We demonstrate that surface ripples with an exceptionally high degree of order can develop when germanium is bombarded with a broad beam of gold ions. In contrast, if silicon is sputtered with an Au− beam, patches of ripples with two distinct wave vectors can emerge. These types of order can be understood if the coupling between the surface morphology and composition is taken into account.
An anisotropic linear combination of hxxxx, hxxyy, and hyyyy in the equation of motion for the surface height was originally obtained by expanding the Sigmund theory of ion sputtering11 to fourth order in the spatial derivatives.10,25 This approach has been criticized because the Sigmund theory is ill defined for short wavelengths.26,27 However, terms of this kind would in general appear in a theory of sputtering that does not suffer from this deficiency. An anisotropic linear combination of hxxxx, hxxyy, and hyyyy is also obtained as a fourth order approximation to a model of ion-induced anisotropic plastic flow.19,20