BIAS-DEPENDENT IMAGING OF THE IN-TERMINATED INAS(001) (4X2) C(8X2) SURFACE BY STM - RECONSTRUCTION AND TRANSITIONAL DEFECT/

We use low-energy electron diffraction (LEED) and scanning tunnelling microscopy (STM) to study the In-terminated InAs(001) surface prepared by argon sputtering and annealing. Characterization by LEED shows the formation of a highly ordered surface with a mixture of (4X2) and c(8 x2) phases. We systematically vary the sample bias in STM to obtain bi as-dependent images over the same surface regions, allowing discrimina tion between topographic and electronic features. Atomic resolution TM images confirm the existence of both (4x2) and c(8X2) phases and iden tify an electronic signature at the transition between the two reconst ructions. Images of (4x2) regions are consistent with a previously pro posed model for this surface in which the unit cell contains one In di mer in the first layer and two In dimers in the third la)er. The c(8X2 ) reconstruction, though similar to the (4x2), is found to arise from a shift in the third- and/or first-layer In dimers by one lattice spac ing. Filled-state imaging at the (4X2)-to-c(8X2) boundary shows two br ight spots positioned midway between the first-layer In dimer rows. In empty states, these spots are entirely absent, underlining their elec tronic origin. These electronic features are explained in terms of a l ocalization of charge due either to a structural defect or to the pres ence of a sulfur doping atom at the transition from (4X2) to c(8X2) re constructions.