Investigation of the electronic structures and associated properties including hyperfine interactions for halogen-adsorbed silicon surfaces: Fluorine through iodine

First-principles self-consistent-field Hartree-Fock cluster investigations have been carried out for the halogen atoms, fluorine through iodine, adsorbed on Si(111) surface. Clusters involving 5, 14, and 27 atoms were employed, where for the first two clusters both all-electron and pseudopotential procedures and for the third cluster pseudopotential procedure alone have been used. The Si-X bond distances for these clusters have been obtained from the minimization of the total energy as a function of adatom position and have been found to compare very well with the available experimental results by surface extended x-ray-absorption fine-structure (SEXAFS) and x-ray standing-wave techniques and with earlier theoretical results. The five-atom cluster corresponds to the SiH3X molecule for which the calculated Si-X bond distance and the nuclear quadrupole hyperfine coupling constants (e2qQ) are in good agreement with the experimental values obtained from microwave measurements. For the surface-adsorbed systems, the location of the adatom, the local density of states (LDOS), the frequencies and the amplitudes associated with the Si-X vibrations, and the quadrupole coupling constants for F*19 (I=(5/2), F20, Cl35, Br79, and I127 nuclei have been studied for the various adsorbed systems. Satisfactory agreement is found between available ultraviolet photoemission spectroscopy (UPS) data for the Cl-adsorbed system and the predictions from our calculated LDOS. Our calculated vibrational amplitude for Br on Si compares well with the available data for Br on Ge surface. It will be very helpful to have experimental data in the future to test our predictions for a number of properties not currently available, especially for the LDOS through UPS measurements and for the e2qQ by the beam-foil and radiative techniques to verify the interesting trends in both these properties in going from F through I and also the significant decrease in e2qQ in going from the SiH3X molecules to the surface-adsorbed systems.