Depth profiling of Fe-implanted Si(100) by means of X-ray reflectivity and extremely asymmetric X-ray diffraction

This article reports on surface density variations that are accompanied by ion-beam-induced pattern formation processes on Si. The density profiles perpendicular to Si(100) surfaces were investigated after off-normal implantation with 5 keV Fe⁺ ions at fluences ranging from 1 × 10¹⁶ to 5 × 10¹⁷ ions cm^-2. Ripple formation was observed for ion fluences above 1 × 10¹⁶ ions cm^-2. X-ray reflectivity (XRR) revealed the formation of a nanometre subsurface layer with incorporated Fe. Using XRR, no major dependence of the surface density on the ion fluence could be found. In order to improve the surface sensitivity, extremely asymmetric X-ray diffraction was applied. Depth profiling was achieved by measuring X-ray rocking curves as a function of the decreasing incidence angle down to 0° using this noncoplanar scattering geometry. The density information was extracted from the dynamical Bragg shift of the diffraction peak caused by refraction of the X-ray beam at the air-sample interface. Simulations based on the dynamical theory of X-ray diffraction revealed a decrease of density for increasing ion fluence in a region close to the surface, caused by the amorphization and surface roughening.