Electric Double Layer Doping and Process Development for Transition-Metal-Dichalcogenide Tunnel Field-Effect Transistors

Electric double layer (EDL) doping of transition metal dichalcogenide (TMD) channels in planar, nanoribbon, and nanotube geometries, have been explored for the realization of tunnel field effect transistors (TFETs). The channel materials included commercially-supplied exfoliated materials, synthesized materials grown by metal organic chemical vapor deposition and nanostructured materials grown by chemical vapor transport (CVT).

The solid polymer, polyethylene oxide with the salt, cesium perchlorate (PEO:CsClO4) was utilized on three different TMDs: WSe2, MoS2 and WS2. On exfoliated WSe2 channels, sheet carrier density and current density as high as (4.9±1.9) x 10^13 cm-2 and 58 μA/μm for electrons, and (3.5±1.9) x 10^13 cm-2 and 50 μA/μm for holes were achieved at a channel length of 3.5 μm, thickness of 6.5 nm, and a drain-source bias of 2 V.

EDL p-i-n junctions were created in exfoliated WSe2 FET channels without the use of side gates. A source/drain bias applied at room temperature creates a channel field which positions Cs+ cations and ClO4− anions at the negative and positive biased contacts respectively. The ions induce electrons and holes respectively to form a p-i-n junction. Cooling below 250 K locks the ions in place for current-voltage measurements. The devices show clear rectification with a forward-to-reverse current ratio of 2000 to 28,000.

As a part of the TFET process development, two techniques were utilized to deposit gate dielectrics on TMD channels: one suitable for planar channel geometries and the other suitable for nonplanar geometries. In the first approach, monolayer titanyl phthalocyanine (TiOPc) was deposited by molecular beam epitaxy (MBE) as a seed layer for atomic layer deposition (ALD) of Al2O3 on WSe2. In the second approach, which was suitable for nonplanar geometries, a gate dielectric was demonstrated by using a low-temperature ALD technique.