Clean, 2 x 1-reconstructed Si(001) surfaces have been exposed, at room temperature, to an increasing flux of diatomic sulfur molecules as produced, within the ultra-high-vacuum vessel, by an all-solid silver sulfide electrolytic tell. The surface properties of the system were studied as a function of the sulfur exposure using low-energy electron diffraction, Auger electron spectroscopy and photoemission yield spectroscopy (PYS). The sulfur adsorption mechanism is characterised by a sticking coefficient between 0.1 and 0.2 up to an S coverage near to one monolayer [on Si(001), 1 ML= 6.7 x 10(14) atoms/cm(2)], then much smaller up to saturation at 1.25 ML, at room temperature. Between 0.5 and 1 hit coverage, the 2 x 1 reconstruction is replaced by a 1 x 1. PYS indicates a significant increase of both the work function and the ionisation energy, together with a removal of the gap surface states, asserting the S monolayer as a topmost (001) plane ending a perfect silicon lattice. (C) 1999 Elsevier Science B.V. All rights reserved.
