A study of chemical beam epitaxy of GaAs using tris-dimethylaminoarsenic

The growth of GaAs by chemical beam epitaxy using triethylgallium and trisdimethylaminoarsenic has been studied. Reflection high-energy electron diffraction (RHEED) measurements were used to investigate the growth behavior of GaAs over a wide temperature range of 300–550°C. Both group III- and group Vinduced RHEED intensity oscillations were observed, and actual V/III incorporation ratios on the substrate surface were established. Thick GaAs epitaxial layers (2–3 μm) were grown at different substrate temperatures and V/III ratios, and were characterized by the standard van der Pauw-Hall effect measurement and secondary ion mass spectroscopy analysis. The samples grown at substrate temperatures above 490°C showed n-type conduction, while those grown at substrate temperatures below 480°C showed p-type conduction. At a substrate temperature between 490 and 510°C and a V/III ratio of about 1.6, the unintentional doping concentration is n ∼2 × 10¹⁵ cm⁻³ with an electron mobility of 5700 cm²/V·s at 300K and 40000 cm²/V·s at 77K.     

Anisotropy in the quantum lifetime in AlGaN/GaN heterostructures

Magnetoresistance measurements were performed on van der Pauw shaped AlGaN/GaN heterostructures grown on either sapphire or silicon carbide. These measurements revealed the presence of Shubnikov-de Haas oscillations. However, the amplitude of the oscillations originating from perpendicular van der Pauw positions were not isotropic. This anisotropy varied from sample to sample and within a sample its magnitude changed with the carrier density which was modulated by illumination as it induced a persistent photocurrent. The results of this study suggest the anisotropy is either a manifestation of electron density inhomogeneities and/or an indication of a nonuniform scattering mechanism arising from nonuniform interface roughness.     

A Network-Failure-Tolerant Message-Passing System for Terascale Clusters

The Los Alamos Message Passing Interface (LA-MPI) is an end-to-end network-failure-tolerant message-passing system designed for terascale clusters. LA-MPI is a standard-compliant implementation of MPI designed to tolerate network-related failures including I/O bus errors, network card errors, and wire-transmission errors. This paper details the distinguishing features of LA-MPI, including support for concurrent use of multiple types of network interface, and reliable message transmission utilizing multiple network paths and routes between a given source and destination. In addition, performance measurements on production-grade platforms are presented.     

Effects of taxol and taxol/hyperthermia treatments on the functional polarization of cytotoxic T lymphocytes

We have assessed gravimetric methods for determination of intravascular water, established whole blood-, plasma- and erythrocyte water reference values in a healthy volunteer group (n = 97, 48 females) and correlated these variables with 30 simultaneous hematological, clinicochemical and body parameters. The water standard was 55.56 mol/kg = 100 mass %. For erythrocyte water determination three methods were evaluated: 2 indirect methods were easy to perform, the third, using a hematocrit centrifuge, was the most reliable. Imprecision (within-batch coefficient of variation (CV), %) was excellent: whole blood 0.2, plasma 0.1, erythrocytes 0.7-2.2 and recoveries (means, %) 99.7-100.1. Serum water was found to be slightly higher than plasma water. Volunteer group, mean reference values, mass %: whole blood water 79.7, plasma water 91.2, erythrocyte water, three methods 66.2, 64.6 and 64.2, respectively. Females had mean 1.6 mass % higher whole blood water and 0.9-1.0 mass % higher erythrocyte water than males with no difference in plasma water. In the volunteer group whole blood water correlated strongly with hematocrit (r = -0.96), hemoglobin (r = -0.94) and erythrocytes (r = -0.85) and centrifuge hematocrit (r = -0.91). Plasma water correlated strongly with plasma total protein (r = -0.74, all correlations P < 0.001). Hemoglobin and hematocrit can serve as surrogate parameters for whole blood water when water determination is not available; total protein reflects plasma water.     

A magnetotransport study of AlGaN/GaN heterostructures on silicon

Resistivity and Hall effect measurements as functions of temperature and magnetic field have been made on AlGaN/GaN heterostructures grown on silicon substrates. Electronic properties are comparable to those of similar structures grown on sapphire or silicon carbide. Persistent photocurrent (PPC) effects induced by illumination with blue and ultraviolet (UV) light-emitting diodes (LEDs) have been investigated. These effects persisted to room temperature. Shubnikov-de Haas (SdH) oscillations were observed in several samples but only after illumination in some samples. Analysis of the mobility dependence on the carrier density suggests small angle scattering is responsible for the suppression of the oscillations prior to illumination in those samples.     

Impact of influenza on acute cardiopulmonary hospitalizations in pregnant women

This study sought to quantify influenza-related serious morbidity in pregnant women, as measured by hospitalizations for or death from selected acute cardiopulmonary conditions during predefined influenza seasons. The study population included women aged 15-44 years who were enrolled in the Tennessee Medicaid program for at least 180 days between 1974 and 1993. In a nested case-control study, 4,369 women with a first study event during influenza season were compared with 21,845 population controls. The odds ratios associated with study events increased from 1.44 (95% confidence interval (CI) 0.97-2.15) for women at 14-20 weeks' gestation to 4.67 (95% CI 3.42-6.39) for those at 37-42 weeks in comparison with postpartum women. A retrospective cohort analysis, which controlled for risk factors identified in the case-control study, identified 22,824 study events during 1,393,166 women-years of follow-up. Women in their third trimester without other identified risk factors for influenza morbidity had an event rate of 21.7 per 10,000 women-months during influenza season. Approximately half of this morbidity, 10.5 (95% CI 6.7-14.3) events per 10,000 women-months, was attributable to influenza. Influenza-attributable risks in comparable nonpregnant and postpartum women were 1.91 (95% CI 1.51-2.31) and 1.16 (95% CI -0.09 to 2.42) per 10,000 women-months, respectively. The data suggest that, out of every 10,000 women in their third trimester without other identified risk factors who experience an average influenza season of 2.5 months, 25 will be hospitalized with influenza-related morbidity.     

Uncooled InAs-GaSb type-II infrared detectors grown on GaAssubstrates for the 8-12-μm atmospheric window

We report on the growth and characterization of type-II infrared detectors with an InAs-GaSb superlattice active layer for the 8-12-μm atmospheric window at 300 K. The material was grown by molecular beam epitaxy on semi-insulating GaAs substrates. Photoconductive detectors fabricated from the superlattices showed 80% cutoff at about 12 μm at room temperature. The responsivity of the device is about 2 mA/W with a 1-V bias (E=5 V/cm) and the maximum measured detectivity of the device is 1.3×10⁸ cm.Hz^1/2/W at 11 μm at room temperature. The detector shows very weak temperature sensitivity. Also, the extracted effective carrier lifetime, τ=26 ns, is an order of magnitude longer than the carrier lifetime in HgCdTe with similar bandgap and carrier concentration     

Influence of an insulating spacer on the V-tcharacteristics of a coaxial gas-insulated cable. I. Study on areduced-scale coaxial conductor

Two different spacer designs, a disc spacer and a composite-profile cone, were evaluated in a coaxial conductor 2.5/7 cm in diameter to study their influence on the V-t characteristic of the conductor when it is subjected to repeated applications of impulse voltages of constant waveshape and increasing magnitude. The results show that an insulating spacer can reduce the critical withstand voltage and yield smaller dispersion in the breakdown voltages. These effects can be minimized by adopting a design that favors breakdown in the gas rather than along the spacer interface     

Influence of an insulating spacer on the V-tcharacteristics of a coaxial gas-insulated cable. II. Tests on EHV buses

For pt.I see ibid., vol.3, no.1, p.16-25 (1988). The V- t characteristics of EHV buses were evaluated for two configurations with rated voltages of 550 and 800 kV. The bus dimensions were 6 m in length and 17.8/53.3 cm and 20.3-61 cm in diameter, respectively. They were each fitted with two tripod-type epoxy spacers. The results show a pronounced influence of the supporting spacers on the V-t characteristic, corresponding to a reduced withstand voltage and less dispersion in the measured breakdown voltages. A statistical method is presented for deriving the V- t characteristics of a gas-insulated system from those of the individual components