Altered photoemission satellites atCaF2- andSrF2-on-Si(111) interfaces

Abstract

Bulk and interface photoemission satellite excitations, measured with x-ray photoelectron spectroscopy and x-ray photoelectron diffraction, are compared for thick, thin, and monolayer films of ${\mathrm{CaF}}_{2}$ and ${\mathrm{SrF}}_{2}$ on Si(111). Intrinsic satellites are observed for excitation of atoms in the first monolayer, both uncovered and at the buried interface, that differ from those associated with bulk atoms. For F 1s excitation, the bulk and interface satellites differ only in width and amplitude; for the cation core excitations (Ca 2p, Sr 3p, and Sr 3d), the observed excitation energies and intensities differ both from the equivalent bulk satellites and among the various core hole states. The results yield new information on the nature of the interface bonding, as well as on the origin of both bulk and interface satellites. Several models are considered, including differential screening, dielectric losses, crystal-field and multiplet effects, and interface excitation. The most likely explanation for the new cation satellites at the ${\mathrm{CaF}}_{2}$/Si(111) [${\mathrm{SrF}}_{2}$/Si(111)] interface is localized excitation of the interface bond, where the interface band gap is controlled by the collapse of the Ca 3d (Sr 4d) level in the presence of the Ca (Sr) core hole. \textcopyright{} 1996 The American Physical Society.