Dynamic Ad-Dimer Twisting Assisted Nanowire Self-Assembly on Si(001)

Based on ab initio total energy calculation, we show that a dynamic ad-dimer twisting assisted (DATA) process plays a crucial role in facilitating a novel structural reconstruction involving surface and subsurface atoms on Si(001). It leads to self-assembly of long nanowires of group-V elements (Bi, Sb) in the trenches of surface dimer vacancy lines (DVLs) with a characteristic double-dimer configuration. The key to this is the lowering of the kinetic barrier by the DATA process in conjunction with a favorable interaction between ad-dimers and step edges in DVLs. The present results provide an excellent account for experimental observations and reveal the atomistic origin and the dynamic transformation path for nanowire self-assembly on Si(001).

Figure 1

Stable configurations for zero to four group-V adatoms (purple balls) on Si(001) with bulklike and odd-membered ring 5-7-5 structures.

Figure 2

Relative surface energy as a function of the adatom chemical potential. Vertical lines divide different phase regions. Symbols of lines correspond to labels in Fig. 1. The corresponding bulk chemical potentials are $-3.90$, $-4.15$, and $-4.70\mathrm{eV}$ per atom for Bi, Sb, and As, respectively.

Figure 3

Top and side views of the atomic configurations along the transformation pathway.

Figure 4

The relative total energy variation in terms of the steps taken along the pathway with (squares) and without (circles) the DATA process.