Adsorbate-Mediated Growth of Rare-Earth Oxides on Silicon

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.