  Lithographically fabricated MoS2 field effect transistors suffer from several
critical imperfections, including low sub-threshold swings, large turn-on gate
voltages (VT), and wide device-to-device variability. The large magnitude and
variability of VT stems from unclean interfaces, trapped charges in the
underlying substrate, and sulfur vacancies created during the mechanical
exfoliation process. In this study, we demonstrate a simple and reliable oxygen
plasma treatment, which mitigates the effects of unintentional doping created
by surface defect cites, such as S vacancies, and surface contamination. This
plasma treatment restores charge neutrality to the MoS2 and shifts the
threshold turn-on voltage towards 0V. Out of the 8 devices measured, all
exhibit a shift of the FET turn-on voltage from an average of -18.7V to -0.9V.
The oxygen plasma treatment passivates these defects, which reduces surface
scattering, causing increased mobility and improved subthreshold swing. For
as-prepared devices with low mobilities (~0.01cm2/V.s), we observe up to a
190-fold increase in mobility after exposure to the oxygen plasma. Perhaps the
most important aspect of this oxygen plasma treatment is that it reduces the
device-to-device variability, which is a crucial factor in realizing any
practical application of these devices.
