Stability of CaF2/Si(111) and Al/CaF2/Si(111) interface systems studied with photoelectron spectroscopy and scanning-tunneling microscopy

Abstract

The stability of CaF2/Si(111) and Al/CaF2/Si(111) interfaces under irradiation by light or electrons and upon thermal annealing was investigated by photoelectron spectroscopy and scanning-tunneling microscopy. Deposition of CaF2 on Si(111)7×7 at 700 °C leads to a shift of the Fermi-level position (EF) towards the Si valence-band maximum (VBM), while subsequent light irradiation results in a back-shift of EF towards its initial value. Upon flashing the irradiated surface at 700 °C, it shifts again towards the VBM. The variation of EF is assigned to a competition of interface states with surface states related to irradiation-induced fluorine vacancies. These surface defects are observed as localized surface charges in the scanning tunneling microscopy images. A thin epitaxial Al layer on top of the CaF2 surface strongly reduces the irradiation-induced effects and leads to a stabilization of EF at 0.30 eV above the VBM.