  We formulate a new (1+1)D step model of potentially unstable vicinal growth
that we call "C+ - C-" model and study the step bunching process in it. The
basic assumption is that the equilibrium adatom concentrations on both sides of
the step are different and this may cause destabilization of the regular step
train. We deduce equations of step motion and numerically integrate them to
obtain the step positions on a discrete time set. New dynamic phenomena are
observed during the bunching process: 1. parts of the crystal surface undergo
temporary evaporation in the course of coalescence of two bunches, the larger
being the one that "goes back", 2. the minimal interstep distance appears in
the beginning rather than in the middle of the bunch. We speculate that the
latter may serve as a diagnostic criterion for a new universality class
constructed from the time- and size-scaling exponents describing the bunching
process in the diffusion-limited regime.
