THEORETICAL-STUDY OF TI ADSORPTION ON SI(001) SURFACES

Ti adsorption on Si(001) surfaces has been investigated by first-princ iples pseudopotential calculations. The -calculations have been perfor med on two adsorption structures, a Ti atom adsorbed at the pedestal s ite and at the dimer vacancy (DV) site, which have been recently obser ved by scanning tunneling microscopy (STM). For both structures, the l ocal density of states is qualitatively in good agreement with the STM image profiles. We have found that the adsorption energies for the pe destal-site and DV-site adsorptions are 4.7 and 5.3 eV/adatom, respect ively, and that the DV-site adsorption is energetically more favorable . The nature of Ti-Si and Si-Si bonds is also discussed in terms of ch arge-density distribution. For the DV-site adsorption, we have found c onsiderable redistribution of the charge density in the substrate, tha t is, the breakage of the bulk Si bonds. This gives an insight into a precursor state of silicidation.