  We consider a vicinal face, where atoms and impurities impinge and evaporate
to a vapor phase, to study how the surface diffusion and evaporation of
impurities affect step bunching induced by impurities. When the lifetime of
impurities on the vicinal face $\tau_\mathrm{imp}$ is long and the surface
diffusion of impurities is neglected, the step bunches induced by impurities
are tight. When $\tau_\mathrm{imp}$ decreases, the size of the step bunches,
which means the number of steps in the bunches, decreases but the separation of
single steps from bunches does not occur. When we take into account fast
surface diffusion of impurities, the separation and collision between single
steps and step bunches occur repeatedly.
