Electronic structure of refractory metal silicide thin films

Abstract

Direct photoemission (XPS and UPS), inverse photoemission (IPE) and SiL2,3 soft X-ray emission have been used to investigate the electronic structure and the chemical bonding in NbSi2, TaSi2, MoSi2 and WSi2 thin films formed on Si(111) clean surfaces. Valence bands and conduction bands reveal strong similarities between NbSi2 and TaSi2 on the one hand, and MoSi2 and WSi2 on the other hand. For the two former silicides, the density of the occupied states fits qualitatively to the calculations of Bisi and Chiao for isoelectronic and isostructural VSi2, while discrepancies are observed for the unoccupied states. Experimental spectra of MoSi2 and WSi2 agree well with the theoretical band structure calculated by Bhattacharyya et al. on both sides of the Fermi level. Moreover, the presence of the Si3d states hybridized with the metal d states in the valence band as predicted by these calculations is confirmed by SiL2,3 emission experiments. Compared to pure metals and silicon the binding energy shifts of the metal and Si core levels are weak and are not simply related to charge transfer. Using the Si "Auger parameter" variation, we find a small charge loss of ∼ 0.1-0.2 electron by each Si atom. This means that the bonding between silicon and the metal is predominantly covalent. © 1989.