Thickness dependence of Shockley-type surface states of Ag(111) ultrathin films on Si(111)7X7 substrates

We studied the surface-state dispersion of ultrathin Ag(111) epitaxial films on Si(111) substrates by analyzing the bias-voltage dependence of surface electron standing-wave patterns using a scanning tunneling microscope. The 40-monolayer (ML)-thick Ag film exhibited a two-dimensional, free-electronlike surface-state dispersion similar to that of the bulk Ag(111) surface. However, the bottom of the surface-state band (E-0) shifted from -51 to + 26 meV with respect to the Fermi level as the Ag film thickness decreased from 40 to 7 ML. The effective mass of the surface electron decreased slightly as film thickness decreased. The shift in E-0 is reasonably attributed to the modification of the electronic structure induced by thickness-dependent misfitstrain relaxation. In contrast to the Ag/Ge(111) system, the surface state hybridizes with neither the substrate bulk band nor the Ag film's quantum-well state in the Ag/Si(111) system.