  Recent observations of the growth of protein crystals have identified two
different growth regimes. At low supersaturation, the surface of the crystal is
smooth and increasing in size due to the nucleation of steps at defects and the
subsequent growth of the steps. At high supersaturation, nucleation occurs at
many places simultaneously, the crystal surface becomes rough and the growth
velocity increases more rapidly with increasing supersaturation than in the
smooth regime. Kinetic roughening transitions are typically assumed to be due
to the vanishing of the barrier for two-dimension nucleation on the surface of
the crystal. We show here, by means of both analytic mean field models and
kinetic Monte Carlo simulations that a transition between different growth
modes reminiscent of kinetic roughening can also arise as a kinetic effect
occurring at finite nucleation barriers.
