Flatband Conditions Observed for Lanthanide-Silicide Monolayers onn-Type Si(111)

We report on a core-level photoemission study of lanthanide silicides epitaxially grown on $n$-type Si(111). In the monolayer regime, an extremely low band bending is observed for the first time, with a Fermi-level position only $0.08\pm 0.05\mathrm{eV}$ below the conduction-band minimum of silicon. With increasing coverage, the Fermi-level position moves towards a final Schottky-barrier height of $0.32\pm 0.05\mathrm{eV}$. This behavior is assigned to the developing metallicity of the silicide overlayer.