter, but with T c values similar to those observed for the samples cooled at p O2 = 1 atm (T c ≈ 88 K). The results are discussed.