Article pubs.acs.org/JPCC
High-Energy X‑ray Photoemission and Structural Study of Ultrapure LaF3 Superionic Conductor Thin Films on Si K. Koshmak,†,‡ A. Banshchikov,‡ T. Vergentev,§ M. Montecchi,†,∥ D. Céolin,⊥ J. P. Rueff,⊥ N. S. Sokolov,‡ and L. Pasquali*,†,∥,# †
Dipartimento di Ingegneria E. Ferrari, Università di Modena e Reggio Emilia, Via Vignolese 905, 41125 Modena, Italy Solid State Physics Division, Ioffe Physical-Technical Institute of Russian Academy of Sciences, 26 Polytechnicheskaya str., 194021 St. Petersburg, Russia § Saint-Petersburg State Polytechnical University, 29 Polytechnicheskaya str., 195251 St. Petersburg, Russia ∥ IOM-CNR, s.s. 14, Km. 163.5 in AREA Science Park, 34149 Basovizza, Trieste, Italy ⊥ Synchrotron SOLEIL, L’Orme des Merisiers, Saint-Aubin, BP 48, F-91192 Gif-sur-Yvette Cedex, France # Department of Physics, University of Johannesburg, PO Box 524, Auckland Park 2006, South Africa ‡
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 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 LaF3 lattice and to the high tendency of defects formation involving F sites.
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INTRODUCTION Lanthanum fluoride (LaF3) is a promising material, both for fundamental studies and for technology. It is classified as a superionic compound1 that is appealing for fabrication of allsolid-state electrolytes.2−5 In this respect, it has been successfully applied as active and transport material in a variety of sensing devices, in both liquid and gaseous environments6−10 and in metal−insulator−semiconductor (MIS) systems.11 Moreover, it presents a high damage resistance to UV irradiation, making it suitable for optical coatings in UV laser applications.12,13 In addition, it also shows good infrared transmission14 for applications in photonic devices. Concerning ionic and superionic behavior, fluoride materials have often been considered as prototypical systems for the study of ion transport. This is due to the possibility to grow planar, highly ordered epitaxial layers15−17 and superlattices,18,19 with the formation of abrupt interfaces between adjacent materials with different conducting properties. This permits one to address the fundamental role of interface boundaries in ion transport. In this work, we studied the growth, morphology, and chemical composition of highly ordered LaF3 epitaxial layers on silicon by high-energy X-ray photoelectron spectroscopy (HAXPES) flanked by conventional XPS, atomic force © 2014 American Chemical Society
microscopy (AFM), and reflection high-energy electron diffraction (RHEED). Indeed, LaF3 thin films have been investigated quite extensively in the literature.2−4,9,11,20−25 Different methods of growth were applied, including sol−gel technique, chemical vapor deposition, vacuum thermal deposition, and molecular beam epitaxy (MBE). Defects, either vacancies or impurities, play a fundamental role in ion conduction, but they also influence optical properties. Different levels of oxygen contamination have often been observed in these systems. This has been largely associated with the growth method or with the growth environment. Compared with other fluorides, because of the high mobility of F− ions and the presence of vacancies in the lattice, LaF3 appears to be considerably reactive with respect to contaminants and especially to oxygen, giving rise to lanthanum oxides and oxofluoride compounds. These compounds, in turn, substantially modify the conductive properties of the material (with a possible increase in electron conductivity, besides ion conductivity).26 Received: February 11, 2014 Revised: April 22, 2014 Published: April 29, 2014 10122
dx.doi.org/10.1021/jp501474e | J. Phys. Chem. C 2014, 118, 10122−10130
The Journal of Physical Chemistry C
Article
Figure 1. RHEED patterns recorded at about 20 nm of coverage along (a) [1−10] and (b) [11−2] directions of the Si(111) substrate. (c) Top view of LaF3 lattice orientation with respect to Si(111): 1 × 1 (inner) cell and hexamolecular (outer) supercell are evidenced, rotated by 30° with respect to each other.
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Here we show that the probe itself and sample treatments after growth can produce sizable modifications in the fluoride composition. This should be carefully considered when studying the microscopic properties of these materials. LaF3 in bulk presents the tysonite crystal structure.27 La forms a hexagonal basal plane, while F occupies three inequivalent sublattices, F1, F2, F3, in the ratio 12:4:2, respectively, with different activation energies for F migration. While conductivity of F1 sites starts at 10 °C, conduction for the other two sublattices begins at ∼190 °C.28 Up to intermediate temperatures
