We have performed frequency-modulated atomic-force microscopy (FM-AFM) on the $c(2\times 8)∕(2\times 4)$ GaAs (001) surface obtained from the $c(8\times 2)∕(4\times 6)$ 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 $\beta 2$ structure, but more rare motifs characteristic of $\alpha 2$ and $\beta$ 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.

• Received 19 April 2007

DOI:https://doi.org/10.1103/PhysRevB.76.245314