ADSORPTION AND DECOMPOSITION OF ACETYLENE ON SI(100)-(2X1)

The adsorption and decomposition of acetylene on Si(100)-(2x1) have been studied in ultrahigh vacuum by Auger electron spectroscopy, temperature-programmed desorption, and changes in the partial pressure of acetylene as measured by a quadrupole mass spectrometer during the formation of a monolayer. Acetylene was found to chemisorb onto Si(100)-(2x1) via a mobile precursor. The difference between the activation energy for desorption from the precursor and that for reaction from the precursor into the chemisorbed state was found to be (E(d) - E(r)) = 1.9 +/- 0.6 kcal mol-1. At a low surface temperature, reaction from the precursor state dominates, giving a chemisorption probability of unity at submonolayer coverages. The saturated acetylene coverage is measured to be one C2H2 per Si2 dimer. Thermochemical arguments are presented, which indicate that acetylene bonds as a di-sigma species to dimer sites in which the Si-Si dimer bond has been cleaved. Chemisorbed acetylene was found to undergo two thermal reactions. A minor pathway (<5%) involves acetylene desorption, and a major pathway (>95%) involves the dissociation of acetylene to produce chemisorbed carbon and H-2 (g). At temperatures above 800 K, the surface carbon begins to diffuse into the bulk of the silicon crystal.