Atomic and electronic structure of ultrathin fluoride barrier layers at the oxide/Si interface

A SrF2 ultrathin barrier layer on Si(001) is used to form a sharp interface and block reactivity and intermixing between the semiconductor and a Yb2O3 overlayer. Yb2O3/Si(001) and Yb2O3/SrF2/Si(001) interfaces grown in ultra high vacuum by molecular beam epitaxy are studied by photoemission and x-ray absorption fine structure. Without the fluoride interlayer, Yb2O3/Si(001) presents an interface reacted region formed by SiOx and/or silicate compounds, which is about 9 °A thick and increases up to 14-15 °A after annealing at 500-700 oC. A uniform single layer of SrF2 molecules blocks intermixing and reduces the oxidized Si region to 2.4 °A after deposition and to 3.5 °A after annealing at 500 oC. In both cases we estimate a conduction band offset and a valence band offset of ~1.7 eV and 2.4 eV between the oxide and Si, respectively. X-ray absorption fine structure measurements at the Yb LIII edge suggest that the Yb oxide films exhibit a significant degree of static disorder with and without the fluoride barrier. Sr K edge measurements indicate that the ultrathin fluoride films are reacted, with the formation of bonds between Si and Sr; the Sr-Sr and Sr-F interatomic distances in the ultrathin fluoride barrier film are relaxed to the bulk value.