Combination of IBA techniques and Raman spectroscopy to study defects in 18O labelled YBaCuO thin films

Abstract

We have studied the annealing conditions leading to the selective 18O labelling of oxygen sites in the Cu-O chain-planes of a-axis oriented superconducting YBaCuO thin films. The labelling is performed by a combination of two processes: (i) 18O uptake due to the reoxidation, and (ii) isotopic 18O ↔ 16O exchange. The annealing conditions (pO2 = 0.13 mbar and T = 400°C), chosen according to the thermodynamical diagram (pO2, T), correspond to an oxygen composition of O6.6. The complete oxygenation to O7 was reached by increasing the 18O pressure to 200 mbar at the same temperature. Due to the reoxidation from the O6.6 composition, it is expected that 40% of the Cu-O chain-planes will be labelled by 18O atoms. Moreover, additional labelling due to the 18O ↔ 16O exchange also takes place at 0.13 and 200 mbar. The composition of the films was determined by RBS and the total 18O content was measured by NRA. The 18O concentration depth profile was determined by the 18O(p, α)15N nuclear reaction resonance at 629 keV. It was found, by combining NRA and Raman spectroscopy, that the 18O atoms are only located in the Cu-O chain-planes. The 18O labelling of the Cu-O chain-planes induced by reoxidation and by isotopic exchange is incomplete and ranges from 58% to 85%. A second annealing in the same conditions leads to obtain a 100% labelling of the Cu-O chain-planes. The defects in the surface region of the 18O labelled Cu-O chain-planes were studied by channeling using the 18O(p, α)15N nuclear reaction resonance at 629 keV. Some correlation was found between the mechanism of 18O labelling and the presence of defects. The results are discussed.