SURFACE-REACTIONS AND LONG-TIME STABILITY OF YBCO THIN-FILMS

The surfaces of c-axis oriented YBa2Cu3O7-delta (YBCO) thin films prep ared by DC-magnetron sputtering on SrTiO3 (100) substrates were studie d by means of X-ray photoelectron spectroscopy (XPS). The measurements were carried out subsequently during a period of a three-months-stora ge of the samples in ambient air as well as immediately after the film deposition without exposing the film surfaces to atmosphere. A chemic al reaction with formation of hydroxides and carbonates on top of the film surface was observed in the case of the air-exposed samples. The thickness of this top layer was found to develop proportionally to the square root of time with a time constant of 0.1 nm/h0.5. Accompanying measurements of the critical temperature (T(c)) and the critical curr ent density (j(c)) show that T(c) and j(c) of the films are normally n ot affected by the top layer formation and remain nearly unchanged ove r periods longer than eight months.