  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.
