Carrier Trapping by Oxygen Impurities in Molybdenum Diselenide

Trapping of photo-excited carriers by oxygen impurities is studied with ultrafast pump-probe spectroscopy. In exfoliated multilayer MoSe₂, oxygen impurities are intentionally created with Ar⁺ plasma irradiation and air exposure. After plasma treatment, the signal of transient absorption shows a signature of defect capturing carriers. In CVD grown monolayer MoSe₂, oxygen impurities are induced during the growth process and confirmed with X-ray photoelectron spectroscopy. For both samples, the observed defect state filling shows a clear saturation at high exciton densities, from which the trapping defect densities are estimated from the transient absorption signal. In CVD grown monolayer MoSe₂, the defect density is around 0.5x10¹²/cm². In plasma treated exfoliated sample, the trapping defect density increases with plasma irradiation time. First principle calculations with density functional theory reveal that oxygen atoms occupying Mo vacancies create mid-gap defect states, which are responsible for the carrier trapping, while oxygen atoms occupying chalcogen vacancies can remove the mid-gap state.