For in-situ measurements of the local electrical conductivity of well-defined crystal surfaces in ultrahigh vacuum, we have developed two kinds of microscopic four-point probe methods. One involves a "four-tip STM prober," in which four independently driven tips of a scanning tunneling microscope (STM) are used for measurements of four-point probe conductivity. The probe spacing can be changed from 500 nm to 1 mm. The other method involves monolithic micro-four-point probes, fabricated on silicon chips, whose probe spacing is fixed around several µm. These probes are installed in scanningelectron-microscopy/electron-diffraction chambers, in which the structures of sample surfaces and probe positions are observed in situ. The probes can be positioned precisely on aimed areas on the sample with the aid of piezoactuators. By the use of these machines, the surface sensitivity in conductivity measurements has been greatly enhanced compared with the macroscopic four-point probe method. Then the conduction through the topmost atomic layers (surface-state conductivity) and the influence of atomic steps on conductivity can be directly measured.
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 be changed from 0.5 µm to 1 mm. The other one is monolithic micro-four-point probes, fabricated on silicon chips, whose probe spacing is fixed around several µm. 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 be positioned precisely on aimed areas on the sample with aid of piezoactuators. 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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