Quaternary, layered, 2D chalcogenide, Mo_1-xW_xSSe: thickness dependent transport properties

Highly oriented, single crystalline, quaternary alloy chalcogenide crystal, Mo_xW_1-xS_2ySe_2(1-y), is synthesized using a high temperature chemical vapor transport technique and its transport properties studied over a wide temperature range. Field effect transistors (FET) with bottom gated configuration are fabricated using Mo_0.5W_0.5SSe flakes of different thicknesses, from a single layer to bulk. The FET characteristics are thickness tunable, with thin flakes (1-4 layers) exhibiting n-type transport behaviour while ambipolar transfer characteristics are observed for thicker flakes (>90 layers). Ambipolar behavior with the dominance of n-type over p-type transport is noted for devices fabricated with layers between 9 and 90. The devices with flake thickness ∼9 layers exhibit a maximum electron mobility 63 ± 4 cm²V^-1s^-1and anI_ON/I_OFFratio >10⁸. A maximum hole mobility 10.3 ± 0.4 cm²V^-1s^-1is observed for the devices with flake thickness ∼94 layers withI_ON/I_OFFratio >10²-10³observed for the hole conduction. A maximumI_ON/I_OFFfor hole conduction, 10⁴is obtained for the devices fabricated with flakes of thickness ∼7-19 layers. The electron Schottky barrier height values are determined to be ∼23.3 meV and ∼74 meV for 2 layer and 94 layers flakes respectively, as measured using low temperature measurements. This indicates that an increase in hole current with thickness is likely to be due to lowering of the band gap as a function of thickness. Furthermore, the contact resistance (R_ct) is evaluated using transmission line model (TLM) and is found to be 14 kohm.μm. These results suggest that quaternary alloys of Mo_0.5W_0.5SSe are potential candidates for various electronic/optoelectronic devices where properties and performance can be tuned within a single composition.