High-performance monolayer MoS2nanosheet GAA transistor.

In this article, a 0.7 nm thick monolayer MoS2nanosheet gate-all-around field effect transistors (NS-GAAFETs) with conformal high-κmetal gate deposition are demonstrated. The device with 40 nm channel length exhibits a high on-state current density of ~410µAµm-1with a large on/off ratio of 6 × 108at drain voltage = 1 V. The extracted contact resistance is 0.48 ± 0.1 kΩµm in monolayer MoS2NS-GAAFETs, thereby showing the channel-dominated performance with the channel length scaling from 80 to 40 nm. The successful demonstration of device performance in this work verifies the integration potential of transition metal dichalcogenides for future logic transistor applications.

High-Performance WSe2 Top-Gate Devices with Strong Spacer Doping.

Because of the lack of contact and spacer doping techniques for two-dimensional (2D) transistors, most high-performance 2D devices have been produced with nontypical structures that contain electrical gating in the contact regions. In the present study, we used chloroauric acid (HAuCl4) as a strong p-dopant for WSe2 monolayers used in transistors. The HAuCl4-doped devices exhibited a record-low contact resistance of 0.7 kΩ·µm under a doping concentration of 1.76 × 1013 cm-2. In addition, an extrinsic carrier diffusion phenomenon was discovered in the HAuCl4-WSe2 system. With a suitably designed spacer length for doping, a normally off, high-performance underlap top-gate device can be produced without the application of additional gating in the contact and spacer regions.

High-Performance Two-Dimensional Electronics with a Noncontact Remote Doping Method.

Because of the intrinsic low carrier density of monolayer two-dimensional (2D) materials, doping is crucial for the performance of underlap top-gated 2D devices. However, wet etching of a high-k (dielectric constant) dielectric layer is difficult to implement without causing performance deterioration on the devices; therefore, finding a suitable spacer doping technique for 2D devices is indispensable. In this study, we developed a remote doping (RD) method in which defective SiOx can remotely dope the underlying high-k capped 2D regions without directly contacting these materials. This method achieved a doping density as high as 1.4 × 1013 cm-2 without reducing the mobility of the doped materials; after 1 month, the doping concentration remained as high as 1.2 × 1013 cm-2. Defective SiOx can be used to dope most popular 2D transition-metal dichalcogenides. The low-k properties of SiOx render it ideal for spacer doping, which is very attractive from the perspective of circuit operation. In our experiments, MoS2 and WS2 underlap top-gate devices exhibited 10× and 200× increases in their on-currents, respectively, after being doped with SiOx. These results indicate that SiOx doping can be conducted to manufacture high-performance 2D devices.

Statistical Assessment of High-Performance Scaled Double-Gate Transistors from Monolayer WS2.

Scaling of monolayer transition metal dichalcogenide (TMD) field-effect transistors (FETs) is an important step toward evaluating the application space of TMD materials. Although some work on ultrashort channel monolayer (ML) TMD FETs has been published, there exist no comprehensive studies that assess their performance in a statistically relevant manner, providing critical insights into the impact of the device geometry. Part of the reason for the absence of such a study is the substantial variability of TMD devices when processes are not carefully controlled. In this work, we show a statistical study of ultrashort channel double-gated ML WS2 FETs exhibiting excellent device performance and limited device-to-device variations. From a detailed analysis of cross-sectional scanning transmission electron microscopy (STEM) images and careful technology computer aided design (TCAD) simulations, we evaluated, in particular, an unexpected deterioration of the subthreshold characteristics for our shortest devices. Two potential candidates for the observed behavior were identified, i.e., buckling of the TMD on the substrate and loss of gate control due to the source geometry and the high-k dielectric between the metal gate and the metal source electrode.

Ultralow contact resistance between semimetal and monolayer semiconductors.

Advanced beyond-silicon electronic technology requires both channel materials and also ultralow-resistance contacts to be discovered1,2. Atomically thin two-dimensional semiconductors have great potential for realizing high-performance electronic devices1,3. However, owing to metal-induced gap states (MIGS)4-7, energy barriers at the metal-semiconductor interface-which fundamentally lead to high contact resistance and poor current-delivery capability-have constrained the improvement of two-dimensional semiconductor transistors so far2,8,9. Here we report ohmic contact between semimetallic bismuth and semiconducting monolayer transition metal dichalcogenides (TMDs) where the MIGS are sufficiently suppressed and degenerate states in the TMD are spontaneously formed in contact with bismuth. Through this approach, we achieve zero Schottky barrier height, a contact resistance of 123 ohm micrometres and an on-state current density of 1,135 microamps per micrometre on monolayer MoS2; these two values are, to the best of our knowledge, the lowest and highest yet recorded, respectively. We also demonstrate that excellent ohmic contacts can be formed on various monolayer semiconductors, including MoS2, WS2 and WSe2. Our reported contact resistances are a substantial improvement for two-dimensional semiconductors, and approach the quantum limit. This technology unveils the potential of high-performance monolayer transistors that are on par with state-of-the-art three-dimensional semiconductors, enabling further device downscaling and extending Moore's law.