  This paper investigates faceting mechanisms induced by electromigration in
the regime where atomic steps are transparent. For this purpose we study
several vicinal orientations by means of in-situ (optical diffraction,
electronic microscopy) as well as ex-situ (AFM, microprofilometry)
visualization techniques. The data show that faceting proceeds in two stages.
The first stage is short and leads to the appearance of a step density wave,
with a wavelength roughly independent of the surface orientation. The second
stage is much slower, and leads to the formation of a hill-and-valley
structure, the period of which depends on the initial surface orientation. A
simple continuum model enables us to point out why the wavelength of the step
density wave does not depend on the microscale details of the surface. The
final wavelength is controlled by the competition between elastic step-step
interaction and facet edge energy cost. Finally, the surface stress angular
dependence is shown to emerge as a coarsed-grained picture from the step model.
