High broadband photoconductivity of few-layered MoS field-effect transistors measured using multi-terminal methods: effects of contact resistance.

Among the layered two dimensional semiconductors, molybdenum disulfide (MoS2) is considered to be an excellent candidate for applications in optoelectronics and integrated circuits due to its layer-dependent tunable bandgap in the visible region, high ON/OFF current ratio in field-effect transistors (FET) and strong light-matter interaction properties. In this study, using multi-terminal measurements, we report high broadband photocurrent response (R) and external quantum efficiency (EQE) of few-atomic layered MoS2 phototransistors fabricated on a SiO2 dielectric substrate and encapsulated with a thin transparent polymer film of Cytop. The photocurrent response was measured using a white light source as well as a monochromatic light of wavelength λ = 400 nm-900 nm. We measured responsivity using a 2-terminal configuration as high as R = 1 × 103 A W-1 under white light illumination with an optical power Popt = 0.02 nW. The R value increased to 3.5 × 103 A W-1 when measured using a 4-terminal configuration. Using monochromatic light on the same device, the measured values of R were 103 and 6 × 103 A W-1 under illumination of λ = 400 nm when measured using 2- and 4-terminal methods, respectively. The highest EQE values obtained using λ = 400 nm were 105% and 106% measured using 2- and 4-terminal configurations, respectively. The wavelength dependent responsivity decreased from 400 nm to the near-IR region at 900 nm. The observed photoresponse, photocurrent-dark current ratio (PDCR), detectivity as a function of applied gate voltage, optical power, contact resistances and wavelength were measured and are discussed in detail. The observed responsivity is also thoroughly studied as a function of contact resistance of the device.