Study on local surface electrical conduction by scanning microscopic four-point probes in ultrahigh vacuum

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We have developed a ultrahigh-vacuum system for electrical conductivity measurements with high surface sensitivity by using monolithic micro-four-point probe method (probe spacing being 4〜40μm) at temperatures ranging from 10 K to room temperature, combined with simultaneous structure analysis by reflection-high-energy electron diffraction (RHEED). This apparatus enables direct measurements of electrical conductivity at the topmost atomic layers on crystal surfaces as a function of temperature. We have made several new findings with this apparatus.(1)A quasi-one-dimensional metallic surface, Si(111)-4×1-In, was found to show a metal-insulator transition around 120 K as revealed by a dramatic increase in resistance by cooling. This is consistent with a picture of Peierls transition accompanied with charge-density waves.(2)A two-dimensional metallic surface, Si(111)-√3×√3-Ag, was found to show a metal-insulator transition around 230 K as revealed by a dramatic increase in resistance by cooling. Combined with photoemission spectroscopy study, the low-temperature phase is not a band insulator. Weak localization may play a role.(3)A quasi-one-dimensional metallic surface, Si(557)-Au, was found to be semiconductive in terms of electrical conduction. This may be due to atomic defects which break up the metallic atomic chains into segments.