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High-resolution low and medium energy ion scattering coupled with photoelectron spectroscopy has revealed unambiguously the atomic structure of NiSi2(001) surface. The NiSi2(001) layers were grown on Si(001) substrates at 600°C for 30 min in ultra-high vacuum and the surface showed a 1×1 RHEED (reflection high energy electron diffraction) pattern. There exist Si adatoms taking H4 (~41 %) and T4 (~10 %) sites above the top Si layer of the bulk-truncated NiSi2(001). The Si adatoms of H4 and T4 sites correspond to the frame and center atoms, respectively observed previously by a scanning tunneling microscope. The ab initio calculations show that the H4 site is energetically preferable on the Si-layer rather than T4 site. A small amount of Si-adatoms located at T4 sites are eventually formed during the silicidation reaction and may reduce the energy by pairing (center atoms). The Si 2p core level spectra observed have three components, which are assigned to (i) the bulk NiSi2 (binding energy EB = 99.8 eV), (ii) the Si adatoms taking T4 site and the top layer Si atoms beneath the Si adatoms of H4 site (EB = 99.2 eV), and to (iii) the Si adatoms of H4 site and the top layer Si atoms beneath the Si adatoms of T4 site (EB = 98.7 eV). |