  In 2D-semiconductor-based field-effect transistors and optoelectronic
devices, metal-semiconductor junctions are one of the crucial factors
determining device performance. The Fermi-level (FL) pinning effect, which
commonly caused by interfacial gap states, severely limits the tunability of
junction characteristics, including barrier height and contact resistance. A
tunneling contact scheme has been suggested to address the FL pinning issue in
metal-2D-semiconductor junctions, whereas the experimental realization is still
elusive. Here, we show that an oxidized-monolayer-enabled tunneling barrier can
realize a pronounced FL depinning in indium selenide (InSe) transistors,
exhibiting a large pinning factor of 0.5 and a highly modulated Schottky
barrier height. The FL depinning can be attributed to the suppression of metal-
and disorder-induced gap states as a result of the high-quality tunneling
contacts. Structural characterizations indicate uniform and atomically thin
surface oxidation layer inherent from nature of van der Waals materials and
atomically sharp oxide-2D-semiconductor interfaces. Moreover, by effectively
lowering the Schottky barrier height, we achieve an electron mobility of 2160
cm$^2$/Vs and a contact barrier of 65 meV in two-terminal InSe transistors. The
realization of strong FL depinning in high-mobility InSe transistors with the
oxidized monolayer presents a viable strategy to exploit layered semiconductors
in contact engineering for advanced electronics and optoelectronics.
