ELSEVIER Materials Science and Engineering A217/218 (1996) 34-37 MATERIALS SCIENCE 8r EWGIWEERING A Local structure and chemical reaction of CGO films on Si( 111)7 x 7 studied by HREELS-STM S. Suto”, A. Kasuya b, C.-W. Hub, A. Wawrob, K. Sakamoto”, T. WakitaZI, T. Gotoa, Y. Nishinab “Department of Physics, Graduate Scl~ool of Science, Tohoh University, Sedai 980-77, Jnpcuz blnstitute for Materials Research, Tohoh University, Set&i 950-77, Japan Abstract We have investigated the thermal stability and the chemical reactions of C6,, thin films on Si( 111)7 x 7 surfaces by the combined measurements of the high-resolution electron-energy-loss spectroscopy (HREELS) and the scanning tunneling microscopy (STM), HREELS-STM. After heating the Si up to 400 “C, the molecules did not align perfectly but made local arrangements. The energy shifts of the inelastic electrons indicate that the electrons in the Si dangling bond transfer to the Cbo molecules. The value of the charge transfer is estimated to be 1 + 1 electron(s). After heating the Si up to 800 “C, the smooth CGO monolayer film aggregates and forms islands. The nearest neighbouring distance between the C6,, molecules is shortened from 10 8, to 9.3 A. The intensity of the 92 and 101 meV peaks drastically increase. These results indicate the formation of a covalent bond between the C,,, molecules. After heating the Si up to 1100 “C, an Sic film grows. Keywords: Silicon; C,, thin films; High resolution electron energy loss spectroscopy; Scanningtunneling microscopy 1. Introduction Extensive experimental and theoretical studies have been carried out to investigate the chemical and physi- cal properties of buckminster fullerene, CGO, and its relatives. It is important to understand the interaction and reaction of the C& molecules on semiconductor surfaces to develop new material functions for C60 molecules and to study their chemical properties. The icosahedral structure of the ChDmolecule has the symmetry of the Ih point group. The C,, molecule has four infrared-active intramolecular vibration modes with T,, symmetry, and ten Raman-active modes with two Ag modes and eight Hg modes [l]. The vibrational modes for bulk CGOare measured at v, = 65.2, v2 = 71.4, v3 = 146.6 and v, = 177.2 meV by infrared absorp- tion spectroscopy [2], and observed at v1 = 66, v2 = 72, v3 = 147 and vq = 178 meV by a high-resolution elec- tron-energy-loss spectroscopy (HREELS) [3,4]. Some peaks of the Raman-active modes are also observed at the disordered CbO films on the Si surfaces using HREELS [4,5]. Recently, Rao et al. have reported that the solid structure of Cd0 can be polymerize under irradiation with visible or ultraviolet light [6], though the cage for C6,, molecules is extremely stable. Once CGO molecules polymerize or form covalent bonds, the sym- metry is reduced from the 11, point group to a lower group. In the case of a dimer, the point group is reduced to D2,,. The reduction in the symmetry induces the new infrared-active and Raman-active modes [7]. The clean Si(l11) surface reconstructs into the 7 x 7 structure with 19 dangling bonds in the unit cell. Since such dangling bonds are chemically active, some charge transfer interaction or chemical reaction is expected between the silicon surface and the CGO molecule. In this article, we report the measurements of the vibrational modes and the structures of the Cc0 thin films on the Si(l11)7 x 7 surface with different anneal- ing temperatures to investigate the chemical reaction and the thermal stability of the C,, films. The combined analysis of the high-resoiution electron-energy-loss spectroscopy and the scanning tunneling microscopy (HREELS-STM) was used. It is possible to correlate the adsorbate bonding features given by HREELS with the adsorbate structure in real space using this method of scanning tunneling microscopy @TM). 0921-5093/96/$15.00 0 1996- Elsevier Science S.A. All rights reserved PII SO921-5093(96)10310-5