Uncovering edge states and electrical inhomogeneity in MoS ₂ field-effect transistors

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 TMD applications in electronics and photonics. Because of the imperfections, electrons moving on these 2D crystals experience a spatially nonuniform 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 MoS 2 field-effect transistors 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 MoS 2 flakes, which can be explained by our firstprinciples 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.MoS 2 | microwave impedance microscopy | edge states | electrical inhomogeneity | metal-insulator transition E lectrostatic gating in the field-effect transistor (FET) configuration has played an essential role in the blooming field of semiconducting transition metal dichalcogenides (TMDs) such as MoS 2 and WSe 2 (1). The electrical control of carrier densities in these naturally formed 2D sheets is crucial for the realization of many intriguing phenomena, such as the metal−insulator transition (2-6), novel spin and valley physics (7-12), and superconducting phases (13-15). In addition, the carrier modulation provides an ideal tuning parameter to study the screening effect, which is particularly important for charge transport in 2D materials that are highly susceptible to local variations of the disorder potential (2-5, 16, 17). As a result, a complete understanding of the electronic properties of TMD FETs at all length scales, i.e., from local defects in the atomic scale, to electronic inhomogeneity in the mesoscale, to device performance in the macroscale, is imperative for both fundamental research on and practical applications of these fascinating materials.Transport and most optical measurements on TMD FETs are inherently macroscopic in nature, in which the sample response is averaged over large areas. TMD films in actual devices, however, are far from electronically uniform. Due to the relatively large amount of intrinsic defects and the inevitable charged states in the substrates, mesoscopic electrical inhomogeneity is not uncommon in TMDs, leading to hopping transport and percolation transition in the devices (6,(16)(17)(18)(19). Little is known, however, about...

Edge states and spin-valley edge photocurrent in transition metal dichalcogenide monolayers

“…Therefore ESs are described by a single parameter a for both spins and valleys (we suppressed spin and valley indexes). However, obtained results are also valid when the two spinpolarised ES branches are described by different parameters a +1,τ = a −1,τ (8), while one can populate only one of them in each valley. We will regard that a ∼ 1, so that ES spectra intersect the gap as shown on Fig.3.…”

One-Dimensional Edge Contacts to a Monolayer Semiconductor

“…This is the most critical part of our fabrication process. As discussed in the main text, etched MoS 2 edges have a large number of dangling bonds that can host in-gap edge states 6 and act as adsorption sites for air molecules (O 2 , H 2 O). These adsorbed species could likely hinder covalent bonding between the contact metal (Ti) and MoS 2 , leaving edge states unpassivated, which subsequently act as traps for injected carriers.…”

Local characterization of mobile charge carriers by two electrical AFM modes: multi-harmonic EFM versus sMIM

“…Considering the following parts are also use MoS 2 as sample, the lumped element model is also applicable for larger than that of three layers (3L) MoS 2 . Besides the influence of layer thickness, local carrier concentration may be influenced by interfacial impurities, strain, disorders and charge transfer [42,43]. Figures 4(d)-(f) are dC/dV amplitude images with different tip bias V G , which are taken in sMIM mode.…”

“…The C-V curves indicate that the carrier concentration of MoS 2 flakes is non-uniform. The carrier concentration difference may be resulting from strain, interfacial impurities, disorders and charge transfer [42,43]. Figures 6(d)-(f) are dC/dV amplitude images with different tip bias V G taken in sMIM mode.…”