  A low Schottky barrier height (SBH) at source/drain contact is essential for
achieving high drive current in atomic layer MoS2 channel based field-effect
transistors. Approaches such as choosing metals with appropriate work functions
and chemical doping are employed previously to improve the carrier injection
from the contact electrodes to the channel and to mitigates the SBH between the
MoS2 and metal. Recent experiments demonstrate significant SBH reduction when
graphene layer is inserted between metal slab (Ti and Ni) and MoS2. However,
the physical or chemical origin of this phenomenon is not yet clearly
understood. In this work, density functional theory (DFT) simulations are
performed, employing pseudopotentials with very high basis sets to get insights
of the charge transfer between metal and monolayer MoS2 through the inserted
graphene layer. Our atomistic simulations on 16 different interfaces involving
five different metals (Ti, Ag, Ru, Au and Pt) reveal that: (i) such a decrease
in SBH is not consistent among various metals, rather an increase in SBH is
observed in case of Au and Pt (ii) unlike MoS2-metal interface, the projected
dispersion of MoS2 remains preserved in any MoS2-graphene-metal system with
shift in the bands on the energy axis. (iii) a proper choice of metal (e.g.,
Ru) may exhibit ohmic nature in a graphene inserted MoS2-metal contact. These
understandings would provide a direction in developing high performance
transistors involving hetero atomic layers as contact electrodes.
