Band dispersion of an interface state: CaF2/Si(111)

The electronic structure of the Ca${\mathrm{F}}_2$/Si(111) interface has been studied using angle-resolved photoelectron spectroscopy with synchrotron radiation. The bonding of the Ca $4s$ electrons with the $3p$ dangling bond orbitals of the Si(111) surface produces an interface state which is located approximately 0.8 eV below the Fermi level in normal emission and has ${\Lambda }_1$ symmetry. This feature disperses downwards by 0.6 eV from $\Gamma$ to $M$ and by 0.8 eV from $\Gamma$ to $K$. The results of this study are qualitatively comparable to a first-principles band calculation of the isoelectronic Na/Si(111) monolayer system with the Na atoms in the threefold hollow site of the Si(111) surface.