Schottky barrier contrasts in single and bi-layer graphene contacts for MoS₂ field-effect transistors

We have investigated single- and bi-layer graphene as source-drain electrodes for n-type MoS₂ transistors. Ti-MoS₂-graphene heterojunction transistors using both single-layer MoS₂ (1M) and 4-layer MoS₂ (4M) were fabricated in order to compare graphene electrodes with commonly used Ti electrodes. MoS₂-graphene Schottky barrier provided electron injection efficiency up to 130 times higher in the subthreshold regime when compared with MoS₂-Ti, which resulted in V_DS polarity dependence of device parameters such as threshold voltage (V_TH) and subthreshold swing (SS). Comparing single-layer graphene (SG) with bi-layer graphene (BG) in 4M devices, SG electrodes exhibited enhanced device performance with higher on/off ratio and increased field-effect mobility (μ_FE) due to more sensitive Fermi level shift by gate voltage. Meanwhile, in the strongly accumulated regime, we observed opposing behavior depending on MoS₂ thickness for both SG and BG contacts. Differential conductance (σ_d) of 1M increases with V_DS irrespective of V_DS polarity, while σ_d of 4M ceases monotonic growth at positive V_DS values transitioning to ohmic-like contact formation. Nevertheless, the low absolute value of σ_d saturation of the 4M-graphene junction demonstrates that graphene electrode could be unfavorable for high current carrying transistors.