A study of oxygen diffusion in and out of YBa2Cu3O7−δ thin films

Abstract

The c-axis oriented YBa2Cu3O7−δ (YBCO) films with superconducting transition temperatures up to 89 K were prepared by ion beam sputter deposition on single crystal (100) MgO and (100) SrTiO3 substrates. The variation of the oxygen concentration δ during annealing in vacuum and in oxygen was measured in situ and in real time by means of spectroscopic ellipsometry. The ellipsometric measurables Δ and Ψ were quantitatively correlated with δ at a photon energy of 4.1 eV. Changes of the oxygen concentration below 1% were resolved. Measurements for different film thicknesses in the range from 15 to 100 nm provided information to elucidate the mechanisms for oxygen out- and in-diffusion. The oxygen out-diffusion rate depends on the layer thickness confirming a bulk diffusion mechanism. The oxygen in-diffusion is relatively independent of layer thickness suggesting that a surface/grain boundary diffusion mechanism is dominant in this case. Additionally, the effect of H2O on the oxygen diffusion was studied. It was found that H2O enhances the oxygen out-diffusion while the in-diffusion remains unchanged or even decreases. The present study shows that ellipsometry provides a powerful tool for the in situ, noninvasive, on-line control of the oxygen content of thin YBCO films.