A PHOTOEMISSION-STUDY OF AMMONIA ADSORPTION ON SI(100)2X1 AND SI(111)2X1 SURFACES

Room temperature NH3 adsorption on Si(100)2 x 1 and Si(111)2 x 1 has b een studied by angle-resolved photoemission using synchrotron radiatio n. The experiments confirmed previous results showing that the adsorpt ion on Si(100)2 x 1 is a dissociative process leading to NH2 and H. A single N2s component of the NH2 species has been identified in the pho toemission spectra at 20 eV below the Fermi level (E(F)). The adsorpti on on the cleaved Si(111)2 x 1 surface is more complex as ascertained by the presence of the same NH2 features as for Si(100), along with a second N2s component at 17 eV and a new valence band feature at 8 eV b elow E(F). The second N2s component at 17 eV is identified by annealin g experiments as due to atomic nitrogen. Thus, a given part of the ads orbed NH3 molecules may be completely dissociated at RT owing to surfa ce states which are specific to the cleaved 2 x 1 reconstructed Si(111 ) surface.