AN STM STUDY OF THE CHEMISORPTION OF C2H4 ON SI(001)(2X1)

The chemisorption of ethylene (C2H4) on Si(001)(2 x 1) at 300 K has been studied by scanning tunneling microscopy (STM) and spectroscopy (STS). Exposure of the surface to C2H4 does not cause large scale rearrangement of the original Si surface atoms. The adsorption of individual molecules and changes in the local structure can, however, be observed. At low coverage, the C2H4 molecules prefer to adsorb on alternate dimer sites creating either a local (2 x 2) or c(2 x 4) structure. The individual domains are relatively small (< 50 angstrom) and the change in reconstruction cannot be detected by any diffraction technique. STS recorded from regions of the surface covered by adsorbates show the removal of the relatively weak pi-bond formed by the dangling bonds and the participation of the dangling bond electrons in stronger sp3-like bonds to the carbon atoms of the molecule after chemisorption. The (2 x 1) dimer surface characteristic of clean Si(001) is restored upon heating the adsorbate covered surface. The pictures are consistent with a model in which the desorbed C2H4 molecules leave restored dimers behind and the molecules which are left remain undissociated on the same type of dimer sites as before.