Interfacial reaction between Ta ultrathin films and Si(111) substrate

We report UPS, work-function and LEED results on the interfacial reaction between evaporated Ta and Si(111)(7×7) surface under ultrahigh vacuum conditions. At room temperature, a dosordered chemisorbed phase is formed at low coverage, θ≲1 monolayer (ML), and is characterized by an UPS Ta 5d peak shifted at about −1.2 eV as compared to bulk Ta. For 1≲θ≲4 ML, although AES and XPS indicate a 1 Ta:2 Si composition, the valence band spectra are still different from the TaSi2 one. At higher coverage, the electronic structure of polycrystalline Ta is progressively recovered. Annealing of the deposits leads to silicide formation by interdiffusion and surface atomic ordering at temperatures from 650°C to 850°C, depending on the thickness. All the deposits in the 0.8–100 ML range, including the smallest ones, are stable until ∼500°C and the silicide formation reaction begins only at higher temperature, reflecting an activation energy barrier. For θ≲5 ML, clustering of the TaSi2 phase occurs. For thicker deposits, continuous TaSi2 overlayers are obtained. The electronic structure of this silicide is discussed in relation with existing models.