Ethylene and coadsorbed hydrogen on Si(100)-(2×1): Structure, bonding, and decomposition

Abstract

The effect of post-adsorbed atomic hydrogen on adsorption, desorption and decomposition of ethylene on Si(100)-(2×1) has been studied using high-resolution electron energy loss spectroscopy (HREELS), temperature programmed desorption (TPD), and low-energy electron diffraction (LEED). HREEL spectra show that ethylene, in the absence of hydrogen, rehybridizes from sp 2 to sp 3 and forms a diσ bonded structure to the surface. After low post-exposures to atomic hydrogen, the observed Si-H mode and the absence of significant change in the chemisorbed ethylene spectrum indicate that the surface dangling bonds still exist after ethylene adsorption, and that the adsorption of ethylene is accompanied by cleavage of the original SiSi dimer bond. In addition, the hydrogen-saturated danging bonds are found to stabilize the diσ bonded ethylene, leading to increased decomposition at higher temperatures. However, higher exposures of atomic hydrogen convert the initial (2×1) reconstruction of the ethylene-saturated surface to a (1×1) structure. Furthermore, the thermal desorption peak of molecular ethylene is shifted up by approximately 100 K. HREELS data show that the CC bond still exists, and, by using atomic deuterium, a C-D mode is observed. We explain our data by an atomic hydrogen-driven conversion of the diσ bonded ethylene to a monoσ bonded surface ethyl. Thermal activation leads to decomposition of about 60% of the initial ethylene in contrast to the mainly molecular desorption in the absence of hydrogen.