Anisotropic oxygen diffusion properties in epitaxial thin films of La₂NiO_4+δ

TLDR

The study develops and validates a new isotopic‑exchange method to determine anisotropic tracer diffusion and surface‑exchange coefficients in high‑quality epitaxial La₂NiO₄+δ thin films, and evaluates how strain from film–substrate mismatch affects oxygen diffusion. Isotopic‑exchange measurements were performed on c‑axis oriented La₂NiO₄+δ films (33–370 nm) grown on SrTiO₃ (100) and NdGaO₃ (110) by pulsed injection metal‑organic chemical vapour deposition, enabling determination of anisotropic diffusion and surface‑exchange coefficients. Tracer diffusion coefficients along the c‑axis and ab plane increase with film thickness (i.e., decreasing stress), while the surface‑exchange coefficient is thickness‑independent; the thickest films exhibit two regions with distinct c‑axis diffusion values, and both diffusion and surface‑exchange coefficients are thermally activated, roughly two orders of magnitude higher in the ab plane, with low activation energies suggesting a vacancy‑mediated mechanism.

Abstract

We report on the development and validation of a new methodology for the determination of anisotropic tracer diffusion and surface exchange coefficients of high quality epitaxial thin films in the two perpendicular directions (transverse and longitudinal), by the isotopic exchange technique. Measurements were performed on c-axis oriented La2NiO4+δ films grown on SrTiO3 (100) and NdGaO3 (110) by pulsed injection metal organic chemical vapour deposition (PIMOCVD), with different thicknesses ranging from 33 to 370 nm. The effect that the strain induced by the film–substrate mismatch has on the oxygen diffusion through the film was evaluated. Both tracer diffusion coefficients, along the c-axis and along the ab plane, were found to increase with film thickness, i.e., as the stress of the film decreases, while the thickness seems to have no effect on the tracer surface exchange coefficient. Best fits were obtained when considering the thickest films composed by two regions with different c-axis tracer diffusion coefficient values, a higher and constant D* close to the film surface and a variable decreasing D* closer to the substrate. As expected, the tracer diffusion and surface exchange coefficients are thermally activated and are approximately two orders of magnitude higher along the ab plane than along the c-axis. The low activation energies of D* compared with bulk values for both directions at low temperatures seem to confirm the contribution of a vacancy mechanism to the ionic conduction.