Hiroshi Sudoh

N–H related defects in GaAsN grown through chemical beam epitaxy

The local vibration modes of N–H related defects in GaAsN are studied using isotopes. When GaAsN is grown through chemical beam epitaxy (CBE) using triethylgallium/tris(dimethylamino)arsenic/monomethylhydrazine gas, there are several local vibration modes (LVMs) in Fourier transform infrared (FTIR) spectra. Signals with stretching mode peaks at 2952, 3098, and 3125 cm−1 are reported, along with new wagging and stretching mode peaks at 960 and 3011 cm−1, which exist only in crystals grown through CBE. When the film is grown using deuterated MMHy as a nitrogen source, new peaks at 2206, 2302, 2318, 2245, and 714 cm−1 appear. This suggests that D related defects are created because of the deuterated MMHy. The ratios of frequencies of these new peaks to those obtained from crystals grown using MMHy are nearly 1.34. This suggests that all defects in GaAsN grown through CBE, which appear as LVMs, are N–H related defects. Especially, those with LVMs at 960 and 3011 cm−1 are new N–H defects only found in GaAsN grown through CBE.

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.

Effects of source gas molecules on N–H- and N–D-related defect formations in GaAsN grown by chemical beam epitaxy

The formation mechanism of N–H-related defects in GaAsN grown by chemical beam epitaxy (CBE) is studied on the basis of the isotope effects on the local vibration modes (LVMs) originating from N–H. When deuterated monomethylhydrazine (MMHy) is used as the N source, LVM signals from the nitrogen–deuterium bond (N–D) are obtained. However, there are still N–H peaks in the IR absorption spectra, which have intensities similar to those of N–D peaks. When the film is grown with deuterated triethylgallium (TEGa), there are no N–D peaks. The peak intensity at 2952 cm−1 increases with increasing tris(dimethylamino)arsenic (TDMAAs) flow rate, and that at 3098 cm−1 is almost constant regardless of the flow rate. These results indicate that H atoms in the N–H-related defects originate from H directly bonded to N in MMHy and CH3 in MMHy and/or H in TDMAAs, not from TEGa.

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.

Atomic Layer Deposition of Thin VNx Film from Tetrakis(diethylamido)vanadium Precursor

Thin vanadium nitride (VNx) films were grown by atomic layer deposition using tetrakis(diethylamido)vanadium as a precursor and NH3 as a reactant. A growth rate of 0.1 nm/cycle without incubation time is obtained. Transmission electron microscopy reveals that a nanocrystalline texture in random orientation is characteristic of the VNx film, which is favorable as an extremely thin barrier application. A low carbon impurity level (~6 at.%) is achieved owing to acceleration of the transamination between the V(NR2)4 precursor and NH3. The lowest resistivity of 120 µΩ cm is successfully achieved for the VNx film prepared under optimized conditions.

Ge2Sb2Te5 Film Fabrication by Tellurization of Chemical Vapor Deposited GeSb

This paper describes stoichiometric Ge2Sb2Te5 (GST) film fabrication by the process based on chemical vapor deposition (CVD). GST films were fabricated by tellurization after GeSb CVD. This two step process enabled to fill high aspect holes. By applying appropriate precursors and process temperature, the surface morphology of the GST film was significantly improved. The moderate tellurization reaction process might contribute GST formation with maintaining the amorphous structure of the CVD GeSb. We believe this technique is useful for phase change memory application.

Effects of arsenic source molecule on N-H related defects formation in GaAsN grown by chemical beam epitaxy

The formation mechanism of N-H related defects in GaAsN grown by chemical beam epitaxy is studied by the isotope effects on the local vibration modes (LVMs). When the deuterated tris(dimethylamino)arsenic (TDMAAs) is used as an arsenic source, new signals appear in the spectrum and they are attributed to the N-D related LVM signals. Therefore, the As source molecule contributes to the formation of N-H related defects in GaAsN. When TDMAAs flow rate increases, the concentration of N-H related defects at 2952cm-1 increases. These results suggest that N-CH3 in the molecule is one of the origins of the N-H at 2952cm-1 in the grown films.