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Charge Transport through Nanostructures of Quantum Materials

Room temperature in-situ measurement of the spin voltage of a BiSbTe3 thin film

One of the hallmarks of topological insulators, the intrinsic spin polarization in the topologically protected surface states, is investigated at room temperature in-situ by means of four-probe scanning tunneling microscopy (STM) for a BiSbTe3 thin film. The spin polarization is measured using the spin-dependent voltage drop between a ferromagnetic nickel tip and a non-magnetic tip. To achieve the required precision of tip positions for measuring a spin signal, a precise positioning method employing STM scans of the local topography with each individual tip is demonstrated. From the four-point transport measurements, the spin polarization in the topological surface states is estimated as p ≃ 0.3 - 0.6, which is close to the theoretical limit. More information can be found in Scientific Reports 10, 2816 (2020).


Conceptual sketch of the electrical measurement setup and the resulting potential along the line of the linearly arranged STM tips. While a NM voltage-probe senses the local spin-averaged potential indicated by the black line, a magnetized FM tip is used to acquire the spin-chemical potential Vs.


Results of the spin voltage measurement. The spin-sensitive four-point resistance is measured as function of the dimensionless distance between the inner voltage-probing tips. Red and blue/cyan data points denote four-point resistances acquired with reversed magnetic polarization directions for the FM tip, respectively. The lines of corresponding color represent fits to a model.

Parasitic conduction channels in topological insulator thin films

Thin films of topological insulators (TI) usually exhibit multiple parallel conduction channels for the transport of electrical current. Beside the topologically protected surface states (TSS), parallel channels may exist, namely the interior of the not-ideally insulating TI film, the interface layer to the substrate, and the substrate itself. To be able to take advantage of the auspicious transport properties of the TSS, the influence of the parasitic parallel channels on the total current transport has to be minimized. Because the conductivity of the interior (bulk) of the thin TI film is difficult to access by measurements, we propose here an approach for calculating the mobile charge carrier concentration in the TI film. To this end, we calculate the near-surface band bending using parameters obtained experimentally from surface-sensitive measurements, namely (gate-dependent) four-point resistance measurements and angle resolved photoelectron spectroscopy (ARPES). While in most cases another parameter in the calculations, i.e. the concentration of unintentional dopants inside the thin TI film, is unknown, it turns out that in the thin-film limit the band bending is largely independent of the dopant concentration in the film. Thus, a well-founded estimate of the total mobile charge carrier concentration and the conductivity of the interior of the thin TI film proves possible. Since the interface and substrate conductivities can be measured by a four-probe conductance measurement prior to the deposition of the TI film, the total contribution of all parasitic channels, and therefore also the contribution of the vitally important TSS, can be determined reliably. More information can be found in Parasitic conduction channels in topological insulator thin films.


Multiple parallel conduction channels in a topological insulator thin film. The current transport can occur through the top and bottom TSS channels, but also through the interior of the TI film, through the interface layer between film and substrate as well as through the substrate itself.

Topological insulators: shedding light on parallel transport channels

Multi-tip STM is used to reveal detailed transport properties of thin films of the promising topological insulator (TI) material BiSbTe3. To study the electronic and transport properties of BiSbTe3 films, scientists from the PGI-3, PGI-6, and PGI-9 institutes at Forschungszentrum Jülich combined angle-resolved photoemission spectroscopy and gated four-tip scanning tunneling microscopy was conducted, gaining access to a comprehensive picture of the transport. A model developed specifically for this analysis enabled them to disentangle the transport occurring through the different channels (the surface states channels on the top and bottom of the sample and the bulk) and to obtain the gate-dependent conductivities, charge carrier concentrations, and mobilities. This combination of experimental techniques and data analysis is of general applicability and should prove useful for studying other samples.

Topological Insulators

Figure caption: Top panel left: Schematic of the measurement geometry and results obtained by photoemission spectroscopy and four-probe measurements. Middle: Scanning electron microscopy image of the four-probe setup with equidistant tip spacing. Scale bar: 100 μm. Right: Photoemission intensity as a function of binding energy with the Dirac cone indicated by dotted red lines. Bottom image: Measurement of the gate-dependent TI film sheet conductivity (black squares) and fit of the transport model, as well as the individual contributions of the model. The colored lines in the inset show current paths through the individual channels. Note that the film thickness d = 10 nm is much smaller than the tip spacing of 100 μm, which are used in the transport measurement. The insets show the band schemes at different voltages.

More information can be found in npj Quantum Materials 3, 46 (2018).

Resistance / dopant profiling along freestanding GaAs nanowires:


Movie of an STM tip moving along a GaAs nanowire measuring a four point probe resistance profile.

The movie above shows a measurement series mapping the (four point) resistance along freestanding GaAs nanowires with a diameter of about 100 nm. The structure of these nanowires grown by a method known as vapor-liquid-solid growth, involving vertical growth of the nanowires on a substrate via a catalytic gold particle, was studied in detail. However, not much is known about the dopant in- corporation and the resulting electrical properties of such freestanding nanowires. This is the case, as the nanowires are still “as grown” upright and attached to the substrate, thus it is not possible to contact nanowires by lithographic techniques.

schematic of four point measurement(a) Schematic of a four-point measurement on a nanowire with three tips contacting the nanowire. (b) SEM image of a freestanding nanowire contacted by three tips. The STM tips act like the test leads of a multimeter, however, contacting objects like the nanowire at the nanoscale.

In the measurement configuration shown in the figure below, the sample is tilted by 45° in order to facilitate optimal SEM imaging of the nanowires, as shown in the right image below. Three tips are brought into contact with a nanowire, realizing a four-point resistance measurement (with the sample as fourth contact). Tip 1 injects the current to the nanowire with the sample acting as current drain, while tip 2 and tip 3 act as voltage probes. In this way a four point measurement is realized. The configuration of the (green) voltage probing tips is analogous to the test leads of a multimeter with the important difference that now the electrical measurements are performed at the nanoscale.

In the STM based approach of nano-contacting, four-point measurements can be performed not only in one single configuration, as it is the case for the lithographic approach, but many configurations can be measured by moving the tips along the nanowire (as seen in the movie above) In this way we can measure a resistance profile along the nanowire with 40 or 50 data points. The figure below shows a resistance profile along a nanowire, which shows a small resistance in the upper part of the wire, while at the nanowire base the resistance becomes very high. This can be correlated to the two-step growth process of the nanowires: An initial high temperature growth step was used in order to nucleate straight vertically growing nanowires, while a lower growth temperature leads to a more efficient incorporation of the doping species. The identification of the undesired very low doping of the base of the nanowires is a very valuable information triggering subsequent efforts towards improving the electrical properties of the nanowires (increasing the doping) by using different growth conditions. More information can be found in Appl. Phys. Lett. 103 , 143104 (2013), J. Phys.: Condens. Matter 31, 074004 (2019) and Nano Res 11, 5924 (2018)

gaas3Resistance profile along a nanowire measured at many different points along the nanowire. The upper part of the nanowire (red) has the desired low resistance.

The SEM movie below shows of a zoom from the macroscopic shape of the STM tips down to the shape of their apex and the nanowires present on the substrate.



How elastic the nanowires are, can be seen thin the following SEM movie, in which a nanowire is bent by the STM tip by more than 90° and subsequently flips back to the straight shape. This bending is used to explore the electric properties of the strained nanowire. More information can be found in Appl. Phys. Lett. 103 , 143104 (2013), and Nano Research 11, 5924 (2018).



SEM movie of an elastic bending of a GaAs nanowire.

Multiprobe measurements on Sb2Te3 nanowires

Here we show an example for contacting Sb2Te3 nanowires of a diameter of about 100 nm, which were grown by CVD [by Prof. Grace Lu (NAMI group – Nanoelectronics and Advanced Materials Innovations) in the University of Southern California, Los Angeles]. Actually, these nanowires have been contacted by gold contacts fabricated using lithographic methods. However, due to technical limitations it was only possible to provide two contacts and to perform two-point resistance measurements. Due to this limitation the question of the influence of a contact resistance, which is unknown in two-point measurements and ads to the device resistance remained as an unsolved problem in this study. We performed a potential measurement along the topological insulator nanowire, as shown in the figure below. The result of this measurement is shown in the diagram. Since the measured three-point resistance at both ends of the TI nanowire corresponded to the two-point resistance, the contact resistance turned out to be negligible, which however was not clear a priori.

Topological Insulator

Topological Insulator

Multiprobe measurement on a freestanding Bi2Se3 topological insulator nanowire

A topological insulator “Nanofinger” (diameter about 500 nm) was etched by FIB from a 1µm Bi2Se3 TI film MBE grown on Si(111) [by P. Schüffelgen and G. Mussler, Jülich]. Two tips contact the “Nanofinger”. Together with the substrate contact, three-point measurements are performed at different positions along the nanowire. The resistance profile along this freestanding “Nanofinger” down to the Si substrate was measured as shown in the diagram below. The topological insulator film has a high conductivity while the Si substrate has a much lower conductivity. The Resistance decreases in the lower (silicon) part due to the larger diameter of the “Nanofinger” in this region.


NanofingerSEM image of the “Nanofinger” contacted by two tips. The diagram on the right shows a resistance profile (three point measurement) along a TI “Nanofinger” showing the TI film with high conductivity and the less conductive Si substrate.

Transition from 2D to 3D charge transport in SrTiO3

The electrical properties of SrTiO3(100) single crystals were investigated in-situ at different stages of thermal reduction by means of a 4-tip STM. Using the tips of the STM as electrical probes, distance-dependent four-point measurements were performed at the surface of the crystal at room temperature after reduction by thermal treatment. For annealing temperatures T = 700°C, charge transport is confined to a surface region < 3mm below the surface. For reduction at T = 900°C a transition from a conducting 2D sheet with insulating bulk to a system with dominant 3D bulk conductivity is found. At an intermediate reduction temperature of T = 800°C, a regime with mixed 2D/3D contributions is observed in the distance-dependent resistance measurements. Describing the depth dependent conductivity with an analytical N-layer model, this regime of mixed 2D/3D conductivity is evaluated quantitatively under the assumption of an exponentially decaying conductivity profile, correlated with the previously observed depth dependent dislocation density in the sample. A non-monotonous temperature dependence of the 3D conductivity in the respective conducting layer is found and the underlying mechanism is discussed. More information can be found in: Scientific Reports 9, 2476 (2019).

Transition from 2D to 3D charge transport in SrTiO3Figure caption: Four-point resistance of a SrTiO3(100) sample measured at room temperature after reduction at T = 800°C as a function of probe distances s and x in the equidistant and non-equidistant configuration. Blue and red curves illustrate exemplary 2D and 3D resistance functions respectively. No matching fit to all the data can be obtained with the 2D and 3D functions due to mixed contributions. The green curve represents the best fit for the conductivity profile shown in the inset.