Anisotropy of ion-induced amorphous-crystalline ripples in silicon

The morphological evolution of surfaces during ion-beam irradiation has attracted significant interest due to possibility of the evelopment and the controlling of self-organization in nanostructures. Pattering and texture on nanometer length scale at metal and semiconductor surfaces has become a topic of intense research. In particular the surface and subsurface ripple formation under 40Ar+ ion-beam irradiation of Si (100) crystal has been studied recently. Based on transmission electron microscopy (TEM), atomic force microscopy (AFM) and x-ray analysis a dramatic effect of the ripple formation was found at an irradiation energy of 60 keV.
Apart from the crystalline part, the amorphization process is very important for understanding the amorphous-crystalline interface and the ripple formation mechanism. The dose of the ion beam was varied in the range from low 3e16 up to high 7e17 ions/cm^2 at an incident angle of 60° and an energy of 60 keV. Keeping the optimized irradiation parameters constant we have measured the degree of amorphization as a function of Ar+ dose by means of x-ray grazing incidence amorphous scattering (GIAS). For irradiated samples we found two broad peaks indicating a short-range ordering of amorphous material which does what with the penetration depth of probing x-ray. GIAS profiles probed at different azimuthal angles display a strong anisotropy of the amorphous scattering. The strong damage of the crystalline structure takes place along particular crystallographic directions. This is for the directional anisotropy: and strongly reveal for low doses, before it becomes completly amorphous and mostly uniform at high doses of implantation.
Such behavior can be explained taking into account the anisotropy of the crystalline material and orientation of the ion beam with respect crystallographic orientation of the substrate material. This work was supported by the DST-DAAD India-Germany Collaborative Program. We are grateful to ID1 beamline staff for the support at ESRF.