Direct real-space imaging of the c(2x8)/(2x4) GaAs (001) surface structure

We have performed frequency-modulated atomic-force microscopy (FM-AFM) on the c(2x8)/(2x4) GaAs (001) surface obtained from the c(8x2)/(4x6) surface by exposing it to 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.