Yoshifumi Ikoma

Growth of Si/3C–SiC/Si(100) heterostructures by pulsed supersonic free jets

We have investigated the epitaxial growth of multilayer structures of Si/3C–SiC/Si(100) by pulsed supersonic free jets of methylsilane (CH3SiH3) for SiC growth and trisilane (Si3H8) for Si growth. Epitaxial Si layers were obtained only on very thin (≈3 nm) 3C–SiC epitaxial layers, while polycrystalline Si was grown on thicker 3C–SiC layers. It was also found that the transition regions with a thickness of ≈1 nm existed at the interface between epitaxial 3C–SiC and Si layers by high-resolution transmission electron microscopy observation. These results suggest that the surface roughness and thickness of the 3C–SiC layer play an important role for epitaxial growth of Si.

Phase transformation and nanograin refinement of silicon by processing through high-pressure torsion

Si(100) wafers were subjected to severe plastic deformation under a pressure of 24 GPa using high-pressure torsion (HPT). Si wafers were plastically deformed at room temperature. HPT-processed samples were composed of metastable body centered cubic Si-III and rhombohedral Si-XII phases in the initial cubic diamond Si-I. The volume fraction of metastable phases increased with increasing plastic strain. Successive annealing at 873 K led to the reverse transformation of metastable phases. A broad photoluminescence peak centered at about 650 nm appears due to the reverse transformation of Si-III/Si-XII nanograins and the reduction of number of defects in Si-I nanograins.

Growth of Ultrathin Epitaxial 3C-SiC Films on Si(100) by Pulsed Supersonic Free Jets of CH3SiH3

We have developed a new method for epitaxial growth of ultrathin (~nm) 3C-SiC films on Si(100) by pulsed supersonic free jets of methylsilane (CH3SiH3). It was found that pit formation at the SiC/Si(100) interface was suppressed by increasing the pulse width and the surface roughness was decreased by decreasing the number of CH3SiH3 jet pulses. A linear relationship was observed between the film thickness and the pulse number in the thin film region of less than ≈40 nm, while the growth rate was decreased and the thickness was eventually saturated for further pulse irradiation.

Nanograin formation of GaAs by high-pressure torsion

Grain refinement of GaAs was achieved by application of severe plastic deformation through high-pressure torsion (HPT). Ultrafine-grained structures including nanograins were produced by the HPT processing under 24 GPa. An intense photoluminescence (PL) peak in the near infrared corresponding to the band gap of GaAs disappeared after the HPT processing but subsequent annealing gave rise not only to restoration of the original band-gap-related peak but also to an additional PL peak in the visible part of the spectrum on account of quantum confinement in the nanograins.

Plastic Deformation of BaTiO3 Ceramics by High-pressure Torsion and Changes in Phase Transformations, Optical and Dielectric Properties

Ceramics are generally brittle at ambient condition and they can hardly be deformed plastically. In this study, severe plastic deformation was successfully imposed on barium titanate ceramic powders by high-pressure torsion. A tetragonal-to-cubic phase transformation occurred, and the fraction and stability of the cubic phase increased by straining because of the formation of nanograins. BaTiO3 exhibited photoluminescence and the yellow intensity increased after straining because of the formation of large fraction of grain boundaries. The dielectric constant of BaTiO3 was unusually increased by nanograin formation while the Curie temperature remained constant.

Epitaxial growth of 3C–SiC on Si(100) by pulsed supersonic free jets of Si(CH3)4 and Si3H8

Heteroepitaxial growth of 3C–SiC on Si(100) by pulsed supersonic free jets of Si(CH3)4 and Si3H8 with various mixture ratios has been investigated. The heteroepitaxy is achieved at the substrate temperature of 900 °C without any carbonization process. The films grown by pure Si(CH3)4 contain inverse pyramidal pits surrounded by the {111} planes of Si, while {311} faceted pits are formed by mixing Si(CH3)4 with a small amount of Si3H8. When the Si3H8/Si(CH3)4 ratio further increases, pit formation is suppressed and instead Si islands are epitaxially grown from the pit region.

Graphite to diamond-like carbon phase transformation by high-pressure torsion

Diamond-like carbon (DLC) with significant fraction of tetrahedral sp3 bonds and amorphous structure is generally produced in the form of thin films by rapid cooling of high-energy carbon atoms in vacuum. This study shows that DLC can be directly formed from bulk samples of graphite by application of severe plastic deformation under high pressures. The formation of DLC is enhanced with increasing the shear strain, pressure, and temperature. It is suggested that the high pressure thermodynamically stabilize DLC and formation of high density of lattice defects by straining reduces the energy barrier for DLC formation.

High-pressure zinc oxide phase as visible-light-active photocatalyst with narrow band gap

Wide band gap of pure ZnO with wurtzite crystal structure (3.1–3.4 eV) limits its photocatalytic activity to the ultraviolet (UV) region of solar spectrum. High-pressure rocksalt polymorph of ZnO can theoretically show narrow band gap; however, the rocksalt phase is unstable at ambient pressure. Herein, rocksalt phase with large fractions of oxygen vacancies is successfully stabilized at ambient conditions by inducing plastic strain in pure ZnO under 6 GPa using the High-Pressure Torsion (HPT) method. Formation of rocksalt phase reduces the band gap of ZnO to 1.8 eV, which is in good agreement with the first-principles calculations, and significantly improves the photocatalytic activity under visible light.

Phase transformation of germanium by processing through high-pressure torsion: strain and temperature effects

Abstract Germanium has been processed by high-pressure torsion (HPT) under a nominal pressure of 24 GPa at room temperature and cryogenic temperature. The samples processed at room temperature were composed of a diamond-cubic Ge-I phase and a metastable tetragonal Ge-III phase. The formation of Ge-III was significantly suppressed and Ge-I and an amorphous phase, in addition to a small amount of body-centred-cubic Ge-IV, appeared in the case of cryogenic HPT processing at 100 K. The Ge-IV phase gradually disappeared at room temperature. These results indicated that shear strain and thermal energy are important for promoting the formation of Ge-III.

Thermal desorption spectroscopy and molecular beam time-of-flight studies of silicon wafer ultraviolet/ozone cleaning

Ultraviolet (UV)/ozone cleaning is a promising way of decontaminating silicon wafers for industrial very large scale integrated processing. We have investigated the cleaning process in high-vacuum conditions utilizing a pulsed supersonic valve and an excimer lamp. With this setup, we are able to supply atomic oxygen onto Si surfaces with and without the simultaneous UV light exposure. We have discovered that thermal desorption spectroscopy analysis of the surface adsorbed species shows marked differences in the wafers processed under different conditions in high vacuum. We have confirmed that UV irradiation is a very essential factor in the UV/ozone cleaning process. In addition, we have carried out time-resolved studies of species coming off the wafer surfaces during UV/ozone cleaning, and found that the main by-products are CO, CO2, and H2O. The hydrocarbon removal reaction is not oxygen-supply limited.