Ichiro Shiraki

Independently driven four-tip probes for conductivity measurements in ultrahigh vacuum

Abstract To measure electrical conductivity of materials in scales ranging from nanometer to millimeter, a four-point probe system was developed and installed in an ultrahigh-vacuum scanning electron microscope (UHV-SEM). Each probe, made of a W tip, was independently driven with piezoelectric actuators and a scanner in XYZ directions to achieve precise positioning in nanometer scales. The SEM was used for observing the tips for positioning, as well as the sample surface together with scanning reflection-high-energy electron diffraction capability. This four-point probe system has two kinds of special devices. One is octapole tube-type scanners for tip scanning parallel to the sample surface with negligible displacements normal to the surface. Another is a pre-amplifier which can be switched in current measurement mode between tunnel contact for scanning tunneling microscopy and direct contact for four-point probe method. The electrical resistance of a silicon crystal with a Si(1xa01xa01)-7×7 clean surface was measured with this machine as a function of probe spacing between 1 mm and 1 μm . The result clearly showed an enhancement of surface sensitivity in resistance measurement by reducing the probe spacing.

Microfour-point probe for studying electronic transport through surface states

Microfour-point probes integrated on silicon chips have been fabricated with probe spacings in the range 4–60 μm. They provide a simple robust device for electrical transport measurements at surfaces, bridging the gap between conventional macroscopic four-point probes and scanning tunneling microscopy. Measurements on Si(111) surfaces in ultrahigh vacuum reveal that the Si(111)–√3×√3–Ag structure induced by a monolayer of Ag atoms has a four-point resistance two orders of magnitude lower than that of the Si(111)–7×7 clean surface. We attribute this remarkable difference to direct transport through surface states, which is not observed on the macroscopic scale, presumably due to scattering at atomic steps.

Direct measurement of surface-state conductance by microscopic four-point probe method

For in situ measurements of local electrical conductivity of well defined crystal surfaces in ultrahigh vacuum, we have developed microscopic four-point probes with a probe spacing of several micrometres, installed in a scanning-electron-microscope/electron-diffraction chamber. The probe is precisely positioned on targeted areas of the sample surface by using piezoactuators. This apparatus enables conductivity measurement with extremely high surface sensitivity, resulting in direct access to surface-state conductivity of the surface superstructures, and clarifying the influence of atomic steps upon conductivity.

ELECTRICAL CONDUCTION THROUGH SURFACE SUPERSTRUCTURES MEASURED BY MICROSCOPIC FOUR-POINT PROBES

For in-situ measurements of the local electrical conductivity of well-dened 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 xed around several m. These probes are installed in scanningelectron-microscopy/electron-dirac tion 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 inuence of atomic steps on conductivity can be directly measured.

Surface-State Bands on Silicon –Si(111)-√3×√3-Ag Surface Superstructure–

After reviewing the atomic and electronic structures of the Si(111)-?3??3-Ag surface, which have recently been clarified after much research, we describe the experimental confirmations of electrical conduction through its surface-state band. A newborn method, micro-four-point probe, is introduced for conductivity measurements with high surface sensitivity.

Transport at surface nanostructures measured by four-tip STM ☆

Abstract 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. The probe spacing can be changed from 500 nm 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 were installed in scanning-electron-microscopy/electron-diffraction chambers, in which the structures of sample surfaces and probe positions were in situ observed. The probes can be 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 the conduction through the topmost atomic layers (surface-state conductivity) and influence of atomic steps upon conductivity could be directly measured. The STM prober is mainly described here.

MICRO-FOUR-POINT PROBES IN A UHV SCANNING ELECTRON MICROSCOPE FOR IN-SITU SURFACE-CONDUCTIVITY MEASUREMENTS

For in-situ measurements of surface conductivity in ultrahigh vacuum (UHV), we have installed micro-four-point probes (probe spacings down to 4 μm) in a UHV scanning electron microscope (SEM) combined with scanning reflection–high-energy electron diffraction (RHEED). With the aid of piezoactuators for precise positioning of the probes, local conductivity of selected surface domains of well-defined superstructures could be measured during SEM and RHEED observations. It was found that the surface sensitivity of the conductivity measurements was enhanced by reducing the probe spacing, enabling the unambiguous detection of surface-state conductivity and the influence of surface defects on the electrical conduction.

Diffusion anisotropy of Ag and In on Si(1 1 1) surface studied by UHV-SEM

Anisotropic features of Ag and In electromigration on clean and Au-precovered Si(1 1 1) surfaces were studied by in situ scanning electron microscopy in ultrahigh vacuum. It was noted that the migration direction of Ag was determined by both applied direct-current direction and step orientation on the substrate surface; on an Si(1 1 1) surface with steps inclined with respect to the current direction, the electromigration direction shows an apparent deviation from the accurate current direction. On clean and Au-precovered Si(1 1 1) surfaces with various coverages of Au (within submonolayer range), the migration behaviors of Ag and In drastically changed with Au coverages and showed different diffusion anisotropy (either thermal diffusion and electromigration) depending on the adsorbate surface structures. Particularly, on a beta-square root of 3 x square root of 3-Au surface of one monolayer Au coverage, In migrated with the highest mobility across the step bands, whereas In showed only a slow movement on the 7 x 7 clean surface due to a migration barrier at step edges. This result implied that the beta-square root of 3 x square root of 3-Au surface phase served as an intermediate layer for In adatoms migration. On the contrary, Ag showed negligible migration on the beta-square root of 3 x square root of 3-Au surface, while the 7 x 7 surface was the substrate for appreciable migration of Ag atoms. The results are discussed in terms of step-edge barriers in migration and on-terrace migration.

Electromigration and phase transformation of Ag on a Cu-precovered Si(1 1 1) surface

Abstract Electromigration and phase transformation of an ultrathin-Ag film patch deposited on a Cu-precovered incommensurate “5×5” phase on Si(1xa01xa01) surface were investigated by in situ ultrahigh-vacuum scanning electron microscopy and microprobe reflection high-energy electron diffraction, and compared with the previous results on the clean and Au-precovered Si(1xa01xa01) surfaces. The film patch spread quite slowly towards the cathode by current apply (and resultant heating), at the initial stage of which the patch area showed a 3 × 3 phase, consisted of Ag–Cu surface alloy, with three-dimensional (3D) islands in it. Later, a phase transformation from the 3 × 3 into a 21 × 21 structure proceeded, accompanied with the 3D islands dissolving. These surface alloy formations play a key role in the migration, which is similar to the case of on Au-precovered Si(1xa01xa01) surfaces.

Substrate-Structure Dependence of Ag Electromigration on Au-Precovered Si(111) Surfaces

Electromigration of Ag on Au-precovered Si(111) surfaces was investigated by in-situ ultrahigh vacuum scanning electron microscopy and µ-probe reflection-high-energy electron diffraction (RHEED). Migration behaviors of a Ag-film patch strongly depended on Au coverage θAu and corresponding surface structures. When θAu 0.7 ML, the patch expansion was greatly reduced. Especially on the β-√3×√3-Au surface (θAu~1.0 ML), the patch showed no directional expansion towards the cathode. But Ag atoms were observed to migrate inside the patches on all substrates (including the β-√3×√3-Au surface) to form 3D islands near terrace edges.