We have performed Monte-Carlo simulations of the growth of GaAs by MBE. We included in our calculations the anisotropy of the migration, the formation of As-dimers and a partial desorption of the As. As an observable we calculated the RHEED signal of the specularly reflected electrons. The results have been compared with experimental data comprising both the damping of the oscillations and the recovery following a growth interrupt. The agreement between experiment and calculations is rather good. Moreover we could identify the mechanisms underlying the fast and the slow component of the recovery.