Influence of sulfur vacancies on the high frequency phonon modes of ALD-grown monolayer MoS₂

Precise control of defects in two-dimensional materials is crucial for their application in functional devices. Raman spectroscopy is a key technique for identifying the number of layers and strain in MoS₂. Changes in the peak area ratio of the Raman active modes A_1g and E_2g¹ measured on monolayer (ML) MoS₂ have, so far, only been correlated to changes in the excitation wavelength of the laser, i.e., resonant effects. However, the role of sulfur vacancies (VS) on this peak area ratio is not well studied. In this study, we show on ALD-grown MoS₂ that changes in the peak area ratio of the A_1g and E_2g¹ Raman modes depend sensibly on the number of VS in the film. Raman measurements carried out with three different excitation wavelengths are combined with detailed x-ray photoelectron spectroscopy measurements to quantify the number of VS in the film and to study their impact on vibrational properties. VS-poor MoS₂ displays a stronger A_1g mode compared to the E_2g¹ mode, whereas the ratio is reversed in VS-rich films. We propose that this effect originates from modifications in the electronic band structure of ML MoS₂. Based on this experimental work, we propose that the peak area ratio of the A_1g and E_2g¹ modes could be used as a fingerprint for the qualitative evaluation of the S deficiency in MoS₂ MLs, which could be an easy-to-apply method for the quality assessment in industrial applications.