Masahiro Yoshimoto

Effects of densification on fluorescence spectra and glass structure of Eu3+‐doped borate glasses

Densified glass specimens of 90 B2O3⋅10 Na2O⋅1 Eu2O3 were obtained by applying hydrostatic pressure up to 6 GPa at various temperatures from 250 to 900u2009°C, and their densities and inhomogeneous bandwidths of Eu3+ fluorescence spectra were determined in order to follow a structural change taking place during densification. The results indicate that the role of hydrostatic pressure is first to eliminate the atomic scale voids usually appearing when quenched from high temperatures and then to increase the fluctuation of local fields around Eu3+ probably due to the distortion of glass network accompanied with a wide variation of bond length. The molecular dynamics simulation of the densified state was also carried out to support the above conclusion.

Preparation of hydrogenated amorphous Si-C alloy films and their properties

Abstract Hydrogenated amorphous Si1−xCx films were prepared by glow discharge decomposition from a mixture of SiH4 and C2H4 at substrate temperatures between room temperature and 260°C under a low r.f. power condition. The deposition rate, hydrogen content, optical band gap and photoluminescence were investigated. These films contain hydrogen atoms at concentrations of 40–70 at.%. The optical band gap and photoluminescence peak energy increase monotonically with x and also with decreasing substrate temperature. Bright bluish white photoluminescence was observed at room temperature in samples of composition Si0.14C0.86. The film structure and the emission mechanism are discussed.

Preferential growth of cubic GaN on sapphire (0001) substrates by metal organic molecular beam epitaxy

Preferential growth of high-quality cubic GaN on sapphire (0001) substrates was realized at 800u2009°C under a Ga-rich condition by metal organic molecular beam epitaxy. Hexagonal GaN was grown under a N-rich condition. On the contrary, under the Ga-rich condition, the growing layer changed from a hexagonal phase to a cubic phase as the growth proceeds, which was verified by in situ reflection high-energy electron diffraction. The low-temperature photoluminescence (PL) of this layer was dominated by a sharp and intense excitonic emission of cubic GaN, indicating high crystal quality. The results of PL and x-ray diffraction indicate that the polytype of GaN can be intentionally controlled by changing V/III ratios without serious degradation of crystal quality. In addition, enhancement of cubic phase growth by using a low-temperature grown buffer layer is also discussed.

Bonding structures in highly photoconductive a-SiC:H films deposited by hybrid-plasma chemical vapor deposition

Abstract Hydrogenated amorphous silicon-carbon (a-Si1 − xCx:H) was deposited by a new deposition method of hybrid-plasma chemical vapor deposition. In this method, CH4 pre-cracked by a microwave plasma is introduced to a chamber where is decomposed by a radio-frequency plasma. A-Si1 − xCx:H films deposited by this method show high-photoconductivity in the range of optical bandgap from 2.0 to 2.3 eV. Bonding structures in the films were analyzed using X-ray photoelectron spectroscopy and Fourier transform infrared absorption spectroscopy. Films contain graphitic Cue5f8C bonds appearing in graphite, and tetrahedral Siue5f8C bonds appearing in crystalline SiC, depending on the deposition conditions. High-performance photoelectric properties are obtained in undoped films with a lot of tetrahedral bonding.

Efficient photo‐enhancement of GaP and AlGaP growth in chemical beam epitaxy

The growth rate of GaP and AlGaP epitaxial layers was efficiently enhanced by N2‐laser irradiation at low substrate temperatures in chemical beam epitaxy. The photo‐enhancement efficiency, defined as the ratio of the number of deposited Ga atoms to the number of irradiating photons per unit area per unit time, is estimated to be 7×10−4. The decomposition of triethylaluminum was also enhanced by the irradiation, but less efficiently.

Laser-assisted atomic layer epitaxy of GaP in chemical beam epitaxy

Abstract A new atomic layer epitaxy using a photo-process without alternate gas supply in chemical beam epitaxy is proposed. The effects of ultraviolet-laser irradiation on GaP growth at low substrate temperatures were studied in detail using triethylgallium (TEGa) and PH3. The growth rate of GaP was enhanced by ultraviolet light emitted from an N2 laser. The growth rate increased with photon number and saturated above ∼1017 photons cm-2 pulse. Under enough light intensity (2.7 × 1017 photons cm-2 pulse), the growth rate was kept constant in a certain range of TEGa flow rate by N2 laser irradiation. This result opens monolayer growth by UV-irradiation under a simultaneous supply of source gases.

Polarization effects in AC electroluminescence of a-Si1−xCx:H

Abstract Visible alternating current (AC) electroluminescence is studied with a sandwich structure of Y2O3/a-Si0.17C0.83:H/Y2O3. The applied voltage should exceed a critical value for initiating electroluminescence, which implies the association of hot carriers. The polarization effects in the electroluminescence suggest the contribution of trapped carriers at the interface between Y2O3 and a-Si0.17C0.83:H.

Carbon background in P-based III-V semiconductors grown by metalorganic molecular beam epitaxy using ethyl-metalorganic sources

Abstract Electrical properties of InP, GaP, InGaP and AlGaP, grown by metalorganic molecular beam epitaxy (MOMBE) using ethyl-metalorganics (triethyl-indium, -gallium and -alluminum), are discussed in connection with the carbon background. More C atoms are incorporated into epilayers in the order of AlGaP, GaP and InP. The C atoms act as acceptors in AlGaP and GaP epilayers, but they probably work as donors in InP. InGaP showed highly resistive or compensated possibly due to the amphoteric nature of C atoms.

In-situ RHEED observation on surface reactions in laser-triggered chemical beam epitaxy of GaP

Abstract The surface reaction triggered by pulsed laser irradiation in chemical beam epitaxy (CBE) for GaP growth using triethylgallium (TEGa) and PH 3 was investigated with reflection high-energy electron diffraction (RHEED). The intensity of RHEED showed an abrupt decrease on laser irradiation followed by gradual recovery. The RHEED intensity recovered faster with a higher PH 3 flow rate, which indicates that the intensity recovery is related with the surface reaction between Ga atoms generated by laser irradiation and P atoms. This result showed that the decomposition of TEGa by laser irradiation and the formation of GaP occur alternately under continuous supply of source gases.

Low-hydrogen-concentration a-Si:H deposited by direct photo-CVD

Abstract a-Si:H films are deposited by a direct VUV (147 nm) photo-CVD method. The films deposited at 200–400°C have a low hydrogen concentration of less than 9 at% and hydrogen bonding configuration of mainly Si-H. The film deposited at 300°C has a low defect-state density of 2.2 × 10 15 cm −3 and Urbach energy of 52 meV, which are estimated from below-gap absorption coefficients. The mobility gap determined by an internal photoemission method is close to the optical gap, which suggests that the width of localized states at band edges is narrow.