Impact collision ion scattering spectrometry studies of thin metal overlayers on Si(111) surfaces

Abstract

The technique of impact-collision ion scattering spectrometry (ICISS) was used to determine the in-plane geometries of the (a) Si(111)−(√3×√3) Ag and (b) Si(111)− Ni surfaces. For Si(111)−(√3×√3) Ag, comparison of experimental data with computer simulation leads to a substitutional trimer (ST) model in which Si honeycombs sit on top of Ag trimers occupying substitutional sites. The separation of the Si and Ag layers is 0.7±0.3 Å and the SiAg bond length is 2.66±0.3 Å. We also observed type-A and type-B arrangements of the Si and Ag atoms on the surface, with type-B being type-A rotated 180° about the surface normal. Our results are consistent with reported scanning tunneling microscopy images and also with a missing top layer (MTL) model proposed from photoemission and x-ray photoelectron diffraction studies. For Si(111)− Ni, the agreement between computer simulations and experimental data indicates that at extremely low Ni coverages, ∼0.1 ML, Ni atoms diffuse into the Si substrate at room temperature and form regions of predominantly type-B NiSi 2.