Conduction bands of oxidized rare-earth-metal–silicon interfaces: An inverse-photoemission study

The unoccupied electronic states at the Gd-Si(111) and Eu-Si(111) interfaces have been investigated by uv inverse-photoemission spectroscopy (IPES) in an isochromat mode (hν=9.5 eV) as a function of oxygen exposure. Complementary results of the occupied states have been obtained by direct uv photoemission spectroscopy (UPS). The IPES spectra emphasize the extended conduction-band states of the systems, since the localized 4f states of the rare-earth atoms have cross sections that are too low at the used isochromat energy to manifest themselves clearly in the spectra. The conduction bands of the oxidized interfaces are compared to those of ${\mathrm{SiO}}_2$ and of rare-earth oxides, and the formation of silicate-type ternary compounds is suggested for the oxidized interfaces. These phases are insulating, and the effective band gaps have been estimated by combining IPES with UPS results. The evolution of the conduction bands with ${\mathrm{O}}_2$ exposure confirms the high reactivity of rare-earth-metal–silicon interfaces towards oxidation, and the dependence of the oxidation rate on the initial metal coverage. IPES data on the oxidation of pure Gd and Eu metals are also presented.