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

TL;DR

This study shows that pattern formation on Si(001) under 2 keV Kr ion bombardment in ultra-high vacuum is primarily influenced by metal impurities and incident angle, with no patterns forming at angles below 45 degrees.

Contribution

It demonstrates that clean Si(001) surfaces do not form patterns under certain ion beam conditions, emphasizing the role of impurities and incident angle in pattern formation.

Findings

No patterns form below 45° incidence in clean conditions.

Metal impurity deposition induces pattern formation.

Pattern formation is strongly dependent on incident angle.

Abstract

We present ion beam erosion experiments performed in ultra high vacuum using a differentially 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 angles below 45 degrees with respect to the global surface normal using 2 keV Kr ions and fluences of 2 x 10^22 ions/m^2. In fact, the ion beam induces a smoothening of preformed patterns. Simultaneous sputter deposition of stainless steel in this angular range creates a variety of patterns, similar to those previously ascribed to clean ion beam induced destabilization of the surface profile. Only for grazing incidence with incident angles between 60 degrees and 83 degrees pronounced ion beam patterns form. It appears that the angular dependent stability of Si(001) against pattern formation under clean ion beam erosion conditions is related to the angular dependence of the sputtering yield, and not primarily to a curvature dependent yield as invoked frequently in continuum theory models.