Seiya Ishihara

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.

Ge homoepitaxial growth by metal–organic chemical vapor deposition using t-C4H9GeH3

We succeeded in growing Ge homoepitaxial films by metal–organic chemical vapor deposition (MOCVD) using tertiarybutylgermane (t-C4H9GeH3). We synthesized and investigated the characteristics of t-C4H9GeH3. The vapor pressure was sufficiently high in the CVD process. The precursor was sufficiently safe as it did not have a pyrophoric and explosive nature. The Ge homoepitaxial growth was achieved at 360 °C under reduced pressure on an appropriately cleaned Ge(001) substrate.

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.

Pattern-dependent anisotropic stress evaluation in SiGe epitaxially grown on a Si substrate with selective Ar+ion implantation using oil-immersion Raman spectroscopy

Biaxial stress states in SiGe stripes on Si substrates fabricated by a novel selective Ar+ ion implantation technique were evaluated by oil-immersion Raman spectroscopy. The oil-immersion technique is appropriate for the measurement of strain induced in nanostructure devices, because it has a higher spatial resolution than conventional Raman spectroscopy and can evaluate anisotropic stress states owing to the excitation of multiple optical phonon modes. Results indicate that quasi-uniaxial stress states exist in SiGe layers in unimplanted Si areas, which depends on the stripe-width ratio of implanted and unimplanted areas, and that quasi-uniaxial stress states are successfully induced in SiGe by the present technique, which can be considered as the channel materials of high-performance transistors.