Instabilities of permeable steps induced by the drift of adatoms

We study the effect of permeability on the two types of step instabilities, bunching and wandering. When the step is permeable for adatom diffusion, t he wandering instability occurs with step-down drift and the bunching insta bility occurs with step-up drift. In contrast, when the step is impermeable for adatom diffusion, both the wandering and the bunching instabilities oc cur with step-down drift. We derive the linear amplification rates of step fluctuation and the critical drift velocities to induce the instabilities. The results of the linear analysis are tested by Monte Carlo simulation.