Experimental verification of SO₂ and S desorption contributing to defect formation in MoS₂ by thermal desorption...

The defect-free surface of MoS₂ is of high importance for applications in electronic devices. Theoretical calculations have predicted that oxidative etching could be responsible for sulfur vacancy formation. No direct experimental evidence, however, points out the role of adsorbed oxygen on sulfur vacancy formation for MoS₂, especially on an insulating SiO₂/Si substrate. Herein, by applying thermal desorption spectroscopy, we found that sulfur loss can be tightly coupled to adsorbed oxygen, as confirmed by observation of SO₂ desorption. With annealing MoS₂, even under ultrahigh vacuum, oxygen molecules adsorbed on MoS₂ assist the sulfur atom in dissociating from MoS₂, and thus, defects are formed as the result of SO₂ desorption from 200 °C to 600 °C. At higher temperatures (over 800 °C), on the other hand, direct sulfur desorption becomes dominant. This finding can be well explained by combining the morphology investigation enabled by atomic layer deposition at defective sites and optical transitions observed by photoluminescence measurements. Moreover, a preannealing treatment prior to exfoliation was found to be an effective method to remove the adsorbed oxygen, thus preventing defect formation.