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

Abstract

LaF 3 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 highenergy 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 LaF 3 lattice and to the high tendency of defects formation involving F sites.

Buried Interfaces Effects in Ionic Conductive LaF3–SrF2 Multilayers

“…In Figure a, the AFM image of the SrF 2 terminal layer of the SrF 2 /LaF 3 multilayer is shown. The morphology of SrF 2 looks similar to the surface of the LaF 3 thin film . The surface morphology is consistent with a layer‐by‐layer type of growth.…”

“…Irradiation effects were studied in detail in ref. (and they are discussed in the Supporting Information) where the emergence of a low binding energy feature analogous to C with irradiation time was observed and it was unambiguously ascribed to the surface damage, with fluorine depletion. In the present case, it can be deduced that either the feature C in Figure a is peculiar of La at the buried interface, and it can be associated to some level of interface defects or lattice modification originated during the growth process, or that even small irradiation doses can produce pronounced effects at the interface, pointing to a sort of intrinsic higher responsiveness of the buried interface to irradiation damage with respect to the single bare LaF 3 film.…”

“…Epitaxial properties and structure of LaF 3 and SrF 2 single layers on Si were widely studied in literature . In this work, SrF 2 /LaF 3 multilayers are addressed, with specific reference to a [SrF 2 6 nm/LaF 3 8 nm] × 5 / Si(111) multilayer.…”

“…In Figure a, HAXPES La 3d 5/2 core level spectrum from a LaF 3 single thick layer (black dotted) and from the SrF 2 /LaF 3 multilayer (red) are compared. Features A and B are characteristic of the pure compound and correspond to states with different f level occupation in the presence of the 3d core hole . Feature C is only present in the multilayer film.…”

“…Their simple crystal structure makes them ideal for fundamental studies on the conduction mechanisms. Good ionic conductors such as LaF 3 are highly responsive toward irradiation and temperature modifications which can be explained by high mobility of the lattice ions. Multilayer/multiphase materials based on LaF 3 further foster the ionic conductivity with respect to the bare materials, due to the role of interfaces between phases.…”