In view of understanding silicon incorporation in the δ doping process of GaAs (001), Si atoms have been deposited, under UHV, on a α(2×4) arsenic terminated substrate. In the low coverage regime, a transition to a less As rich (3×2) reconstructed Si–GaAs (001) surface was observed whose atomic structure has been investigated by grazing incidence X-ray diffraction performed in situ. Silicon is found to occupy not only a Ga substitutional site, precursor of a donor dopant but also to form nuclei for neutral clusters, on a template made by the (3×2) GaAs (001) reconstructed surface observed by Martrou et al. [Phys. Rev. B 72 (2005) 241307®]. The maximum surface concentration of donor-like silicon is estimated at 1.04×10 14 cm −2 (1/6th monolayer).