Scaling limits and generality of asymmetrical metal-contact scaling in MoS2 field-effect transistors

As transistors based on two-dimensional (2D) semiconductors such as monolayer MoS2 approach the 10 nm regime, metal contacts — not the channel — increasingly limit performance. The source study (Cheng et al., Advanced Materials 2023; preprint arXiv:2209.04144) introduced asymmetrical contact measurements (ACMs), which compare electron injection through contacts of different lengths sharing one identical channel, and found an experimental transfer length $L_{\mathrm{T}}<$ 30 nm (interface-quality dependent) versus a clean-interface quantum-transport prediction of about 5 nm, a marked asymmetry between scaled source and scaled drain contacts, negative differential resistance (NDR) when the short contact serves as the source, and a yield drop below $L_{\mathrm{c}}\approx 40$ nm. The authors posed a connected set of open questions: the ultimate experimental lower bound of $L_{\mathrm{T}}$ and its intrinsic-versus-extrinsic origin; whether the source/drain scaling asymmetry generalizes to other contact metals and to p-type channels; the transport mechanisms at scaled contacts (temperature-dependent behavior and Schottky-barrier heights versus contact length); whether NDR can be suppressed by pulsed or frequency-domain measurement; contact simulations including dissipation, self-heating, and interface disorder; systematic contact-gating effects; and manufacturable yield of scaled contacts. A 2026 Nature study has since directly measured $L_{\mathrm{T}}\approx 2$–3 nm for bismuth-contacted monolayer MoS2 by operando cross-sectional scanning tunneling microscopy, resolving the headline lower-bound question for an optimized semimetal contact, and contact gating has been quantified; the generality of the asymmetrical scaling behavior, dynamic NDR suppressibility, temperature-dependent barrier extraction at scaled contacts, and realistic dissipative modeling remain open.

Background

Two-dimensional (2D) semiconductors, such as the transition-metal dichalcogenide molybdenum disulfide (MoS2), are atomically thin channel candidates for field-effect transistors (FETs) because their thickness suppresses short-channel effects — the loss of gate control as device dimensions shrink. Device scaling, however, requires shrinking every dimension, including the metal contacts. When current is injected from a metal electrode into a semiconductor channel, most of the injection occurs near the edge of the contact over a characteristic distance called the transfer length $L_{\mathrm{T}}$ (a phenomenon known as current crowding). If the physical contact length $L_{\mathrm{c}}$ falls below $L_{\mathrm{T}}$, the contact can no longer deliver the full current and the contact resistance rises sharply, so $L_{\mathrm{T}}$ sets the ultimate footprint limit of the contact. $L_{\mathrm{T}}$ is classically extracted with the transfer length method (TLM), which fits contact resistance and channel resistivity measured across a series of devices with different geometries — a method that suffers from device-to-device variation, and reported $L_{\mathrm{T}}$ values for MoS2 spanned a wide and inconsistent range. The metal–MoS2 interface usually forms a Schottky barrier, a potential barrier for charge injection whose height controls thermionic emission, and contacts made of semimetals such as bismuth (Bi) or antimony (Sb) were shown (Shen et al., Nature 2021) to reach nearly barrier-free, ultralow-resistance contacts on monolayer MoS2. Cheng et al. (Advanced Materials 2023; preprint arXiv:2209.04144) introduced the asymmetrical contact measurement (ACM): a device in which the same MoS2 channel is contacted on one side by a long reference contact and on the other by a scaled contact (a short contact; in the source study, $L_{\mathrm{c}}$ below $\approx 40$ nm, the regime in which device yield and high-bias performance degrade), so that swapping source and drain directly compares injection through different $L_{\mathrm{c}}$ on the exact same channel, eliminating channel-to-channel variation. They found that devices with short contacts (Ni contacts fabricated with $L_{\mathrm{c}}$ decreasing from 120 nm down to 30 nm) show larger variation, smaller drain currents at high drain–source voltage, and — when the short contact serves as the source — frequent early saturation and negative differential resistance (NDR, a regime in which the drain current decreases as the drain voltage increases). Their ballistic quantum-transport simulation (density-functional-theory-based, no dissipation, clean interface) predicted $L_{\mathrm{T}}$ as short as about 5 nm for the Ni–MoS2 interface, while the experimental behavior indicated $L_{\mathrm{T}}<$ 30 nm with a strong dependence on local metal–2D interface quality. Crucially, charge injection proved asymmetric: a scaled source contact limits the drain current, whereas a scaled drain contact does not. Whether the gate overlaps the contacts also matters — contact gating, the modulation of the semiconductor underneath the source/drain metal by the gate potential, changes the effective injection barrier but had not been systematically quantified for scaled 2D transistors.

Problem Statement

Determine the ultimate scaling limit and the generality of the contact-scaling physics of metal contacts to 2D-semiconductor transistors, as posed by the asymmetrical contact measurements of Cheng et al. (Adv. Mater. 2023). Parts (a)–(g) are independent sub-goals, each judged on its own rather than jointly required: a part counts as resolved only when its stated measurement, experiment, or simulation is carried out and reported, and where a part notes an already-reported partial result, resolving it requires going beyond that result (for (a), bounding conventional-metal contacts with disordered interfaces; for (f), extending beyond monolayer-MoS2 dual-gate studies to other materials and geometries; for (g), establishing a general, high-yield, low-variability process rather than an individual demonstration). Concretely: (a) establish the experimental lower bound of the carrier transfer length $L_{\mathrm{T}}$ and its intrinsic versus extrinsic (interface-quality) origin — since substantially advanced by a direct operando cross-sectional scanning-tunneling-microscopy (STM) measurement (imaging the contact cross-section of a working, biased device) giving $L_{\mathrm{T}}\approx 2$–3 nm for bismuth contacts on monolayer MoS2, which settles the optimized-semimetal case but leaves conventional-metal contacts with disordered interfaces to be bounded; (b) test whether the asymmetric scaling behavior — a scaled source contact limiting drain current while a scaled drain contact does not — generalizes across contact metals, semimetals, and two-dimensional van der Waals (2D-vdW) contact materials (electrodes made of layered crystals whose surfaces bond to the channel without dangling bonds), and to p-type 2D transistors; (c) characterize the transport mechanisms at scaled contacts through temperature-dependent measurements (below 300 K) and Schottky-barrier-height extraction as a function of contact length $L_{\mathrm{c}}$; (d) determine experimentally whether the negative differential resistance observed when the scaled contact serves as the source can be suppressed by pulsed or frequency-domain measurement, i.e., whether it is dynamically suppressible; (e) extend quantum-transport contact simulations to include dissipation, self-heating, and realistic interface disorder so that the clean-interface prediction ($L_{\mathrm{T}}\approx 5$ nm for Ni–MoS2) is bridged to variable real interfaces; (f) quantify systematically how contact gating and substrate effects modify contact-scaling behavior for gated versus ungated contacts (largely addressed for monolayer MoS2 by dual-gate studies, remaining open for other materials and geometries); and (g) achieve manufacturable, high-yield, low-variability scaled contacts (substantially advanced by composite-electrode and ultrasonic-scaled-contact demonstrations, not yet general).

References

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