ADSORPTION OF SATURATED-HYDROCARBONS ON THE SI(111)-7X7 SURFACE STUDIED BY PHOTOELECTRON AND PHOTON-STIMULATED DESORPTION SPECTROSCOPIES

Photoelectron spectroscopy (PES) and photon stimulated desorption (PSD ) experiments were carried out to follow the thermal chemistry of meth ane- (CH4), neopentane- (C5H12), and adamantane- (C10H16)-dosed-Si(111 )-7 x 7 surfaces. Both methane and adamantane adsorb molecularly on th e 7 x 7 reconstructed Si surface at temperatures of 30 and 85 K, respe ctively. In contrast, at low coverages (<7 x 10(15) molecules/cm(2), 8 5 K; 0.4 ML), a fraction of the neopentane adlayer adsorbs dissociativ ely; at higher coverages, neopentane adsorption is predominantly molec ular. Conversely, the adamantane and neopentane adlayers desorb at tem peratures of similar to 200 and 115 K, respectively, for heating rates of similar to 1 K/min. No desorption temperature was determined for m ethane, but the methane adlayer was observed to desorb below 100 K. As determined by both PES and PSD, annealed adamantane- and neopentane-d osed surfaces react to form nearly identical surfaces. Si 2p core-leve l spectra show chemically shifted components of 0.48+/-0.05, 1.00+/-0. 05, and 1.50+/-0.05 eV with respect to the bulk component. This demons trates formation of Si(CHy)(x) (x=1-3; y=0-3) type surface species. Th e H+ PSD spectra of the thermally reacted surfaces were measured and s how chemical shifts of similar to 0.7 eV with respect to bulk Si and t wo sharp resonances at 100.7 and 101.3 eV. The edge shift and associat ed structure highlights the chemical specificity of PSD and demonstrat es its utility for following surface chemical reactions.