21 research outputs found
Defect filtering for thermal expansion induced dislocations in III-V lasers on silicon
Epitaxially integrated III-V semiconductor lasers for silicon photonics have
the potential to dramatically transform information networks, but currently,
dislocations limit performance and reliability even in defect tolerant InAs
quantum dot (QD) based lasers. Despite being below critical thickness, QD
layers in these devices contain previously unexplained misfit dislocations,
which facilitate non-radiative recombination. We demonstrate here that these
misfit dislocations form during post-growth cooldown due to the combined
effects of (1) thermal-expansion mismatch between the III-V layers and silicon
and (2) precipitate and alloy hardening in the active region. By incorporating
an additional sub-critical thickness, indium-alloyed misfit dislocation
trapping layer, we leverage these mechanical hardening effects to our
advantage, successfully displacing 95% of misfit dislocations from the QD layer
in model structures. Unlike conventional dislocation mitigation strategies, the
trapping layer reduces neither the number of threading dislocations nor the
number of misfit dislocations. It simply shifts the position of misfit
dislocations away from the QD layer, reducing the defects' impact on
luminescence. In full lasers, adding a misfit dislocation trapping layer both
above and below the QD active region displaces misfit dislocations and
substantially improves performance: we measure a twofold reduction in lasing
threshold currents and a greater than threefold increase in output power. Our
results suggest that devices employing both traditional threading dislocation
reduction techniques and optimized misfit dislocation trapping layers may
finally lead to fully integrated, commercially viable silicon-based photonic
integrated circuits.Comment: 9 pages, 6 figure
Dislocation-induced structural and luminescence degradation in InAs quantum dot emitters on silicon
We probe the extent to which dislocations reduce carrier lifetimes and alter
luminescence and growth morphology in InAs quantum dots (QD) grown on silicon.
These heterostructures are key ingredients to achieving a highly reliable
monolithically integrated light source on silicon necessary for photonic
integrated circuits. We find up to 20-30% shorter carrier lifetimes at
spatially resolved individual dislocations from both the QD ground and excited
states at room temperature using time-resolved cathodoluminescence
spectroscopy. These lifetimes are consistent with differences in the intensity
measured under steady-state excitation suggesting that trap-assisted
recombination limits the minority carrier lifetime, even away from
dislocations. Our techniques also reveal the dramatic growth of misfit
dislocations in these structures under carrier injection fueled by
recombination-enhanced dislocation glide and III-V/Si residual strain. Beyond
these direct effects of increased nonradiative recombination, we find the
long-range strain field of misfit dislocations deeper in the defect filter
layers employed during III-V/Si growth alter the QD growth environment and
introduce a crosshatch-like variation in the QD emission color and intensity
when the filter layer is positioned close to the QD emitter layer. Sessile
threading dislocations generate even more egregious hillock defects that also
reduce emission intensities by altering layer thicknesses, as measured by
transmission electron microscopy and atom probe tomography. Our work presents a
more complete picture of the impacts of dislocations relevant for the
development of light sources for scalable silicon photonic integrated circuits.Comment: 15 pages, 6 figure
Homozygosity for a missense mutation in the 67 kDa isoform of glutamate decarboxylase in a family with autosomal recessive spastic cerebral palsy: parallels with Stiff-Person Syndrome and other movement disorders
Background
Cerebral palsy (CP) is an heterogeneous group of neurological disorders of movement and/or posture, with an estimated incidence of 1 in 1000 live births. Non-progressive forms of symmetrical, spastic CP have been identified, which show a Mendelian autosomal recessive pattern of inheritance. We recently described the mapping of a recessive spastic CP locus to a 5 cM chromosomal region located at 2q24-31.1, in rare consanguineous families.
Methods
Here we present data that refine this locus to a 0.5 cM region, flanked by the microsatellite markers D2S2345 and D2S326. The minimal region contains the candidate gene GAD1, which encodes a glutamate decarboxylase isoform (GAD67), involved in conversion of the amino acid and excitatory neurotransmitter glutamate to the inhibitory neurotransmitter γ-aminobutyric acid (GABA).
Results
A novel amino acid mis-sense mutation in GAD67 was detected, which segregated with CP in affected individuals.
Conclusions
This result is interesting because auto-antibodies to GAD67 and the more widely studied GAD65 homologue encoded by the GAD2 gene, are described in patients with Stiff-Person Syndrome (SPS), epilepsy, cerebellar ataxia and Batten disease. Further investigation seems merited of the possibility that variation in the GAD1 sequence, potentially affecting glutamate/GABA ratios, may underlie this form of spastic CP, given the presence of anti-GAD antibodies in SPS and the recognised excitotoxicity of glutamate in various contexts
The genomes of two key bumblebee species with primitive eusocial organization
Background: The shift from solitary to social behavior is one of the major evolutionary transitions. Primitively eusocial bumblebees are uniquely placed to illuminate the evolution of highly eusocial insect societies. Bumblebees are also invaluable natural and agricultural pollinators, and there is widespread concern over recent population declines in some species. High-quality genomic data will inform key aspects of bumblebee biology, including susceptibility to implicated population viability threats. Results: We report the high quality draft genome sequences of Bombus terrestris and Bombus impatiens, two ecologically dominant bumblebees and widely utilized study species. Comparing these new genomes to those of the highly eusocial honeybee Apis mellifera and other Hymenoptera, we identify deeply conserved similarities, as well as novelties key to the biology of these organisms. Some honeybee genome features thought to underpin advanced eusociality are also present in bumblebees, indicating an earlier evolution in the bee lineage. Xenobiotic detoxification and immune genes are similarly depauperate in bumblebees and honeybees, and multiple categories of genes linked to social organization, including development and behavior, show high conservation. Key differences identified include a bias in bumblebee chemoreception towards gustation from olfaction, and striking differences in microRNAs, potentially responsible for gene regulation underlying social and other traits. Conclusions: These two bumblebee genomes provide a foundation for post-genomic research on these key pollinators and insect societies. Overall, gene repertoires suggest that the route to advanced eusociality in bees was mediated by many small changes in many genes and processes, and not by notable expansion or depauperation
GWAS meta-analysis of intrahepatic cholestasis of pregnancy implicates multiple hepatic genes and regulatory elements
Intrahepatic cholestasis of pregnancy (ICP) is a pregnancy-specific liver disorder affecting 0.5–2% of pregnancies. The majority of cases present in the third trimester with pruritus, elevated serum bile acids and abnormal serum liver tests. ICP is associated with an increased risk of adverse outcomes, including spontaneous preterm birth and stillbirth. Whilst rare mutations affecting hepatobiliary transporters contribute to the aetiology of ICP, the role of common genetic variation in ICP has not been systematically characterised to date. Here, we perform genome-wide association studies (GWAS) and meta-analyses for ICP across three studies including 1138 cases and 153,642 controls. Eleven loci achieve genome-wide significance and have been further investigated and fine-mapped using functional genomics approaches. Our results pinpoint common sequence variation in liver-enriched genes and liver-specific cis-regulatory elements as contributing mechanisms to ICP susceptibility
Glide of threading dislocations in (In)AlGaAs on Si induced by carrier recombination: Characteristics, mitigation, and filtering
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Glide of threading dislocations in (In)AlGaAs on Si induced by carrier recombination: Characteristics, mitigation, and filtering
III-V optoelectronics grown epitaxially on Si substrates have large networks of dislocations due to a lattice constant mismatch between the device layers and the substrate. Recombination-enhanced dislocation glide (REDG) allows these dislocations to move and increase in length during device operation, which degrades performance. In this paper, we study REDG dynamics of threading dislocations in situ in (In)AlGaAs double heterostructures grown on Si substrates using scanning electron microscopy cathodoluminescence. The driving force for REDG arises due to coefficient of thermal expansion differences between Si and the III-V layers leading to large residual strains in the films. Tracking of threading dislocations as moving dark spot defects reveals glide characteristics that vary based on the nature of the dislocation. Remarkably, the alloying of a few atom percent of indium using metamorphic structures arrests threading dislocation glide by more than two orders of magnitude. Finally, we present REDG-based filtering as a pathway to reducing the threading dislocation density in select areas, removing a large fraction of the mobile dislocations. Together, these techniques will enable the understanding of dislocation-dislocation and carrier-dislocation interactions that have so far remained elusive during device operation, leading to reliable III-V integrated optoelectronics on silicon
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Glide of threading dislocations in (In)AlGaAs on Si induced by carrier recombination: Characteristics, mitigation, and filtering
III-V optoelectronics grown epitaxially on Si substrates have large networks of dislocations due to a lattice constant mismatch between the device layers and the substrate. Recombination-enhanced dislocation glide (REDG) allows these dislocations to move and increase in length during device operation, which degrades performance. In this paper, we study REDG dynamics of threading dislocations in situ in (In)AlGaAs double heterostructures grown on Si substrates using scanning electron microscopy cathodoluminescence. The driving force for REDG arises due to coefficient of thermal expansion differences between Si and the III-V layers leading to large residual strains in the films. Tracking of threading dislocations as moving dark spot defects reveals glide characteristics that vary based on the nature of the dislocation. Remarkably, the alloying of a few atom percent of indium using metamorphic structures arrests threading dislocation glide by more than two orders of magnitude. Finally, we present REDG-based filtering as a pathway to reducing the threading dislocation density in select areas, removing a large fraction of the mobile dislocations. Together, these techniques will enable the understanding of dislocation-dislocation and carrier-dislocation interactions that have so far remained elusive during device operation, leading to reliable III-V integrated optoelectronics on silicon
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Nucleation control and interface structure of rocksalt PbSe on (001) zincblende III-V surfaces
Data on the densification during sintering of binder jet printed samples made from water- and gas-atomized alloy 625 powders
Binder jet printing (BJP) is a metal additive manufacturing method that manufactures parts with complex geometry by depositing powder layer-by-layer, selectively joining particles in each layer with a polymeric binder and finally curing the binder. After the printing process, the parts still in the powder bed must be sintered to achieve full densification (A. Mostafaei, Y. Behnamian, Y.L. Krimer, E.L. Stevens, J.L. Luo, M. Chmielus, 2016; A. Mostafaei, E. Stevens, E. Hughes, S. Biery, C. Hilla, M. Chmielus, 2016; A. Mostafaei, Y. Behnamian, Y.L. Krimer, E.L. Stevens, J.L. Luo, M. Chmielus, 2016) [1–3]. The collected data presents the characterization of the as-received gas- and water-atomized alloy 625 powders, BJP processing parameters and density of the sintered samples. The effect of sintering temperatures on the microstructure and the relative density of binder jet printed parts made from differently atomized nickel-based superalloy 625 powders are briefly compared in this paper. Detailed data can be found in the original published papers by authors in (A. Mostafaei, J. Toman, E.L. Stevens, E.T. Hughes, Y.L. Krimer, M. Chmielus, 2017) [4]