Initial stages of growth and electronic properties of epitaxial SrF₂ thin films on Ag(1 1 1)

Molecular beam epitaxy (MBE) is used to grow ultrathin SrF₂ layers at different temperatures on Ag(1 1 1) epitaxial films prepared on mica. The electronic properties, structure and morphology of the films are probed by ultraviolet and x-ray photoelectron spectroscopies (UPS and XPS) supported by ab-initio calculations, reflection high energy electron diffraction (RHEED) and atomic force microscopy (AFM). The SrF₂ films are composed of crystallites with their [1 1 1] direction parallel to the substrate normal, thereby reproducing the symmetry of the substrate. Twinned domains are also observed. At the Ag/SrF₂ interface, fluoride molecules do not dissociate and the interaction with the substrate is weak. At room temperature, fluoride crystallites merge together, resulting in a continuous film covering the entire substrate when it reaches a nominal thickness of 5 nm. Growth at higher temperature induces the formation of triangular 3D islands, leaving sizable portions of the Ag substrate uncovered. At 400 °C, also a small fraction of SrF₂(0 0 1)-oriented crystallites are observed, with their [1 −1 0] axis rotated by about 30° with respect to the [1 −1 0] substrate direction.