Clustering of charged adsorbates: Scanning tunneling microscopy observations of chlorine on gallium-rich GaAs(001)-c(8 x 2)

The forces between adsorbates on metals are often repulsive due to dipole-d ipole interactions. Electronegative adsorbates on semiconductors, however, sometimes form clusters, exhibiting attractive interactions even though the same dipole-dipole model should apply. Here we report observations with sc anning tunneling microscopy of the clustering of chlorine chemisorbed on th e gallium arsenide(001)-c(8 x 2) surface. For this system, the adsorbate ha s been found to have an anomalously high mobility; the clustering is theref ore clearly energetically favored, rather than kinetically frozen. The clus tering can be understood on a purely electrostatic basis. Because the Cl-Ga bonds are not normal to the surface, clusters more closely resemble an ion ic crystal than an array of dipoles. Charging of the second-neighbor Ga of the Cl adsorbate enhances this effect.