Electron beam evaporated Au islands as a nanoscale etch mask on few-layer MoS₂ and fabrication of top-edge hybrid contacts for field-effect transistors

Metal contacts to two-dimensional layered semiconductors are crucial to the performance of field-effect transistors (FETs) and other applications of layered materials in nanoelectronics and beyond. In this work, the wetting behavior of very thin Au films on exfoliated MoS₂ flakes was studied and evaluated as a nanoscale, self-assembled dry etch mask. Etching nanoscale pits into MoS₂ flakes prior to metallization from the top of the flake forms edge sites that contribute some fraction of edge contacts in addition to top contacts for additional carrier injection and lower contact resistance. The morphology and thickness of Au islands and MoS₂ were studied with scanning electron microscopy and atomic force microscopy before and after etching with low-power plasmas. A Cl₂ plasma etch of 10 s with a Au island mask of 6 nm (nominal) showed the best resulting morphology among the plasma conditions studied. Back-gated MoS₂-based FETs on SiO₂/p ⁺-Si with Ti/Au contacts were fabricated using a Cl₂ etch of only the contact regions, and they yielded devices with ON currents of 100s µA/µm, ON/OFF ratios ⩾10⁶, and contact resistance <10 kΩ µm. The best set of devices had a very low contact resistance of ∼1 kΩ µm with almost no dependence of contact resistance on gating. Using nanoscale etch masks made from metal islands could be highly customizable and shows promise for engineering FETs with low contact resistance. (paper)