Supercapacity (>1000 fJF/cm2) charge release in a CVD-grown WSe2 FET incorporating a PEO: CsCI04 side gate

Abstract

We report extraordinarily high capacitance density in tungsten diselenide (WSe2) field-effect transistors (FETs) capped with polyethylene oxide (PEO) and ion doped with cesium perchlorate (CsCI04). The FETs have 3–4 monolayer WSe2 channels grown by chemical vapor deposition (CVD) using W(CO) 6 and H2Se at 800 °C for 30 minutes [1]. A stepper-based process was used to fabricate the transistors and an 11 nm Al 2 O 3 atomic layer deposited (ALD) gate dielectric was formed on the WSe2 with a low temperature nucleation layer, 1 nm Ah03, deposited at 110 °C to facilitate nucleation followed by a 10 nm Ah03 deposited at 200 °C, after Kwak [2]. The n-FET achieves an ON/OFF ratio of 106 with noise-floor-limited gate current. Most prior studies of WSe2 FETs have been on exfoliated materials and to our knowledge there is only one prior report of a top-gated CVD WSe2 p-FET [3]. Following FET characterization PEO: CsCI04 was drop-cast and annealed at 90 °C for 3 minutes in Ar after Xu [4], yielding a layer thickness of 400 nm as measured by transmission electron microscopy (TEM). Ion gating using a side gate ( $6 \mu\mathrm{m}$ from the WSe2 mesa edge) produces ambipolar transfer characteristics with approximately equal ON-currents of $2 \mu\mathrm{A}/\mu\mathrm{m}$ attributed to the multi-work-function source/drain contacts [5]. ON/OFF current ratio exceeds 106 with field-effect electron and hole mobilities of 10 and 25 cm2/Vs respectively and subthreshold swings of 260 and 180 mY/decade respectively. Atomic force microscope images show that the WSe2 nucleates in triangles which are apparent in Fig. 1; FET characteristics after testing of the transistor are shown in Fig. 2 along with TEM.