Demonstration of Ambipolar Transport in WS₂ via a Heterojunction-Based Charge Transfer Doping

Although higher device integration density is required, Si-based electronics reached the physical limits of their electrical property. The down-scaling of Si-based devices is restrained by several problems: punch-through, drain-induced barrier lowering, etc. Increment of the surface area-to-volume ratio of semiconductor devices and maintaining robust mechanical and electrical properties are prerequisites for next-generation materials.<br/>To overcome the limitation of Si, atomically thin 2D transition-metal dichalcogenides (TMDs) have gained significant attraction due to their unique mechanical, optical, and electrical properties. TMDs layers are held by the weak van der Waals interaction and can be mechanically exfoliated into thin layers without surface dangling bonds. WS₂ belongs to the group of TMDs, which feature bandgaps that vary depending on the number of layers present (bulk: 1.44 eV, monolayer: 2.13 eV). WS₂ undergoes indirect-to-direct bandgap transitions as its thickness decreases from bulk to monolayer, which is appropriate for optoelectronic device applications. WS₂ possesses electron mobility of ∼234 cm²/Vs and hole mobility of ~39 cm²/Vs. Ambipolar transport in WS₂ allows the modulation of both n-type and p-type channel operation under gate voltage. However, the n-dominant property exerted in intrinsic WS₂ results from the sulfur vacancy-induced trap states near the conduction band level.<br/>In this study, a p-doping method for WS₂ is proposed using WSe₂ as a charge transfer layer. Back-gated WS₂ FETs with Ti/Au source and drain electrodes were fabricated, and WSe₂ flakes were dry-transferred onto the WS₂ FET channel. The surface of the WS₂/WSe₂ FET was oxidized by UV-ozone treatment. Previous studies have reported degenerate p-doping of WSe₂ through UV-ozone exposure. The tungsten oxide (WO_X) layer with high electron affinity effectively induces p-type doping in the underlying WS₂/WSe₂ upon oxidation. Ambipolar transport was observed with an on/off ratio of ~10⁸ for the p-branch and ~10⁷ for the n-branch and the hole mobility of 143 cm²/Vs after oxidation. WS₂/WSe₂ FETs with varying WS₂/WSe₂ channel coverage ratios were fabricated to examine the behavior of the formed WO_X on the WSe₂ surface. Electrical characteristics were measured using 3-terminal measurements with back gate modulation. Crystallinity and lattice structures of the oxidized channel layers were analyzed using micro-Raman spectroscopy and high-resolution transmission electron microscopy (HR-TEM), respectively. The current fabrication of Si-based CMOS devices involves multiple processes of thermal annealing and diffusion doping. Ambipolar materials such as WS₂ can be expected to reduce process complexity with a single-channel CMOS device structure. Therefore, the proposed p-doping methods in this study enable the fabrication of complementary metal-oxide semiconductor (CMOS) devices with down-scaled architectures.