ALBERT

Notes: The Energetic Gamma-Ray Experiment Telescope (EGRET) has observed gamma-rays bursts with the highest energy gamma-rays and the longest high energy emission to date. EGRET measures the high energy gamma-rays with its large NaI scintillator (1 to 200 MeV) and its spark chamber (30 MeV to 30 GeV). The spark chamber also measures time and arrival directions of individual photons allowing locations for the energetic bursts to be determined. Since the Compton Gamma Ray Observatory launch in 1991, EGRET has observed five bursts in the spark chamber with several having gamma-ray energies grater than 1 GeV. The recording breaking burst, GRB940217, had gamma-rays up to 18 GeV and lasted over 5000 seconds. The results for the energetic bursts are presented. The high energies observed from these gamma-ray bursts set constraints for the burst distances.

Notes: The incorporation of the group V components in In1−xGaxAsyP1−y, grown lattice matched to InP by gas source molecular beam epitaxy, has been studied over the entire alloy range, 0≤y≤1, as a function of the group V source composition, the V/III beam flux ratio, and the substrate surface orientation. Several aspects of the group V incorporation are most easily understood in terms of a simple model involving a constant incorporation coefficient and an As "underpressure'' condition. An improved description of the results at lower values of the V/III flux ratio is provided by a thermodynamic model based on equilibrium reactions for the formation of the binary constituents, and using the bulk properties of the solid solution. However, the thermodynamic model is quantitatively incorrect for large values of the V/III flux ratio. Furthermore, the results for different surface orientations reveal additional weaknesses in the thermodynamic model and suggest the need to account for the surface bonding configurations in describing the group V incorporation in epitaxial growth. © 1996 American Institute of Physics.

Notes: He+ and N+ ion irradiation of epitaxial p-type In0.76Ga0.24As0.58P0.42 and In0.53Ga0.47As was performed at 300 K to obtain high-resistivity regions. In both the ternary and quaternary samples the resistivity first increases with ion dose. A maximum is reached at a critical dose depending on the ion species and initial doping concentration. Above this dose the conductivity converts to n type and the resistivity steadily decreases to ∼102 Ω cm in InGaAsP and ∼2 Ω cm in InGaAs. After thermal annealing the type converted samples revert to p type. However, for ion doses ≥1013 cm−2 the high resistivities remain stable up to 700 K. The results suggest that simple point defects, rather than complexes are responsible for the changes in the electrical properties of the samples. © 1995 American Institute of Physics.

Notes: The thermal desorption of ultraviolet-ozone oxide on InP substrates prepared for molecular-beam epitaxy has been performed with overpressures of P2, As2, and As4. Surface analysis using reflection high-energy electron diffraction, Auger electron spectroscopy, and x-ray photoelectron spectroscopy and thermodynamic calculations indicate that thermal desorption proceeds via a reaction between the oxide and atomic phosphorus from the substrate to produce volatile phosphorus oxides such as P2O3. The overpressure species serves to stabilize the substrate against surface dissociation once the oxide is removed. In the case of an arsenic overpressure the desorption of the final monolayer of oxide is slowed, relative to the case of phosphorus overpressure, due to the formation of InAs. © 1995 American Institute of Physics.

Notes: Hydrogen incorporation into Si-doped InP grown by gas-source molecular beam epitaxy was studied. P-H sites were identified by infrared spectroscopy. Proton-implanted reference samples were used to quantify the infrared results. Approximately 0.1 at. % hydrogen was found to be incorporated into InP:Si. Hall measurements indicated that most of the Si atoms were electrically active as donors. Rapid thermal annealing at 600 °C removed most of the bonded hydrogen from the samples. However, this resulted in relatively little change in either the room-temperature free-carrier concentration or Hall mobility. © 1995 American Institute of Physics.

Notes: Room-temperature, polarization-resolved photoluminescence from a (001) surface has been used to investigate InP/InGaAs/InP quantum wells grown by gas source molecular beam epitaxy. The degree of polarization of photoluminescence from a (001) surface, DOP001, is a direct measure of the anisotropy of polarization of luminescence between [110] and [11¯ 0] directions. DOP001 is observed to be strongly dependent on the quantum well thickness, composition (strain), and the gas switching time at the growth-interrupted interface. Results show that the anisotropy of polarization may be due to an effect of an anisotropic strain field that is associated with strained bonds at the interfaces of the quantum well. © 1997 American Institute of Physics.

Notes: Six annual legumes were evaluated as components of cereal-grass-legume intercrops in two experiments at two sites differing in elevation by 789 m. Barley (Hordeum vulgare L.) and Westerwolds rye-grass (Lolium multiflorum Lam.) were seeded on all intercrop plots. Dry-matter (DM) yield, crude protein (CP) and organic matter digestibility (OMD) were measured. DM yield and N content were used to estimate legume N fixation. Experiment 1 was conducted at both sites. At the lower site, Persian clover (Trifolium resupinatum L.) and annual alfalfa (Medicago sativa L.) accounted for 70% of the DM yield in harvest 1 (July), increased CP and OMD, but did not affect intercrop yield. They increased harvest 2 (August/September) intercrop yield by 263% and CP concentration by 65 g kg−1 DM. They increased harvest 3 (October) yield by 275% and CP concentration by 78 g kg−1 DM. Inclusion of striate lespedeza (Lespedeza striata) did not affect intercrop yield or quality. Annual legumes failed to establish at the higher elevation site and therefore had no effect on DM yield or forage quality. In Experiment 2, in which the performance of Westerwolds ryegrass was also compared with that of Italian ryegrass, and conducted at the lower site only, Persian clover and berseem clover (T. alexandrinum L.) increased CP of all three of the year's harvests. These two species contributed 29% of the DM yield in the first harvest (July) but did not affect total intercrop yield. They increased harvest 2 (August) yield by 313%. Persian clover increased harvest 3 (October) yield by 318% and berseem clover increased harvest 3 yield by 405%. Barrel medic (Medicago truncatula) and snail medic (M. scutellata) contributed 29% of harvest 1 yield, and increased both DM yield and CP content. Medics did not regrow. Aubade Westerwolds ryegrass contributed a greater percentage of the DM yield than did Maris Ledger Italian ryegrass at harvests 1 and 2. Ryegrass type did not affect total DM yield but did affect forage quality; intercrops containing the Italian ryegrass had higher CP at harvest 2 and higher OMD at harvest 3 than those containing the Westerwolds ryegrass. Over both experiments, at the lower elevation site, stands with Persian clover, berseem clover or alfalfa produced 80% of the yield of barley-ryegrass receiving 250 kg N ha−1, and 165% of the yield of unfertilized barley-ryegrass. Berseem and Persian clover fixed about the same amount of N over the growing season; 188 kg N ha−1 in Experiment 1 and 134 kg N ha−1 in Experiment 2.

Notes: A method for quantifying the degree to which the uneven carrier distribution affects the operation of multiple quantum well (MQW) lasers is developed by comparing the net gains of wells in mirror image asymmetric MQW structures. The uneven carrier distribution is found to affect the performance of devices with as few as two quantum wells and decreases the net gain for wells on the n side of a ten quantum well structure by more than a factor of two. © 1999 American Institute of Physics.

Notes: InP and related quaternaries (InGaAsP) have been grown by conventional gas source molecular beam epitaxy while simultaneously exposing the growth surface to a He plasma stream generated by electron cyclotron resonance. For growth temperatures from 400 to 450 °C, the InP produced by this process displays greatly increased resistivity, as high as 105 Ω cm, compared to growth without plasma where resistivities are typically less than 1 Ω cm. An InGaAsP quaternary, with band-gap wavelength of 1.55 μm, grown with the plasma displays a sharp band edge and fast photoresponse (15 ps). © 1996 American Institute of Physics.