Localized excess negative charges in surface states of the clean Ga-rich GaAs(100)c(8x2)/4x2 reconstruction as imaged by scanning tunneling microscopy

Scanning tunneling microscopy images of the Ga-rich GaAs(100)c(8x2)/(4x2) surface exhibit vivid long-range patterns consisting of bright spots ("ghosts") which are attributed to localized excess charge rather than atomic clusters. The nearly planar geometry of the sp(2)-hybridized gallium dimer atoms results in localized pi states made up of a combination of the Ga p(z) orbitals. These states in the upper half of the band gap form 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 leads to the observed "ghost" pattern. (C) 2000 American Institute of Physics. [S0021-9606(00)71230-X].