Two-Stage Rotation Mechanism for Group-V Precursor Dissociation on Si(001)

We report ab initio identification of initial dissociation pathways for ${\mathrm{Sb}}_4$ and ${\mathrm{Bi}}_4$ tetramer precursors on Si(001). We reveal a two-stage double piecewise rotation mechanism for the tetramer to ad-dimer conversion involving two distinct pathways: one along the surface dimer row via a rhombus intermediate state and the other across the surface dimer row via a rotated rhombus intermediate state. These two-stage double piecewise rotation processes play a key role in lowering the kinetic barrier by establishing and maintaining energetically favorable bonding between adatoms and substrate atoms. These results provide an excellent account for experimental observations and elucidate their underlying atomistic origin that may offer useful insights for other surface reaction processes.

Figure 1

Optimized ad-dimer and tetramer configurations and their energies (in eV) defined in the text for $\mathrm{Bi}/\mathrm{Sb}$ on Si(001). Large (red) and small (gray) circles represent the adatoms and the Si atoms in the top two layers of the substrate, respectively. Lines are drawn to indicate the electronic bonds.

Figure 2

The two-stage dissociation pathway and the corresponding energies for ${\mathrm{Sb}}_4$ and ${\mathrm{Bi}}_4$ on Si(001) with the double piecewise rotation process, first from $T_A$ to $P$ and then from $P$ to $AB2$ across the surface dimer row.

Figure 3

The two-stage dissociation pathway and the corresponding energies for ${\mathrm{Sb}}_4$ and ${\mathrm{Bi}}_4$ on Si(001) with the double piecewise rotation process, first from $T_B$ to $R$ and then from $R$ to $AB1$ along the surface dimer row.

Figure 4

The pathways and relative energies for the diffusion of the ad-dimers on Si(001) with a rolling-over process along the surface dimer row from $B$ to $B$ and with a piecewise rotation process from $B$ to $A$ (or $AB$ to $AA$).