  Two-dimensional transitional metal dichalcogenide (TMD) field-effect
transistors (FETs) are promising candidates for future electronic applications,
owing to their excellent transport properties and potential for ultimate device
scaling. However, it is widely acknowledged that substantial contact resistance
associated with the contact-TMD interface has impeded device performance to a
large extent. It has been discovered that O2 plasma treatment can convert WSe2
into WO3-x and substantially improve contact resistances of p-type WSe2 devices
by strong doping induced thinner depletion width. In this paper, we carefully
study the temperature dependence of this conversion, demonstrating an oxidation
process with a precise monolayer control at room temperature and multilayer
conversion at elevated temperatures. Furthermore, the lateral oxidation of WSe2
under the contact revealed by HR-STEM leads to potential unpinning of the metal
Fermi level and Schottky barrier lowering, resulting in lower contact
resistances. The p-doping effect is attributed to the high electron affinity of
the formed WO3-x layer on top of the remaining WSe2 channel, and the doping
level is found to be dependent on the WO3-x thickness that is controlled by the
temperature. Comprehensive materials and electrical characterizations are
presented, with a low contact resistance of ~528 ohm-um and record high
on-state current of 320 uA/um at -1V bias being reported.
