Electronic structure of the CaF2/Si(111) interface

Abstract

High-resolution core-level and Auger-electron spectroscopy, polarization-dependent near-edge x-ray absorption, and angle-resolved photoemission are used to study the electronic structure and the bonding at the CaF2/Si(111) interface. Si core-level shifts of +0.4 and -0.8 eV show that both Ca and F bond to Si and that the interface is atomically sharp. Interface-derived Ca and F core-level and Auger-electron shifts are found indicating layer-by-layer growth. The interface Ca 2p,3p,3s core-level shifts are about 2 eV and the Ca 2p Auger energy shift is 4.5 eV. The F 1s,2s core levels show no interface shift but a shift of 1.7-2 eV in the initial adsorption regime indicating a re- arrangement of F after the completion of the first layer. The F 1s Auger electrons show an interface shift of 2.0 eV. Initial-state and relaxation contributions to the shifts are considered. In the Ca 2p and F 1s near-edge x-ray-absorption fine-structure (NEXAFS) spectra several unoccupied Ca- and F-derived interface states are found. The orientation of the corresponding orbitals is revealed by the polarization dependence. The oxidation state of the Ca atoms at the interface is found to be changed to 1+. The CaF2 valence bands start to form at 2 layers with an overall bandwidth of 3.3 eV. An occupied interface state is found at 1.2 eV. The Fermi level shifts by 0.6-0.65 eV when 2 CaF2 layers are deposited and a new pinning position is established at the Si valence-band maximum. A bonding model for the interface is proposed. © 1986 The American Physical Society.