Christian Leth Petersen

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.

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.

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.

Oxidation of clean silicon surfaces studied by four-point probe surface conductance measurements

Abstract We have investigated how the conductance of Si(100)−(2 × 1) and Si(111)−(7 × 7) surfaces change during exposure to molecular oxygen. A monotonic decrease in conductance is seen as the (100) surfaces oxidizes. In contrast to a prior study, we propose that this change is caused by a decrease in surface mobility due to surface roughening. The oxidation of the (111) surfaces induces a more complex conductance variation. This can be explained by changes in the band bending which occur during the reaction. Our results show a two-stage reaction, the first stage being completed after one monolayer of oxygen has been absorbed.

Two-dimensional electron gases in the quantum hall regime : Analysis of the circulating current in contactless corbino geometry

Abstract We have investigated the magnitude of the circulating bulk current density in Corbino geometry with high-impedance capacitive contacts using a two-dimensional distributed model. For parameters suited for GaAlAs GaAs heterostructure samples we find that local current densities can attain values larger than 1 A/m, at which a breakdown of the quantum Hall effect might be expected. The circulating current is found to be sufficiently large to allow inductive detection, and we suggest this as a method to measure σxy.

The diagonal and off-diagonal AC conductivity of two-dimensional electron gases with contactless Corbino geometry in the quantum Hall regime

We have investigated the AC conductivity elements in the quantum Hall regime of two‐dimensional electron gases coupled capacitively to electrodes with Corbino geometry. The samples are GaAlAs/GaAs single heterostructures, and the measurements are made at low frequencies, up to 20 kHz. The diagonal conductivity is derived from magnetocapacitance measurements. It increases with increasing frequency according to a power law at integer filling factors. The exponent of the power law depends on both temperature and filling factor. Ratios between Hall conductivities at different filling factors are obtained by inductive measurements of the circulating current. They are found to agree with quantization in multipla of e2/h at the integer filling factors.