Contribution of IBA techniques to the study of YBaCuO thin films with anomalous c-axis lattice parameter

Abstract

In this paper we report further results concerning the expansion of the c-axis lattice parameter of superconducting YBaCuO thin films. Our films were deposited, on MgO single crystals heated to about 750°C, by inverted cylindrical magnetron sputtering of a stoichiometric Y1Ba2Cu3O7 target, at relatively high oxygen partial pressure (pO2 = 0.25 mbar) and then cooled to room temperature at the same pO2. Compared with films prepared in the same conditions, but cooled at high oxygen pressure (1 atm), the c-axis lattice parameter shows a significant increase Δc. The films prepared at low pO2 have unexpected high critical transition temperatures (Tc ≈ 88 K). However, according to the relationship between the c-axis lattice parameter and the oxygen stoichiometry, these films would be oxygen deficient. The oxygen content measurements, performed by nuclear reaction analysis and Rutherford backscattering spectrometry, show that the films are fully oxygenated (i.e. O6.8–O7.0), independently of the oxygen pressure during cooling. Therefore, for the films cooled at low pO2, the usual relationship between the c-axis lattice parameter and the oxygen content does not hold, showing the anomalous expansion of the c-axis lattice parameter. The c-axis expansion is not due to oxygen deficiencies at the chain sites, but it seems rather that the films grow with an oxygen content higher than that expected by thermodynamical considerations, because of the presence of atomic oxygen during the film formation. In order to study the effect of the substrate, a series of YBaCuO thin films were prepared in the same conditions, but using LaAlO3 as substrate. The results also show a significant difference Δc in the c-axis lattice parameter between the films prepared at low and high pO2. While Δc is the same for films deposited on MgO and LaAlO3, the oxygen content deduced from the c-axis lattice parameter is shifted to higher oxygen stoichiometries in the case of LaAlO3 substrates. The results are discussed.