Westlake News ACADEMICS

Soft-lock Drawing of Super-aligned Carbon Nanotube Bundles for Nanometer Electrical Contacts

Enzheng Shi
18, 2022

Email: zhangchi@westlake.edu.cn
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Assembling single-walled carbon nanotubes (CNTs) into high-density horizontal arrays is strongly desired for practical applications, but challenges remain despite much research. Therefore, we developed a nondestructive soft-lock drawing method to achieve ultraclean single-walled CNT arrays with a very high degree of alignment (angle standard deviation of ~0.03 degree). These arrays contained a large portion of nanometer-sized CNT bundles, yielding a high packing density (~400 per micrometer) and high current carrying capacity (~1.8´108 amperes per square centimeter). This alignment strategy can be generally extended to diverse substrates or sources of raw single-walled CNTs. Significantly, the assembled CNT bundles were used as nanometer electrical contacts of high-density monolayer MoS2 transistors, exhibiting high current density (~38 µA/µm), low contact resistance (~1.6 kΩ·µm), excellent device-to-device uniformity and highly reduced device areas (0.06 µm2/device), demonstrating their potential for future electronic devices and advanced integration technologies.

Main text

Implementing individual semiconducting carbon nanotubes as transistor channels and individual metallic CNTs as gate electrodes in field-effect transistors (FETs) has the potential to promote transistor miniaturization, which is far superior to state-of-the-art commercial channels and gate lengths. The van der Waals interaction between parallel nanotubes leads to the formation of bundles with high-packing density and reduced transport resistance. Compared to conventional metals, metallic bundles are ideal candidates for next-generation microelectronic interconnects or wiring given the strong carbon-to-carbon covalent bonds that resist electromigration as well as high electrical and thermal conductivities. In addition, metallic CNT bundles possess excellent mechanical capacity (tensile strength ~80 GPa) as well as current carrying capacity (ampacity).

Despite the development of numerous alignment and assembly methods for CNTs, controllable formation of highly aligned arrays of CNT bundles has not been realized. Horizontally aligned CNT arrays can be directly synthesized by chemical vapor deposition (CVD), but the yield and density of aligned CNTs remain low. Outside of CVD, efforts have been focused on using solution processing to achieve aligned CNTs from random CNTs, including “bubble-blowing”, slow vacuum infiltration, solid-state drawing and domino pushing from vertically aligned multi-walled CNT (MWCNT) forests, Langmuir-Schaefer methods, DNA-assisted alignment, and dimension-limited self-alignment. Although these methods can effectively transform randomly oriented CNTs into relatively aligned CNTs, the increased density of defects and impurities (such as surfactants), the barely satisfactory alignment, and the short length of CNTs (generally <10 μm) make it difficult to achieve aligned CNT bundles with remarkable mechanical and electrical properties.

We report a soft-lock drawing method to align random single-walled CNT (SWNT) networks into high-density and ultraclean super-aligned CNT arrays free of any surfactants. As shown in the low-magnification image from a scanning electron microscope, the CNTs were aligned throughout the drawing region and the alignment process was nondestructive. As quantified by the angular standard deviation σ and the length/diameter ratio, the soft-lock drawing method exhibited a much higher degree of alignment comparable to previously reported alignment methods. In addition, this method is compatible with many commonly used industrial substrates. Furthermore, diverse sources of random CNTs, such as commercially available SWNT fragments, (6, 5) rich SWNT powders, and Bucky paper could be well-aligned with the soft-lock drawing method.

To understand the mechanism of the soft-lock drawing, we propose a flexible “Gluttonous Snake” model to depict the alignment process. The rationality of the “Gluttonous Snake” model is evidenced by the experimental results. When we intentionally shifted the direction of applied force following a wavy path, the aligned CNTs also smoothly followed a wavy path, which could be useful for the fabrication of on-chip nanoscale inductors with extremely reduced area. Based on the above discussion, our alignment process and mechanisms are distinctly different from previous reports.

For high-performance electronics, aligned CNTs are desired in high-packing density. During our soft-lock drawing alignment process, aligned CNTs may spontaneously form bundles when the local density of CNTs is high. As the average CNT diameter was calculated as 1.41 nm based on statistics from the Raman spectra, a high CNT density of ~400/µm was achieved.

An additional benefit of the soft-lock drawing method is the removal of surface contamination, such as amorphous carbon and catalysts, from raw CNTs after alignment. From the TEM and SEM images, the random CNTs were initially coated with a thick amorphous sheath, which was later detached after the alignment.

In fact, the demonstrated alignment uniformity, surface cleanness and excellent conductance of the CNT bundles already make them a promising contact material for electronic devices. Here, we made the first experimental demonstration of top-gated monolayer MoS2 transistors based on CNT nanoscale contacts. We demonstrated the transfer characteristic of 20 functional devices from the 25-device array. All these transistors demonstrated good device uniformity. In addition, the outstanding linearity of the output curves illustrated a low-contact resistance of the nanoscale CNT contacts, which is better than most of the demonstrated metallic contact materials for MoS2.

The MoS2 transistors with CNT bundle contacts possess advantages over silicon devices of similar channel length in terms of device area due to highly reduced contact area. We also observed good device-to-device uniformity among different transistor arrays, demonstrating the possibility of larger-scale integration by fabricating more devices in one large array or making more transistor arrays with few devices.

In this work, we developed a nondestructive soft-lock drawing method to achieve ultraclean SWNT arrays with a very high degree of alignment, which allows both straight and wavy alignment, and works for both metallic and semiconducting CNTs. We experimentally demonstrated MoS2 transistor arrays based on metallic CNT bundle contacts as well as field-effect transistors with aligned semiconducting CNT channels, and hypothesized highly scaled on-chip inductors made by wavy CNT bundles. The proposed technology, based on the integration of metallic and semiconducting low-dimensional nanomaterials, represents a new paradigm for the fabrication of future nanoelectronics. By further improving the control of the spatial location of all individual CNTs and the spacing between neighboring CNTs the realized clean and low-resistance CNT contact could provide a way to make nanoscale contacts for 2D material or other material systems to the ultimate scaling of transistor contacts. The extremely small thickness of the CNT bundles also ensures device thickness on a nanometer scale. With more optimizations, the metallic CNT bundles could be used as highly scaled sources/drain electrodes of both logic and memory devices in BEOL integrations or 3D integrations with low-dimensional material systems.