Eels Study of Bulk Nickel Silicides and the Nickel DISILICIDE/SILICON(111) Interface.

The unoccupied density of states (DOS) of bulk nickel silicides (Ni_2Si, NiSi and NiSi_2) and of the NiSi _2/Si(111) interface is studied by Electron Energy Loss Spectroscopy (EELS) in a Scanning Transmission Electron Microscope (STEM). The core loss spectra recorded in bulk silicide samples were compared to those recorded in pure Ni. The differences are interpreted in terms of the hybridization between the Ni d and the Si sp electrons. The presence of the Ni L_{2,3} white lines in the bulk silicides indicates that there are significant amount of empty Ni d-states which are well localized in energy above E_{F} . The change in the edge profiles and the shift in binding energy from Ni to NiSi_2 are consistent with the increasing Ni-Si hybridization as Si concentration increases. Compared to metallic Ni, the collective excitation of the valence electrons in the silicides becomes much more free-electron-like. The EELS study carried out at the NiSi_2/Si(111) interface shows little change in the Ni L_ {2,3} edges as compared to those in bulk NiSi_2. The Si L_ {2,3} edges recorded at the interface show downward shifts in the binding energy, 0.13 +/- 0.03 eV in A-type and 0.03 +/- 0.03 eV in B-type, compared to bulk Si. The magnitude of the shifts is consistent with the different Schottky barrier heights at the two types of interface. The energy of the interfacial plasmon matches the prediction of the free electron model very well.