H2S assisted contact engineering: a universal approach to enhance hole conduction in all TMD Field-Effect Transistors and achieve ambipolar CVD MoS2 Transistors

Unlike Si, 2-dimensional (2D) Transition Metal Dichalcogenides (TMDs) offer atomically thin channels for carrier transport in FETs. Despite advantages like superior gate control, steep sub-threshold swing and high carrier mobility offered by 2D FET channels, process related challenges like lack of selective doping techniques like implantation and CMOS compatible process for fabrication of 2D TMD based FETs hinder the anticipated viability of 2D semiconductor technology for future electronic applications. In this letter, we demonstrate a process oriented approach to realize superior ambipolarity in 2D FETs based on TMDs like Molybdenum disulfide (MoS2), Tungsten disulfide (WS2), Molybdenum diselenide (MoSe2) and Tungsten diselenide (WSe2) by enhancing hole current by multiple orders of magnitude in otherwise strong N-type transistors. The method involves Hydrogen Sulfide (H2S gas) assisted contact engineering of N-type FETs to introduce surface states that alter device behavior. Based on material characterization and bandstructure calculations, physical insights have been developed to understand the effect of such a contact engineering technique. Subsequently, this technique has been demonstrated to alter device behavior by enhancing hole conduction in originally N-type exfoliated (MoS2, WS2, MoSe2 and WSe2) and CVD grown (MoS2) TMD samples to confirm its potential towards enabling the feasibility of 2D semiconductor device technology.