[en] Device performance of two dimensional (2D) material based field effect transistors is severely limited by the relatively high contact resistance encountered at the contact-channel interface. Metal-graphene hybrid contacts have been previously used to improve the contact resistance of devices based on thick exfoliated materials. Here we report a novel 2D FET fabrication process entailing the transfer of metal-graphene hybrid contacts on top of 3 monolayer-thick chemical vapor deposition (CVD) MoS₂, enabling a lithography free contacting strategy, with respect to MoS₂. Three different metal-graphene stacks consisting of Ni, Pd and Ru, have been fabricated, transferred onto MoS₂ and characterized extensively using electrical and physical characterization techniques. We find strong correlation between the measured electrical characteristics and physical characterization of the contact interface. From Raman spectra measurement, maximum charge transfer of 1.7 × 10¹³ cm⁻² is observed between graphene and Ru, leading to an improved contact resistance for MoS₂ devices with Ru-Gr contacts. Ru-Gr contact shows the lowest contact resistance of 9.34 kΩ · µm among the three metal-graphene contact stacks reported in this article. This contact resistance is also the best among reported CVD grown graphene contacted MoS₂ devices. Using more than 400 devices, we study the impact of the different metal-graphene contacts on other electrical parameters such as hysteresis, sub-threshold swing and threshold voltage. The metal-graphene contact stack transfer technique represents a technologically relevant contacting approach which can be further up-scaled to larger wafer areas. (paper)

[en] 2D materials offer a pathway for further scaling of CMOS technology. However, for this to become a reality, both n-MOS and p-MOS should be realized, ideally with the same (standard) material. In the specific case of MoS₂ field effect transistors (FETs), ambipolar transport is seldom reported, primarily due to the phenomenon of Fermi level pinning (FLP). In this study we identify the possible sources of FLP in MoS₂ FETs and resolve them individually. A novel contact transfer technique is used to transfer contacts on top of MoS₂ flake devices that results in a significant increase in the hole branch of the transfer characteristics as compared to conventionally fabricated contacts. We hypothesize that the pinning not only comes from the contact-MoS₂ interface, but also from the MoS₂-substrate interface. We confirm this by shifting to an hBN substrate which leads to a 10 fold increase in the hole current compared to the SiO₂ substrate. Furthermore, we analyse MoS₂ FETs of different channel thickness on three different substrates, SiO₂, hBN and Al₂O₃, by correlating the p-branch I _ON/I _OFF to the position of oxide defect band in these substrates. FLP from the oxide is reduced in the case of Al₂O₃ which enables us to observe ambipolar transport in a bilayer MoS₂ FET. These results highlight that MoS₂ is indeed an ambipolar material, and the absence of ambipolar transport in MoS₂ FETs is strongly correlated to its dielectric environment and processing conditions. (paper)