  Variability and lack of control in the nature of contacts between metal/MoS2
interface is a major bottleneck in the realisation of high-performance devices
based on layered materials for several applications. In this letter, we report
on the reduction in Schottky barrier height at metal/MoS2 interface by
engineering the surface states through sulphur treatment. Electrical
characteristics for back-gated MoS2 field effect transistor structures were
investigated for two high work-function metal contacts Ni and Pd. Contacts on
MoS2 treated with sulphur exhibited significant improvements in Ohmic nature
with concomitant reduction in variability compared to those on untreated MoS2
films leading to a 2x increase in extracted mobility. X-ray Photoelectron
Spectroscopy (XPS) measurements, Raman Spectroscopy and comparison of threshold
voltages indicated absence of additional doping or structural changes due to
sulphur treatment. The Schottky barrier heights were extracted from
temperature-dependent transfer characteristics based on the thermionic current
model. A reduction in barrier height of 80 and 135 meV extracted for Ni/MoS2
and Pd/MoS2 contacts respectively is hence attributed to the increase in
surface states (or stronger Fermi level pinning) due to sulphur treatment. The
corresponding charge neutrality levels at metal/MoS2 interface, were extracted
to be 0.16 eV (0.17 eV) below the conduction band before (after) Sulphur
treatment. This first report of surface states engineering in MoS2 leading to
superior contacts is expected to significantly benefit the entire class of
devices based on layered 2D materials.
