Uncovering edge states and electrical inhomogeneity in MoS2 field-effect transistors

TL;DR

This study uses microwave impedance microscopy to map conductance in MoS2 transistors, revealing edge conduction initially, inhomogeneity due to disorder, and providing insights for improving device performance.

Contribution

It uncovers the spatial evolution of conductance in MoS2 FETs, highlighting the role of edge states and disorder in charge transport through mesoscopic conductance mapping.

Findings

Edge conduction appears before bulk conduction during device turn-on.

Edge states significantly contribute to conductance near threshold voltage.

Conductance inhomogeneity correlates with disorder potential fluctuations.

Abstract

The understanding of various types of disorders in atomically thin transition metal dichalcogenides (TMDs), including dangling bonds at the edges, chalcogen deficiencies in the bulk, and charges in the substrate, is of fundamental importance for their applications in electronics and photonics. Because of the imperfections, electrons moving on these two-dimensional (2D) crystals experience a spatially non-uniform Coulomb environment, whose effect on the charge transport has not been microscopically studied. Here, we report the mesoscopic conductance mapping in monolayer and few-layer MoS2 field-effect transistors (FETs) by microwave impedance microscopy (MIM). The spatial evolution of the insulator-to-metal transition is clearly resolved. Interestingly, as the transistors are gradually turned on, electrical conduction emerges initially at the edges before appearing in the bulk of MoS2 flakes, which can be explained by our first-principles calculations. The results unambiguously confirm that the contribution of edge states to the channel conductance is significant under the threshold voltage but negligible once the bulk of the TMD device becomes conductive. Strong conductance inhomogeneity, which is associated with the fluctuations of disorder potential in the 2D sheets, is also observed in the MIM images, providing a guideline for future improvement of the device performance.