Screening and delocalization effects in Schottky barrier formation

Abstract

In order to assess the role of metallic screening at the metal–semiconductor interface, we have investigated the effects of Ag deposition on the band bending of GaAs(110) surfaces previously ‘‘pinned’’ with vanadium atoms. Their coverages ranged from 0.05 to 1.5 monolayers (ML), which positioned the surface Fermi level (EF)∼0.85–0.6 eV above the valence band maximum (VBM). For subsequent depositions of Ag additional changes in EF of up to 200 meV towards the VBM were observed on both n- and p-type GaAs. The onset of these changes, which occurs for mean Ag coverages of ∼0.1 ML, coincides with the appearance of metallic characteristics in the Ag clusters. These changes in EF can be explained in terms of a new model in which the interface impurity levels in the band gap of the semiconductor become delocalized through interaction with states of the metal overlayer. A functional expression is developed which relates the Fermi level to the metal and semiconductor parameters, and to an effective charge in each delocalized level. These levels, actually broadened resonances of the impurity-metal system, accommodate the charge densities required to equalize EF throughout the system. In analogy to a partially filled band, EF is determined by the average effective charge residing in each resonance. The resonances are nearly neutral and can accept or donate electronic charge as needed to compensate for the conductivity type of the semiconductor. Calculated changes of EF based on this model agree well with the experimental observations.