Oxidation kinetics of YBa2Cu3O7−x thin films in the presence of atomic oxygen and molecular oxygen by in-situ resistivity measurements

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Abstract
The kinetics of oxidation in YBa2Cu3O7−x thin films in the presence of molecular and atomic oxygen ambients have been studied. The resistivity of c‐axis‐, a‐axis‐, and mixed a+c‐axis‐oriented films, deposited in situ by off‐axis magnetron sputtering, was measured as a function of time subsequent to a change in the ambient conditions. Atomic oxygen was produced by a well‐characterized, compact electron cyclotron resonance source that is compatible with molecular‐beam epitaxy deposition. The resistivity of a film in the presence of atomic oxygen is shown to be determined by the flux of atomic oxygen on the film surface. The activation energy for the decomposition of O2 on the surface of the film was determined to be ∼1.3 and ∼2.1 eV for mixed a+c‐axis‐ and c‐axis‐oriented films, respectively. The oxidation process is shown to be thermally activated and can be characterized by a diffusion model with an activation energy which varies from approximately 1.2 eV in the presence of molecular oxygen to 0.6 eV for a flux of 2×1015 oxygen atoms/cm2 s. In both cases, diffusivity is found to be insensitive to oxygen stoichiometry, but the rate of oxidation is found to be sensitive to the microstructure and orientation of the films. The lower activation energy observed for the atomic oxygen case is postulated to be due to the large oxygen concentration gradient present during the oxidation.