Oxygen-Driven Growth Regulation and Defect Passivation in Chemical Vapor Deposited MoS2 Monolayers

Abstract

Due to the lowest formation energies, sulfur vacancies are inevitable in the vapor-phase chemical vapor deposition (CVD) of MoS2, which act as deep donors and induce midgap defect states, making the material intrinsically n-type. The postgrowth oxygen passivation of such defects has been the subject of a large number of recent studies because passivation of defects augments the photoluminescence quantum yield by several orders. In this study, by introducing an SiO2/Si wafer in close proximity to the growth substrate, we were able to supply trace oxygen in situ during the growth while simultaneously enabling chemisorption of oxygen at defect sites on the basal plane of large-area MoS2 monolayers. Low-temperature photoluminescence spectroscopy allowed us to distinguish clearly the nature of oxygen bonding in defective MoS2 grown with and without the trace oxygen. Chemisorption of oxygen enabled elimination of defect-related bound exciton emission at the near band edge transition of MoS2, leading to about 300% enhancement in the photoluminescence. We observed unusual splitting of the first-order A1g Raman mode in monolayer MoS2 films when the sulfur vacancies are not compensated by oxygen. The present study provides new experimental evidence to better distinguish between chemisorption and physisorption of oxygen and may serve as an effective way to tune the optical properties of van der Waals crystals during the large-area CVD process.