Role of Hole Trap Sites in MoS 2 for Inconsistency in Optical and Electrical Phenomena.

Because of strong Coulomb interaction in two-dimensional van der Waals layered materials, the trap charges at the interface strongly influence the scattering of the majority carriers, and thus often degrade their electrical properties. However, the photogenerated minority carriers can be trapped at the interface, modulate the electron-hole recombination, and eventually influence the optical properties. In this study, we report the role of the hole trap sites on the inconsistency in the electrical and optical phenomena between two systems with different interfacial trap densities, which are monolayer MoS-based field-effect-transistors on hexagonal boron nitride (h-BN) and SiO substrates. Electronic transport measurements indicate that the use of h-BN as a gate insulator can induce a higher n-doping concentration of the monolayer MoS by suppressing the free electron transfer from the intrinsically n-doped MoS to the SiO gate insulator. Nevertheless, optical measurements show that the electron concentration in MoS/SiO is heavier than that in MoS/h-BN, manifested by the relative red-shift of the A Raman peak. The inconsistency in the evaluation of the electron concentration in MoS by electrical and optical measurements is explained by the trapping of the photogenerated holes in the spatially modulated valence-band edge of the monolayer MoS caused by the local strain from the SiO/Si substrate. This photo-induced electron doping in MoS/SiO is further confirmed by the development of the trion component in the power-dependent photoluminescence spectra and negative-shift of the threshold voltage of the field-effect transistor after the illumination.