  We study further the recently introduced [Ranguelov et al., Comptes Rendus de
l'Acad. Bulg. des Sci. 60, 4 (2007) 389] "C+-C-" model of step flow crystal
growth over wide range of model parameters. The basic assumption of the model
is that the reference ("equilibrium") densities used to compute the
supersaturation might be different on either side of a step. We obtain the
condition for linear stability of the whole step train in the form CL/CR>1 (L/R
stands for left/right in a descending from left to right step train). Further
we integrate numerically the equations of step motion to monitor the bunching
process in the long times limit. Thus we obtain the exact size- and time-
scaling of the step bunches including the numerical prefactors. We show that in
a broad range of parameters the morphology is characterized with appearance of
the minimal interstep distance in the bunch in the beginning of the bunches (at
the trailing edge of the bunch) and may be described by a single universality
class, different from those already generated by continuum theories [Krug et
al., PRB 71, 045412].
