Acetylene adsorption on the Si(111)-(7X7) surface: Ultraviolet photoemission and high-resolution electron-energy-loss spectroscopies

The room temperature adsorption and the thermal reaction of acetylene on the Si(111)-(7 x 7) surface has been investigated by means of ultraviolet-photoemission spectroscopy and high-resolution electron-energy-loss spectroscopy (HREELS). In the first stage of the adsorption process, the evolution of the restatom- and adatom-related electronic states as a function of acetylene exposure is well described by a modified di-sigma -bonding model, in which the metallic character of the (7 x 7) surface plays a fundamental role. For higher exposure, all the restatom. dangling bonds are saturated and the 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 dehydrogenation and partial desorption occur, as demonstrated by the disappearance of the C-H modes in HREELS and the reappearance of the adatom. and restatom dangling-bond states in valence-band spectra. At 700 degreesC the formation of a Si-C compound is observed. Atomic-force-microscopy topographic images taken ex situ after annealing to 800 degreesC show that the Si-C interface is constituted by islands with lateral dimension of similar to 300 Angstrom, while the overall surface roughness is 10-20 Angstrom.