Acetylene adsorption on the Si(111)-(7X7) surface: Ultraviolet photoemission and high-resolution electron-energy-loss spectroscopies - art. no. 155305

The room temperature adsorption and the thermal reaction of acetylene on th e Si(111)-(7 x 7) surface has been investigated by means of ultraviolet-pho toemission spectroscopy and high-resolution electron-energy-loss spectrosco py (HREELS). In the first stage of the adsorption process, the evolution of the restatom- and adatom-related electronic states as a function of acetyl ene exposure is well described by a modified di-sigma -bonding model, in wh ich the metallic character of the (7 x 7) surface plays a fundamental role. For higher exposure, all the restatom. dangling bonds are saturated and th e adsorption process continues through saturation of the remaining adatoms. In this phase, a molecular tilting is observed, possibly accompanied by a surface atom rearrangement. Upon annealing to 600 degreesC, acetylene dehyd rogenation and partial desorption occur, as demonstrated by the disappearan ce of the C-H modes in HREELS and the reappearance of the adatom. and resta tom dangling-bond states in valence-band spectra. At 700 degreesC the forma tion of a Si-C compound is observed. Atomic-force-microscopy topographic im ages taken ex situ after annealing to 800 degreesC show that the Si-C inter face is constituted by islands with lateral dimension of similar to 300 Ang strom, while the overall surface roughness is 10-20 Angstrom.