  Ultrathin cerium oxide films have been deposited on chlorine, gallium, and
silver passivated Si(111) by reactive molecular beam epitaxy in a comparative
study. The crystallinity of these films has been characterized by x-ray
standing waves while the oxidation state of the rare-earth oxide (REOx) films
and the chemical interface composition have been revealed by hard x-ray
photoelectron spectroscopy. The use of Cl as passivating agent results in the
epitaxial growth of highly crystalline REOx films with the RE metal in the 3+
oxidation state while effectively suppressing silicate and silicon oxide
formation at the interface. In contrast, Ga and Ag preadsorption yield films of
inferior quality, in the case of Ag of even lower crystallinity than without
passivation. Further investigations show that Cl-passivation also results in
ultrathin lanthana films of superior quality, which facilitate the growth of
well-ordered ceria on lanthana REOx multilayers.
