Understanding the electrical properties of nanoscale contacts is paramount in small-scale devices,  including probe-based microscopies [1], nanomanufacturing techniques [2] and  micro/nano-electromechanical systems (M/NEMS) [3]. In many cases, the electrical transport properties of the contact determines the device’s functionality, and yet the behavior of the contact conductance is multi-faceted and not easily characterized. There has been extensive characterization of the electrical properties of ultra-small contacts using mechanically controllable break junctions and scanning probe techniques [4]. However, in these techniques the shape, size, and atomic structure of the contacting bodies and the contact itself are typically unknown. Thus, confounding factors such as the presence of oxide films and contaminants; the evolving shapes of the bodies due to inelastic deformation; and inaccurate estimation of contact sizes causes uncertainty in experimental measurements based on contact properties. In situ transmission electron microscopy (TEM) measurements of electrical contacts can overcome these limitations. While investigations have been performed using in situ electrical measurements inside a TEM before – including on single-atom-width nanocontacts in gold [5] – these methods typically require specially prepared contacts and are limited to a range of materials and geometries. In this study we show initial results obtained with a new in situ TEM electrical characterization tool that contains a movable probe, which allows to make site-specific electrical contact measurements to study device-related nanoscale electrical contacts (Fig. 1). The flexibility of the present in situ tool rests in its unique removable sample cartridge that enables simple, repeatable and accurate probe positioning, high-resolution imaging, and accommodates a wide range of nanoscale contact samples.

Two contact configurations that are common to conductive scanning probe microscopy were recreated in situ in the TEM. Namely, a W substrate was contacted by a sharp nanoscale tip that is composed either of Pt/Ir or of doped Si. We demonstrate that current-voltage sweeps can be performed while real-time images of the nanoscale contact are acquired. As shown in Fig. 2(a), the metal/metal contact is ohmic (resistance 730 ohms). By contrast, the metal/semiconductor contact of Fig. 2(b) has a highly asymmetrical IV curve, displaying Schottky-type behavior – as commonly seen in conductive probe microscopy with doped-silicon tips [6].

As an example of the benefits of in situ imaging we compute the contact resistivity of the metal/metal contact. From the images of the contact we estimate a contact radius of 9.8 nm. The resistivity can be calculated using the classical (Maxwell), ballistic (Sharvin), or intermediate (Knudsen) limits [7]. The mean free path for W (estimated from the Fermi velocity and the bulk conductivity [8]) is close to 15 nm. Because this value is on the order of the contact radius, the intermediate resistivity limit is appropriate, leading to a value of r_Knudsen = 620 mW-cm. By having a direct measure of the contact area – obviating the reliance on continuum contact models – we can compute the contact’s resistivity directly. It should be noted that this value is much larger than the bulk resistivity of W which is 4.82 mW-cm [9]. This is attributable to the presence of insulating surface films (such as oxide or contamination). 

References:

[1]JY Park et al, Materials today, 38 (2010), p. 38.

[2] C Cen et al, Nature Materials, 7 (2008), p. 298.

[3] OY Loh, HD Espinosa, Nature Nanotechnology, 7 (2012), p. 283.

[4] N Agrait, AL, Yeyati, JM van Ruitenbeek, Physics Reports, 377 (2003), p. 81.

[5] H Ohnishi, Y Kondo, K Takayanagi, Nature 395 (1998), p. 780.

[6] MA Lantz, SJ O’Shea, ME Welland, Review of scientific instruments 69 (1998), p. 1757.

[7] Wiesendanger, Scanning Probe Microscopy and Spectroscopy, Cambridge U. Press (1994).

[8] Ashcroft & Mermin, Solid State Physics, Brooks Cole (1976).

[9] WM Haynes, ed. CRC handbook of chemistry and physics, CRC press (2014).

[10] The authors thank Julio A. Rodríguez-Manzo for his input and review of the abstract. T.D.B.J. acknowledges support from National Science Foundation under award

No. #CMMI-1536800. 

Figures:

Figure 1: Schematic of in situ nano-manipulator biasing TEM holder tip with sample/probe and wiring configuration. The Si or Pt/Ir coated AFM cantilevers for the experiments presented here are located at the sample location and are probed with the a blunt W tip.

Figure 2: In-situ TEM electrical data was collected during TEM imaging of (a) a Pt/Ir tip contacting a W substrate and (b) a Si tip contacting a W substrate. The electrical measurements for each are shown in the insets. The direct imaging of the real-time apparent contact radius provides additional insights into the origin of the measured contact properties.

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