Thickness dependence of the surface plasmon dispersion in ultrathin aluminum films on silicon

The collective excitation in Al films deposited on Si(1 1 1)-7 × 7 surface was investigated by high-resolution electron-energy-loss spectroscopy (HREELS), X-ray photoelectron spectroscopy (XPS) and scanning tunneling microscopy (STM). At the Al film thickness d < 10 ML, the surface plasmon of Al film has only a small contribution to the observed energy-loss peaks in the long wavelength limit ( q ∥ ≈ 0 ) , while its contribution becomes significant for q ∥ > d - 1 . More interestingly, for thin Al films, the initial slope of the surface plasmon dispersion curve is positive at q ∥ ∼ 0 , in a sharp contrast to bulk Al surface where the energy dispersion is negative. These observations may be explained based on the screening interaction of the space charge region at the Al–Si interface.