  We studied the step dynamics during sublimation and growth in the presence of
electromigration force acting on the adatoms. In the limit of fast surface
diffusion and slow kinetics of atom attachment-detachment at the steps we
formulate a model free of the quasi-static approximation in the calculation of
the adatom concentration on the terraces. Numerical integration of the
equations for the time evolution of the adatom concentrations and the equations
of step motion reveals two different step bunching instabilities: 1) step
density waves (small bunches which do not manifest any coarsening) induced by
the kinetic memory effect and 2) step bunching with coarsening when the
dynamics is dominated by the electromigration. The model developed in this
paper also provides very instructive illustrations of the Popkov-Krug dynamical
phase transition during sublimation and growth of a vicinal crystal surface.
