  The morphology of a growing crystal surface is studied in the case of an
unstable two-dimensional step flow. Competition between bunching and meandering
of steps leads to a variety of patterns characterized by their respective
instability growth rates. The roughness exponent is shown to go from 1/2 to 1,
between the pure bunching to the meandering regimes. Using numerical
simulations, we observe that generically, a transition between the two regimes
occurs. We find surface shapes and roughness time evolution in quantitative
agreement with experiments.
