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(8x2) phases. We systematically vary the sample bias in STM to obtain bias-dependent images over the same surface regions, allowing discrimination between topographic and electronic features. Atomic resolution TM images confirm the existence of both (4x2) and c(8X2) phases and identify an electronic signature at the transition between the two reconstructions. Images of (4x2) regions are consistent with a previously proposed model for this surface in which the unit cell contains one In dimer 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 spacing. Filled-state imaging at the (4X2)-to-c(8X2) boundary shows two bright spots positioned midway between the first-layer In dimer rows. In empty states, these spots are entirely absent, underlining their electronic origin. These electronic features are explained in terms of a localization of charge due either to a structural defect or to the presence of a sulfur doping atom at the transition from (4X2) to c(8X2) reconstructions.