Structural properties of epitaxialNiSi2on Si(111) investigated with x-ray standing waves

Abstract

We have investigated the structure of epitaxial layers of A-type and B-type ${\mathrm{NiSi}}_2$ on Si(111) with x-ray standing waves. Several samples of each type were used for the measurements with thicknesses of the silicides ranging from 6.3 to 97.6 nm which corresponds to 20 to 314 layers of ${\mathrm{NiSi}}_2$. Our results agree with a model, where the ${\mathrm{NiSi}}_2$ bonds, via the Si atoms, to the Si(111) surface dangling bonds, which leads to a sevenfold coordination of the Ni atoms in the first silicide layer. The distance of the first layer of nickel atoms from the Si(111) surface diffraction plane was determined with high accuracy for all samples. This distance is significantly shorter than the value of 0.352 nm calculated from bulk bond lengths for A-type epitaxy and B-type epitaxy for which values of 0.337 and 0.346 nm, respectively, were determined. This substantial difference and the amount of relaxation at the interface may promote understanding of the Schottky-barrier heights observed at both types of interfaces.