Screened Coulomb interaction at a semiconductor surface

The static electronic screening at Si(111)2 × 1 is calculated using a model which focuses on the contribution of surface states and includes the effect of the presence of the surface on transitions between bulk states. Surface states give an enhancement of bulk screening of the order of 50% at short distances and a considerable reduction of bulk screening at intermediate distances. This antiscreening contribution is shown to be a geometric effect. We also discuss the validity of the classical approximation for the dielectric function.