An Atomically-Resolved STM Study of the Thermal Decomposition of Disilane on Si(001)

Scanning Tunneling Microscopy STM has been used to study the adsorption and thermal dissociation of disilane Si2H6 on the SiOO1 surface. At low coverage, disilane adsorbs dissociatively to produce adsorbed SiH3 groups which are randomly distributed on the surface. STM images show that these SiH3 groups spontaneously dissociate into SiH2 and H within several minutes of adsorption at room temperature, and images depicting individual dissociation events have been obtained. Thermal annealing leads to diffusion and dissociation of the SiH2 groups through an intermediate which consists of a hydrogenated dimer which has its dimer axis parallel to those of the underlying substrate, instead of rotated. The structure and chemical identity of the intermediates are discussed in terms of the overall dissociation mechanism. Diffusion of hydrogen atoms leads to the formation of segregated surface phase of clean Si and the Si001-2x1 monohydride structure, and bias-dependent STM imaging permits these species to be identified on a dimer-by-dimer basis. At low temperatures the monohydride distribution is non-statistical, showing a preference for flat terraces over small epitaxial islands. The diffusion, desorption, segregation of hydrogen and its role in blocking diffusion of silicon atom is also discussed.