Enhanced sulfurization reaction of molybdenum using a thermal cracker for forming two-dimensional MoS2 layers

Abstract

We propose a method to fabricate two-dimensional (2D) molybdenum disulfide (MoS₂) layers to overcome issues in typical fabrication processes by promoting the sulfurization reaction of molybdenum (Mo). A thin sputtered-Mo layer was sulfurized using a sulfur (S) thermal cracker to form 2D MoS₂ layers. The effects of key process parameters such as cracking-zone temperature (T_C-zone), thickness of the sputtered-Mo layer, and Ar pressure during deposition of the Mo layer were systematically investigated. The degree of thermal treatment of evaporated S vapor is controlled by varying T_C-zone. The higher T_C-zone enabled easy formation of thin MoS₂ layers at a low substrate temperature of 250 °C due to the greatly enhanced sulfurization reaction. The thickness of the final MoS₂ layers was controlled by changing the initial thickness of the sputtered-Mo film. Ultra-thin MoS₂ film about 2-layers-thick was obtained by sulfurizing a 2 Å-thick Mo film. The chemical state of the MoS₂ layers largely depended on the Ar pressure during the sputtering process of the initial Mo. Lower Ar pressure enhanced MoS₂ formation due to more efficient substitution of the MoS₂ phase for the MoO₃ phase. By using the S thermal cracker, we demonstrate a method to easily fabricate 2D MoS₂ layers, excluding some problematic issues such as toxic and expensive reactants, non-vacuum conditions susceptible to contamination, and high substrate temperature.