Empty electronic states at rare-earth-metal-silicon interfaces: Inverse photoemission of Gd and Eu on Si(111)7 x 7.

Abstract

The unoccupied electronic states at Gd/Si(111)7\ifmmode\times\else\texttimes\fi{}7 and Eu/Si(111)7\ifmmode\times\else\texttimes\fi{}7 interfaces have been investigated by inverse-photoemission spectroscopy (IPES) in an isochromat mode at h\ensuremath{\nu}=9.5 eV. The evolution of the conduction bands has been tracked as a function of metal coverage and during annealing of the reacted room-temperature interfaces. The metal-rich silicide-type phases, which are formed after room-temperature metal deposition, are different for Gd and Eu: Whereas the Gd/Si surface phase is metallic as evidenced by the high density of states (DOS) at the Fermi level, the Eu/Si interface shows a more semimetallic behavior with low DOS at ${\mathit{E}}_{\mathit{F}}$. The spectral changes observed upon annealing are consistent with transitions from metal-rich to Si-rich silicide phases. The present results of the interfaces are compared to data and calculations on bulk Gd and Ca silicides. The epitaxially ordered disilicides, which are formed at elevated temperature, show dispersive features in k-resolved IPES due to direct transitions, and this confirms the structural order in these phases.