Phosphine dissociation and diffusion on Si(001) observed at the atomic scale

A detailed atomic-resolution scanning tunneling microscopy (STM) and density functional theory study of the adsorption, dissociation, and surface diffusion of phosphine (PH₃) on Si(001) is presented. Adsorbate coverages from ∼0.01 monolayer to saturation are investigated, and adsorption is performed at room temperature and 120 K. It is shown that PH₃ dissociates upon adsorption to Si(001) at room temperature to produce both PH₂ + H and PH + 2H. These appear in atomic-resolution STM images as features asymmetric-about and centered-upon the dimer rows, respectively. The ratio of PH₂ to PH is a function of both dose rate and temperature, and the dissociation of PH₂ to PH occurs on a time scale of minutes at room temperature. Time-resolved in situ STM observations of these adsorbates show the surface diffusion of PH₂ adsorbates (mediated by its lone pair electrons) and the dissociation of PH₂ to PH. The surface diffusion of PH₂ results in the formation of hemihydride dimers on low-dosed Si(001) surfaces and the ordering of PH molecules along dimer rows at saturation coverages. The observations presented here have important implications for the fabrication of atomic-scale P dopant structures in Si, and the methodology is applicable to other emerging areas of nanotechnology, such as molecular electronics, where unambiguous molecular identification using STM is necessary