Bias-dependent imaging of the In-terminated InAs(001)(4×2)c(8×2)surface by STM: Reconstruction and transitional defect

We use low-energy electron diffraction (LEED) and scanning tunneling 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 (4×2) and $c(8\mathrm{}\times 2)$ 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 STM images confirm the existence of both (4×2) and $c(8\mathrm{}\times 2)$ phases and identify an electronic signature at the transition between the two reconstructions. Images of (4×2) 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 layer. The $c(8\mathrm{}\times 2)$ reconstruction, though similar to the (4×2), 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 (4×2)-to-$c(8\mathrm{}\times 2)$ 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 (4×2) to $c(8\mathrm{}\times 2)$ reconstructions.