ALBERT

Notes: Annihilation characteristics of positrons in SiO2/Si structure were studied by using a monoenergetic positron beam in the temperature range between 50 K and room temperature. In the SiO2 film, positrons formed positronium (Ps) and they annihilated from localized states in open spaces. Below 100 K, the Ps formation was found to be suppressed. This fact was attributed to the trapping of positrons by point defects in the SiO2 film at low temperature. The depth distribution of such traps was not homogeneous; its concentration in the central region of the SiO2 film was higher than that in the regions near the surface or the interface. The potential of monoenergetic positrons for the detection of the point defects provides unique information for depth distributions of hole traps in SiO2 films. © 1995 American Institute of Physics.

Notes: The annihilation characteristics of positrons in SiO2 films grown on Si substrates were studied by using monoenergetic positron beams. Doppler broadening profiles of the annihilation radiation and lifetime spectra of positrons were measured as a function of incident positron energy for SiO2 (166 nm)/Si specimens fabricated by thermal oxidation. From the measurements, it was found that about 90% of positrons implanted into the SiO2 film annihilate from positronium (Ps) states. This fact was due to the trapping of positrons by open-space defects and a resultant enhanced formation of Ps in such regions. For the SiO2 film grown at 650 °C, the lifetime of ortho-Ps was found to be shorter than that in the film grown at 1000 °C. This result suggests that the volume of open-space defects in the SiO2 film decreased with decreasing the growth rate of the SiO2 film.

Notes: Frequency dependence of power loss of MnZn ferrite was investigated in relation with the absolute complex permeability, μa(=μa−jμa″), in the frequency range 10 kHz–5 MHz and the temperature range 23–120 °C. Residual loss, Pr contributes more than half of the total power loss, Pc, at a frequency above 1 MHz and exhibits a close relationship with μa″. The specimen with grain size of 4 μm and ZnO content of 0 mol % indicates μa′ of 1280 and Pc of 140 kW/m3 at 1 MHz and 50 mT. In the second measurement at the remanent state after the dc magnetization of 400 A/m, these values, however, increased and then gradually recovered with time. This behavior is considered to be related to the diffusion of Fe2+ through cation vacancy. © 2000 American Institute of Physics.

Notes: Heteroepitaxial layers of InP with thickness D ranging from 0.1 to 6.0 μm were grown by low-pressure metalorganic chemical vapor deposition on (001) surfaces of GaAs substrates. Their dislocation structure, induced strains, and nature of the radiative recombinations were investigated as a function of D with transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy. For D〈2 μm, the films are highly dislocated with a tangle of interfacial and threading dislocations above the heterointerface. The spatial extent of the interfacial dislocations and the density of threading dislocations increase with increasing D. For D(approximately-greater-than)2 μm the portion of the layers away from the heterointerface by more than 1.5 μm shows a decrease in the density of threading dislocations and a dramatic improvement in the crystalline quality with increasing D.Typical dislocation densities in the neighborhood of the top surface are in the mid 107 cm−2 range when D surpasses 4.0 μm. Concomitant with the improved crystalline quality, the following observations are made. Firstly, the full width at half maximum of the x-ray rocking curves diminishes from values larger than 500 arcsec for D〈1.0 μm to about 200 arcsec for D(approximately-greater-than)4.0 μm. Secondly, the near-band-edge photoluminescence transitions, which for D〈2.0 μm are predominantly determined by defect-induced band tailing, display excitonic character. Thirdly, below-band-gap transitions due to interfacial defects decrease in intensity. Biaxial compressive strain is present in the layers because of lattice mismatch and differences in linear thermal expansion with the substrate.The strain removes the degeneracy between the light- and heavy-hole states at the top of the valence band, and consequently with increasing temperature above 12 K recombinations from the conduction to the split valence bands are observed in the photoluminescence spectra for all D. The identification of such transitions follows from their temperature dependence and the activation energy yield for the thermalization of the holes. The measured valence-band splitting decreases from 12.5 meV for D=0.3 μm to saturation values of 5.6 meV for D(approximately-greater-than)3.0 μm, indicating strain relaxation with D in qualitative agreement with x-ray determinations. Quantitative differences between both methods are realized and are attributed to a temperature dependence of the differential linear thermal expansion. The contribution to the strain from the lattice mismatch is much larger than expected from equilibrium models. The dislocation generation at different stages during the growth is inferred from the strain relaxation against D and the observed location of the dislocations throughout the layers.

Notes: GaAs layers grown on (001) InP surfaces by low-pressure metalorganic chemical vapor deposition were investigated with photoluminescence spectroscopy, x-ray diffraction, Raman scattering, and transmission electron microscopy. Correlations between the optoelectronic properties, the strain relaxation, and the structural defects were established for layer thickness D ranging between 0.1 and 3.0 μm. A comparison with the case of InP layers grown on GaAs substrates is presented. Radiative recombinations to split light- and heavy-hole valence bands near the zone center are seen at 12 K in the photoluminescence spectra. The splitting is due to a biaxial tensile strain. With increasing temperature, the heavy-hole transitions gain intensity and at around 140 K they are the only features in the spectra. In the 12–50 K temperature range the intensity ratio between the heavy- and light-hole transitions also depends on laser power. The hole activation energy determined from the temperature dependence of the intensity ratio above 50 K agrees with the valence-band splitting. The strain for D(approximately-greater-than)0.3 μm arises from differences in linear thermal expansion and has contributions from the lattice mismatch in thinner layers. The strain values yielded by x-ray diffraction are smaller than those obtained from the valence-band splitting measured with photoluminescence. The difference is attributed to a temperature dependence of the linear thermal expansion, which was corroborated by the shifts of the longitudinal optical phonon frequencies measured with Raman spectroscopy at 300 and 12 K. A comparison is made of the absolute magnitude of the strain and the x-ray diffraction linewidths for heteroepitaxial GaAs and InP layers on InP and GaAs substrates, respectively. The contribution to the strain from the lattice mismatch relaxes in GaAs faster than in InP and the GaAs x-ray linewidths are narrower for D〈1.0 μm. These results are understood in terms of the growth habit and the behavior of threading and interfacial dislocations.

Notes: Heteroepitaxial InP layers were grown by metalorganic chemical vapor deposition on novel GaAs buffer layers on Si substrates. The GaAs buffers were prepared by molecular beam epitaxy and show a reduced threading dislocation density achieved by inserting one nanometer thick Si interlayers. The structural and optoelectronic properties of the heteroepitaxial InP layers, which were investigated by transmission electron microscopy, x-ray diffraction, and photoluminescence spectroscopy, show improvements attributed to the optimized GaAs buffer layers.

Notes: Self-sustained pulsating optical power and kink level in AlGaAs semiconductor lasers are remarkably improved by introducing a multiquantum well (MQW) structure in the active layer. Stable fundamental transverse mode operation at output power up to 50 mW and self-sustained pulsation at output power up to 40 mW are obtained simultaneously for MQW lasers with antireflective and high-reflective coatings on the facets. Low-noise characteristics (relative intensity noise of less than 10−13 Hz−1 under 3–4% optical feedback) are obtained in the output power range from 7 to 17 mW in MQW lasers with high-reflective coating on the rear facet. These results suggest that low-noise high-power characteristics can be achieved in self-sustained pulsating lasers with a MQW active layer.

Notes: The application of microbubbles of pressured CO2 greatly increased CO2 concentration in the solution treated. By treatment at 6 MPa, 35°C and average residence time 15 min, L. brevis was completely inactivated at the level of dissolved CO2,γ≧ 11 (γ, Kuenen's gas absorption coefficient). E. coli and S. cerevisiae required γ≧ 17, and T. versatilis required γ≧ 21 for complete inactivation. Z. rouxii could be sterilized at 20 MPa and 26. A comparison of the continuous and batch method showed that L. brevis was inactivated completely under pressured CO2 〉 0.16 g/cm3 with the continuous method and 〉0.9 g/cm3 with the batch method.

Notes: There have been many reports suggesting the involvement of reactive oxygen species (ROS), including superoxide anion (O2.–), in salt stress. Herein, direct evidence that treatments of cell suspension culture of tobacco (Nicotiana tabacum L.; cell line, BY-2) with various salts of trivalent, divalent and monovalent metals stimulate the immediate production of O2.– is reported. Among the salts tested, LaCl3 and GdCl3 induced the greatest responses in O2.– production, whereas CaCl2 and MgCl2 showed only moderate effects; salts of monovalent metals such as KCl and NaCl induced much lower responses, indicating that there is a strong relationship between the valence of metals and the level of O2.– production. As the valence of the added metals increased from monovalent to divalent and trivalent, the concentrations required for maximal responses were lowered. Although O2.– production by NaCl and KCl required high concentrations associated with hyperosmolarity, the O2.– generation induced by NaCl and KCl was significantly greater than that induced simply by hyperosmolarity. Since an NADPH oxidase inhibitor, diphenyleneiodonium chloride, showed a strong inhibitory effect on the trivalent and divalent cation-induced generation of O2.–, it is likely that cation treatments activate the O2.–-generating activity of NADPH oxidase.