Electric-double-layer Doping of WSe2 Field-effect Transistors using Polyethylene-oxide Cesium Perchlorate
The channel conductivity of WSe2 FETs is controlled between n and p-type doping to degenerate levels using electric double layers (EDLs). In the absence of doping, remarkable agreement of the I-V characteristic with Schottky theory is obtained by assuming that the transport of electrons and holes from the metal to the WSe2 channel is decreased by a constant transmission factor of approximately 100 relative to what is expected from thermionic emission. Using this assumption barrier heights extracted from temperature-dependence are comparable with the barrier-heights extracted from I-V measurements. A dual work-function Ti/Pd contact is used to form the Schottky contacts with Ti as the n-contact and Pd as the p-contact and these are evaporated in a single evaporation. Using EDL doping technique, sheet carrier density and current is as high as (4.9±1.9) x 10(13) and 58 μA/μm for n-doping and (3.5 ±1.9) x 1013 and 50 μA/μm for p-doping for the highest channel conductivities. The weak temperature dependence of the transfer characteristics at high doping levels reveals that the current in the Schottky contacts are dominated by tunneling with a contact resistance of 1 kΩ μm for the p-branch and 3.4 kΩ μm for the n-branch, comparable to the best WSe2 FET reports. At the highest carrier densities the temperature coefficient of the conductivity becomes negative as the mobility of the channel controls the temperature dependence.