Unpinning of the Fermi level at erbium silicide/silicon interfaces

Fermi-level positions at nearly perfect interfaces of epitaxial erbium silicide on silicon are measured by internal photoemission as a function of the thickness and oxygen contamination of the silicide. Thicknesses in excess of 30 silicide monolayers (about 120 {}) and oxygen concentration lower than 0.3% are necessary to obtain the lowest value of 0.28 eV for the barrier height on n-type Si. For thinner silicide films or higher oxygen contents, the barrier height increases and shifts the Fermi level downwards across 45% of the Si band gap. Such an unpinning of the Fermi level is believed to be the consequence of the simultaneous occurrence of the high structural perfection of these interfaces, of the low electronegativity of the silicide, and of the low density of metal-induced gap states near the Fermi level.