Bonding at theCaF2/Si(111) interface from tight-binding cluster and band theory

The Si-Ca bonding at ${\mathrm{CaF}}_2$/Si(111) interfaces is characterized through the use of tight-binding cluster and band models. Strong covalent single bonds form from the overlap of higher-lying ${\mathrm{Ca}}^{+}$-radical orbitals with lower-lying Si-radical orbitals. These bonds also have a substantial charge-transfer (ionic) component. It is suggested that the type-B Si/CaF⋅${\mathrm{CaF}}_2$ interface structure forms in part because the initial type-B Si/${\mathrm{CaF}}_2$ interface is slightly more stable than the corresponding type-A interface, but more importantly because it provides an interstitial pathway for F to enter bulk Si.