High-performance transistors with polycrystalline 2D material channels: The influence of gold electrode crystallinity and the layer number of molybdenum disulfide channels

By using gold (Au) as the source/drain electrodes with a deposition temperature 100 °C, compatible performances with the device with Sb/Au electrodes can be observed for mono-layer MoS₂ transistors. Through the sequential MoS₂ film transferring, enhanced field-effect mobility values from 4.1 to 56.4 cm²·V⁻¹·s⁻¹ are observed from mono- to quad- layer MoS₂ transistors. The gradual threshold voltage shift to negative gate biases suggests increasing electron densities are observed with increasing MoS₂ layer numbers, which also result in a low contact resistance 1.8 × 10² Ω∙$\mu$m at the Au electrode/quad-layer MoS₂ interface. By further reducing the channel width/length to 500/850 nm through the e-beam lithography, a high field-effect mobility 105.8 cm²·V⁻¹·s⁻¹ and ON/OFF ratios > 10⁵ are observed for the quad-layer MoS₂ transistor. The drain current density of the device also reaches 130 μA/μm. The increasing electron densities with increasing MoS₂ layer numbers suggest that the electron storage will take place in between MoS₂ layers, which will help to screen the influence of defects in the polycrystalline MoS₂ films and reduce the contact resistance at the Au/MoS₂ interface. First-principles calculations indicate that the effective conduction channel number depends on the allocated electron density under the bias voltage.