Two-step GISAXS characterization of NiSi₂ nanoplates and Ni nanocrystals embedded in a silicon wafer covered with a silica thin film

Here, an experimental grazing-incidence small-angle X-ray scattering (GISAXS) study of the nanostructure of a sample composed of a Si(001) wafer covered by a Ni-doped SiO₂ thin film and thermally treated at high temperature is described. Previous studies indicated that this type of composite contains Ni nanocrystals mainly inside the thin film and NiSi₂ nanoplates buried in the Si wafer. To achieve accurate determinations of low-resolution structural parameters of the nanoparticles derived from the experimental results (shape, volume and size), two GISAXS patterns were recorded. The first pattern was produced by the Si wafer covered by a nanoporous Ni-doped SiO₂ thin film and the second pattern was recorded after removal of the SiO₂ thin film by chemical etching. By using the procedure of best fitting of a modeled isotropic GISAXS intensity to a set of 1D GISAXS patterns recorded before thin-film removal, the average radius and radius dispersion of a size polydisperse set of spherical Ni nanocrystals were determined. The GISAXS pattern recorded after removal of the SiO₂ thin film was almost completely due to the scattering intensity produced by oriented NiSi₂ nanoplates located inside the Si wafer. By fitting a theoretical function for the X-ray scattering intensity produced by oriented nanoplates to a set of experimental 1D GISAXS patterns, the maximum diameters and average thickness of the hexagonal NiSi₂ nanoplates were determined. The GISAXS intensity pattern produced by the sample after thin-film removal is essentially anisotropic (with only a weak isotropic contribution from Ni nanocrystals), thus allowing for precise quantitative determinations of the relevant sizes of the NiSi₂ nanoplates.