Semiconducting rhenium silicide thin films on Si(111)

Abstract

The crystallographic, electronic, and optical properties of thin ReSi2 films (∼20–300 Å) have been investigated in situ by low energy electron diffraction (LEED) and photoelectron spectroscopy (XPS and UPS), ex situ by glancing incidence x-ray diffraction (GIXD), and optical absorption measurements. Thin Re layers were evaporated under ultrahigh vacuum on Si(111) (7×7) surfaces, maintained at room temperature, or heated at 650 °C. The films were subsequently annealed at increasing high temperature and the silicide formation was followed by in situ surface techniques. For very thin films (≲35 Å) LEED shows a faint (1×1) pattern after annealing at 750 °C, which improves slightly up to ∼900 °C. For thick films (∼50–300 Å) only a bright background is observed. XPS indicates that the ReSi2 composition is attained upon annealing at 600 °C. In the Re-Si bonding the charge transfer is negligible: the energy positions of the corelevels (Si 2p and Re 4f) are the same in the compound and in the elements. As the energy shift of the Si KLL Auger is negligible also, the extra-atomic relaxation energy for Si atoms is the same in silicide as in silicon, indicating that ReSi2 has a semiconducting character. UPS results confirm this assumption: the density of states near EF decreases strongly upon ReSi2 formation and at the same time the valence band edge shifts from EF to lower binding energy. GIXD gives sharp diffraction peaks, characteristic of ReSi2 (110) in epitaxy on Si(111). This technique also reveals that the films present an additional orientation near the interface. Optical absorption measurements performed on ReSi2 films of ∼300 Å thick indicate that this silicide is a semiconductor with an indirect energy gap of 0.15 eV, in agreement with previous studies.