  Metal contacts to two-dimensional (2D) semiconductors are ubiquitous in
modern electronic and optoelectronic devices. Such contacts are, however, often
plagued by strong Fermi level pinning (FLP) effect which reduces the tunability
of the Schottky barrier height (SBH) and degrades the performance of
2D-semiconductor-based devices. In this work, we show that monolayer
MoSi$_2$N$_4$ and WSi$_2$N$_4$ - a recently synthesized 2D material class with
exceptional mechanical and electronic properties - exhibit strongly suppressed
FLP and wide-range tunable SBH when contacted by metals. An exceptionally large
SBH slope parameter of S=0.7 is obtained, which outperform the vast majority of
other 2D semiconductors. Such surprising behavior arises from the unique
morphology of MoSi$_2$N$_4$ and WSi$_2$N$_4$. The outlying Si-N layer forms a
native atomic layer that protects the semiconducting inner-core from the
perturbance of metal contacts, thus suppressing the FLP. Our findings reveal
the potential of MoSi$_2$N$_4$ and WSi$_2$N$_4$ monolayers as a novel 2D
material platform for designing high-performance and energy-efficient 2D
nanodevices.
