Is keV ion-induced pattern formation on Si(001) caused by metal impurities?

Abstract

We present ion beam erosion experiments performed in ultrahigh vacuum using adifferentially pumped ion source and taking care that the ion beam hits the Si(001)sample only. Under these conditions no ion beam patterns form on Si for anglesϑ≤45° with respect to the global surface normal using 2 keVKr+ andfluences of ≈ 2 × 1022 ions m−2. In fact, the ion beam induces a smoothening of preformed patterns. Simultaneoussputter deposition of stainless steel in this angular range creates a varietyof patterns, similar to those previously ascribed to clean ion-beam-induceddestabilization of the surface profile. Only for grazing incidence with60°≤ϑ≤83° do pronounced ion beam patterns form. It appears that the angular-dependent stability ofSi(001) against pattern formation under clean ion beam erosion conditions isrelated to the angular dependence of the sputtering yield, and not primarily to acurvature-dependent yield as invoked frequently in continuum theory models.