Nanoscale patterns produced by ion erosion of a solid with codeposition of impurities: The crucial effect of compound formation

We advance a theory of ripple formation on the surface of an initially elemental material that is subjected to concurrent oblique-incidence impurity deposition and normal-incidence ion bombardment. The theory applies when the target atoms and impurities react to produce a chemical compound, as in many experiments that have been carried out in which the target material is silicon and the impurities are metallic. We show that the formation of a compound can destabilize a surface that would otherwise be stable. If ripples do form, they are oriented with their wave vector parallel to the projected impurity deposition direction and they propagate counter to the deposited flux. Our theory also predicts how the ripple wavelength behaves for impurity fluxes just above the threshold value necessary for ripple formation.