Growth, microstructure, and electrochemical oxidation of MBE-grown c-axis La2CuO4 thin films

The growth, microstructure, and electrochemical oxygen intercalation of c-axis La(2)CuO4(+delta) thin films on substrates with different lattice mismatch [SrTiO3 (001) and SrLaAlO4 (001) substrates] are compared. Except for the absence of planar defects in the latter case, the microstructural properties of both film types are very similar. For films on SrTiO3, oxygen can be intercalated electrochemically into the grown c-axis thin film with a high diffusion coefficient (D-ox=10(-13)-10(-14) cm(2)/s), and subsequently additional (001) reflections, l=2n+1, are observed by x-ray diffraction measurements. A double transition with a resistivity drop at similar or equal to 55 - 58 K, suggesting a more strongly oxidized phase, and a zero-resistance state at similar or equal to 42 K are found. For films on SrLaAlO4, the value of T-c could not be raised further, as the films decompose during the anodic polarization. This comparison reveals the role of planar defects, and we propose an electrochemical oxidation mechanism that occurs in two steps: First oxygen is transported into the film by intercalation into the planar defects, and then a slower oxygen diffusion into the interstitial sites occurs along the ab planes.