Influence of direct deposition of dielectric materials on the optical response of monolayer WS₂

We investigate the effects of direct deposition of different dielectric materials (AlO_x, SiO_x, SiN_x) onto atomically thin TMDC WS₂ on its optical response using atomic layer deposition (ALD), electron beam evaporation (EBE), plasma-enhanced chemical vapor deposition (PECVD), and magnetron sputtering. The photoluminescence measurements reveal quenching of the excitonic emission after all deposition processes, which is linked to the increased level of charge doping and associated rise of the trion emission and/or the localized (bound) exciton emission. Furthermore, Raman spectroscopy allows us to clearly correlate the observed changes in excitonic emission with the increased levels of lattice disorder and defects. In particular, we show that the different doping levels in a monolayer WS₂ capped by a dielectric material are strongly related to the defects in the WS₂ crystal introduced by all capping methods, except for ALD. The strong charge doping in the ALD-capped sample seems to be caused by other factors, such as deviations in the dielectric layer stoichiometry or chemical reactions on the monolayer surface, which makes ALD distinct from all other techniques. Overall, the EBE process results in the lowest level of doping and defect densities and in the largest spectral weight of the exciton emission in the PL. Sputtering is revealed as the most aggressive dielectric capping method for WS₂, fully quenching its optical response. Our results demonstrate and quantify the effects of direct deposition of dielectric materials onto monolayer WS₂, which can provide valuable guidance for the efforts to integrate monolayer TMDCs into functional optoelectronic devices.