Variable Stoichiometry of the GaAs (001) - c(4x4): In Situ X-Ray Scattering Study

The atomic structure of the c(4x4) reconstruction of GaAs(001) has been studied by grazing incidence X-Ray diffraction using synchrotron radiation on surfaces prepared in-situ by molecular beam epitaxy. A total of 46 independent reflexions has been collected among which 40 pure surface peaks with fractional indices carry the information on the atomic structure in the surface layer and 6 spectra with integer indices characterize the registry of the reconstructed layer with respect to the bulk. The actual surface arrangement is described as a mixture of two limiting structures having identical centered square unit cells but different As coverages: 40% of the reconstructed area correspond to an ordered array of 4 As-atom clusters chemisorbed on the As bulk termination layer (coverage 0.5) whereas 60% show building blocks of 6 As-atom clusters (coverage 0.75).