Multi-tip Scanning Tunneling Microscope Research Articles

Nanoscale tip positioning with a multi-tip scanning tunneling microscope using topography images.

Multi-tip scanning tunneling microscopy (STM) is a powerful method to perform charge transport measurements at the nanoscale. With four STM tips positioned on the surface of a sample, four-point resistance measurements can be performed in dedicated geometric configurations. Here, we present an alternative to the most often used scanning electron microscope imaging to infer the corresponding tip positions. After the initial coarse positioning is monitored by an optical microscope, STM scanning itself is used to determine the inter-tip distances. A large STM overview scan serves as a reference map. Recognition of the same topographic features in the reference map and in small scale images with the individual tips allows us to identify the tip positions with an accuracy of about 20nm for a typical tip spacing of ∼1μm. In order to correct for effects such as the non-linearity of the deflection, creep, and hysteresis of the piezoelectric elements of the STM, a careful calibration has to be performed.

Direct measurement of anisotropic conductivity in a nanolaminated (Mn0.5Cr0.5)2GaC thin film

The direct and parameter-free measurement of anisotropic electrical resistivity of a magnetic Mn+1AXn (MAX) phase film is presented. A multitip scanning tunneling microscope is used to carry out 4-probe transport measurements with variable probe spacing s. The observation of the crossover from the 3D regime for small s to the 2D regime for large s enables the determination of both in-plane and perpendicular-to-plane resistivities ρab and ρc. A (Cr0.5Mn0.5)2GaC MAX phase film shows a large anisotropy ratio ρc/ρab=525±49. This is a consequence of the complex bonding scheme of MAX phases with covalent M–X and metallic M–M bonds in the MX planes and predominately covalent, but weaker bonds between the MX and A planes.

Multi-Probe Electrical Characterization of Nanowires for Solar Energy Conversion

Catalysis-assisted vapor–liquid–solid nanowire (NW) growth offers opportunities to prepare versatile, axial, and radial III–V homo- and hetero-structures, which combine multiple scientific and economic benefits including applications in innovative solar energy conversion. For an essential and suitable optoelectronic analysis of NW heterocontacts, we have established a sophisticated multi-tip scanning tunneling microscope (STM) used as a four-point prober, which is in vacuo combined with state-of-the-art preparation, enabling an individual characterization of free-standing NWs with no contamination after preparation and with highest spatial resolution. We apply the superior capabilities of the ultra-high-vacuum-based multi-tip STM to perform an in-depth study of gallium arsenide NW structures, incorporating an axial p-n junction. Two- and four-point I–V characteristics of the diode are recorded non-destructively, enabling the determination of a local ideality factor. Four-point-probe measurements at different NW positions result in an axial resistance profile, allowing the calculation of the doping concentration of p- and n-doped parts. Around the p-n junction, a 500-nm-width region of low conductance was detected, indicating a compensation effect of dopants during growth. By recording electron-beam-induced current images, the position of the charge separating contact was confirmed.

Spatially resolved analysis of dopant concentration in axial GaAs NW pn-contacts

The efficiency of novel opto-electronic devices e.g. solar cells crucially depends on controlling the doping levels in the device. Decreasing the size of the structure by employing nanowires is attractive for several reasons, for instance reduced material costs, but makes the characterization more challenging. There is still a lack of a precise and simultaneously rapid characterization technique for doping concentrations in nanostructures. In this work axial pn-junctions of GaAs-NWs were investigated electrically, by the utilization of a multi-tip scanning tunneling microscope (MT-STM) as a four point prober. Additionally, these NWs were probed optically at room-temperature by photoluminescence (PL) and cathodoluminescence (CL) microscopy spectroscopy. With both approaches we were able to achieve complementary information of the nanostructure and have developed a complete picture. By using a transport model (for MT-STM) and semi-empirical equations (for PL & CL) which take the Burstein-Moss shift and the band gap narrowing into account, the doping concentrations of the NWs were evaluated and compared. An axial variation in the electron concentration of the n-doped NW-top was resolved as well as a constant hole concentration in the p-doped NW-base. We demonstrate that the combination of both techniques will be necessary to improve the development of a suitable and precise model for the estimation of doping profiles by luminescence data.

Invited Review Article: Multi-tip scanning tunneling microscopy: Experimental techniques and data analysis.

In scanning tunneling microscopy, we witness in recent years a paradigm shift from "just imaging" to detailed spectroscopic measurements at the nanoscale and multi-tip scanning tunneling microscope (STM) is a technique following this trend. It is capable of performing nanoscale charge transport measurements like a "multimeter at the nanoscale." Distance-dependent four-point measurements, the acquisition of nanoscale potential maps at current carrying nanostructures and surfaces, as well as the acquisition of I - V curves of nanoelectronic devices are examples of the capabilities of the multi-tip STM technique. In this review, we focus on two aspects: How to perform the multi-tip STM measurements and how to analyze the acquired data in order to gain insight into nanoscale charge transport processes for a variety of samples. We further discuss specifics of the electronics for multi-tip STM and the properties of tips for multi-tip STM, and present methods for a tip approach to nanostructures on insulating substrates. We introduce methods on how to extract the conductivity/resistivity for mixed 2D/3D systems from four-point measurements, how to measure the conductivity of 2D sheets, and how to introduce scanning tunneling potentiometry measurements with a multi-tip setup. For the example of multi-tip measurements at freestanding vapor liquid solid grown nanowires, we discuss contact resistances as well as the influence of the presence of the probing tips on the four point measurements.

In situ disentangling surface state transport channels of a topological insulator thin film by gating

In the thin film limit, the surface state of a three-dimensional topological insulator gives rise to two parallel conduction channels at the top and bottom surface of the film, which are difficult to disentangle in transport experiments. Here, we present gate-dependent multi-tip scanning tunneling microscope transport measurements combined with photoemission experiments all performed in situ on pristine BiSbTe3 thin films. To analyze the data, we develop a generic transport model including quantum capacitance effects. This approach allows us to quantify the gate-dependent conductivities, charge carrier concentrations, and mobilities for all relevant transport channels of three-dimensional topological insulator thin films (i.e., the two topological surface state channels, as well as the interior of the film). For the present sample, we find that the conductivity in the bottom surface state channel is minimized below a gate voltage of Vgate = −34 V and the top surface state channel dominates the transport through the film.

Four-point probe measurements using current probes with voltage feedback to measure electric potentials

We present a four-point probe resistance measurement technique which uses four equivalent current measuring units, resulting in minimal hardware requirements and corresponding sources of noise. Local sample potentials are measured by a software feedback loop which adjusts the corresponding tip voltage such that no current flows to the sample. The resulting tip voltage is then equivalent to the sample potential at the tip position. We implement this measurement method into a multi-tip scanning tunneling microscope setup such that potentials can also be measured in tunneling contact, allowing in principle truly non-invasive four-probe measurements. The resulting measurement capabilities are demonstrated for and samples.

Comparative analysis on resistance profiling along tapered semiconductor nanowires: multi-tip technique versus transmission line method

The detection of doping dependent values like contact- and path resistances along nanowires (NWs) still proves to be rather challenging compared to planar structures. Unfortunately, the usually used and well established TLM (transmission line measurement) setup exhibits some drawbacks. Complex preliminary preparation steps and the necessity of ohmic contacts limit the investigation to certain semiconductor materials. The simultaneous determination of contact- and path resistances with an unknown distribution makes an analysis on complex structures like tapered nanowires very challenging. Our approach is the utilization of a multi-tip scanning tunneling microscope (MT-STM) as a four point prober, which allows the investigation of freestanding nanowires with an increased spatial resolution. Here, the used measurement setup allows a local separation of current injection and potential measurement and thus a highly precise determination of path resistances. Tapered p-doped GaAs-NWs were used to compare both techniques. Whereas the evaluation of the axial doping profile by MT-STM was rather simple, correction factors had to be introduced for the TLM measurement to calculate the specific resistances and transfer length. By comparing the results of both methods for the very same NW-sample, the precision and accuracy of MT-STM measurements was demonstrated. We found an agreement, which allows the conclusion that both methods exhibit advantages; however the MT-STM was determined as the more precise setup, which enables additional characterization capabilities, such as surface, temperature or light dependent measurements.

Surface conductivity of Si(100) and Ge(100) surfaces determined from four-point transport measurements using an analyticalN-layer conductance model

An analytical N-layer model for charge transport close to a surface is derived from the solution of Poisson's equation and used to describe distance-dependent electrical four-point measurements on the microscale. As the N-layer model comprises a surface channel, multiple intermediate layers and a semi-infinite bulk, it can be applied to semiconductors in combination with a calculation of the near-surface band-bending to model very precisely the measured four-point resistance on the surface of a specific sample and to extract a value for the surface conductivity. For describing four-point measurements on sample geometries with mixed 2D-3D conduction channels often a very simple parallel-circuit model has so far been used in the literature, but the application of this model is limited, as there are already significant deviations, when it is compared to the lowest possible case of the N-layer model, i.e. the 3-layer model. Furthermore, the N-layer model is applied to published distance-dependent four-point resistance measurements obtained with a multi-tip scanning tunneling microscope (STM) on Germanium(100) and Silicon(100) with different bulk doping concentrations resulting in the determination of values for the surface conductivities of these materials.

Surface and Step Conductivities on Si(111) Surfaces.

Four-point measurements using a multitip scanning tunneling microscope are carried out in order to determine surface and step conductivities on Si(111) surfaces. In a first step, distance-dependent four-point measurements in the linear configuration are used in combination with an analytical three-layer model for charge transport to disentangle the 2D surface conductivity from nonsurface contributions. A termination of the Si(111) surface with either Bi or H results in the two limiting cases of a pure 2D or 3D conductance, respectively. In order to further disentangle the surface conductivity of the step-free surface from the contribution due to atomic steps, a square four-probe configuration is applied as a function of the rotation angle. In total, this combined approach leads to an atomic step conductivity of σ(step)=(29±9) Ω(-1) m(-1) and to a step-free surface conductivity of σ(surf)=(9±2)×10(-6) Ω(-1)/□ for the Si(111)-(7×7) surface.

Resistance and dopant profiling along freestanding GaAs nanowires

Resistance profiles along as-grown GaAs nanowires were measured with a multi-tip scanning tunneling microscope used as a nanoprober. The nanowires were grown in the vapor-liquid-solid growth mode in a two-temperature-step mode and doped with Zn. Using a transport model, the resistance profile was converted to a dopant profile. The dopant distribution along the nanowires was found to correlate with the temperature during different phases of nanowire growth. The nanowire base grown at higher temperature exhibits a decreased dopant concentration. Mechanical stress by intentional bending of a nanowire was shown not to influence nanowire conductance.

Ultra compact multitip scanning tunneling microscope with a diameter of 50 mm

We present a multitip scanning tunneling microscope (STM) where four independent STM units are integrated on a diameter of 50 mm. The coarse positioning of the tips is done under the control of an optical microscope or scanning electron microscopy in vacuum. The heart of this STM is a new type of piezoelectric coarse approach called KoalaDrive. The compactness of the KoalaDrive allows building a four-tip STM as small as a single-tip STM with a drift of less than 0.2 nm/min at room temperature and lowest resonance frequencies of 2.5 kHz (xy) and 5.5 kHz (z). We present as examples of the performance of the multitip STM four point measurements of silicide nanowires and graphene.

A nanopositioner for scanning probe microscopy: The KoalaDrive

We present a new type of piezoelectric nanopositioner called KoalaDrive which can have a diameter less than 2.5 mm and a length smaller than 10 mm. The new operating principle provides a smooth travel sequence and avoids shaking which is intrinsic to nanopositioners based on inertial motion with sawtooth driving signals. In scanning probe microscopy, the KoalaDrive can be used for the coarse approach of the tip or sensor towards the sample. Inserting the KoalaDrive in a piezo tube for xyz-scanning integrates a complete scanning tunneling microscope (STM) inside a 4 mm outer diameter piezo tube of <10 mm length. The use of the KoalaDrive makes the scanning probe microscopy design ultracompact and accordingly leads to a high mechanical stability. The drive is UHV, low temperature, and magnetic field compatible. The compactness of the KoalaDrive allows building a multi-tip STM as small as a single tip STM.

Variable-temperature independently driven four-tip scanning tunneling microscope

The authors have developed an ultrahigh vacuum (UHV) variable-temperature four-tip scanning tunneling microscope (STM), operating from room temperature down to 7 K, combined with a scanning electron microscope (SEM). Four STM tips are mechanically and electrically independent and capable of positioning in arbitrary configurations in nanometer precision. An integrated controller system for both of the multitip STM and SEM with a single computer has also been developed, which enables the four tips to operate either for STM imaging independently and for four-point probe (4PP) conductivity measurements cooperatively. Atomic-resolution STM images of graphite were obtained simultaneously by the four tips. Conductivity measurements by 4PP method were also performed at various temperatures with the four tips in square arrangement with direct contact to the sample surface.

In situ resistance measurements of epitaxial cobalt silicide nanowires on Si(110)

We have performed in situ resistance measurements for individual epitaxial CoSi2 nanowires (NWs) (approximately 60 nm wide and 5μm long) formed on a Si(110) surface. Two- and four-point probe measurements were done with a multitip scanning tunneling microscope at room temperature. The NWs were well isolated from the substrate by a Schottky barrier with zero-bias resistance of 107Ω. The resistivity of the NWs was 30μΩcm, which is similar to that for high-quality epitaxial films. The NW resistance was essentially unchanged after exposure to air.

Resistance measurements of metallic silicide nanowires on a Si substrate with a four-tip scanning tunneling microscope

We have measured electrical resistance of individual epitaxial CoSi2 nanowires (NWs) formed on a Si(110) surface in situ in ultrahigh vacuum, by two-, three-, and four-point probe methods using a multi-tip scanning tunneling microscope at room temperature. The NWs were electrically isolated from the substrate by a Schottky barrier in-between with zero-bias resistance of approximately 107 Ω. Contact resistance between a W tip and a NW was as small as several tens Ω. The resistivity of the NWs was ca. 30 μΩcm, which is similar to that of high-quality epitaxial CoSi2 films. The oxidation of the NW surface had negligible influence on the resistance, meaning marginal carrier scattering at the NW surface. [DOI: 10.1380/ejssnt.2005.362]

Microscope for Direct Measurements of Nanoscale Properties: Multi-tip Scanning Tunneling Microscope

Microscope for Direct Measurements of Nanoscale Properties: Multi-tip Scanning Tunneling Microscope