Deciphering the Atomistic Mechanism of Si(111)‑7 × 7 Surface Reconstruction Using a Machine-Learning Force Field

While the complex 7 × 7 structure that arises upon
annealing
the Si(111) surface is well-known, the mechanism underlying this unusual
surface reconstruction has remained a mystery. Here, we report molecular
dynamics simulations using a machine-learning force field for Si to
investigate the Si(111)-7 × 7 surface reconstruction from the
melt. We find that there are two possible pathways for the formation
of the 7 × 7 structure. The first path arises from the growth
of a faulted half domain from the metastable 5 × 5 phase to the
final 7 × 7 structure, while the second path involves the direct
formation of the 7 × 7 reconstruction. Both pathways involve
the creation of dimers and bridged five-membered rings, followed by
the formation of additional dimers and the stabilization of the triangular
halves of the unit cell. The corner hole is formed from the joining
of several five-member rings. The insertion of atoms below the adatoms
to form a dumbbell configuration involves extra atom diffusion or
rearrangement during the evolution of triangular halves and dimer
formation along the unit cell boundary. Our findings may provide insights
for manipulating the surface structure by introducing other atomic
species.