Halogen chemisorption, the pairwise diffusion of I, and trapping by defects on Si(1Â 0Â 0)

Halogen molecules dissociatively chemisorb on Si(1 0 0)-(2 × 1), and the bonding structures that they adopt can be elucidated with scanning tunneling microscopy. Of the Cl, Br, and I group, Cl has the highest single atom diffusion barrier, and both single and paired adatoms are observed at 295 K. The barrier is smaller for Br, and the adatoms can interrogate the surface until they form pairs, which are then immobile, or are trapped at C-type defects. The barrier is smallest for I, allowing the formation of pairs and trapped states, but the pairs are mobile at ambient temperature. Their motion is thermally activated, the events are random, and the diffusivities along and across the dimer row are ∼0.42 and ∼0.17 Å2/s at 295 K. The respective energy barriers for pairwise diffusion are ∼0.76 and ∼0.82 eV, assuming an attempt frequency of 1012 s−1. Studies over long times reveal that pairwise diffusion at low coverage is ultimately quenched by the increasing density of C-type defects, i.e. the increasing amounts of dissociated H2O.