Oxygen content of YBa2Cu3O6+x thin films during growth by pulsed laser deposition

The oxidation ability of the laser plasma plume during in situ formation of YBa2Cu3O6+x (YBaCuO) thin films has been studied as a function of the deposition conditions. A quenching technique has been used immediately after termination of growth to avoid any oxygen in or out-diffusion during the cooling down step. It is shown that superconducting YBaCuO thin films can be formed without any post-oxygenation procedure, contrary to that expected from the (P-O2, T) thermodynamic diagram. This is due to the production of oxygen activated species in the plume, which significantly increases the oxygen potential and, therefore, the oxidation state of the films during deposition. Moreover, it is demonstrated that there exits an optimal position of the substrate respecting to the visible luminous plume for each O-2 pressure, which leads to the highest T-c and to the best structural and morphological properties of quenched films. It is concluded that the presence of active oxygen, which is not homogeneously distributed over space, enhances the surface reaction kinetics and, thus, plays an important role in the mechanism of growth of laser ablated YBaCuO films. (C) 1998 Elsevier Science B.V.