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

Abstract

The growth, microstructure, and electrochemical oxygen intercalation of c-axis ${\mathrm{La}}_{2}$${\mathrm{CuO}}_{4+\mathrm{\ensuremath{\delta}}}$ thin films on substrates with different lattice mismatch [${\mathrm{SrTiO}}_{3}$ (001) and ${\mathrm{SrLaAlO}}_{4}$ (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 ${\mathrm{SrTiO}}_{3}$, oxygen can be intercalated electrochemically into the grown c-axis thin film with a high diffusion coefficient (${\mathit{D}}_{\mathrm{ox}}$=${10}^{\mathrm{\ensuremath{-}}13}$--${10}^{\mathrm{\ensuremath{-}}14}$ ${\mathrm{cm}}^{2}$/s), and subsequently additional (00l) reflections, l=2n+1, are observed by x-ray diffraction measurements. A double transition with a resistivity drop at \ensuremath{\simeq}55-58 K, suggesting a more strongly oxidized phase, and a zero-resistance state at \ensuremath{\simeq}42 K are found. For films on ${\mathrm{SrLaAlO}}_{4}$, the value of ${\mathit{T}}_{\mathit{c}}$ 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. \textcopyright{} 1996 The American Physical Society.