Progress in the preparation and device research of two-dimensional metallic transition metal dichalcogenides for electrical contact applications

With the continuous scaling down of electronic device sizes, the electrical contact becomes increasingly important for determining the devices performances, even beyond the semiconductor material itself. Contacts are the communication links between two-dimensional (2D) channel materials and external circuits, which play decisive roles in the performance of electronic and photoelectric devices. The traditional metal electrode preparation usually involves a deposition process of metals on the surfaces of 2D channel materials. However, the bombardments of "high-energy" metal atoms or clusters onto the surfaces of 2D channel materials, along with the strong local heating caused by the evaporation process, often lead to some problems, such as defects in metal-semiconductor interfaces, metal atoms diffusion, and chemical bonding, etc. These problems will give rise to the generation of defect-induced gap states (DIGS), which will induce the accumulation of a large number of electrons or holes, and ultimately result in the Fermi level pinning effect. To make things worse, the Schottky barrier between metal-semiconductor interfaces will bring about high contact resistance, seriously affecting the devices performance. Therefore, it is of great significance to explore effective strategies to construct atomically clean and sharp contact interfaces and achieve satisfying electrical contacts for improving device performance. Up to now, many efforts have been made to elevate the contact quality of the metal-semiconductor interfaces, including fabricating heterophase (metal-semiconductor) homojunctions based on the same material possessing different phases, constructing van der Waals contacts, designing novel metal electrodes, integrating the 2D semiconductor material with metallic counterpart and so on. Among them, the chemical vapor deposition (CVD) preparation of metallic transition metal dichalcogenides (MTMDCs) and their applications as contact materials in electronic devices have aroused great attention. The CVD method has been proven to enable the controllable preparation of high-quality 2D MTMDCs materials possessing electrical conductivities as high as that of copper. By facilely adjusting the experimental parameters (type and amount of precursors, growth substrates, growth temperature, etc.) in the CVD process, the layer thicknesses, phases, morphologies, domain sizes, and orientations of 2D MTMDCs materials can be accurately controlled. More importantly, MTMDCs and semiconductor TMDCs (STMDCs) have similar lattice structures and complementary electrical properties, which make the metal-semiconductor heterostructures based on two-dimensional MTMDCs/STMDCs show great potential in improving the contact quality in related devices. This paper reviews the recent progress in the preparation of some typical MTMDCs materials and their applications in the electrical contact fields. 2D MTMDCs materials containing three metal elements (i.e., V, Nb, and Ni) will be introduced in detail. V-based dichalcogenides (VX2) with high stabilities are insusceptible to react with oxygen and water in the air, which is the prerequisite for their practical applications as electrode materials in 2D-STMDCs-related devices. Theoretical calculations have demonstrated that both the H and T phases of monolayer VX2 materials show metallic properties and similar crystal structures with STMDCs, making VX2 promising electrical contact materials towards improving the performances of two-dimensional STMDCs-based FET devices. For Nb-based dichalcogenides (NbX2), it has been experimentally proven that NbS2, as an electrical contact material, can effectively improve the performance of the related electronic devices, and can even show different electrical characteristics (e.g., current rectification behavior) from traditional metal-contact devices. However, the instabilities of NbX2-related materials hinder their further research and applications. Improving the stabilities of NbX2 or developing effective encapsulation strategies should be possible solutions. For Ni-based dichalcogenides (NiX2), the representative member NiTe2 hosts topologically protected electronic states and have been proven to exhibit novel physical properties. Additionally, the intrinsic layered feature and high conductivity also endow NiTe2 with good application prospects in the electrical contact-related field. Finally, the challenges and opportunities in the preparation of the 2D MTMDCs and their applications as electrical contacts of 2D STMDCs-based devices are also discussed and prospected.