For in -situ measurements of local electrical conductivity of well-defined crystal surfaces in ultrahigh vacuum, we have developed two kinds of microscopic four-point probe methods. One is a 'four-tip STM prober', in which independently driven four tips of scanning tunneling microscope (STM) are used for four-point probe conductivity measurements, whose probe spacing can he changed from 0.5 mu m to 1 mm. The other one is monolithic micro-four-point probes, fabricated on silicon chips, whose probe spacing is fixed around several mu m. These probes are installed in scanning-electron-microscopy/ electron-diffraction chambers, in which the structures of sample surfaces and probe positions are in situ observed. The probe can he positioned precisely on aimed areas on the sample with aid of piezo-actuators. With these machines, the surface sensitivity in conductivity measurements has been greatly enhanced compared with macroscopic four-point probe method. Then surface-state conductivity and influence of atomic steps upon conductivity can be directly measured.
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