Takumi Ohashi

Multi-layered MoS2 film formed by high-temperature sputtering for enhancement-mode nMOSFETs

A multi-layered MoS2 film was formed on a SiO2 film by high-temperature sputtering, which is one of the alternative methods of Si LSI technology. It was found that the carrier density of a sputter-deposited MoS2 film is 1000 times smaller than that of an exfoliated one. By sputtering, two different orientations, namely a layer lateral to a SiO2/Si substrate and a layer perpendicular to the substrate, were formed. The lateral layer showed a lower carrier density than the perpendicular layer because of the decrease in the number of sulfur vacancies, as commonly discussed in several research studies. However, the vacancies are not sufficient for describing this significant reduction in carrier density. It is considered that a sodium ion functioning as an interface trapped charge is one of the main origins of carriers. Sputtering, which enables us to determine the sodium contamination level, can be seen as appropriate for reducing the carrier density; hence, this method is considered to be efficient in realizing enhancement-mode MoS2 MOSFETs. In addition, sputtering also enable us to form large-scale MoS2 films up to a wafer size. Therefore, a sputter-deposited MoS2 film is a promising material for post-silicon devices.

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.

High-mobility and low-carrier-density sputtered MoS2 film formed by introducing residual sulfur during low-temperature in 3%-H2 annealing for three-dimensional ICs

We investigate the low-temperature formation of MoS2 films by radio frequency (RF) sputtering. This work is focused on reducing the number of sulfur defects and the improving electrical characteristics of sputtered MoS2 films by low-temperature annealing in various atmospheres. 10 nm MoS2 films were synthesized by the RF sputtering at 300 °C and followed by annealing in nitrogen or forming gas (FG: 3% hydrogen in N2) at 200–400 °C. As a result, the compensation for sulfur defects in FG anneal process using residual sulfur gave better results that in N2 annealing. Eventually, a high Hall-effect mobility of 36 cm2 V−1 s−1 and a low carrier density of 1014 cm−3 were achieved.

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.

Resistivity reduction of low-carrier-density sputtered-MoS 2 film using fluorine gas

High-performance and low-power LSIs have been achieved by 3D transistor such as FinFET in logic and DRAM using crystalline-silicon channel and 3D-stacked devices in NAND flash using poly-crystalline-silicon channel [1]. For the future, monolithic transistors using atomic-layer materials are expected above the silicon devices [2]. A transition-metal di-chalcogenide (TMDC) such as molybdenum di-sulfide (MoS2) is one of the promising candidates because of not only a higher mobility but also attractive functionalities such as flexibility and transparency [3]. However, it is very difficult to reduce a contact resistance, especially using doping [4-7].

Low-carrier density sputtered-MoS 2 film by H 2 S annealing for normally-off accumulation-mode FET

We investigate low-temperature formation process of sputtered-MoS<inf>2</inf> film. The MoS2 film was formed by radio frequency (RF) sputtering. Then the sputtered-MoS<inf>2</inf> was annealed in H2S at from 200 to 400°C. We find that the hydrogen sulfur (H<inf>2</inf>S) annealing compensate for sulfur defects at low temperature significantly, resulting in a lower carrier density of 2–10¹⁶ cm⁻³.

Crystallinity improvement using migration-enhancement methods for sputtered-MoS 2 films

We investigate crystallinity of sputtered MoS<inf>2</inf> films formed in various sputtering conditions to enhance the migration. We found that high substrate temperature, high radio frequency (RF) power and long throw were effective for crystallinity improvement of sputtered MoS2 films and in these conditions higher Hall-effect mobility of 12 cm²/V-s and lower carrier density of 10¹⁸ cm⁻³ were achieved.