THE INTERACTION OF METHYL-CHLORIDE WITH A SILICON(111) SURFACE

In the technical Muller-Rochow synthesis, dimethyl dichlorsilane (DMD) is fabricated from methyl chloride which interacts with a 'contact ma ss' consisting of elemental Si and Cu (Cu being a catalyst). In order to better understand some of the elementary steps of this reaction, we have followed the adsorption of methyl chloride on a (7x7) reconstruc ted Si(111) surface in the temperature range between 100 and 1000 K by means of LEED, Auger electron spectroscopy (AES), temperature-program med thermal desorption (TPD), vibrational loss spectroscopy (HREELS), and work function change (Delta Phi) measurements. CH3Cl adsorbs effec tively in a single weakly chemisorbed state. The adsorption energy is strongly coverage-dependent and decreases from initially similar to 40 kJ/mol to similar to 30 kJ/mol near the monolayer. During adsorption, the work function of the Si(111) surface first increases sharply by 0 .36 eV and saturates, after an intermediate maximum of similar to 40 e V near the monolayer, at similar to 0.61 eV. CH3Cl-derived vibrational losses appear at 88, 176, 340, and 370 meV and can be associated with known gas phase vibrations of CH3Cl. An additional band at 260 meV is tentatively ascribed to the Si-H stretching vibration, pointing to co mpeting dissociation of adsorbed CH3Cl into hydrogen and methylene chl oride CH2Cl. Otherwise, CH3Cl does not react with Si(111) under our UH V conditions (CH3Cl partial pressures less than or equal to 10(-5) mba r, 100 less than or equal to T less than or equal to 300 K), as the ab sence of any Cl- and/or Si-containing mass fragments in the TPD spectr a proves. We deduce from our investigation that the combined chlorinat ion and methylation of Si in the technical Muller-Rochow process is di ctated by processes which run only at much larger (atmospheric) pressu res and/or elevated temperatures.