AN INVESTIGATION OF THE SI(111)-(SQUARE-ROOT-3XSQUARE-ROOT-3)R30-PERCENT-AG SURFACE BY LI+ IMPACT COLLISION ION-SCATTERING SPECTROSCOPY

The Si(111)-(square-root 3 x square-root 3)R30-degrees-Ag surface has been investigated using the technique of Li+ impact collision ion scattering spectroscopy. Typical LEED square-root 3 domain sizes were estimated to be on the order of 150 angstrom for a 1 ML coverage of Ag, with the square-root 3 structure persisting for coverages of Ag up to 35 ML. Silver islanding was found to influence the appearance of the 5 keV Li+ ICISS angular scans even for 1 ML coverages of Ag deposited at 480-degrees-C. A detailed structural analysis of the Si(111)-square-root 3-Ag surface (0.25 ML deposition) involved the comparison of 5 keV Li+ ICISS experimental data along the [112BAR, [1BAR10] and [23BAR1] azimuths with computer simulations of the scattered ion intensities based on previously proposed models for the square-root 3 surface. Nine structurally different models were tested, and only the missing-top-layer (MTL) and the honeycomb-chained-trimer (HCT) models were found to be consistent with all the experimental results. An estimate of 0.4 angstrom for the maximum downward vertical displacement of the Ag atoms with resect to the surface Si atoms in the MTL model is made. The effects of increased thermal vibrational amplitude in the simulation of Si-Ag shadowing effects is also discussed. The interpretations of previous noble gas ICISS results are shown to be inconsistent with the present alkali metal ICISS study of the square-root 3 surface.