Asymmetric Chemical Functionalization of Top‐Contact Electrodes: Tuning the Charge Injection for High‐Performance MoS₂ Field‐Effect Transistors and Schottky Diodes

Abstract

The fabrication of high‐performance (opto‐)electronic devices based on 2D channel materials requires the optimization of the charge injection at electrode–semiconductor interfaces. While chemical functionalization with chemisorbed self‐assembled monolayers has been extensively exploited to adjust the work function of metallic electrodes in bottom‐contact devices, such a strategy has not been demonstrated for the top‐contact configuration, despite the latter being known to offer enhanced charge‐injection characteristics. Here, a novel contact engineering method is developed to functionalize gold electrodes in top‐contact field‐effect transistors (FETs) via the transfer of chemically pre‐modified electrodes. The source and drain Au electrodes of the molybdenum disulfide (MoS2) FETs are functionalized with thiolated molecules possessing different dipole moments. While the modification of the electrodes with electron‐donating molecules yields a marked improvement of device performance, the asymmetric functionalization of the source and drain electrodes with different molecules with opposed dipole moment enables the fabrication of a high‐performance Schottky diode with a rectification ratio of ≈103. This unprecedented strategy to tune the charge injection in top‐contact MoS2 FETs is of general applicability for the fabrication of high‐performance (opto‐)electronic devices, in which asymmetric charge injection is required, enabling tailoring of the device characteristics on demand.

Recent Progress in Contact Engineering of Field-Effect Transistor Based on Two-Dimensional Materials

“…Han et al reported the use of molecular functionalization to change the work function of gold electrodes. Then, they fabricated top-contact FETs via the transfer of these pre-modified electrodes to tune the charge injection in MoS 2 FETs [ 32 ], demonstrating the modulation of the Schottky barrier. This method has also been used in 2D-materials-based resistive random-access memory, leading to a stable resistive switching performance [ 33 ].…”

Isomer Discrimination via Defect Engineering in Monolayer MoS2

“…Figure S2a,d show the optical microscopy images of monolayer MoS 2 , while Figure S1 displays the AFM topographical image. The pristine monolayer MoS 2 crystal exhibits an atomically flat surface with a root-mean-square roughness ( R RMS ) of approximately 0.5 nm, due to its easy-to-peel van der Waals layered structure and the absence of dangling bonds on the surface. , Figure S2b and e confirm the absence of changes in the thickness and roughness of MoS 2 upon the introduction of defects. However, after molecular functionalization, a considerable increase in the average thickness of the MoS 2 monolayer has been observed.…”

“…We counted dozens of devices and found that after functionalizing with 26DFBT, the V th decreased by 20.3 ± 6.3 V, while the V th increased by 18.8 ± 8.5 V after functionalizing with 35DFBT. The charge in the carrier density change (Δ n ) within the channel can be quantified by the change of V th with the equation below: Δ n = C o x Δ V th e = ε o x Δ V th t o x e = 7.99 × 10 10 Δ V th cm − 2 where C ox is the capacitance per unit area of dielectric layer, Δ V th is the change of V th , e is the elementary charge, ε ox is the dielectric constant of the dielectric layer, and t ox is the thickness of dielectric layer. The value of Δ n is indicative of the molecule’s dipole-induced doping capability.…”

Selective Isolation of Mono- to Quadlayered 2D Materials via Sonication-Assisted Micromechanical Exfoliation

“…In another device with a trilayer flake with 7 fingered device, as shown in Figure S12, also showed a gate dependency with the longitudinal resistance R xx = 108–119 kΩ at V G = 30 V. The resistance is relatively small and the estimated contact resistance is ∼100 kΩ. The issues on the high contact resistance are still in challenge to realize 2D electronics. − …”