In Situ Study of Molecular Doping of Chlorine on MoS₂ Field Effect Transistor Device in Ultrahigh Vacuum Conditions

We report a precise measurement of the sensor behavior of the field effect transistor (FET) formed with the MoS₂ channel when the channel part is exposed to Cl₂ gas. The gas exposure and the electrical measurement of the MoS₂ FET were executed with in situ ultrahigh-vacuum (UHV) conditions in which the surface analysis techniques were equipped. This makes it possible to detect how much sensitivity the MoS₂ FET can provide and understand the surface properties. With the Cl₂ gas exposure to the channel, the plot of the drain current versus the gate voltage (I _d-V _g curve) shifts monotonically toward the positive direction of V _g, suggesting that the adsorbate acts as an electron acceptor. The I _d-V _g shifts are numerically estimated by measuring the onset of I _d (threshold voltage, V _th) and the mobility as a function of the dosing amounts of the Cl₂ gas. The behaviors of both the V _th shift and the mobility with the Cl₂ dosing amount can be fitted with the Langmuir adsorption kinetics, which is typically seen in the uptake curve of molecule adsorption onto well-defined surfaces. This can be accounted for by a model where an impinging molecule occupies an empty site with a certain probability, and each adsorbate receives a certain amount of negative charge from the MoS₂ surface up to the monolayer coverage. The charge transfer makes the V _th shifts. In addition, the mobility is reduced by the enhancement of the Coulomb scattering for the electron flow in the MoS₂ channel by the accumulated charge. From the thermal desorption spectroscopy (TDS) measurement and density functional theory (DFT) calculations, we concluded that the adsorbate that is responsible for the change of the FET property is the Cl atom that is dissociated from the Cl₂ molecule. The monotonic shift of V _th with the coverage suggests that the MoS₂ device sensor has a good sensitivity to detect 10^-3 monolayers (ML) of adsorption corresponding to the ppb level sensor with an activation time of 1 s.