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 A1g 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.
Keywords: Raman; atomic force microscopy; persistent photoconductivity; photogenerated holes; photoluminescence; threshold voltage shift; trapping.