Gap state distribution and Fermi level pinning in monolayer to multilayer MoS₂ field effect transistors

Gap states and Fermi level pinning play an important role in all semiconductor devices, but even more in transition metal dichalcogenide-based devices due to their high surface to volume ratio and the absence of intralayer dangling bonds. Here, we measure Fermi level pinning using Kelvin probe force microscopy, extract the corresponding electronic state distribution within the band gap, and present a systematic comparison between the gap state distribution obtained for exfoliated single layer, bilayer and thick MoS2 FET samples. It is found that the gap state distribution in all cases decreases from the conduction band edge and is in the order of 1019 eV-1 cm-3 and slightly decreases with increasing channel thickness. Strong Fermi level pinning is observed near the conduction band edge, and it decreases as it approaches the middle and lower part of the bandgap.