  Monolayer WSe2 is a two dimensional (2D) semiconductor with a direct bandgap,
and it has been recently explored as a promising material for electronics and
optoelectronics. Low field effect mobility is the main constraint preventing
WSe2 from becoming one of the competing channel materials for field-effect
transistors (FETs). Recent results have demonstrated that chemical treatments
can modify the electrical properties of transition metal dichalcogenides
(TMDCs) including MoS2 and WSe2. Here, we report that controlled heating in air
significantly improves device performance of WSe2 FETs in terms of on-state
currents and field-effect mobilities. Specifically, after heating at optimized
conditions, chemical vapor deposition grown monolayer WSe2 FETs showed an
average FET mobility of 31 cm2/Vs and on/off current ratios up to 5*108. For
few-layer WSe2 FETs, after the same treatment applied, we achieved a high
mobility up to 92 cm2/Vs. These values are significantly higher than FETs
fabricated using as-grown WSe2 flakes without heating treatment, demonstrating
the effectiveness of air heating on the performance improvements of WSe2 FETs.
The underlying chemical processes involved during air heating and the formation
of in-plane heterojunctions of WSe2 and WO3-x, which is believed to be the
reason for the improved FET performance, were studied by spectroscopy and
transmission electron microscopy. We further demonstrated that by combining air
heating method developed in this work with supporting 2D materials on BN
substrate, we achieved a noteworthy field effect mobility of 83 cm2/Vs for
monolayer WSe2 FETs. This work is a step towards controlled modification of the
properties of WSe2 and potentially other TMDCs, and may greatly improve device
performance for future applications of 2D materials in electronics and
optoelectronics.
