Surface Science 251/252 (1991) 175-179 North-Holland 175 zyxwvut Electronic structure of epitaxial / 3-FeSi 2 on Sit 111) M. De Crescenti, G. Gaggiotti, N. Motta, F. Patella, A. Balzarotti Dipartimento di Fisica, II Universitli di Roma, 00173 Rama, Italy G. Mattogno Istituto di Teoria e Struttura Eletiron~ea del CNR, Area della Ricerea di Rama, Manierotondo Stazione, 00016 Roma, Italy and J. Derrien Centre de Recherche sur les Mecanismes de la Croissance Cristalline-CNRS, Campus Luminy, I3288 Marseille, France Received 1 October 1990; accepted for publication 16 December 1990 We have applied a combination of surface spectroscopy techniques (XPS, Auger, electron energy loss and bremsstrahhmg isochromat spectroscopy) to investigate the chemical bonding of /3-FeSia thin films epitaxially grown on the Si(ll1) surface. The local structure of the films has been investigated with the extended BIS fine structure technique. The semiconducting nature of the films is evidenced by a shift of the empty d-band to higher binding energies and by the lowering of the density of the states close to E, as compared to pure Fe films. The dielectric functions extracted from the electron energy loss spectra strongly support this picture. We measured an energy gap of 1.0 f 0.2 eV which compares favourably with density of states calculations for the b-FeSi* semiconduct- ing phase. zyxwvutsrqponmlkjihgfedcbaZYXWVUTSRQPONMLKJIHGFEDCBA 1. Introduction The electronic and structural properties of sili- tides have been investigated extensively in the last two decades for f~d~ental and practical pur- poses [l]. Of particular interest for optoelectronic devices are the transition metal s&ides with a direct semiconducting energy gap [2]. Among them, &FeSi, has attracted much interest [3-71 because its alloy with other transition metal opens the possibility of band gap engineering in silicide-silicon heterostructures. Recent calcula- tions [8] of the the~od~a~c stability of the Fe, _$o.Ji, alloy indicates a phase separation below 150 K suggesting the possibility of forma- tion of a mixed silicide even at room temperature. In this paper we report on the electronic struc- ture characterization of &FeSi, films measured by means of surface techniques, i.e. XPS (X-ray photoelectron spectroscopy), BIS (bremsstrahlung isochromat spectroscopy), Auger and EELS (elec- tron energy loss spectroscopy). The aim is to correlate the changes of the Fe and Si spectral features upon formation of the /3-FeSi, compound. The energy region above E, analyzed by the BIS spectroscopy provides clear evidence of the opening of a gap when the /3-FeSi, phase is formed on Si(ll1) upon annealing at 600°C a 100 A thick Fe overlayer. The combina- tion of XPS valence band and BIS data leads to an estimated value for the energy gap of 1.0 & 0.2 eV in good agreement with optical absorption data [2,7]. The Auger spectroscopy is used to evidence possible correlation effects both at the Fe and Si sites. From the Kramers-Kronig analysis of the 0039~6028/91/$03.50 0 1991 - Elsevier Science Publishers B.V. (North-Holland)