Cluster calculations of the Si(111) 7×7 dimer–adatom–stacking-fault structure

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Abstract
We have studied the dimer–adatom–stacking-fault (DAS) 7×7 reconstruction on Si(111) using quantum-chemical calculations of large triangular clusters representing the two halves of the surface unit cell (SUC). We calculate the DAS structure to be 0.17 eV per 1×1 SUC more favorable than the 2×1 chain reconstruction. It is found that the electronic structure is quite sensitive to vertical displacements of the surface atoms, even though the surface energy remains relatively undisturbed. We associate the phenomenon with the pairing of the rest atoms and the central adatoms which resembles the Jahn-Teller effect. We then show that the effect is important for a realistic model of the DAS reconstruction energetics, which explains the occurrence of 7×7 reconstruction rather than other odd-by-odd surface patterns. Regarding the bonding configuration, we find (somewhat delocalized) bonds between the adatom and first-layer atoms, and also between these and the second-layer atom directly below the adatom. However, we find there is no bond between the adatom and the second-layer atom. Instead, there is an s-like dangling-bond orbital on the adatom. We expect the adatom backbonds to be weakened single bonds.