Metallic and Semimetallic Silicon $⟨100⟩$ Nanowires

Silicon nanowires grown along the $⟨100⟩$ direction with a bulk Si core are studied with density-functional calculations. Two surface reconstructions prevail after exploration of a large fraction of the phase space of nanowire reconstructions. Despite their energetical equivalence, one of the reconstructions is found to be strongly metallic while the other one is semimetallic. This electronic-structure behavior is dictated by the particular surface states of each reconstruction. These results imply that doping is not required in order to obtain good conducting Si nanowires.

Figure 1

$\{100\}$-facet reconstructions. (a) $1c$ and (b) $2c$ dimerization of the surface dangling bonds. Lattice vectors of the facet unit cell are sketched to underline the different periodicity along $⟨100⟩$ axis ($c$ is the bulk lattice parameter).

Figure 2

Band structure diagrams. (a) The $1c$ reconstruction has four metallic states [(i) to (iv)] clearly crossing the Fermi level. (b) The $2c$ reconstruction has rather a single semimetallic state. For comparison, both diagrams refer to the $2c$ unit cell.

Figure 3

Cross-section view of the wave functions at $\Gamma$ point of (a) the four metallic states of the $1c$ reconstruction [(i) to (iv)] and (b) the semimetallic state of the $2c$ reconstruction. Labeling follows the scheme of Fig. 2. These wave functions give a false impression of ${\mathrm{C}}_{2v}$ symmetry. The pseudo ${\mathrm{C}}_{2v}$ symmetry explains why bands (ii) and (iii) are almost degenerate [surface states (a)-ii and (a)-iii in the figure].