Atomic scale oxidation of silicon nanoclusters on silicon carbide surfaces

The initial stage adsorption of molecular oxygen on the 6H-SiC(0001)-3x3 surface has been studied using scanning tunneling microscopy (STM) and X-ray photoelectron spectroscopy. STM filled-state imaging reveals the sequential appearances of dark sites upon initial oxygen exposure at room temperature, followed by the appearances of bright sites at higher oxygen exposures. The changes in the STM images are interpreted on the basis of changes in the local density-of-states of the silicon adatom cluster site following the sequential development of different oxygen chemisorption states, beginning with the attachment of oxygen on the dangling bond site to its subsequent insertion in the Si-Si back-bond. Periodic density functional theory calculations supported our interpretations because the most stable chemisorption site was found to occur on the reactive adatom cluster site, as opposed to the adlayer coplanar bonds. The interaction of atomic 0 beam with the 3 x 3 surface results in the formation of silicon oxide nanoclusters.