  Electromigration-induced step bunching in the presence of sublimation or
deposition is studied theoretically in the attachment-limited regime. We
predict a phase transition as a function of the relative strength of kinetic
asymmetry and step drift. For weak asymmetry the number of steps between
bunches grows logarithmically with bunch size, whereas for strong asymmetry at
most a single step crosses between two bunches. In the latter phase the
emission and absorption of steps is a collective process which sets in only
above a critical bunch size and/or step interaction strength.
