Impact of humidity on long-term stability of HfS₂ grown on sapphire substrate by chemical vapor deposition and strategies to prevent native oxidation

Two-dimensional transition metal dichalcogenides (TMDs) are susceptible to native oxidation by oxygen and/or moisture, which can completely alter their physical and chemical properties. Hafnium disulfide (HfS₂), which is expected to exhibit superior properties, also faces significant limitations in long-term device operation due to rapid oxidation into HfS_2-xO_x, resulting in material quality degradation. In this paper, we focus on the native oxidation caused by atmospheric humidity, confirming that the A_1g mode peak of HfS₂ grown by chemical vapor deposition (CVD) completely disappear within a week under high humidity (70 % RH) at room temperature and atmospheric pressure. As countermeasures against oxidation, we deposited polymethyl methacrylate (PMMA) and Al₂O₃ on the HfS₂ surface and evaluated their ability to prevent oxidation by comparing them with native HfS₂ under high and low humidity conditions. Thermodynamic modeling further showed that HfS₂ reacts with O₂ and H₂O to form HfO₂, but not with Al₂O₃, indicating Al₂O₃ effectively protects against HfS₂ oxidation. Our observations give guidance in choosing a protective layer for TMDs to prevent native oxidation.