Yusuke Hibino

Improving crystalline quality of sputtering-deposited MoS2 thin film by postdeposition sulfurization annealing using (t-C4H9)2S2

A sputtered MoS2 thin film is a candidate for realizing enhancement-mode MoS2 metal–oxide–semiconductor field-effect transistors (MOSFETs). However, there are some sulfur vacancies in the film, which degrade the device performance. In this study, we performed postdeposition sulfurization annealing (PSA) on a sputtered MoS2 thin film in order to complement sulfur vacancies, and we investigated the fundamental properties of the MoS2 film. As a result, a high-quality crystalline 10-layer MoS2 film with an ideal stoichiometric composition was obtained at a relatively low process temperature (500 °C). The MoS2 film had an indirect bandgap of 1.36 eV and a high Hall mobility compared with the as-deposited sputtered MoS2 film.

Properties of single-layer MoS2 film fabricated by combination of sputtering deposition and post deposition sulfurization annealing using (t-C4H9)2S2

The fabrication of a high-quality single-layer MoS2 film was achieved at a sufficiently low temperature of 500 °C by the combination of sputtering deposition and post deposition sulfurization annealing. Fabrication only by sputtering produces unintentionally sulfur-deficient nonstoichiometric films with poor crystalline quality in nature, making it difficult to fabricate atomically thin sputtered MoS2 films, especially with a single layer. From the results of the sulfurization annealing, sulfur deficiencies in the film were fully complemented and the crystalline quality, especially in-plane symmetry, was dramatically improved. The quasi-layered structure of the sputtered-MoS2 film led to the success in achieving low-temperature sulfurization annealing. Moreover, the film had large area uniformity, accurate thickness controllability, a direct bandgap of 1.86 eV, and an extremely high visible transmittance of more than 97%. Therefore, we consider that the fabrication technique will contribute to realizing MoS2 display applications such as a low-power-consumption thin-film-transistor liquid crystal display.

Quantitative relationship between sputter-deposited-MoS2 properties and underlying-SiO2 surface roughness

Substrate roughness affects the physical and electrical properties of deposited layered materials. However, the quantitative relationship is unknown. In this work, a quantitative analysis of sputter-deposited MoS2 films on an SiO2 substrate was conducted. Flattening the substrate helped realize an MoS2 structure closer to the ideal honeycomb structure and a Hall mobility of ~26 cm2/(Vs) and a carrier density of ~1016 cm−3 (less than that of exfoliated MoS2 by 104). These results stress the necessity of considering even roughness of the order of angstroms to improve the physical and electrical properties of atomically layered functional devices.