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Charge Transport at Surfaces

We acquire maps of the potential landscape which is generated by a current flowing along the surface or through a nanostructure (scanning tunneling potentiometry). These maps give insight into fundamental transport properties of quantum materials, such as the influence of defects on the local electric transport.

The increasing importance of the surface conductance (compared to that of the bulk) in modern nanoelectronic devices calls for a reliable determination of the surface conductivity in order to minimize the influence of undesired leakage currents on the device performance or to use surfaces as functional units.

Disentangling surface conductivity from semiconductor bulk conductivity is achieved by distance-dependent electrical four-point measurements using a multi-tip STM. The results of these measurements, combined with a theoretical model of the charge transport, are used to disentangle surface conductivity from parallel conductance channels through the space charge layer and the bulk [1].

We use the multi-tip scanning tunneling potentiometry variant, developed in Jülich [2] to analyze the resistance of different kinds of defects at surfaces of topological insulators. The largest localized voltage drop we find at domain boundaries in the topological insulator film, with a resistivity about four times higher than that of a step edge. As shown in the figure, we also resolve resistivity dipoles located around nanoscale voids in the sample surface. The influence of such defects on the resistance of the topological surface state is analyzed by means of a resistor network model [3].

Nanoscale PotentiometryFigure (a) shows an STM image of a typical void in a BiSbTe3 topological insulator thin film surface. Scale bar: 5 nm. (b) Corresponding potential map showing a dipole shaped feature centered at the defect. (c) Resistor network model mask with indicated schematic of the resistors. (d) Calculated potential distribution around the defect resulting from the resistor network model shown in (c). (e) Cross sections of the images in (a) - (d).