  Two-dimensional (2D) layered materials are promising for replacing Si to
overcome the scaling limit of recent ~5 nm-length metal-oxide-semiconductor
field-effect transistors (MOSFETs). However, the insulator/2D channel interface
severely degrades the performance of 2D-based MOSFETs, and the origin of the
degradation remains largely unexplored. Here, we present the full energy
spectra of the interface state densities (Dit) for both n- and p- MoS2 FETs,
based on the comprehensive and systematic studies, i.e., thickness range from
monolayer to bulk and various gate stack structures including 2D
heterostructure with h-BN as well as typical high-k top-gate structure. For
n-MoS2, Dit around the mid gap is drastically reduced to 5*10^11 cm-2eV-1 for
the heterostructure FET with h-BN from 5*10^12 cm-2eV-1 for the high-k top-gate
MoS2 FET. On the other hand, Dit remains high, ~10^13 cm-2eV-1, even for the
heterostructure FET for p-MoS2. The systematic study elucidates that the strain
induced externally through the substrate surface roughness and high-k
deposition process is the origin for the interface degradation on the
conduction band side, while sulfur-vacancy-induced defect-states dominate the
interface degradation on the valance band side. The present understanding on
the interface properties provides the key to further improving the performance
of 2D FETs.
