Quantum-well states in ultrathin Ag(111) films deposited onto H-passivated Si(111)-(1×1) surfaces

Ag(111) films were deposited at room temperature onto H-passivated $\mathrm{Si}(111)\ensuremath{-}(1\ifmmode\times\else\texttimes\fi{}1)$ substrates, and subsequently annealed at $300\ifmmode^\circ\else\textdegree\fi{}\mathrm{C}.$ An abrupt nonreactive Ag/Si interface is formed, and very uniform nonstrained Ag(111) films of 6\char21{}12 ML have been grown. Angle-resolved photoemission spectroscopy was used to study the valence band electronic properties of these films. Well-defined Ag $\mathrm{sp}$ quantum-well states (QWS's) have been observed at discrete energies between 0.5\char21{}2 eV below the Fermi level, and their dispersions have been measured along the $\overline{\ensuremath{\Gamma}K},$ $\overline{\ensuremath{\Gamma}{M(M}^{\ensuremath{'}})},$ and $\ensuremath{\Gamma}L$ symmetry directions. QWS's show a parabolic bidimensional dispersion, with in-plane effective mass of $(0.38\char21{}{0.50)m}_{o},$ along the $\overline{\ensuremath{\Gamma}K}$ and $\overline{\ensuremath{\Gamma}{M(M}^{\ensuremath{'}})}$ directions, whereas no dispersion has been found along the $\ensuremath{\Gamma}L$ direction, indicating the low-dimensional electronic character of these states. The binding energy dependence of the QWS as a function of the Ag film thickness has been analyzed in the framework of the phase accumulation model. A good agreement between experimental data and the above-mentioned model is obtained for the Ag/H/Si(111)-(1\ifmmode\times\else\texttimes\fi{}1) system. Hydrogen at the interface not only enhances the Ag film uniformity, but also acts as a barrier modifying the phase change of the $\mathrm{Ag}\ensuremath{-}sp$ electron wave upon reflection at the Ag/Si interface.