New insights on metallization of semiconductor-alkali interfaces

The problem of metallization of semiconductor surfaces by deposition of a layer of alkali atoms has originated a recent controversy about the ionicity of the alkali-semiconductor bond and the associated charge transfer from the alkali layer to the empty dangling bonds of the reconstructed semiconductor surface. In this work we report calculations about the 2x1 reconstructed Si(100)/K interface (symmetric dimers model) for coverages θ = 1 and 2 potassium atoms per dimer. Two new ingredients are introduced: (1) The empty band of Si dangling bonds is a narrow Hubbard band (large U/W ratio) of antibonding orbitals and (2) the low dimensionality (1D or 2D) of the Si-supported potassium band may also play an important role. Large charge transfers are hindered by the large U/W ratio. Although the electron charge sits mainly on the Si surface for small U, it tends to be distributed evenly between both the Si surface and the K overlayer as U increases. For θ = 1 the system is metallic because both components metallize. In the θ = 2 case, however, a gap which develops at the Fermi level for large U confers the interface its observed semiconducting character.