Electronic states around and within the bandgap for clusters on GaAs

Recently two types of states have been observed for Sb, Bi, and other adsorbates on GaAs(110). For ordered overlayers, photoemission and inverse photoemission show adsorbate-induced states of the kinds that one might expect for systems with good two-dimensional order. On the other hand, scanning tunneling spectroscopy measurements also show states associated with various types of imperfections. The former states are consistent with no Fermi-level pinning, whereas the latter states can provide a mechanism for Fermi-level pinning. Here we report theoretical studies of the electronic states associated with Sb and Bi clusters, around and within the GaAs band gap. Incomplete bonding leads to gap states, but the Sb clusters with the most nearly complete adatom-adatom bonding show semiconducting behavior.