Surface properties of Si(111)7×7 upon H and NH2 adsorption: A local-density-functional study

The structural and electronic properties of H and NH2 adsorbed onto the Si(111)7×7 surface are studied using first-principles local-density total-energy and atomic force calculations carried out for cluster models. The results show that different adsorption sites, i.e., the center adatom site, the corner adatom site, and the first-layer ''rest-atom'' site, exhibit different chemisorption properties. The results obtained from energy minimization for H and NH2 on these possible sites deduced from the Auger electron spectroscopy and photoemission yield spectroscopy studies confirm the model of a two-step adsorption process. The electronic structures of the adsorption systems were determined and a spectrum of orbital overlap population PO versus energy was calculated that is found to bear close relationship with images obtained from scanning tunneling microscope (STM) studies. We conclude from the properties of PO that H and NH2 adsorbed on the rest-atom site may not be detected by the STM but are still visible when absorbed on the adatom sites.