Effect of immobile impurities on motion of steps on a vicinal face.

We carry out Monte Carlo simulations and study the dependence of the behaviors of steps on impurities on a vicinal face. We assume that the impurities are immobile and the lifetime of them is much longer than that of adatoms. During growth, the impurities in front of a step prevent the step from advancing. If a wide terrace appears due to the fluctuations of step positions, the number of impurities on the terrace is more than other terraces. The step at the upper side of the wide terrace is slower than other steps, so that an equidistant train of steps becomes unstable. In our Monte Carlo simulations, with steps gathering and separating repeatedly, the bunches of steps are formed on the unstable surface.