Improving Contact Interfaces in Fully Printed Carbon Nanotube Thin-Film Transistors

Abstract

Single-walled carbon nanotubes (CNTs) printed into thin films have been shown to yield high mobility, thermal conductivity, mechanical flexibility, and chemical stability as semiconducting channels in field-effect, thin-film transistors (TFTs). Printed CNT-TFTs of many varieties have been studied; however, there has been limited effort toward improving overall CNT-TFT performance. In particular, contact resistance plays a dominant role in determining the performance and degree of variability in the TFTs, especially in fully printed devices where the contacts and channel are both printed. In this work, we have systematically investigated the contact resistance and overall performance of fully printed CNT-TFTs employing three different printed contact materials-Ag nanoparticles, Au nanoparticles, and metallic CNTs-each in the following distinct contact geometries: top, bottom, and double. The active channel for each device was printed from the dispersion of high-purity (>99%) semiconducting CNTs, and all printing was carried out using an aerosol jet printer. Hundreds of devices with different channel lengths (from 20 to 500 μm) were fabricated for extracting contact resistance and determining related contact effects. Printed bottom contacts are shown to be advantageous compared to the more common top contacts, regardless of contact material. Further, compared to single (top or bottom) contacts, double contacts offer a significant decrease (>35%) in contact resistance for all types of contact materials, with the metallic CNTs yielding the best overall performance. These findings underscore the impact of printed contact materials and structures when interfacing with CNT thin films, providing key guidance for the further development of printed nanomaterial electronics.

Determination of individual contact interfaces in carbon nanotube network-based transistors

“…The plot of R total vs. 1/( V GS  –  V T ) demonstrated that the extracted R D  +  R S was expected to be more accurate than the conventional R total extrapolation from the plot for R total vs. V GS because of the ease of fitting to the R D  +  R S value. Moreover, the CNT-TFTs with a higher CNT density tend to have lower combined contact resistances from the S/D electrodes, i.e., R D  +  R S , which is primarily attributed to an increased contact area between the CNT network film with a higher CNT density and the electrode 22 . A summary of the results of the averaged R D  +  R S from a total of 3–4 CNT-TFTs normalized to the width of the CNT network channel with different CNT densities is shown in Fig.…”

“…The functionalized wafer was then rinsed with DI water to remove the unattached monomers and blown dry thoroughly using N 2 gas 22 . Subsequently, the substrate was immersed in highly purified 99% semiconducting enriched CNT solution (concentration: 0.01 mg/ml, average diameter: 1.4 nm, average length: 1–2 µm, type of purification: density gradient ultracentrifugation, provided by NanoIntergris, Inc.).…”

“…Despite these efforts, there are still factors limiting the ultimate performance of CNT-TFTs, such as the contact interface, as quantified by the contact resistance, which is difficult to reduce, even in the ballistic transport regime for application to logic devices 20–22 . Therefore, the contact interface plays a particularly dominant role in determining the performance and degree of variability in CNT-TFTs and should thus be further optimized and investigated in depth.…”

Ultralow voltage operation of biologically assembled all carbon nanotube nanomesh transistors with ion-gel gate dielectrics

“…Field effect transistors (FETs) based entirely on carbon nanotubes (CNTs) have several advantages such as simple device design, improved contact at the channel-to source(S)/drain(D) interface, good optical transmittance and excellent mechanical flexibility 3–9 . The use of CNTs as electrodes instead of metallic electrodes provides not only mechanical flexibility and transparency but also good electrical contacts to an CNT channel due to ideally the similar work function of the CNT electrodes and channel 7, 10 .…”

“…The use of CNTs as electrodes instead of metallic electrodes provides not only mechanical flexibility and transparency but also good electrical contacts to an CNT channel due to ideally the similar work function of the CNT electrodes and channel 7, 10 . Improved electrical contact at the channel-to-S/D interface results in a small contact resistance, which could lead to high on-current 3 . Various approaches such as transferring, printing and self-assembly process have been developed to fabricate high performance CNT-based transistors 4, 5, 7–9, 11–16 .…”