Role of lattice mismatch and surface chemistry in the formation of epitaxial semiconductor-insulator interfaces

The formation of SrF₂/Si(111) and Ge/CaF₂/Si(111) interfaces is studied with photoemission and compared to previous results for the CaF₂/Si(111) interface. The interface between SrF₂ and Si(111) is found to be nonstoichiometric, similar to the interface between CaF₂ and Si(111): the bonding is between Si and the cation, with a layer of fluorine missing at the interface. In the case of Ge growth on CaF₂/Si(111), a variety of effects are noted: The CaF₂/Si(111) valence-band offset is reduced by about 1 eV upon deposition of Ge at room temperature. The sticking coefficient of the Ge is significantly increased by preparing the CaF₂ surface with electron bombardment to remove the top layer of fluorine. For both the irradiated and nonirradiated cases, annealing of thin room-temperature-deposited films resulted in Ge island formation.