Atomic Structure of Hydrogen-Terminated Si(111) Surfaces by Hydrofluoric Acid Treatments

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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 Å and the corrugation amplitude of 0.5 Å. The origin of these dots is ascribed to the H-related states of the trihydride phase (SiH 3 ): the surface dangling bonds are terminated by the SiH 3 radicals which are rotated by 30° 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 Å and the corrugation amplitude of less than 0.1 Å. 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.