Electronic properties of Si(111)-7 x 7 and related reconstructions: Density functional theory calculations

The 7 x 7 reconstruction of Si(111) has the interesting property of being metallic despite bulk Si being a semiconductor. This surface has a complex reconstruction that takes on a dimer-adatom stacking fault (DAS) structure composed of adatoms, rest atoms, and several other key features. It is believed that the dangling bonds of the adatoms play a crucial role in the high conductivity and that this is predominantly a surface-state band effect. To elucidate the details of this mechanism, we investigate a set of related Si(111) reconstructions of increasing complexity in order to resolve the effect of the different DAS features on the electronic and transport properties of the Si(111)-7 x 7 surface. Density functional theory calculations are carried out on the root 3 x root 3-R30 degrees, 2 x 2, 5 x 5, and 7 x 7 reconstructions of Si(111). Since these surfaces are modeled as two-dimensional slabs, a careful investigation is carried out to determine the slab thickness needed to capture the structural and electronic properties of these systems. The densities of states (DOSs) projected on different atoms in these surfaces are then compared, revealing that the root 3 x root 3, 5 x 5, and 7 x 7 surfaces are metallic, while the 2 x 2 surface is semiconducting. Finally, the DOSs for Si(111)-7 x 7 are related to scanning tunneling microscope data to offer an explanation for different adatom prominence trends depending on Si sample doping.