STABILITY OF CAF2 SI(111) AND AL/CAF2/SI(111) INTERFACE SYSTEMS STUDIED WITH PHOTOELECTRON-SPECTROSCOPY AND SCANNING-TUNNELING-MICROSCOPY/

The stability of CaF2/Si(111) and Al/CaF2/Si(111) interfaces under irr adiation by light or electrons and upon thermal annealing was investig ated by photoelectron spectroscopy and scanning-tunneling microscopy. Deposition of CaF2 on Si(111)7x7 at 700 degrees C leads to a shift of the Fermi-level position (E(F)) towards the Si valence-band maximum (V BM), while subsequent light irradiation results in a back-shift of E(F ) towards its initial value. Upon flashing the irradiated surface at 7 00 degrees C, it shifts again towards the VBM. The variation of E(F) i s assigned to a competition of interface states with surface states re lated to irradiation-induced fluorine vacancies. These surface defects are observed as localized surface charges in the scanning tunneling m icroscopy images. A thin epitaxial Al layer on top of the CaF2 surface strongly reduces the irradiation-induced effects and leads to a stabi lization of E(F) at 0.30 eV above the VBM. (C) American Vacuum Society .