Effect of Two-Dimensionality on Step Bunching Induced by the Drift of   Adatoms
  We study the effect of two-dimensionality on step bunching induced by drift of adatoms. When anisotropy of the diffusion coefficient changes alternately on consecutive terraces like a Si(001) vicinal face, bunching occurs with the drift of adatoms. If the fluctuation of step bunches is neglected as in the one-dimensional model, the bunching with step-down drift is faster than that with step-up drift in contradiction with the experiment by Latyshev and coworkers. In a two-dimensional model, the step bunches wander heavily with step-up drift and recombination with neighboring bunches occur more frequently than those with step-down drift and the bunching is accelerated. When the difference of kinetic coefficients between two types of steps is taken into account, the bunching with step-up drift can be faster than that with step-down drift. 
Masahide Sato, Makio Uwaha, Yukio Hirose