Unconventional Behavior in Low-Resistance Metal Contacts to High-Purity, hBN-Encapsulated Monolayer Semiconductors

Two-dimensional (2D) monolayer semiconductors such as transition metal dichalcogenides (TMDs) have attracted intense attention in electronics and optoelectronics due to properties such as high mobility, direct optical bandgap, and mechanical flexibility. For most applications, optimum performance is attained using high-purity TMDs encapsulated with hexagonal boron nitride (hBN) to minimize extrinsic disorder. However, achieving high-quality contacts to hBN-encapsulated ultrapure monolayer TMDs remains challenging. Here we first report a well-developed flux growth strategy that could produce a series of centimeter-sized single-crystal TMDs with record low defect densities of ~10⁹-10¹⁰cm^-2, including 2H-WSe₂, 2H-MoSe₂, Td-WTe₂, 2H-MoTe₂ and 1T’-MoTe₂. Next, taking the ultraclean semiconductor WSe₂ as an example, we demonstrate a process for mechanical transfer of metal contacts embedded within hBN onto ultraclean monolayer TMDs that minimizes doping, strain, and interfacial roughness. Using this process, we further show efficient contacts to hBN-encapsulated ultrapure monolayer WSe₂. This method achieves excellent electric performance with room temperature contact resistance <i>R</i>_c near 5 , and enables hysteresis-free transistors with record-high mobility of 655 cm²/Vs at room temperature. Furthermore, these contacts show an ultralong transfer length of 1 m, placing them in a regime distinct from previous reports for contacts to TMDs and bulk semiconductors. This work provides a general method of making strain- and doping-free hBN-encapsulated vdW contacts and highlights critical challenges for device applications.