High-Resolution Van der Waals Stencil Lithography for 2D Transistors

Wafer-scale and universal van der Waals metal semiconductor contact

Van der Waals (vdW) metallic contacts have been demonstrated as a promising approach to reduce the contact resistance and minimize the Fermi level pinning at the interface of two-dimensional (2D) semiconductors. However, only a limited number of metals can be mechanically peeled and laminated to fabricate vdW contacts, and the required manual transfer process is not scalable. Here, we report a wafer-scale and universal vdW metal integration strategy readily applicable to a wide range of metals and semiconductors. By utilizing a thermally decomposable polymer as the buffer layer, different metals were directly deposited without damaging the underlying 2D semiconductor channels. The polymer buffer could be dry-removed through thermal annealing. With this technique, various metals could be vdW integrated as the contact of 2D transistors, including Ag, Al, Ti, Cr, Ni, Cu, Co, Au, Pd. Finally, we demonstrate that this vdW integration strategy can be extended to bulk semiconductors with reduced Fermi level pinning effect.

Strain Releasing of Flexible 2D Electronics through van der Waals Sliding Contact

Two-dimensional (2D) materials have demonstrated promising potential for flexible electronics, owning to their atomic thin body thickness and dangling-bond-free surface. Here, we report a sliding contact device structure for efficient strain releasing. By fabricating a weakly coupled metal-2D junction with a van der Waals (vdW) gap in between, the applied strain could be effectively released through their interface sliding; hence minimized strain is transferred to the 2D lattice. Therefore, we observed stable device behavior with electrodes stretching over 110%, much higher than 2D devices using evaporated metal contacts. Furthermore, through multicycle straining-releasing measurements, we found the electrodes still form intimate contact with nearly constant contact resistance during sliding, confirming the optimization of device flexibility and electrical properties at the same time. Finally, we demonstrate this vdW sliding contact is a general device geometry and could be well-extended to various 2D or 3D bulk materials, leading to devices with much higher strain tolerance.

Ice-assisted electron-beam lithography for MoS2 transistors with extremely low-energy electrons

Ice-assisted electron-beam lithography (iEBL) by patterning ice with a focused electron-beam has emerged as a green nanofabrication technique for building nanostructures on diverse substrates. However, materials like atomically thin molybdenum disulfide (MoS₂), can be easily damaged by electron irradiation. To ensure the performance of devices based on sensitive materials, it is critical to control electron-beam induced radiolysis in iEBL processes. In this paper, we demonstrate that electron-beam patterning with extremely low-energy electrons followed by a heating process can significantly reduce the damage to substrate materials. A thin film of water ice not only acts as a sacrificial layer for patterning but also becomes a protecting layer for the underlying materials. As a result, MoS₂ field effect transistors with back-gate configuration and ohmic contacts have been successfully fabricated. Moreover, the presence or absence of such a protecting layer can lead to the retention or destruction of the underlying MoS₂, which provides a flexible method for creating electrical insulation or connection on 2D materials.

Molecule-Upgraded van der Waals Contacts for Schottky-Barrier-Free Electronics

The applications of any ultrathin semiconductor device are inseparable from high-quality metal-semiconductor contacts with designed Schottky barriers. Building van der Waals (vdWs) contacts of 2D semiconductors represents an advanced strategy of lowering the Schottky barrier height by reducing interface states, but will finally fail at the theoretical minimum barrier due to the inevitable energy difference between the semiconductor electron affinity and the metal work function. Here, an effective molecule optimization strategy is reported to upgrade the general vdWs contacts, achieving near-zero Schottky barriers and creating high-performance electronic devices. The molecule treatment can induce the defect healing effect in p-type semiconductors and further enhance the hole density, leading to an effectively thinned Schottky barrier width and improved carrier interface transmission efficiency. With an ultrathin Schottky barrier width of ≈2.17 nm and outstanding contact resistance of ≈9 kΩ µm in the optimized Au/WSe₂ contacts, an ultrahigh field-effect mobility of ≈148 cm²  V^-1 s^-1 in chemical vapor deposition grown WSe₂ flakes is achieved. Unlike conventional chemical treatments, this molecule upgradation strategy leaves no residue and displays a high-temperature stability at >200 °C. Furthermore, the Schottky barrier optimization is generalized to other metal-semiconductor contacts, including 1T-PtSe₂ /WSe₂ , 1T'-MoTe₂ /WSe₂ , 2H-NbS₂ /WSe₂ , and Au/PdSe₂ , defining a simple, universal, and scalable method to minimize contact resistance.

Dry Exfoliation of Large-Area 2D Monolayer and Heterostructure Arrays

Two-dimensional (2D) semiconductors have attracted considerable attention in recent years. However, to date, there is still no effective approach to produce large-scale monolayers while retaining their intrinsic properties. Here, we report a simple mechanical exfoliation method to produce large-scale and high-quality 2D semiconductors, by designing an atomically flat Au-mesh film as the peeling tape. Using our prefabricated mesh tape, the limited contact region (between the 2D crystal and Au) could provide enough adhesion to mechanically exfoliate uniform 2D monolayers, and the noncontact region (between the mesh holes and monolayers) ensures weak interaction to mechanically release the 2D monolayers on desired substrates. Together, we demonstrate a scalable method to dry exfoliate various 2D monolayer arrays onto different substrates without involving any solutions or contaminations, representing the optimization between material yield, scalability, and quality. Furthermore, detailed optical and electrical characterizations are conducted to confirm their intrinsic quality. With the ability to mechanically exfoliate various 2D arrays and further restacking them, we have demonstrated large-scale van der Waals heterostructure arrays through layer-to-layer assembling. Our study offers a simple and scalable method for dry exfoliating 2D monolayer and heterostructure arrays with intrinsic material quality, which could be crucial to accelerate fundamental investigations as well as practical applications of proof-of-concepts devices.