ATOMIC-STRUCTURE OF HYDROGEN-TERMINATED SI(111) SURFACES BY HYDROFLUORIC-ACID TREATMENTS

Characterization of "ultra-clean" Si(111) surfaces has been made with atomic-scale resolution by means of scanning tunneling microscopy (STM). The STM images on the flat part of the hydrogen-(H-)terminated Si(111) surface prepared by dipping into the dilute (1%) HF solution exhibit regular dots with threefold symmetry, the distance of 2.2 angstrom and the corrugation amplitude of 0.5 angstrom. The origin of these dots is ascribed to the H-related states of the trihydride phase (SiH3): the surface dangling bonds are terminated by the SiH3 radicals which are rotated by 30-degrees from the ideal configuration. In contrast, on the surface prepared by repeating boiling in hot water following 1% HF dipping, we obtain regular dots with threefold symmetry, the distance of 3.8 angstrom and the corrugation amplitude of less than 0.1 angstrom. These originate from the H-related states of the monohydride phase (SiH): a single H atom terminates each surface bond by locating on the top site of the surface Si atom.