Charge-Transfer in Silicon Governs the Pattern of Dissociative Attachment of Hydrogen Halides: HCl, HBr, and HI

The dissociative attachment of hydrogen halides, HCl, HBr, and HI, on Si(100) was studied by scanning tunneling microscopy, and modeled by molecular dynamics computations based on density functional theory. The relative yields of on-dimer (OD), inter-dimer (ID), and inter-row (IR) products, reported here for the first time, were unaltered by temperature change, indicative of barrier-free reaction. Interdimer reaction was found experimentally to be overwhelmingly the favored reaction path at all temperatures, 175-300 K. This is a preference that theory accounts for by a combination of kinematics favoring the initial approach of H, and subsequent charge-displacement directing the halogen atom to an inter-dimer site. Molecular dynamics, with statistical sampling of the initial collision-geometry at the surface, showed that the light H-atom invariably was the first to engage with the surface, forming H Si. This bond-formation resulted in directional charge-transfer to a neighboring Si dimer-pair of the same dimer-row. As a consequence this neighboring Si became attractive to the incoming electrophilic halogen-atom, accounting for the observed strong preference for the inter-dimer outcome.