Impact of thermal annealing in forming gas on the optical and electrical properties of MoS 2 monolayer.

Technological applications involving 2D MoS ₂ require transfer of chemical vapor deposition (CVD) grown material from its original substrate and subsequent lithographic processes. Inevitably, those steps contaminate the surface of the 2D material with polymeric residues affecting the electronic and optical properties of the MoS ₂ . Annealing in forming gas is considered an efficient treatment to partially remove such residues. However, hydrogen also interacts with MoS ₂ creating or saturating sulfur vacancies. Sulfur vacancies are known to be at the origin of n-doping evident in the majority of as-grown MoS ₂ samples. In this context, investigating the impact of thermal annealing in forming gas on the electronic and optical properties of MoS ₂ monolayer is technologically important. In order to address this topic, we have systematically studied the evolution of CVD grown MoS ₂ monolayer using Raman spectroscopy, photoluminescence, x-ray photoelectron spectroscopy and transport measurements through a series of thermal annealing in forming gas at temperatures up to 500 °C. Efficient removal of the polymeric residues is demonstrated at temperatures as low as 200 °C. Above this value, carrier density modulation is identified by photoluminescence, x-ray photoelectron spectroscopy and electrical characterization and is correlated to the creation of sulfur vacancies. Finally, the degradation of the MoS ₂ single layer is verified with annealing at or above 350 °C through Raman and photocurrent measurements.
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