The initial stages of MBE growth of PbSe on CaF2/Si(111) were studied by in situ XPS. The surface reaction between CaF2 and elemental Se is also studied as a comparison. Ca 2p, F 1s, Pb 4f and Se 3d peaks all shift to lower binding energies (1.2 – 1.6 eV) with increasing PbSe coverage. This indicates a strong interaction between the PbSe overlayer and the underlying CaF2 and the formation of Pb-F and Ca-Se interfacial bonds. No chemically graded reaction products are detected at the PbSe/CaF2 interface. A transition layer of about 6 ML is observed which can be partially accounted for by the open 3D structure of CaF2 where three layers of atoms are available for bonding with the deposited material. This structure allows interpenetration of the PbSe and CaF2 layers which necessarily leads to a thin transition layer. Based on our XPS results, we propose a model for the bonding arrangements at the PbSe/CaF2(111) interface.
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