Torben Mikael Hansen

Scanning microscopic four-point conductivity probes

A method for fabricating microscopic four-point probes is presented. The method uses silicon-based microfabrication technology involving only two patterning steps. The last step in the fabrication process is an unmasked deposition of the conducting probe material, and it is thus possible to select the conducting material either for a silicon wafer or a single probe unit. Using shadow masking photolithography an electrode spacing (pitch) down to 1.1 μm was obtained, with cantilever separation down to 200 nm. Characterisation measurements have shown the microscopic probes to be mechanically very flexible and robust. Repeated conductivity measurements on polythiophene films in the same surface area are reproduced within an accuracy of 3%. Automated nanoresolution position control allows scanning across millimetre sized areas, in order to create high spatial resolution maps of the in-plane conductivity.

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.

Scanning nanoscale multiprobes for conductivity measurements

We report fabrication and measurements with two- and four-point probes with nanoscale dimensions, for high spatial resolution conductivity measurements on surfaces and thin films. By combination of conventional microfabrication and additive three-dimensional nanolithography, we have obtained electrode spacings down to 200 nm. At the tips of four silicon oxide microcantilevers, narrow carbon tips are grown in converging directions and subsequently coated with a conducting layer. The probe is placed in contact with a conducting surface, whereby the electrode resistance can be determined. The nanoelectrodes withstand considerable contact force before breaking. The probe offers a unique possibility to position the voltage sensors, as well as the source and drain electrodes in areas of nanoscale dimensions.

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.

Comparative study of size dependent four-point probe sheet resistance measurement on laser annealed ultra-shallow junctions

In this comparative study, the authors demonstrate the relationship∕correlation between macroscopic and microscopic four-point sheet resistance measurements on laser annealed ultra-shallow junctions (USJs). Microfabricated cantilever four-point probes with probe pitch ranging from 1.5to500μm have been used to characterize the sheet resistance uniformity of millisecond laser annealed USJs. They verify, both experimentally and theoretically, that the probe pitch of a four-point probe can strongly affect the measured sheet resistance. Such effect arises from the sensitivity (or “spot size”) of an in-line four-point probe. Their study shows the benefit of the spatial resolution of the micro four-point probe technique to characterize stitching effects resulting from the laser annealing process.

Fast and direct measurements of the electrical properties of graphene using micro four-point probes

We present measurements of the electronic properties of graphene using a repositionable micro four-point probe system, which we show here to have unique advantages over measurements made on lithographically defined devices; namely speed, simplicity and lack of a need to pattern graphene. Measurements are performed in ambient, vacuum and controlled environmental conditions using an environmental scanning electron microscope (SEM). The results are comparable to previous results for microcleaved graphene on silicon dioxide (SiO(2)). We observe a pronounced hysteresis of the charge neutrality point, dependent on the sweep rate of the gate voltage; and environmental measurements provide insight into the sensor application prospects of graphene. The method offers a fast, local and non-destructive technique for electronic measurements on graphene, which can be positioned freely on a graphene flake.

Resolution enhancement of scanning four-point-probe measurements on two-dimensional systems

A method to improve the resolution of four-point-probe measurements of two-dimensional (2D) and quasi-2D systems is presented. By mapping the conductance on a dense grid around a target area and postprocessing the data, the resolution can be improved by a factor of approximately 50 to better than 1/15 of the four-point-probe electrode spacing. The real conductance sheet is simulated by a grid of discrete resistances, which is optimized by means of a standard optimization algorithm, until the simulated voltage-to-current ratios converges with the measurement. The method has been tested against simulated data as well as real measurements and is found to successfully deconvolute the four-point-probe measurements. In conjunction with a newly developed scanning four-point probe with electrode spacing of 1.1 μm, the method can resolve the conductivity with submicron resolution.

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.

Sensitivity study of micro four-point probe measurements on small samples

The authors calculate the sensitivities of micro four-point probe sheet resistance and Hall effect measurements to the local transport properties of nonuniform material samples. With in-line four-point probes, the measured dual configuration sheet resistance is more sensitive near the inner two probes than near the outer ones. The sensitive area is defined for infinite film, circular, square, and rectangular test pads, and convergent sensitivities are observed for small samples. The simulations show that the Hall sheet resistance RH in micro Hall measurements with position error suppression is sensitive to both local carrier density and local carrier mobility because the position calculation is affected in the two pseudo-sheet-resistance measurements needed for the position error suppression. Furthermore, they have also simulated the sensitivity for the resistance difference ΔRBB′ of two specific configurations to clarify the effect of the calculated position, which results in an unexpected sensitivity to...

Integration, gap formation, and sharpening of III-V heterostructure nanowires by selective etching

Epitaxial growth of heterostructure nanowires allows for the definition of narrow sections with specific semiconductor composition. The authors demonstrate how postgrowth engineering of III-V heterostructure nanowires using selective etching can form gaps, sharpening of tips, and thin sections simultaneously on multiple nanowires. They investigate the potential of combining nanostencil deposition of catalyst, epitaxial III-V heterostructure nanowire growth, and selective etching, as a road toward wafer scale integration and engineering of nanowires with existing silicon technology. Nanostencil lithography is used for deposition of catalyst particles on trench sidewalls and the lateral growth of III-V nanowires is achieved from such catalysts. The selectivity of a bromine-based etch on gallium arsenide segments in gallium phosphide nanowires is examined, using a hydrochloride etch to remove the III-V native oxides. Depending on the etching conditions, a variety of gap topologies and tiplike structures are observed, offering postgrowth engineering of material composition and morphology.