Effect of the Surface Diffusion and Evaporation of Impurities on Step Bunching Induced by Impurities.

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.