The ABC model of recombination reinterpreted: Impact on understanding carrier transport and efficiency droop in InGaN/GaN light emitting diodes

The efficiency of light emitting diodes remains a topic of great contemporary interest due to their potential to reduce the amount of energy consumed in lighting. The current consensus is that electrons and holes distribute themselves through the emissive region by a drift-diffusion process which results in a highly non-uniform distribution of the light emission and can reduce efficiency. In this paper the measured variations in external quantum efficiency of a range of InGaN/GaN LEDs with different numbers of quantum wells are shown to compare closely with the predictions of a revised ABC model in which it is assumed that the electrically injected electrons and holes are uniformly distributed through the multi-quantum well region, or nearly so, and hence carrier recombination occurs equally in all the quantum wells. The implications of the reported results are that drift-diffusion plays a far lesser role in cross-well carrier transport than previously thought; that the dominant cause of efficiency droop is intrinsic to the quantum wells and that reductions in the density of non-radiative recombination centers in the MQW would enable the use of more QWs and thereby reduce Auger losses by spreading carriers more evenly across a wider emissive region

Analysis of defect-related inhomogeneous electroluminescence in InGaN/GaN QW LEDs

The inhomogeneous electroluminescence (EL) of InGaN/GaN quantum well light emitting diode structures was investigated in this study. Electroluminescence hyperspectral images showed that inhomogeneities in the form of bright spots exhibited spectrally blue-shifted and broadened emission. Scanning electron microscopy combined with cathodoluminescence (SEM-CL) was used to identify hexagonal pits at the centre of approximately 20% of these features. Scanning transmission electron microscopy imaging with energy dispersive X-ray spectroscopy (STEM-EDX) indicated there may be p-doped AlGaN within the active region caused by the presence of the pit. Weak beam dark-field TEM (WBDF-TEM) revealed the presence of bundles of dislocations associated with the pit, suggesting the surface features which cause the inhomogeneous EL may occur at coalescence boundaries, supported by trends in the number of features observed across the wafer.The European Research Council has provided financial support under the European Community’s Seventh Framework Programme/ ERC grant agreement no. 279361 (MACONS).This is the author accepted manuscript. The final version is available from Elsevier via http://dx.doi.org/10.1016/j.spmi.2016.03.03

The partition bundle of type A_{N-1} (2, 0) theory

Six-dimensional (2, 0) theory can be defined on a large class of six-manifolds endowed with some additional topological and geometric data (i.e. an orientation, a spin structure, a conformal structure, and an R-symmetry bundle with connection). We discuss the nature of the object that generalizes the partition function of a more conventional quantum theory. This object takes its values in a certain complex vector space, which fits together into the total space of a complex vector bundle (the `partition bundle') as the data on the six-manifold is varied in its infinite-dimensional parameter space. In this context, an important role is played by the middle-dimensional intermediate Jacobian of the six-manifold endowed with some additional data (i.e. a symplectic structure, a quadratic form, and a complex structure). We define a certain hermitian vector bundle over this finite-dimensional parameter space. The partition bundle is then given by the pullback of the latter bundle by the map from the parameter space related to the six-manifold to the parameter space related to the intermediate Jacobian.Comment: 15 pages. Minor changes, added reference

A life cycle stakeholder management framework for enhanced collaboration between stakeholders with competing interests

This is a postprint version of the Book Chapter. Information regarding the official publication is available from the link below - Copyright @ 2011 SpringerImplementation of a Life Cycle Sustainability Management (LCSM) strategy can involve significant challenges because of competing or conflicting objectives between stakeholders. These differences may, if not identified and managed, hinder successful adoption of sustainability initiatives. This article proposes a conceptual framework for stakeholder management in a LCSM context. The framework identifies the key sustainability stakeholder groups and suggests strategic ambiguity as a management tool to harness dysfunctional conflict into constructive collaboration. The framework is of practical value as it can be used as a guideline by managers who wish to improve collaboration with stakeholders along the supply chain. The article also fills a gap in the academic literature where there is only limited research on sustainability stakeholder management through strategic ambiguity

A genome-wide linkage and association scan reveals novel loci for autism

Although autism is a highly heritable neurodevelopmental disorder, attempts to identify specific susceptibility genes have thus far met with limited success. Genome-wide association studies using half a million or more markers, particularly those with very large sample sizes achieved through meta-analysis, have shown great success in mapping genes for other complex genetic traits. Consequently, we initiated a linkage and association mapping study using half a million genome-wide single nucleotide polymorphisms (SNPs) in a common set of 1,031 multiplex autism families (1,553 affected offspring). We identified regions of suggestive and significant linkage on chromosomes 6q27 and 20p13, respectively. Initial analysis did not yield genome-wide significant associations; however, genotyping of top hits in additional families revealed an SNP on chromosome 5p15 (between SEMA5A and TAS2R1) that was significantly associated with autism (P = 2 x 10(-7)). We also demonstrated that expression of SEMA5A is reduced in brains from autistic patients, further implicating SEMA5A as an autism susceptibility gene. The linkage regions reported here provide targets for rare variation screening whereas the discovery of a single novel association demonstrates the action of common variants

Charge Lattices and Consistency of 6D Supergravity

We extend the known consistency conditions on the low-energy theory of six-dimensional N = 1 supergravity. We review some facts about the theory of two-form gauge fields and conclude that the charge lattice Gamma for such a theory has to be self-dual. The Green-Schwarz anomaly cancellation conditions in the supergravity theory determine a sublattice of Gamma. The condition that this sublattice can be extended to a self-dual lattice Gamma leads to a strong constraint on theories that otherwise appear to be self-consistent.Comment: 15 pages. v2: minor changes; references, additional example added; v3: minor corrections and clarifications added, JHEP versio

Game semantic analysis of equivalence in IMJ

Using game semantics, we investigate the problem of verifying contextual equivalences in Interface Middleweight Java (IMJ), an imperative object calculus in which program phrases are typed using interfaces. Working in the setting where data types are non-recursive and restricted to finite domains, we identify the frontier between decidability and undecidability by reference to the structure of interfaces present in typing judgments. In particular, we show how to determine the decidability status of problem instances (over a fixed type signature) by examining the position of methods inside the term type and the types of its free identifiers. Our results build upon the recent fully abstract game semantics of IMJ. Decidability is proved by translation into visibly pushdown register automata over infinite alphabets with fresh-input recognition

The impact of trench defects in InGaN/GaN light emitting diodes and implications for the "green gap" problem

The impact of trench defects in blue InGaN/GaN light emitting diodes (LEDs) has been investigated. Two mechanisms responsible for the structural degradation of the multiple quantum well (MQW) active region were identified. It was found that during the growth of the p-type GaN capping layer, loss of part of the active region enclosed within a trench defect occurred, affecting the top-most QWs in the MQW stack. Indium platelets and voids were also found to form preferentially at the bottom of the MQW stack. The presence of high densities of trench defects in the LEDs was found to relate to a significant reduction in photoluminescence and electroluminescence emission efficiency, for a range of excitation power densities and drive currents. This reduction in emission efficiency was attributed to an increase in the density of non-radiative recombination centres within the MQW stack, believed to be associated with the stacking mismatch boundaries which form part of the sub-surface structure of the trench defects. Investigation of the surface of green-emitting QW structures found a two decade increase in the density of trench defects, compared to its blue-emitting counterpart, suggesting that the efficiency of green-emitting LEDs may be strongly affected by the presence of these defects. Our results are therefore consistent with a model that the “green gap” problem might relate to localized strain relaxation occurring through defects.This is the accepted manuscript version. The final version is available from AIP at http://scitation.aip.org/content/aip/journal/apl/105/11/10.1063/1.4896279?showFTTab=true&containerItemId=content/aip/journal/apl

Altering fatty acid availability does not impair prolonged, continuous running to fatigue: evidence for carbohydrate dependence

We determined the effect of suppressing lipolysis via administration of nicotinic acid (NA) on fuel substrate selection and half-marathon running capacity. In a single-blinded, Latin square design, 12 competitive runners completed four trials involving treadmill running until volitional fatigue at a pace based on 95% of personal best half-marathon time. Trials were completed in a fed or overnight fasted state: 1) carbohydrate (CHO) ingestion before (2 g CHO·kg−1·body mass−1) and during (44 g/h) [CFED]; 2) CFED plus NA ingestion [CFED-NA]; 3) fasted with placebo ingestion during [FAST]; and 4) FAST plus NA ingestion [FAST-NA]. There was no difference in running distance (CFED, 21.53 ± 1.07; CFED-NA, 21.29 ± 1.69; FAST, 20.60 ± 2.09; FAST-NA, 20.11 ± 1.71 km) or time to fatigue between the four trials. Concentrations of plasma free fatty acids (FFA) and glycerol were suppressed following NA ingestion irrespective of preexercise nutritional intake but were higher throughout exercise in FAST compared with all other trials (P < 0.05). Rates of whole-body CHO oxidation were unaffected by NA ingestion in the CFED and FAST trials, but were lower in the FAST trial compared with the CFED-NA trial (P < 0.05). CHO was the primary substrate for exercise in all conditions, contributing 83-91% to total energy expenditure with only a small contribution from fat-based fuels. Blunting the exercise-induced increase in FFA via NA ingestion did not impair intense running capacity lasting ∼85 min, nor did it alter patterns of substrate oxidation in competitive athletes. Although there was a small but obligatory use of fat-based fuels, the oxidation of CHO-based fuels predominates during half-marathon running