Impact of N-plasma and Ga-irradiation on MoS₂ layer in molecular beam epitaxy

Recent interest in two-dimensional materials has resulted in ultra-thin devices based on the transfer of transition metal dichalcogenides (TMDs) onto other TMDs or III-nitride materials. In this investigation, we realized p-type monolayer (ML) MoS₂, and intrinsic GaN/p-type MoS₂ heterojunction by the GaN overgrowth on ML-MoS₂/c-sapphire using the plasma-assisted molecular beam epitaxy. A systematic nitrogen plasma (N 2) and gallium (Ga) irradiation studies are employed to understand the individual effect on the doping levels of ML-MoS₂, which is evaluated by micro-Raman and high-resolution X-Ray photoelectron spectroscopy (HRXPS) measurements. With both methods, p-type doping was attained and was verified by softening and strengthening of characteristics phonon modes E 2 g 1 and A 1 g from Raman spectroscopy. With adequate N 2-irradiation (3 min), respective shift of 1.79 cm^-1 for A 1 g and 1.11 cm^-1 for E 2 g 1 are obtained while short term Ga-irradiated (30 s) exhibits the shift of 1.51 cm^-1 for A 1 g and 0.93 cm^-1 for E 2 g 1. Moreover, in HRXPS valence band spectra analysis, the position of valence band maximum measured with respect to the Fermi level is determined to evaluate the type of doping levels in ML-MoS₂. The observed values of valance band maximum are reduced to 0.5, and 0.2 eV from the intrinsic value of 1.0 eV for N 2- and Ga-irradiated MoS₂ layers, which confirms the p-type doping of ML-MoS₂. Further p-type doping is verified by Hall effect measurements. Thus, by GaN overgrowth, we attained the building block of intrinsic GaN/p-type MoS₂ heterojunction. Through this work, we have provided the platform for the realization of dissimilar heterostructure via monolithic approach.