ELECTRONIC AND VIBRATIONAL PROPERTIES OF SEMICONDUCTING CRYSTALLINE FESI2 LAYERS GROWN ON SI(111)

Semiconducting FeSi2 thin layers have been grown on Si(111) by solid state reaction under ultrahigh vacuum (UHV) conditions. The different reaction steps in the temperature range 380-750-degrees-C were followed in situ by Auger electron spectroscopy (AES) and electron energy-loss spectroscopy (EELS). FeSi2 is formed between 550 and 680-degrees-C. At higher temperatures the silicide thin film disrupts forming islands as shown by EELS and ex situ medium-energy ion scattering (MEIS). The low energy excitation spectra of the thin FeSi2 layers were measured by high-resolution electron energy-loss spectroscopy (HREELS). Surface phonon excitations at 50 meV energy are observed and explained in the framework of dielectric theory of surface scattering. This is the first time that Fuchs-Kliewer surface phonons, thpical for heteropolar crystals such as GaAs, ZnO, etc. are so clearly observed on a silicide surface by HREELS. Their presence gives evidence of the semiconducting character of the FeSi2 overlayer. A broad loss structure occurs in the higher energy range corresponding to the excitation of electronic interband transitions. The onset of this structure at approximately 0.8 eV gives an indication of the band gap energy of the silicide. Its intensity together with the small experimental q parallel-to transfer is an evidence of a direct band gap for the FeSi2 layer.