Molecular mechanisms for disilane chemisorption on Si(100)-(2x1)

The dissociative chemisorption of disilane is an important elementary process in the growth of silicon films. Although factors governing the rate of film growth such as surface temperature and disilane flux have been extensively studied experimentally by a large number of groups, the molecular mechanism for disilane adsorption is not well established. In particular, although it is generally held that chemisorption occurs via silicon-silicon bond dissociation, there have been a number of suggestions that silicon-hydrogen bond dissociation also occurs. We consider this issue in detail hereby examining a number of different paths that disilane can take to chemisorb. In addition to silicon-silicon bond dissociation paths, we examine three different mechanisms for silicon-hydrogen bond dissociation, for each path considering both adsorption at interdimer and intradimer sites. The calculated barriers are critically compared to experimental data. We conclude that silicon-hydrogen bond dissociation is likely, finding two zero barrier paths for chemisorption at interdimer sites, and a precursor-mediated path with a low barrier. We also find two precursor states, and show that each can lead to chemisorption via either silicon-silicon or silicon-hydrogen bond dissociation. Finally, we calculated the barriers for reaction of coadsorbed disilyl and hydrogen to form gas phase silane. Our calculations are performed using density-functional theory within a planewave ultrasoft pseudopotential methodology. We traced the reaction paths with the nudged-elastic band technique.