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Charge Transport through Nanostructures

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:

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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).

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.

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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).

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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.

finger

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.



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