Full Energy Spectra of Interface State Densities for n- and p-type MoS₂ Field-Effect Transistors

2D materials are promising to overcome the scaling limit of Si field-effect transistors (FETs). However, the insulator/2D channel interface severely degrades the performance of 2D FETs, and the origin of the degradation remains largely unexplored. Here, the full energy spectra of the interface state densities (D_it) are presented for both n- and p- MoS₂ FETs, based on the comprehensive and systematic studies, i.e., full rage of channel thickness and various gate stack structures with h-BN as well as high-k oxides. For n-MoS₂, D_it around the mid-gap is drastically reduced to 5 × 10¹¹ cm⁻² eV⁻¹ for the heterostructure FET with h-BN from 5 × 10¹² cm⁻² eV⁻¹ for the high-k top-gate. On the other hand, D_it remains high, ≈10¹³ cm⁻² eV⁻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 conduction band side, while sulfur-vacancy-induced defect states dominate the interface degradation on valance band side. The present understanding of the interface properties provides the key to further improving the performance of 2D FETs.