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

The surfaces of c-axis oriented YBa2Cu3O7-delta (YBCO) thin films prepared by DC-magnetron sputtering on SrTiO3 (100) substrates were studied by means of X-ray photoelectron spectroscopy (XPS). The measurements were carried out subsequently during a period of a three-months-storage of the samples in ambient air as well as immediately after the film deposition without exposing the film surfaces to atmosphere. A chemical 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 current density (j(c)) show that T(c) and j(c) of the films are normally not affected by the top layer formation and remain nearly unchanged over periods longer than eight months.