Metallic Phases on a Pristine Si(111) Surface

We present the surface structures of pristine silicon with novel structural and electronic properties based on first-principles calculations. The surfaces are composed of connected armchair ribbons, resulting in quadrilateral, hexagonal, and elongated octagonal structures, in which the constituent Si atoms are threefold (dangling bonds) or fourfold coordinated. Some of the subsurface Si atoms below the surface layer are fivefold-coordinated (floating bonds). Despite having higher formation energies than the conventional Si(111) surfaces (e.g., 0.5 eV/(1 × 1) higher than the Pandey chain model), the structures have unique structural and electronic properties. One of the interesting structural properties is that the surface is nearly flat (average height difference of ∼0.02 Å) unlike the typical Si surfaces with large atomic bucklings. The calculated electronic structures show that the surfaces are metallic, as in topological insulators, in which the insulating bulk is surrounded by metallic surfaces.