Reaction paths of phosphine dissociation on silicon (001)

Using density functional theory and guided by extensive scanning tunneling microscopy (STM) image data, we formulate a detailed mechanism for the dissociation of phosphine (PH₃) molecules on the Si(001) surface at room temperature. We distinguish between a main sequence of dissociation that involves PH₂+H, PH+2H, and P+3H as observable intermediates, and a secondary sequence that gives rise to PH+H, P+2H, and isolated phosphorus adatoms. The latter sequence arises because PH₂ fragments are surprisingly mobile on Si(001) and can diffuse away from the third hydrogen atom that makes up the PH₃ stoichiometry. Our calculated activation energies describe the competition between diffusion and dissociation pathways and hence provide a comprehensive model for the numerous adsorbate species observed in STM experiments.