Model for electrostatic screening by a semiconductor with free surface carriers
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For free carriers on a semiconductor surface, we model screening with a local but unspecified relationship between the chemical potential and the density. The carriers can reside either in surface states or in a thin conducting overlayer. The semiconductor is taken to have a dielectric constant due to polarizable bound charge, and cases both with and without bulk free carriers are considered. At small and intermediate distances from a source charge on the surface, the surface potential is similar to a screened Coulomb potential, with a characteristic surface screening length; however, the asymptotic vacuum potential is dipolar and vanishes on the surface, and the asymptotic surface potential is quadrupolar. This model may be relevant to the short screening length inferred from surface-vacancy interactions in recent scanning tunneling microscopy experiments on p-type InP with a (110) surface at a relatively high surface vacancy concentration. Assuming that such vacancies cause occupancy of surface free carrier states, a reasonable value for the surface free carrier density yields good agreement with the experimentally inferred surface screening length. 2002 The American Physical Society.
