Two-dimensional (2D) materials offer exciting possibilities for numerous applications, including next-generation sensors and field-effect transistors (FETs). With their atomically thin form factor, it is evident that molecular activity at the interfaces of 2D materials can shape their electronic properties. Although much attention has focused on engineering the contact and dielectric interfaces in 2D material-based transistors to boost their drive current, less is understood about how to tune these interfaces to improve the long-term stability of devices. In this work, we evaluated molybdenum disulfide (MoS 2 ) transistors under continuous electrical stress for periods lasting up to several days. During stress in ambient air, we observed temporary threshold voltage shifts that increased at higher gate voltages or longer stress durations, correlating to changes in interface trap states (ΔN it ) of up to 10 12 cm −2 . By modifying the device to include either SU-8 or Al 2 O 3 as an additional dielectric capping layer on top of the MoS 2 channel, we were able to effectively reduce or even eliminate this unstable behavior. However, we found this encapsulating material must be selected carefully, as certain choices actually amplified instability or compromised device yield, as was the case for Al 2 O 3 , which reduced yield by 20% versus all other capping layers. Further refining these strategies to preserve stability in 2D devices will be crucial for their continued integration into future technologies.

Tailoring the properties of two-dimensional (2D) crystals is important for both understanding the material behavior and exploring new functionality. Here we demonstrate the alteration of MoS 2 and metal-MoS 2 interfaces using a convergent ion beam. Different beam energies, from 60 eV to 600 eV, are shown to have distinct effects on the optical and electrical properties of MoS 2. Defects and deformations created across different layers were investigated, revealing an unanticipated improvement in the Raman peak intensity of multilayer MoS 2 when exposed to a 60 eV Ar + ion beam, and attenuation of the MoS 2 Raman peaks with a 200 eV ion beam. Using cross-sectional scanning transmission electron microscopy (STEM), alteration of the crystal structure after a 600 eV ion beam bombardment was observed, including generated defects and voids in the crystal. We show that the 60 eV ion beam yields improvement in the metal-MoS 2 interface by decreasing the contact resistance from 17.5 kΩ • µm to 6 kΩ • µm at a carrier concentration of n 2D = 5.4 × 10 12 cm −2. These results advance the use of low-energy ion beams to modify 2D materials and interfaces for tuning and improving performance in applications of sensors, transistors, optoelectronics, and so forth.

Two-dimensional (2D) van der Waals materials are subject to mechanical deformation and thus forming bubbles and wrinkles during exfoliation and transfer. A lack of interfacial “flatness” has implications for interface properties, such as those formed by metal contacts or insulating layers. Therefore, an understanding of the detailed properties of 2D interfaces, especially their flatness under different conditions, is of high importance. Here we use cross-sectional scanning transmission electron microscopy (STEM) to investigate various 2D interfaces (2D-2D and 3D-2D) under the effects of stacking, atomic layer deposition (ALD), and metallization. We characterize and compare the flatness of the hBN-2D and metal-2D interfaces down to angstrom resolution. It is observed that the dry transfer of hexagonal boron nitride (hBN) can dramatically alter the interface structure. When characterizing 3D metal-2D interfaces, we find that Ni-MoS2 interfaces are more uneven and have larger nanocavities compared to other metal-2D interfaces. The electrical characteristics of a MoS2-based field-effect transistor are correlated to the interfacial transformation in the contact and channel regions. The device transconductance is improved by 40% after the hBN encapsulation, likely due to the interface interactions at both the channel and contacts. Overall, these observations reveal the intricacy of 2D interfaces and their dependence on the fabrication processes.

The research community has invested heavily in semiconducting two-dimensional (2D) materials, such as transition metal dichalcogenides (TMDs). Their stability when scaled down to a few atoms thick makes them attractive candidates to replace or supplement silicon in many future technologies. Although this sentiment is prevalent, demonstrations of 2D field-effect transistors (FETs) often do not present their data in a way that enables a straightforward comparison. For example, some papers solely use mobility as the figure of merit, while others focus on unnormalized device on-current. Here, we benchmark the performance of a selection of 2D FETs with field-corrected metrics that allow a more accurate projection of their potential; while the demonstrated methods are by no means comprehensive, they provide insight into improved benchmarking of 2D FETs going forward. Importantly, we show that appropriate benchmarking requires consideration of the specific application, with the three dominant potential application areas of front-end-of-line (FEOL) high-performance FETs, back-end-of-line (BEOL) 3D-integrated FETs, and low-cost thin-film FETs (or TFTs) each demonstrated. We find that 2D materials have the potential to compete with silicon as the channel in scaled FEOL high-performance devices. Meanwhile, in BEOL applications, FETs from in situ synthesized 2D materials have performance limited by their low crystal quality – a result of the stringent thermal budget of BEOL fabrication, which necessitates the use of transferred 2D materials. In the TFT area, 2D materials are simpler to fabricate than their silicon-based counterparts and they are competitive with other material alternatives. As promising as these findings are, there remain many hurdles for 2D materials to overcome, including poor reliability, performance variability, and fabrication scalability. Continuous research effort, combined with appropriate benchmarking, is strongly encouraged.

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