H. F. Wilson

Split-off dimer defects on the Si ( 001 ) 2 × 1 surface

Dimer vacancy (DV) defect complexes in the Si(001)2×1 surface are investigated using high-resolution scanning-tunneling microscopy and first-principles calculations. We find that under low-bias filled-state tunneling conditions, isolated “split-off” dimers in these defect complexes are imaged as pairs of protrusions, while the surrounding Si surface dimers appear as the usual “bean-shaped” protrusions. We attribute this to the formation of π-bonds between the two atoms of the split-off dimer and second-layer atoms, and present charge density plots to support this assignment. We observe a local brightness enhancement due to strain for different DV complexes and provide the first experimental confirmation of an earlier prediction that the 1+2-DV induces less surface strain than other DV complexes. Finally, we present a previously unreported triangular shaped split-off dimer defect complex that exists at S-type step edges, and propose a structure for this defect involving a bound Si monomer.

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.

Towards the routine fabrication of P in Si nanostructures: Understanding P precursor molecules on Si(001)

The ability to controllably position individual phosphorus dopant atoms in silicon surfaces is a critical first step in creating nanoscale electronic devices in silicon, for example a phosphorus in silicon quantum computer. While individual P atom placement in Si(001) has been achieved, the ability to routinely position P atoms in Si for large-scale device fabrication requires a more detailed understanding of the physical and chemical processes leading to P atom incorporation. Here we present an atomic-resolution scanning tunneling microscopy study of the interaction of the P precursor molecule phosphine (PH3) with the Si(001) surface. In particular, we present the direct observation of PH3 dissociation and diffusion on Si(001) at room temperature and show that this dissociation is occasionally complete, leaving a P monomer bound to the surface. Such surface bound P monomers are important because they are the most likely entry point for P atoms to incorporate into the substrate surface at elevated temperature.