Sub-10 nm two-dimensional transistors: Theory and experiment

Ultrathin 2D semiconductor devices are considered to have beyond‐silicon potential but are severely troubled by the high Schottky barriers of the metal–semiconductor contacts, especially for p‐type semiconductors. Due to the severe Fermi‐level pinning effect and the lack of conventional semimetals with high work functions, their Schottky hole barriers are hardly removed. Here, an all‐van‐der‐Waals barrier‐free hole contact between p‐type tellurene semiconductor and layered 1T′‐WS2 semimetal is reported, which achieves a zero Schottky barrier height of 3 ± 9 meV and a high field‐effect mobility of ≈1304 cm2 V–1 s–1. The formation of such contacts can be attributed to the higher work function of ≈4.95 eV of the 1T′‐WS2 semimetal, which is in sharp contrast with low work function (4.1–4.7 eV) of conventional semimetals. The study defines an available strategy for eliminating the Schottky barrier of metal–semiconductor contacts, facilitating 2D‐semiconductor‐based electronics and optoelectronics to extend Moore's law.

All‐van‐der‐Waals Barrier‐Free Contacts for High‐Mobility Transistors

Recently, a novel two-dimensional (2D) MoSi2N4 has been successfully synthesized and features high carrier mobility, moderate bandgap, and outstanding ambient stability (Science 369, 670, 2020). Through ab initio quantum transport simulations, we investigated the performance limits of the monolayer (ML) MoSi2N4 metal-oxide-semiconductor field-effect transistors (MOSFETs). We found that the optimized n- and p-type ML MoSi2N4 transistors can well satisfy the key criteria of the International Technology Roadmap for Semiconductors (ITRS) for high performance (HP) applications at a 3 nm gate length, while the p-type devices can also fulfill the requirement of low power (LP) applications at a 5 nm gate length. By taking advantage of the negative capacitance (NC) effect, both the n- and p-type MOSFETs can achieve the ITRS LP goal when the gate length is decreased to 3 nm. The ML MoSi2N4 MOSFETs generally outperform their MoS2 counterparts. Hence, MoSi2N4 is a potential alternative to MoS2 to expand Moore's law beyond the sub-5 nm scale.

Performance limit of monolayer MoSi2N4 transistors

The deployment of two-dimensional (2D) materials for solar energy conversion requires scalable large-area devices. Here, we present the design, modeling, fabrication, and characterization of monolayer MoS2-based lateral Schottky-junction photovoltaic (PV) devices grown by using chemical vapor deposition (CVD). The device design consists of asymmetric Ti and Pt metal contacts with a work function offset to enable charge separation. These early stage devices show repeatable performance under 1 sun illumination, with VOC of 160 mV, JSC of 0.01 mA/cm2, power conversion efficiency of 0.0005%, and specific power of 1.58 kW/kg. An optoelectronic model for this device is developed and validated with experimental results. This model is used to understand loss mechanisms and project optimized device designs. The model predicts that a 2D PV device with ∼70 kW/kg of specific power can be achieved with minimum optimization to the current devices. By increasing the thickness of the absorber layer, we can achieve even higher performance devices. Finally, a 25 mm2 area solar cell made with a 0.65 nm thick MoS2 monolayer is demonstrated, showing VOC of 210 mV under 1 sun illumination. This is the first demonstration of a large-area PV device made with CVD-grown scalable 2D materials.

Large-Area, High-Specific-Power Schottky-Junction Photovoltaics from CVD-Grown Monolayer MoS2

Flexible electronics and optoelectronics of 2D van der Waals materials

Monolayer two-dimensional (2D) semiconductors are emerging as top candidates for the channels of the future chip industry due to their atomically thin body and superior immunity to short channel effect. However, the low saturation current caused by the high contact resistance (Rc) in monolayer MoS2 field-effect transistors (FETs) limits ultimate electrical performance at scaled contact lengths, which seriously hinders application of monolayer MoS2 transistors. Here we present a scalable strategy with a clean end-bond contact scheme that leads to size-independent electrodes and ultralow contact resistance of 2.5 kΩ·µm to achieve record high performances of saturation current density of 730 µA·µm−1 at 300 K and 960 µA·µm−1 at 6 K. Our end-bond contact strategy in monolayer MoS2 FETs enables the great potential for atomically thin integrated circuitry.

Kuanglei Chen

Papers

Record-high saturation current in end-bond contacted monolayer MoS2 transistors