SURFACE-PROPERTIES OF SI(111)7X7 UPON H AND NH2 ADSORPTION - A LOCAL DENSITY-FUNCTIONAL STUDY

The structural and electronic properties of H and NH, adsorbed onto th e Si(111)7 x 7 surface are studied using first-principles local-densit y total-energy and atomic force calculations carried out for cluster m odels. The results show that different adsorption sites, i.e., the cen ter adatom site, the corner adatom site, and the first-layer ''rest-at om'' site, exhibit different chemisorption properties. The results obt ained from energy minimization for H and NH2 on these possible sites d educed from the Auger electron spectroscopy and photoemission yield sp ectroscopy studies confirm the model of a two-step adsorption process. The electronic structures of the adsorption systems were determined a nd a spectrum of orbital overlap population P(o)(E) versus energy epsi lon was calculated that is found to bear close relationship with image s obtained from scanning tunneling microscope (STM) studies. We conclu de from the E properties of P(o)(E) that H and NH2 adsorbed on the res t-atom site may not be detected by the STM but are still visible when absorbed on the adatom sites.