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Abstract:
The evolution of core-level photoemission spectra has been studied during the initial stages of Cs and In overlayer growth on thin SiO2 layers on Si(111) substrates to probe the influence of the metal-SiO2 interaction on the charge state of the resulting structure. The Si 2p soft-x-ray photoelectron spectra on the SiO2 and the Si substrate, which have been recorded simultaneously, show substantially different shifts to higher binding energy for the two components of the structure for Cs adsorption on the SiO2 surface, but only small uniform shifts for In overlayers. The Si 2p core-level spectra have been analyzed by simulating the spectra with a model that separates the Cs-covered SiO2 structure into thin layers with a constant dipole field gradient applied across them. Accordingly, the metal-induced Si 2p core-level shifts of the Si substrate are interpreted in terms of Si band-bending changes as a result of charge transport from the metal layer to the SiO2/Si interface. The differential Si 2p shifts, which lead to a larger separation of substrate and oxide peaks on cesiated SiO2/Si than on the pristine SiO2/Si structure, are discussed in terms of a Cs-induced dipole field across the SiO2 layer. It is proposed that the different behavior of Cs and In overlayers may be related to the different electronegativity and, consequently, to the different adsorbate dipoles of the two metals.
