Accuracy of Measuring Intensity Through Means of Repetition in Reserve in Trained College Females

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Deep-level defects in n-type GaAsBi alloys grown by molecular beam epitaxy at low temperature and their influence on optical properties

Deep-level defects in n-type GaAs1-x Bi x having 0 ≤ x ≤ 0.023 grown on GaAs by molecular beam epitaxy at substrate temperature of 378 °C have been injvestigated by deep level transient spectroscopy. The optical properties of the layers have been studied by contactless electroreflectance and photoluminescence. We find that incorporating Bi suppresses the formation of GaAs-like electron traps, thus reducing the total trap concentration in dilute GaAsBi layers by over two orders of magnitude compared to GaAs grown under the same conditions. In order to distinguish between Bi- and host-related traps and to identify their possible origin, we used the GaAsBi band gap diagram to correlate their activation energies in samples with different Bi contents. This approach was recently successfully applied for the identification of electron traps in n-type GaAs1-x N x and assumes that the activation energy of electron traps decreases with the Bi (or N)-related downward shift of the conduction band. On the basis of this diagram and under the support of recent theoretical calculations, at least two Bi-related traps were revealed and associated with Bi pair defects, i.e. (VGa+BiGa)(-/2-) and (AsGa+BiGa)(0/1-). In the present work it is shown that these defects also influence the photoluminescence properties of GaAsBi alloys

Identifying invasive species threats, pathways, and impacts to improve biosecurity

Managing invasive species with prevention and early-detection strategies can avert severe ecological and economic impacts. Horizon scanning, an evidence-based process combining risk screening and consensus building to identify threats, has become a valuable tool for prioritizing invasive species management and prevention. We assembled a working group of experts from academic, government, and nonprofit agencies and organizations, and conducted a multi-taxa horizon scan for Florida, USA, the first of its kind in North America. Our primary objectives were to identify high-risk species and their introduction pathways, to detail the magnitude and mechanism of potential impacts, and, more broadly, to demonstrate the utility of horizon scanning. As a means to facilitate future horizon scans, we document the process used to generate the list of taxa for screening. We evaluated 460 taxa for their potential to arrive, establish, and cause negative ecological and socioeconomic impacts, and identified 40 potential invaders, including alewife, zebra mussel, crab-eating macaque, and red swamp crayfish. Vertebrates and aquatic invertebrates posed the greatest invasion threat, over half of the high-risk taxa were omnivores, and there was high confidence in the scoring of high-risk taxa. Common arrival pathways were ballast water, biofouling of vessels, and escape from the pet/aquarium/horticulture trade. Competition, predation, and damage to agriculture/forestry/aquaculture were common impact mechanisms. We recommend full risk analysis for the high-risk taxa; increased surveillance at Florida's ports, state borders, and high-risk pathways; and periodic review and revision of the list. Few horizon scans detail the comprehensive methodology (including list-building), certainty estimates for all scoring categories and the final score, detailed pathways, and the magnitude and mechanism of impact. Providing this information can further inform prevention efforts and can be efficiently replicated in other regions. Moreover, harmonizing methodology can facilitate data sharing and enhance interpretation of results for stakeholders and the general public.</p

Piezoelectric field and confinement effects on the dielectric function spectrum of InGaN/GaN quantum wells

In(0.13)Ga(0.87)N/GaN double heterostructures and quantum wells (QWs) have been studied by room-temperature photoluminescence (PL) and spectroscopic ellipsometry (SE). PL revealed the presence of strong piezoelectric fields, which strongly influence the luminescence properties for InGaN QW widths in the 3-12 nm range. The pseudodielectric function spectrum derived from the SE measurements were analyzed using a multilayer approach, describing the dielectric function of the individual layers by a parametric oscillator model. The fundamental band-gap resonance in the InGaN dielectric-function spectrum was found to broaden for an InGaN layer width of 12 nm, as compared to bulk-like InGaN layers, due to piezoelectric field effects. For a much narrower QW width of 1.7 nm, however, quantum confinement was found to dominate over piezoelectric-field effects, resulting in a much sharper band-gap resonance shifted to higher energies and an increased oscillator strength

Temperature dependence of excitonic photoluminescence and residual shallow donors in high-purity GaN/Al2O3

The temperature dependence of free and bound exciton lines in high-purity, undoped wurtzite GaN layers has been studied by photoluminescence (PL) between 2 and 300 K. Below 30 K the neutral donor bound exciton D deg X produces the strongest near band gap PL signal whereas free A and B excitons dominate the spectrum at room temperature. A deconvolution of the asymmetric D deg X line shape provides strong evidence for two residual shallow donors differing in ionisation energies by a factor of 1.5. The origin of a PL line occurring at Eg = 116 meV is discussed in two alternative models

Origin of defect-related photoluminescence bands in doped and nominally undoped GaN

The efficient room-temperature photoluminescence bands of wurtzite GaN, which are peaked in the red (1.8 eV), the yellow (2.2 eV), and the blue (2.8 eV) spectral range, have been studied as a function of doping (species and concentration) and excitation power density (PD). It is shown that the yellow and the blue band are induced by Si and Mg doping, respectively, while codoping with Si and Mg generates the red band. At high-doping levels, the yellow and the blue band reveal strong peak shifts to higher energy with increasing PD providing very strong evidence for their distant donor-acceptor (DA) pair recombination character. The deep centers involved in DA recombination having electrical activity opposite to that of the shallow level of the dopant, are suggested to arise from self-compensation and to be vacancy-dopant associates. Self-compensation is found to be weak in the case of Si doping, but significant for Mg doping. A recombination model is presented, which accounts for the ess ential properties of all three bands in deliberately doped GaN. These results also suggest that the yellow and the blue bands in nominally undoped GaN arise from distant DA pairs involving residual Si and Mg impurities, respectively, as well as their respective vacancy associates

Nature of the 2.8 eV photoluminescence band in Mg doped GaN

The blue Mg induced 2.8 eV photoluminescence (PL) band in metalorganic chemical vapor deposition grown GaN has been studied in a large number of samples with varying Mg content. It emerges near a Mg concentration of 1x10(exp 19) cm(-3) and at higher concentrations dominates the room temperature PL spectrum. The excitation power dependence of the 2.8 eV band provides convincing evidence for its donor-acceptor (D-A) pair recombination character. It is suggested that the acceptor A is isolated Mg(Ga) while the spatially separated, deep donor (430 meV) D is attributed to a nearest-neighbor associate of a Mg(Ga) acceptor with a nitrogen vacancy, formed by self-compensation

Consequences of Fixing 3 Parallel Coplanar Double-bonds In Close Proximity With Different Geometries - Synthesis and Spectral Parameters of Syn-sesquinorbornatriene and Anti-sesquinorbornatriene

syn-Sesquinorbornatriene, possibly the extreme example of x-bond pyramidalization, has been synthesized and characterized. Cycloaddition of (Z)-1,2-bis(phenylsulfonyl)ethylene with tricyclo[5.2.1 .02-6]deca-2,5,8-triene proceeds with at least 95% below-plane stereoselectivity to give the syn [4 + 21 adduct. Mild reductive desulfonylation of this product leads to the above hydrocarbon. Its I3C NMR shifts are particularly instructive when compared to syn-sesquinorbornadiene, which was prepared in comparable fashion. Peracid oxidation of the original adduct, followed by comparable treatment with I-2% sodium amalgam, affords the structurally related diene epoxide. Sequential photocyclization and periodic acid cleavage of this intermediate delivers [4]peristylane-2,4-dione and constitutes an expedient route to this hemispherical compound. By comparison, the Diels-Alder reaction of tricyclo[5.2.1 .@6]deca-2,5,8-triene with (E)- 1,2-bis(phenylsulfonyl)ethylene is sterically controlled and proceeds with predominant above-plane dienophile capture. Reductive desulfonylation of this adduct affords anti-sesquinorbornatriene. Although this hydrocarbon is sensitive to air oxidation, it is somewhat less so than its syn counterpart. Nonetheless, the 13C chemical shift of its internal olefinic carbon (173.23 ppm) appears slightly to lower field than that of the syn isomer (172.14 ppm). The possible contributory causes to the extreme deshielding of these signals are discussed in light of comparison with lesser unsaturated congeners and orbital interaction patterns. Finally, heating tricyclo[5.2.1 .0296]- deca-2,5,8-triene with the (E)-disulfone in toluene leads efficiently to an angular above-plane adduct. Removal of the sulfonyl groups in this instance give s rise to a structural isomer of the above pair of trienes that does not share their high reactivity toward triplet oxyge