Charge Injection in High-κ Gate Dielectrics of Single-Walled Carbon Nanotube Thin-Film Transistors

We investigate charge injection into the gate dielectric of single-walled carbon nanotube thin-film transistors (SWCNT-TFTs) having Al₂O₃ and HfO₂ gate dielectrics. We demonstrate the use of electric field gradient microscopy (EFM) to identify the sign and approximate the magnitude of the injected charge carriers. Charge injection rates and saturation levels are found to differ between electrons and holes and also vary according to gate dielectric material. Electrically, Al₂O₃ gated devices demonstrate smaller average hysteresis and notably higher average on-state current and p-type mobility than those gated by HfO₂. These differences in transfer characteristics are attributed to the charge injection, observed <i>via</i> EFM, and correlate well with differences in tunneling barrier height for electrons and holes formed in the conduction and valence at the SWCNT/dielectric interface, respectively. This work emphasizes the need to understand the SWCNT/dielectric interface to overcome charge injection that occurs in the focused field region adjacent to SWCNTs and indicates that large barrier heights are key to minimizing the effect

Carbon Nanotube Thin-Film Antennas

Multiwalled carbon nanotube (MWCNT) and single-walled carbon nanotube (SWCNT) dipole antennas have been successfully designed, fabricated, and tested. Antennas of varying lengths were fabricated using flexible bulk MWCNT sheet material and evaluated to confirm the validity of a full-wave antenna design equation. The ∼20× improvement in electrical conductivity provided by chemically doped SWCNT thin films over MWCNT sheets presents an opportunity for the fabrication of thin-film antennas, leading to potentially simplified system integration and optical transparency. The resonance characteristics of a fabricated chlorosulfonic acid-doped SWCNT thin-film antenna demonstrate the feasibility of the technology and indicate that when the sheet resistance of the thin film is >40 ohm/sq no power is absorbed by the antenna and that a sheet resistance of <10 ohm/sq is needed to achieve a 10 dB return loss in the unbalanced antenna. The dependence of the return loss performance on the SWCNT sheet resistance is consistent with unbalanced metal, metal oxide, and other CNT-based thin-film antennas, and it provides a framework for which other thin-film antennas can be designed

Radiation-Hard Complementary Integrated Circuits Based on Semiconducting Single-Walled Carbon Nanotubes

Increasingly complex demonstrations of integrated circuit elements based on semiconducting single-walled carbon nanotubes (SWCNTs) mark the maturation of this technology for use in next-generation electronics. In particular, organic materials have recently been leveraged as dopant and encapsulation layers to enable stable SWCNT-based rail-to-rail, low-power complementary metal-oxide-semiconductor (CMOS) logic circuits. To explore the limits of this technology in extreme environments, here we study total ionizing dose (TID) effects in enhancement-mode SWCNT-CMOS inverters that employ organic doping and encapsulation layers. Details of the evolution of the device transport properties are revealed by <i>in situ</i> and <i>in operando</i> measurements, identifying <i>n</i>-type transistors as the more TID-sensitive component of the CMOS system with over an order of magnitude larger degradation of the static power dissipation. To further improve device stability, radiation-hardening approaches are explored, resulting in the observation that SWNCT-CMOS circuits are TID-hard under dynamic bias operation. Overall, this work reveals conditions under which SWCNTs can be employed for radiation-hard integrated circuits, thus presenting significant potential for next-generation satellite and space applications

Enhanced Electrical Transport in Carbon Nanotube Thin Films through Defect Modulation

The electrical properties of single-wall carbon nanotube (SWCNT) thin films were enhanced through defect introduction and subsequent thermal annealing in forming gas. The defect density in the SWCNT thin films was modulated using ion irradiation with 150 keV ¹¹B⁺ over the fluence range of 1 × 10¹³ and 1 × 10¹⁵ ions/cm². Following thermal annealing at 1000 °C in forming gas, partial recovery in the optical absorbance and Raman spectra is observed at all fluences studied, with 100% recovery observed in samples exposed to a fluence less than 5 × 10¹³ ions/cm². By comparison, annealing yields near complete recovery of the electrical conductivity at all fluences studied (up to 1 × 10¹⁵ ions/cm²). Remarkably, radiation exposure up to a fluence of 1 × 10¹⁴ ions/cm² followed by thermal annealing improves the electrical conductivity, exceeding the as-purified value by as much as ∼4×. These results implicate the origin of the enhanced SWCNT network conductance with the formation of transport-enhancing inter-SWCNT bridges that decrease inter-SWCNT junction resistance, thereby enhancing the overall network connectivity

Nitrogen-Doped Graphene and Twisted Bilayer Graphene <i>via</i> Hyperthermal Ion Implantation with Depth Control

We investigate hyperthermal ion implantation (HyTII) as a means for substitutionally doping layered materials such as graphene. In particular, this systematic study characterizes the efficacy of substitutional N-doping of graphene using HyTII over an N⁺ energy range of 25–100 eV. Scanning tunneling microscopy results establish the incorporation of N substituents into the graphene lattice during HyTII processing. We illustrate the differences in evolution of the characteristic Raman peaks following incremental doses of N⁺. We use the ratios of the integrated D and D′ peaks, <i>I</i>(D)/<i>I</i>(D′) to assess the N⁺ energy-dependent doping efficacy, which shows a strong correlation with previously reported molecular dynamics (MD) simulation results and a peak doping efficiency regime ranging between approximately 30 and 50 eV. We also demonstrate the inherent monolayer depth control of the HyTII process, thereby establishing a unique advantage over other less-specific methods for doping. We achieve this by implementing twisted bilayer graphene (TBG), with one layer of isotopically enriched ¹³C and one layer of natural ¹²C graphene, and modify only the top layer of the TBG sample. By assessing the effects of N-HyTII processing, we uncover dose-dependent shifts in the transfer characteristics consistent with electron doping and we find dose-dependent electronic localization that manifests in low-temperature magnetotransport measurements

Tunable Radiation Response in Hybrid Organic–Inorganic Gate Dielectrics for Low-Voltage Graphene Electronics

Solution-processed semiconductor and dielectric materials are attractive for future lightweight, low-voltage, flexible electronics, but their response to ionizing radiation environments is not well understood. Here, we investigate the radiation response of graphene field-effect transistors employing multilayer, solution-processed zirconia self-assembled nanodielectrics (Zr-SANDs) with ZrO_<i>x</i> as a control. Total ionizing dose (TID) testing is carried out in situ using a vacuum ultraviolet source to a total radiant exposure (RE) of 23.1 μJ/cm². The data reveal competing charge density accumulation within and between the individual dielectric layers. Additional measurements of a modified Zr-SAND show that varying individual layer thicknesses within the gate dielectric tuned the TID response. This study thus establishes that the radiation response of graphene electronics can be tailored to achieve a desired radiation sensitivity by incorporating hybrid organic–inorganic gate dielectrics