Substitution of In for Si adatoms and In-induced charge redistribution of the Si(1 1 1)-(7×7) surface

The adsorption mechanism and the origin of the In-induced surface electronic states of the Si(1 1 1)-(7×7) surface have been studied using bias-dependent scanning tunneling microscopy (STM) and scanning tunneling spectroscopy (STS). At very low In coverages, bias-dependent STM images show that some of the adatoms have either slightly higher or lower apparent height in the filled-state topographic images, while the empty-state images are completely identical to those of the clean Si(1 1 1)-(7×7). These peculiar features are interpreted as being caused by three different adsorption mechanisms: the substitution of In for Si adatoms in the 7×7 unit cell; In atom adsorption on Si adatoms; and charge redistribution due to the adsorption of In atoms on Si rest-atom dangling bonds (DBs). These mechanisms involve covalent bonding which removes intrinsic metallic surface states caused by partially occupied Si adatom DBs, opening a band gap at the surface and producing a metal–insulator transition.