A Study of the PbSe/CaF2 Interface Grown on Si(111) by MBE

Epitaxial growth of high quality PbSe/CaF_2/Si(111) heterostructures is an important materials technology for fabricating infrared detector arrays and tunable diode lasers. While the CaF_2/Si(111) interface has been extensively investigated, little has been reported on the PbSe/CaF₂ interface. In this work, we use in situ XPS to study the initial stages of MBE growth of PbSe on CaF_2/Si(111). 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--1.6eV) 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 6ML 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 models for the bonding arrangements at the PbSe/CaF_2(111) interface. This work is supported by NSF grant no. OSR-9550478.