It was recalled that, although strongly bound chemisorbates readily diffused at low coverages on metal surfaces at 300K, they did not generally diffuse on semiconductor surfaces because of large corrugations of the adsorbate-surface interaction potential. It was noted that Cl, when chemisorbed on Ga-rich (001)-c(8 x 2) surfaces exhibited anomalously rapid diffusion even though the chemisorption state was tightly bound and highly specific. Hartree-Fock total-energy calculations suggested that the diffusion of strongly bound adsorbates could occur at 300K because there were multiple nearly-degenerate adsorbate sites. That is, for a single Cl atom, there were 2 almost-degenerate adsorption sites. One dangling-bond site had an adsorption energy of 2.42eV. Insertion of the Cl atom into the Ga dimer formed a bridge-bond with an angle of 130º. Another site had an adsorption energy of 2.50eV. In both cases, there was a charge of –0.5e on the Cl atom, and a charge of 0.32e on the nearest Ga atoms.

Anomalous Mobility of Strongly Bound Surface Species: Cl on GaAs(001)-c(8 x 2). I.P.McClean, P.Krüger, J.Guo-Ping, H.E.Ruda, A.C.Kummel: Physical Review Letters, 2000, 85[7], 1488-91