  Stepped GaN(0001) surface is studied by the kinetic Monte Carlo method and
compared with the model based on Burton-Carbera-Frank equations. Successive
stages of surface pattern evolution during high temperature sublimation process
are discussed. At low sublimation rates clear, well defined step bunches form.
The process happens in the absence or for very low Schwoebel barriers at the
ideal surface. Bunches of several steps are well separated, move slowly and are
rather stiff. Character of the process changes for more rapid sublimation
process where double step formations become dominant and together with meanders
and local bunches assemble into the less ordered surface pattern. Solution of
the analytic equations written for one dimensional system confirms that step
bunching is induced by the particle advection caused by step-flow anisotropy.
This anisotropy becomes important when due to the low Schwoebel barrier both
sides of step are symmetric. Simulations show that in the opposite limit of
very high Schwoebel barrier steps fracture and rough surface builds up.
