Electrical contacts to individual SWCNTs: A review

Abstract

Owing to their superior electrical characteristics, nanometer dimensions and definable lengths, single-walled carbon nanotubes (SWCNTs) are considered as one of the most promising materials for various types of nanodevices. Additionally, they can be used as either passive or active elements. To be integrated into circuitry or devices, they are typically connected with metal leads to provide electrical contacts. The properties and quality of these electrical contacts are important for the function and performance of SWCNT-based devices. Since carbon nanotubes are quasi-one-dimensional structures, contacts to them are different from those for bulk semiconductors. Additionally, some techniques used in Si-based technology are not compatible with SWCNT-based device fabrication, such as the contact area cleaning technique. In this review, an overview of the investigations of metal-SWCNT contacts is presented, including the principle of charge carrier injection through the metal-SWCNT contacts and experimental achievements. The methods for characterizing the electrical contacts are discussed as well. The parameters which influence the contact properties are summarized, mainly focusing on the contact geometry, metal type and the cleanliness of the SWCNT surface affected by the fabrication processes. Moreover, the challenges for widespread application of CNFETs are additionally discussed.

Keywords: CNFET; SWCNT; charge carrier transport; electrical contact; metal–SWCNT interface.

Figure 1

Contact geometries: (a) side-bonded and (b) end-bonded contact configurations.

Figure 2

The role of the Fermi level pinning effect at the metal–semiconductor interface. a) Simulation of band bending at the metal–SWCNT (semiconducting SWCNT, diameter: 1.4 nm) interface for a low work function metal. The dotted, dash-dotted, dashed, and solid lines correspond to the different pinning strength 0, 0.01, 0.1, and 1 state/(atom eV), respectively. The pinning effect for the planar junction is shown in the inset for comparison using the same simulation parameters. b) The experimental results of the on current as a function of CNT diameter with different metals. The clear dependence of the Schottky barrier height on the type of metal and the SWCNT diameter is observed. This indicates that no Fermi level pinning effect exists at the metal–SWCNT interface. Figure reprinted with permission from (a) [14] copyright 2000 American Physical Society, (b) [15] copyright 2005 American Chemical Society.

Figure 3

The energy band diagram of a CNFET. a) The band bending effect at the metal–SWCNT interface for a metal with a low work function. A positive voltage bias is applied between drain–source electrodes and the gate bias is set to zero. b) The transfer characteristics of an n-type CNFET. Inset sketches show the Schottky barrier width modulation with respect to the different bias conditions from the gate electrodes.

Figure 4

The procedures for determining the Schottky barrier height. (a) CNFET transfer characteristics measured at different temperatures. (b) Arrhenius plot with linear fits for different gate biases, V_gs. For the better visualization, only the data points obtained for five different gate voltages: 4 V, −1.15 V, −1.25 V, −1.35 V, −4 V are shown. (c) Activation energy (E_a) as a function of gate bias (V_gs) estimated from (a). The highlighted data points represent the activation energies derived from the corresponding slopes of linear fits in (b).

Figure 5

Schematics of different approaches for characterizing the electrical contacts of CNFETs. a) An optimized configuration for the four-terminal measurement by using MWCNTs as the voltage sensing electrode according to the reference [26]. b) Multiple contacts are defined on the same individual SWCNT with different channel lengths. Potential drops can be measured between any two electrodes.

Figure 6

Needle-like side-bonded contact. Reprinted with permission from [53]. Copyright 2010 Macmillan Publisher.

Figure 7

TEM images for (a) metal deposition on suspended, as-grown SWCNTs. Ti displays the best wettability to the SWCNT. Compared to Au, Pd showed better wettability to the SWCNT. (b) The graphitic carbon interfacial layer formed under a Ni layer through annealing treatment. Figure reprinted with permission from (a) [57] copyright 2000 American Institute of Physics and (b) [64] copyright IEEE Electron Devices Society.

Figure 8

Inspection of the cleanliness of SWCNTs. a) AFM image obtained after developing a PMMA resist from the SiO₂-supported SWCNTs. b) AFM image obtained after PMMA and following alumina removal from the contact area. c) SEM image of a Pd contact on a suspended SWCNT using a shadow mask process. d) The cumulative distribution function of on-resistance for n-type CNFETs. Red circles and blue triangles represent the experimental results with and without using the protective layer, respectively. e) The distribution of the hysteresis width for n-type CNFETs. Figure reprinted with permission from (c) [53] copyright 2010 Macmillan Publisher and (d,e) [71] copyright 2014 Elsevier.

Figure 9

Improving the contact performance by post-metallization annealing. a) The process of removing contamination from the sidewalls of SWCNTs by annealing at modest temperatures. b) The TEM image shows the TiC formation through annealing Ti–SWCNT contacts above 800 ºC. c) The schematic of contact design using the Joule heating method for local annealing of the metal–SWCNT contact (using Pd as the contact metal) by pulsed current. Figure reprinted with permission from (a) [70] copyright 2011 Author(s) and (b) [50] copyright 1999 The American Association for the Advancement of Science.