Short-Channel Field Effect Transistors with 9-Atom and 13-Atom wide Graphene Nanoribbons

Abstract: Bottom-up synthesized GNRs and GNR heterostructures have promising electronic properties for high performance field effect transistors (FETs) and ultra-low power devices such as tunnelling FETs. However, the short length and wide band gap of these GNRs have prevented the fabrication of devices with the desired performance and switching behaviour. Here, by fabricating short channel (Lch ~20 nm) devices with a thin, high-k gate dielectric and a 9-atom wide (0.95 nm) armchair GNR as the channel material, we demonstrate FETs with high on-current (Ion >1 uA at Vd = -1 V) and high Ion/Ioff ~10^5 at room temperature. We find that the performance of these devices is limited by tunnelling through the Schottky barrier (SB) at the contacts and we observe an increase in the transparency of the barrier by increasing the gate field near the contacts. Our results thus demonstrate successful fabrication of high performance short-channel FETs with bottom-up synthesized armchair GNRs.

• The paper presents the fabrication and performance evaluation of short-channel FETs using bottom-up synthesized 9- and 13-atom GNRs with band gaps of ~2.10 eV and ~2.35 eV.
• It employs advanced synthesis techniques to achieve atomically precise GNRs with uniform edges, significantly enhancing electronic homogeneity.
• Electrical characterization shows on-currents exceeding 1 μA and improved switching via optimized gating, mitigating Schottky barrier effects.

Short-Channel Field Effect Transistors with Graphene Nanoribbons

This paper explores the fabrication and characterization of short-channel field effect transistors (FETs) utilizing graphene nanoribbons (GNRs) synthesized through a bottom-up approach. The study primarily focuses on the development and performance evaluation of devices that incorporate 9-atom wide armchair GNRs (9AGNRs) and 13-atom wide armchair GNRs (13AGNRs) as channel materials, investigating their potential for integration into high-performance electronic applications.

The research emphasizes the intrinsic properties of GNRs, including their electronic, optical, and magnetic characteristics, which are highly tunable based on their width and edge configurations. Traditional GNR fabrication techniques, like unzipping carbon nanotubes or lithographic approaches, commonly lead to rough edges that degrade electronic properties. In contrast, the bottom-up synthesis method, advanced in this study, offers atomically precise GNRs with uniform width and well-defined edges, fostering significant electronic homogeneity, crucial for the development of GNR-based FETs suitable for digital circuits.

A notable advancement in this work is the synthesis and implementation of 9AGNRs and 13AGNRs, having respective band gaps of approximately 2.10 eV and 2.35 eV. The paper details the synthesis method involving oligo-phenylene oligomers, which polymerize on a Au(111) surface under high vacuum and subsequent cyclodehydrogenation to form planar GNRs. Raman spectroscopy is employed to confirm the structural integrity of the GNRs post-transfer onto insulating substrates.

The fabricated GNRFETs exhibit significant performance metrics. For instance, devices with a ~20 nm channel length, employing a high-k gate dielectric, demonstrate an on-current (I_on) exceeding 1 μA at a drain voltage (V_d) of -1 V. The study identifies tunneling through the Schottky barrier at metal-GNR interfaces as a limiting factor in device performance, with the barrier’s transparency increasing upon enhancing the gate field at contacts—a key observation leading to the demonstration of high performance in these devices.

Electrical characterization of both 9AGNR and 13AGNR devices reveals similar transport characteristics largely due to their comparable band gaps. The observed non-linear I_d-V_d characteristics at low bias confirm the presence of a Schottky Barrier, with tunneling—rather than thermionic emission—as the dominant transport mechanism, supported by weak temperature dependence in current-voltage measurements.

To address the contact resistance issue, the paper explores ionic liquid (IL) gating, specifically using DEME-TFSI, enhancing electrostatic coupling at the Pd-GNR interface. This approach achieves a marked increase in I_on under reduced gate voltages, highlighting improved transistor switching efficiency.

When transitioned to solid gate dielectrics, particularly using thin HfO₂ with an effective oxide thickness of 1.5 nm, the devices maintain high performance, achieving I_on > 1 μA and substantial I_on/I_off ratios. This structure allows the sub-nanometer GNRs to outperform other top-down fabricated GNR devices, presenting them as viable candidates for logic applications.

In conclusion, the paper illustrates the potential of atomically precise, bottom-up synthesized GNRs in FET applications, signifying advancements in low-dimensional material electronics. Future work could focus on overcoming the Schottky barrier limitations further and exploring these structures within more complex or novel device architectures, such as tunneling FETs, leveraging atomically precise GNR heterostructures. The progress in aligning and growing narrow band gap GNRs further amplifies the prospects for integrating such nanostructures into advanced semiconductor technologies.