Scanning tunnelling microscopy images of the Ga-rich GaAs(100)c(8 x 2)/(4 x 2) surface exhibited vivid long-range patterns consisting of bright spots ("ghosts") which were attributed to localized excess charge rather than atomic clusters. The nearly planar geometry of the sp2-hybridized gallium dimer atoms results in localized π states made up of a combination of the Ga pz orbitals. These states in the upper half of the band gap formed the lowest unoccupied band. Surface or bulk defects lead to excess negative charge flowing into these localized states. Repulsion between the trapped negative excess charges led to the observed so-called ghost pattern.

Localized Excess Negative Charges in Surface States of the Clean Ga-Rich GaAs(100)c(8 x 2)/4 x 2 Reconstruction as Imaged by Scanning Tunnelling Microscopy. P.Kruse, J.G.McLean, A.C.Kummel: Journal of Chemical Physics, 2000, 113[6], 2060