Direct real-space imaging of thec(2×8)∕(2×4)GaAs (001) surface structure

Abstract

We have performed frequency-modulated atomic-force microscopy (FM-AFM) on the $c(2\ifmmode\times\else\texttimes\fi{}8)∕(2\ifmmode\times\else\texttimes\fi{}4)$ GaAs (001) surface obtained from the $c(8\ifmmode\times\else\texttimes\fi{}2)∕(4\ifmmode\times\else\texttimes\fi{}6)$ surface by exposing it to ${\mathrm{As}}_{2}$ gas and annealing. Highly resolved interaction patterns reflect prevailing surface dimer pairs consistent with a so-called $\ensuremath{\beta}2$ structure, but more rare motifs characteristic of $\ensuremath{\alpha}2$ and $\ensuremath{\beta}$ structures are also seen. Atoms of the dimers interact with the atomic force microscope tip repulsively and appear as sharp features on a smooth background when imaged in constant-height mode. An analysis of the interaction decay length and lateral size of the atomic features indicates that the surface atoms are visualized through a core-core repulsion mechanism. In this imaging mode, the FM-AFM can be regarded as a true surface structure tool, since the observed features are, in the absence of significant lateral relaxation, associated with surface atoms directly.