Characteristics of Sputtered p-Type MoS₂ Thin Films and Their TFT Application

Abstract

Two-dimensional transition-metal dichalcogenides have recently emerged as a promising material system. In particular, molybdenum (IV) sulfide (MoS₂) films have been widely investigated as one of the most promising devices for next-generation electronic transistor applications. In this study, an MoS₂ film consisting of small crystalline particles and containing a small amount of oxygen atoms substituted for sulfur atoms is deposited by radiofrequency sputtering. The film's optical bandgap is ~1.25 eV, and it exhibits p-type conduction properties. According to first-principles calculations, the oxygen doping produces a new state immediately above the valence band edge, which might explain the p-type conductivity of the as-sputtered MoS₂ film. We fabricated p-channel thin-film transistors (TFTs). Simulation and Kelvin probe analysis of the TFT indicated that Cu is the most suitable material to make ohmic contact between the source and drain contacts among the metals we compared. The mobility of the TFT was found to be ~31.47 cm²/(V·s) initially. After the TFT was annealed at 200°C for 1 h in air, its mobility increased to 52.42 cm²/(V·s). Analyses by Raman spectroscopy and X-ray photoelectron spectroscopy indicated that the sputtered film contained numerous sulfur defects. To compensate for the defects, we carried out oxidization by hot-wire. As a result, the mobility increased to 62.73 cm²/(V·s). Moreover, to reduce the number metal–MoS₂ interfaces at sulfur defects, a sulfide treatment was conducted at 200°C for 1 h. An ON/OFF ratio of 2.85×10⁻⁶ and mobility of 54.92 cm²/(V·s) were achieved after the sulfurization.