Interface contact and modulated electronic properties by in-plain strains in a graphene-MoS2 heterostructure

Designing a specific heterojunction by assembling suitable two-dimensional (2D) semiconductors has shown significant potential in next-generation micro-nano electronic devices. In this paper, we study the structural and electronic properties of graphene-MoS2 (Gr-MoS2) heterostructures with in-plain biaxial strain using density functional theory. It is found that the interaction between graphene and monolayer MoS2 is characterized by a weak van der Waals interlayer coupling with the stable layer spacing of 3.39 Å and binding energy of 0.35 J m-2. In the presence of MoS2, the linear bands on the Dirac cone of graphene are slightly split. A tiny band gap about 1.2 meV opens in the Gr-MoS2 heterojunction due to the breaking of sublattice symmetry, and it could be effectively modulated by strain. Furthermore, an n-type Schottky contact is formed at the Gr-MoS2 interface with a Schottky barrier height of 0.33 eV, which can be effectively modulated by in-plane strain. Especially, an n-type ohmic contact is obtained when 6% tensile strain is imposed. The appearance of the non-zero band gap in graphene has opened up new possibilities for its application and the ohmic contact predicts the Gr-MoS2 van der Waals heterojunction nanocomposite as a competitive candidate in next-generation optoelectronics and Schottky devices.

High performance sub-bandgap photodetection via internal photoemission based on ideal metal/2D-material van der Waals Schottky interface

Two-dimensional (2D) materials have been demonstrated to be promising candidates to design high performance photodetectors owing to their strong light-matter interaction. However, the performance of 2D material photodetectors is still unsatisfactory, such as slow response speed due to defects and vulnerable contact interface, which impede their rapid development in the field of optoelectronics. In this paper, we obtained the ideal and large photosensitive van der Waals Schottky interface by the laminating-flipping method. Hence, a fast response speed (<1 ms) and high detectivity (>10¹² Jones) are observed on the van der Waals Schottky junction photodiode. More importantly, benefiting from the flat Schottky interface (the roughness ∼0.6 nm), a sub-bandgap light response modulated by the Schottky barrier height (cut-off edge at 1050 nm) has been detected based on the large Au/MoSe₂ sensitive Schottky interface internal photoemission. As a result, a universal strategy for the sub-bandgap near-infrared van der Waals Schottky junction detector of 2D materials was obtained.

Electrical contacts to few-layer MoS2 with phase-engineering and metal intercalation for tuning the contact performance

Due to Fermi-level pinning in metal-two-dimensional MoS₂ junctions, improving the performance of MoS₂-based electrical devices is still under extensive study. The device performance of few-layer MoS₂ depends strongly on the number of layers. In this work, via density-functional theory calculations, a comprehensive understanding from the atomistic view was reached for the interlayer interaction between metal and few-layer MoS₂ with phase-engineering and intercalation doping, which are helpful for improving the contact performance. These two methods are probed to tune the performance of few-layer MoS₂-based field-effect transistors, and both of them can tune the Schottky barrier height. Phase-engineering, which means that the MoS₂ layer in contact with metal is converted to the T phase, can transform the Schottky barrier from n- to p-type. Intercalation doping, which takes advantage of annealing and results in metal atom interaction in between MoS₂ layers, makes the MoS₂ layers become quasi-freestanding and converts the indirect bandgap into direct bandgap. Our atomistic insights help improve the performance of few-layer MoS₂-based electronic devices.

Direct Optoelectronic Imaging of 2D Semiconductor-3D Metal Buried Interfaces

The semiconductor-metal junction is one of the most critical factors for high-performance electronic devices. In two-dimensional (2D) semiconductor devices, minimizing the voltage drop at this junction is particularly challenging and important. Despite numerous studies concerning contact resistance in 2D semiconductors, the exact nature of the buried interface under a three-dimensional (3D) metal remains unclear. Herein, we report the direct measurement of electrical and optical responses of 2D semiconductor-metal buried interfaces using a recently developed metal-assisted transfer technique to expose the buried interface, which is then directly investigated using scanning probe techniques. We characterize the spatially varying electronic and optical properties of this buried interface with <20 nm resolution. To be specific, potential, conductance, and photoluminescence at the buried metal/MoS₂ interface are correlated as a function of a variety of metal deposition conditions as well as the type of metal contacts. We observe that direct evaporation of Au on MoS₂ induces a large strain of ∼5% in the MoS₂ which, coupled with charge transfer, leads to degenerate doping of the MoS₂ underneath the contact. These factors lead to improvement of contact resistance to record values of 138 kΩ μm, as measured using local conductance probes. This approach was adopted to characterize MoS₂-In/Au alloy interfaces, demonstrating contact resistance as low as 63 kΩ μm. Our results highlight that the MoS₂/metal interface is sensitive to device fabrication methods and provide a universal strategy to characterize buried contact interfaces involving 2D semiconductors.

Electron Irradiation of Metal Contacts in Monolayer MoS2 Field-Effect Transistors

Metal contacts play a fundamental role in nanoscale devices. In this work, Schottky metal contacts in monolayer molybdenum disulfide (MoS2) field-effect transistors are investigated under electron beam irradiation. It is shown that the exposure of Ti/Au source/drain electrodes to an electron beam reduces the contact resistance and improves the transistor performance. The electron beam conditioning of contacts is permanent, while the irradiation of the channel can produce transient effects. It is demonstrated that irradiation lowers the Schottky barrier at the contacts because of thermally induced atom diffusion and interfacial reactions. The simulation of electron paths in the device reveals that most of the beam energy is absorbed in the metal contacts. The study demonstrates that electron beam irradiation can be effectively used for contact improvement through local annealing.

Insights into Multilevel Resistive Switching in Monolayer MoS2

The advent of two-dimensional materials has opened a plethora of opportunities in accessing ultrascaled device dimensions for future logic and memory applications. In this work, we demonstrate that a single layer of large-area chemical vapor deposition-grown molybdenum disulfide (MoS₂) sandwiched between two metal electrodes can be tuned to show multilevel nonvolatile resistive memory states with resistance values separated by 5 orders of magnitude. The switching process is unipolar and thermochemically driven requiring significant Joule heating in the reset process. Temperature-dependent electrical measurements coupled with semiclassical charge transport models suggest that the transport in these devices varies significantly in the initial (pristine) state, high resistance state, and low resistance state. In the initial state, the transport is a one-step direct tunneling (at low voltage biases) and Fowler Nordeim tunneling (at higher bias) with an effective barrier height of 0.33 eV, which closely matches the Schottky barrier at the MoS₂/Au interface. In the high resistive state, trap-assisted tunneling provides a reasonable fit to experimental data for a trap height of 0.82 eV. Density functional theory calculations suggest the possibility of single- and double-sulfur vacancies as the microscopic origins of these trap sites. The temperature-dependent behavior of the set and reset process are explained by invoking the probability of defect (sulfur vacancy) creation and mobility of sulfur ions. Finally, conductive atomic force microscopy measurements confirm that the multifilamentary resistive memory effects are inherent to a single-crystalline MoS₂ triangle and not necessarily dependent on grain boundaries. The insights suggested in this work are envisioned to open up possibilities for ultrascaled, multistate, resistive memories for next-generation digital memory and neuromorphic applications.