Simultaneous reflection high-energy electron diffraction and reflectance difference spectroscopy measurements are used-to monitor characteristic changes in the short-and long-range order that accompany Si incorporation on GaAs(001) and Si segregation during GaAs overgrowth. A new difference-function approach between reflectance difference spectra taken from Si-covered and from bare GaAs(001) surfaces allow us to detect not only As and Ga dimers but also Si dimers and As dimers on Si. Transients measured at characteristic photon energies provide a detailed picture of the surface kinetics and its dependence on the misorientation. For Si deposition on singular surfaces with (2X4)beta reconstruction the nucleation occurs at randomly distributed sites, due to Si incorporation on unoccupied Ga sites in the trenches of this structure, and the phase transitions occur at higher coverages than for pulsed Si supply on a well prepared vicinal surface with (2X4)cu reconstructed terraces. In the latter case well-ordered structures are developed at well defined coverages. These differences in the incorporation kinetics explain the diverging findings of carrier concentration saturation in delta-doped samples reported in the literature.