Impact of N-plasma and Ga-irradiation on MoS_2 layer in molecular beam epitaxy

摘要

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) MoS_2, and intrinsic GaN/p-type MoS_2 heterojunction by the GaN overgrowth on ML-MoS_2/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-MoS_2, 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_(2g)~1 and A_(1g) from Raman spectroscopy. With adequate N_2~*-irradiation (3 min), respective shift of 1.79cm~(-1) for A_(1g) and 1.11cm~(-1) for E_(2g)~1 are obtained while short term Ga-irradiated (30s) exhibits the shift of 1.51 cm~(-1) for A_(1g) and 0.93cm~(-1) for E_(2g)~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-MoS_2. The observed values of valance band maximum are reduced to 0.5, and 0.2 eV from the intrinsic value of ≈1.0eV for N_2~*- and Ga-irradiated MoS_2 layers, which confirms the p-type doping of ML-MoS_2. Further p-type doping is verified by Hall effect measurements. Thus, by GaN overgrowth, we attained the building block of intrinsic GaN/p-type MoS_2 heterojunction. Through this work, we have provided the platform for the realization of dissimilar heterostructure via monolithic approach.