Atomically-thin Ohmic Edge Contacts Between Two-dimensional Materials

Abstract: With the decrease of the dimensions of electronic devices, the role played by electrical contacts is ever increasing, eventually coming to dominate the overall device volume and total resistance. This is especially problematic for monolayers of semiconducting transition metal dichalcogenides (TMDs), which are promising candidates for atomically thin electronics. Ideal electrical contacts to them would require the use of similarly thin electrode materials while maintaining low contact resistances. Here we report a scalable method to fabricate ohmic graphene edge contacts to two representative monolayer TMDs - MoS2 and WS2. The graphene and TMD layer are laterally connected with wafer-scale homogeneity, no observable overlap or gap, and a low average contact resistance of 30 k$\Omega$ $\mu$m. The resulting graphene edge contacts show linear current-voltage (IV) characteristics at room temperature, with ohmic behavior maintained down to liquid helium temperatures.

• The paper demonstrates a scalable technique for lateral stitching of graphene with TMDs, achieving low contact resistance (~30 kΩµm) for efficient carrier injection.
• It employs precise CVD and MOCVD processes to form one-dimensional edge contacts, minimizing van der Waals gaps and multilayer issues.
• The findings pave the way for integrating atomically-thin contacts in flexible and nanoscale electronics, promising enhanced device performance.

Overview of Atomically-thin Ohmic Edge Contacts Between Two-dimensional Materials

The paper "Atomically-thin Ohmic Edge Contacts Between Two-dimensional Materials" addresses the essential challenge of engineering low-resistance, high-efficiency electrical contacts for two-dimensional (2D) semiconductor materials, specifically transition metal dichalcogenides (TMDs) like MoS₂ and WS₂. The work introduces a scalable fabrication method for ohmic edge contacts using monolayer graphene electrodes, thereby offering significant advancements in the field of atomically-thin electronics.

Introduction and Problem Definition

As the dimensions of electronic devices continue to shrink, the efficiency and integration of contacts become increasingly critical. This is particularly crucial for monolayer TMDs, which represent a promising class of materials for future electronics due to their atomic thickness and semiconducting properties. Traditional top contacts using 3D metallic electrodes impose limitations due to their volume and potential van der Waals gaps, which elevate contact resistance. Thus, optimizing the contact interface is imperative to fully leverage the potential of 2D materials.

Experimental Approach

The authors present a technique involving the lateral stitching of graphene with TMDs to form heterostructures with one-dimensional graphene (1DG) edge contacts. The method utilizes chemical vapor deposition (CVD) for graphene, combined with metal-organic CVD (MOCVD) for the TMD layer, ensuring precise control over the growth environment to avoid overlap or multilayer regions. Through this approach, they achieve low average contact resistance values of approximately 30 kΩµm, with stable ohmic behavior extending to low temperatures.

Results and Discussion

The research highlights key outcomes including:

• Graphene edge contacts promoting efficient carrier injection into TMDs without significant volume or overlap, leading to superior performance metrics compared to conventional top contacts.
• Empirical comparisons reveal that graphene electrodes maintain lower contact resistance than traditional contacts despite the reduced volume, substantiated by a higher two-probe conductance and enhanced field-effect mobility.
• Through a systematic study, the paper delineates the influence of process parameters, especially the precursor partial pressures during MOCVD, to prevent TMD nucleation on graphene, thereby optimizing the contact interfaces.

The study includes comprehensive characterization using dark-field electron microscopy (DF-TEM) and various spectroscopic techniques to confirm the uniformity and structural integrity of these lateral heterostructures across the substrate. The robustness of the contact behavior is verified over multiple independent samples and different experimental runs.

Implications and Future Prospects

The development of one-dimensional ohmic edge contacts is poised to significantly impact the field of nanoscale electronics. The presented methodology not only reduces contact resistance but also minimizes electrode volume, which is crucial for the integration of flexible and optically transparent electronic devices. These findings can facilitate further miniaturization of electronic components without sacrificing performance.

Future research could expand on enhancing the versatility of this contact design with various TMDs and explore integration with other 2D materials or advanced optoelectronic devices. The implications extend to potential innovations in wearable technology, sensor networks, and quantum devices, driven by ongoing developments in the precision fabrication of atomically-thin circuitry.

In conclusion, the paper convincingly demonstrates the viability and advantages of using graphene edge contacts for monolayer TMDs, paving the way for advancing 2D material-based electronics with improved efficiency and scalability.