Large-area niobium disulfide thin films as transparent electrodes for devices based on two-dimensional materials

Direct contacts of a metal with atomically thin two-dimensional (2D) transition metal dichalcogenide (TMDC) semiconductors have been found to suppress device performance by producing a high contact resistance. NbS₂ is a 2D TMDC and a conductor. It is expected to form ohmic contacts with 2D semiconductors because of its high work function and the van der Waals interface it forms with the semiconductor, with such an interface resulting in weak Fermi level pinning. Despite the usefulness of NbS₂ as an electrode, previous synthesis methods could not control the thickness, uniformity, and shape of the NbS₂ film and hence could not find practical applications in electronics. Here, we report a patternable method for carrying out the synthesis of NbS₂ films in which the number of NbS₂ layers formed over a large area was successfully controlled, which is necessary for the production of customized electrodes. The synthesized NbS₂ films were shown to be highly transparent and uniform in thickness and conductivity over the large area. Furthermore, the synthesized NbS₂ showed half the contact resistance than did the molybdenum metal in MoS₂ field effect transistors (FETs) on a large transparent quartz substrate. The MoS₂ device with NbS₂ showed an electron mobility as high as 12.7 cm² V^-1 s^-1, which was three times higher than that found for the corresponding molybdenum-contacted MoS₂ device. This result showed the high potential of the NbS₂ thin film as a transparent electrode for 2D transition metal dichalcogenide (TMDC) semiconductors with low contact resistance.