Shuichi Emura

Infrared study of amorphous B1−xCx films

Amorphous boron carbide (a‐B1−xCx) is believed to have an icosahedron‐based random network. In this paper, vibrational properties of a‐B1−xCx films are studied by IR and Raman spectra, placing particular emphasis on the interpretation of the most prominent 1100‐cm−1 band associated with the B–C bond. The 1100‐cm−1 band appears in both IR and Raman spectra, and the frequency variation and the intensity as a function of the C content are examined, together with evaluation of the absolute absorption coefficients. Within the framework of the impurity‐induced vibration theory, the 1100‐cm−1 band is characterized. The estimation of the force constants by the observed frequencies leads to a conclusion that the vibration can be classified as an extrinsic and preferably a local mode. This vibration is well described as the stretching mode of a localized two‐center bond between the B and C atoms. In this sense, the C atom in amorphous B1−xCx is not regarded as a network constituent. The frequency shift with the C c...

Electronic structure of Eu@C60 studied by XANES and UV-VIS absorption spectra

Eu endohedral C60, Eu@C60, has been extracted with aniline from soot prepared by arc-heating of a graphite/Eu2O3 composite rod and obtained at high concentration by combining sublimation and repeated high-performance liquid chromatography. The laser desorption time-of-flight mass spectrum showed a pronounced peak of Eu@C60+. The UV–VIS absorption spectrum of this sample has a red-shift in the onset (>900 nm) in comparison with those for C60 and C70, as expected from electron transfer from the Eu atom to the C60 cage. The valence state of the Eu atom in Eu@C60 has been determined to be +2 by Eu LIII-edge XANES.

Exafs study of the fluorite and pyrochlore compounds in the system ZrO2-Gd2O3

Abstract The local structures of the fluorite and pyrochlore compounds in the system (1-x)ZrO2-xGdO1.5 (0.20≤×0.62) have been studied by means of EXAFS spectroscopy. The EXAFS results indicate that trivalent Gd ions are 8-coordinated in both the fluorite-type and the pyrochlore-type solid solutions in the compositional range 0.2≤×0.5, and that the mean coordination number of tetravalent Zr ions decreases from eight to six with increasing Gd content. This indicates that the oxygen vacancies are located adjacent to tetravalent Zr ions. Cluster models which are a part of the pyrochlore structure, are proposed in the region of the fluorite phase.

Raman spectra of AlxIn1−xAs grown by molecular‐beam epitaxy

Raman study of the lattice vibrations of a ternary alloy AlxIn1−xAs on InP substrate over the whole range of compositions is reported. The Raman spectra show a two‐mode‐type behavior involving AlAs‐ and InAs‐like longitudinal optical‐phonon modes. The frequency of the AlAs‐like mode strongly depends on the composition, but frequency of the InAs‐like mode is almost independent of composition. A disorder‐induced scattering by acoustic vibrations is also observed.

Lattice vibrations and the bonding nature of boron carbide

The vibrations of boron carbide have been studied by the valence force model. There have been so far several ambiguities in the interpretation of experimental spectra, which made theoretical study difficult. This paper solves the main difficulties, that is, characterization of the highest band and most of the Raman bands, providing reasonable interpretations for low-frequency modes. The band is ascribed to the anti-symmetric stretching of the linear chain. The necessity of large angle-bending forces of carbon atom arose for better fitting, which fact can be regarded as evidence for formation of strong covalent bonds around this atom. The present analysis also throws a new light on the Longuet-Higgins interpretation of the role of carbon incorporation. The coexistence of the strong covalent bonds of carbon atoms and the relatively weak icosahedral bonds results in such a balance of bonding that the intra-icosahedral and rhombohedral bonds are further weakened, and the covalent bonds of the carbon chain are instead reinforced.

Large magnetization in high Gd concentration GaGdN and Si-doped GaGdN grown at low temperatures

GaGdN layers were grown at temperatures below 300°C by radio-frequency plasma-assisted molecular-beam epitaxy on sapphire substrates. GaGdN samples with high Gd concentration as high as 12.5% were obtained. X-ray diffraction results showed no obvious secondary phase, which means that the phase separation can be suppressed by the growth at low temperatures. Local structure around the Gd atom was investigated by extended x-ray absorption fine structure measurement using GdLIII edge. It was shown that Gd atoms were mainly incorporated into the Ga sites in the GaGdN layers. All the samples grown at low temperatures exhibited ferromagnetic characteristics. It is considerable that electrons coming from defects, especially, nitrogen vacancy, stabilize ferromagnetism, and that the carrier-induced ferromagnetism occurs in the low-temperature-grown GaGdN. Furthermore, Si was doped into GaGdN layers during growth in order to increase carrier density, and larger magnetization was observed.

Study of Ga ion implantation damage and annealing effect in Sn‐doped InP using Raman scattering

The damage of Sn‐doped InP by Ga ion implantation with fluences ranging from 1×1013 to 5×1014 cm−2 and annealing effects (150–650 °C) are investigated by means of Raman scattering. The shift and asymmetrical broadening of a longitudinal optical phonon peak and the appearance of transverse optical mode and disorder‐activated acoustic modes show that the damage effect by Ga ion implantation is very large, and the crystalline structure becomes amorphouslike at a fluence as low as 1×1014 cm−2. The damaged state is investigated in terms of the spatial correlation model and quantitatively estimated by comparing the Raman peaks for longitudinal optical and transverse acoustic phonon modes with the theoretical calculations. It is shown that the correlation lengths at 1×1013 cm−2 are 36 and 29 A, respectively. This largely damaged structure is found to be recovered by annealing at as low as 250 °C.

Gas source molecular beam epitaxy growth of short period GaP/AlP(001) superlattices

Short period GaP/AlP superlattices are grown on GaP and GaAs substrates at 600 °C by gas source molecular beam epitaxy with growth interruption. Alternating monolayer growth of GaP and AlP is confirmed by the observation of the reflection high‐energy electron diffraction intensity oscillations during growth. The formation of short period superlattice structures and the zone‐folded LO phonons are observed in the x‐ray diffraction rocking curves and Raman spectra, respectively.

EXAFS study of the fluorite-type compounds in the systems ( 1 − x ) ZrO2−xYbO1.5 ( x = 0.18 ≦ x ≦ 0.5 ) and Zr2Ln2O7 (Ln = Tb, Dy, Ho, Er, and Yb)

Abstract The local structures of the fluorite-type compounds with a composition of Zr 2 Ln 2 O 7 ( Ln = Gd, Dy, Ho, Er and Yb) have been studied by means of EXAFS spectroscopy. The EXAFS results indicate that Ln 3+ ions are essentially 8-coordinated by oxide ions but Zr 4+ ions are 6-coordinated. The distinct difference in coordination number between Zr 4+ and Ln 3+ ions means that the local structures of the fluorite-type Zr 2 Ln 2 O 7 are similar to the pyrochlore-type structure, even though both Zr 4+ and Ln 3+3 ions are located statistically at the cation sites (4a site) in the fluorite-type lattice. The local structure of the fluorite-type solid solution in the system ( 1 − x )ZrO 2 − x YbO 1.5 (0.18≦ x ≦0.50) has been also studied by EXAFS spectroscopy. The result indicates that the coordination numbers of Zr 4+ and Yb 3+ ions are 6 and 8, respectively, in the solid solution in the compositional range 0.18≦ x ≦0.50, and that the oxygen vacancies are located adjacent to Zr 4+ ions.

RAMAN SCATTERING STUDY OF THERMAL INTERDIFFUSION IN INGAAS/INP SUPERLATTICE STRUCTURES

Interdiffusion process of InGaAs/InP superlattice structures by thermal annealing of 700–850 °C is studied by Raman spectroscopy. Peak intensities and peak energies of InAs‐, GaAs‐, and InP‐like longitudinal optical (LO) phonon modes change with thermal annealing temperature and time. Depth profiles of the group III and group V atoms are estimated quantitatively by measuring the variations of the peak energies of the LO phonon modes and the ratios of the mode intensities. The energy shift of the GaAs‐like LO peak showing a small broadening, the existence of nonenergy shift InP LO phonon peak and the emerging of InP‐like LO peak in the lower energy region indicate that the interdiffused superlattice consists of uniform compositional InGaAsP well and InP barrier layers and sharp interfaces. It is found that the resulting InGaAsP quaternary alloy is roughly lattice‐matched to InP (<±0.5%). It is also found that the diffusion coefficient in the well region is larger than that in the barrier region, and that t...