High-Energy X-ray Photoemission and Structural Study of Ultrapure LaF3 Superionic Conductor Thin Films on Si

Abstract

LaF3 films in the 5–40 nm thickness range were grown on Si(111) by molecular beam epitaxy. The substrates were kept at 450 °C during deposition. The films were investigated by high-energy X-ray photoemission flanked by conventional X-ray photoemission, reflection high-energy electron diffraction, and atomic force microscopy. The film growth was layer-by-layer. The surface of the films presented flat terraces, ∼100 nm wide, separated by monatomic steps, reproducing the morphology of the substrate. La 3d, F 1s, O 1s, and Si 2p core levels and valence band were measured by high-energy photoemission to investigate the reactivity of the system and the surface and bulk composition of the films, following varying sample treatments (X-ray irradiation, sputtering, heating). The fresh prepared films resulted of high purity, with no traces of reaction or intermixing at the buried interface between the substrate and the trifluoride. The X-ray beam was seen to induce F depletion at the surface and promote oxide formation. F depletion enhancement was obtained through Ar ion sputtering. An irreversible variation of the film composition was finally observed for samples heated above 300 °C, with the development of La oxides and oxofluorides. These effects were related to the high mobility of F ions in the LaF3 lattice and to the high tendency of defects formation involving F sites.