Role of Hole Trap Sites in MoS₂ 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 MoS₂-based field-effect transistors (FETs) on hexagonal boron nitride (h-BN) and SiO₂ 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 MoS₂ by suppressing the free-electron transfer from the intrinsically n-doped MoS₂ to the SiO₂ gate insulator. Nevertheless, optical measurements show that the electron concentration in MoS₂/SiO₂ is heavier than that in MoS₂/h-BN, manifested by the relative red shift of the A_1g Raman peak. The inconsistency in the evaluation of the electron concentration in MoS₂ by electrical and optical measurements is explained by the trapping of the photogenerated holes in the spatially modulated valence band edge of the monolayer MoS₂ caused by the local strain from the SiO₂/Si substrate. This photoinduced electron doping in MoS₂/SiO₂ 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 FET after illumination.