117 research outputs found
GaN Homoepitaxy on Freestanding (11Μ00) Oriented GaN Substrates
We report homoepitaxialGaNgrowth on freestanding (11Μ00) oriented (M-plane GaN) substrates using low-pressure metalorganic chemical vapor deposition.Scanning electron microscopy,atomic-force microscopy, and photoluminescence were used to study the influence of growth conditions such as the V/III molar ratio and temperature on the surface morphology and optical properties of the epilayers. Optimized growth conditions led to high quality (11Μ00) oriented GaN epilayers with a smooth surface morphology and strong band-edge emission. These layers also exhibited strong room temperature stimulated emission under high intensity pulsed optical pumping. Since for III-N materials the (11Μ00) crystal orientation is free from piezoelectric or spontaneous polarization electric fields, our work forms the basis for developing high performance III-N optoelectronic devices
Nitrogen ion beam synthesis of InN in InP (100) at elevated temperature
InN phase is grown in crystalline InP(100) substrates by 50 keV N+
implantation at an elevated temperature of 400 deg C followed by annealing at
525 deg C in N2 ambient. Crystallographic structural and Raman scattering
studies are performed for the characterization of grown phases. Temperature-
and power-dependent photoluminescence studies show direct band-to-band
transition peak ~1.06 eV at temperatures <=150K. Implantations at an elevated
temperature with a low ion beam current and subsequent low temperature
annealing step are found responsible for the growth of high-quality InN phase.Comment: 11 pages, 4 figures, Journa
Polarization Effects in Photoluminescence of C- and M-Plane GaN/AlGaN Multiple Quantum Wells
Polarizationeffects have been studied in GaN/AlGaN multiple quantum wells(MQWs) with different c-axis orientation by means of excitation-dependent photoluminescence(PL) analysis. Quantum structures were grown on [0001]-oriented sapphire substrates (C plane) and single-crystalline [11Μ00]-oriented freestanding GaN (M plane) using the metalorganic chemical vapor deposition technique. Strong PL spectrum line blueshifts (up to 140 meV) which are correlated with the excitation intensity have been obtained for C-plane MQWs, whereas no shift has been observed for M-plane MQWs.Theoretical calculations and comparison with the PL data confirm that the built-in electric field for C-plane structures is much stronger than the field present for M-plane MQWs. In the former case, the excitation-induced blueshift of the PL line is due to the screening of the built-in electric field by photoinjected carriers, which is consistent with the field strength of 1.23 MV/cm in the absence of excitation
Visual Information Alone Changes Behavior and Physiology during Social Interactions in a Cichlid Fish (Astatotilapia burtoni)
Social behavior can influence physiological systems dramatically yet the sensory
cues responsible are not well understood. Behavior of male African cichlid fish,
Astatotilapia burtoni, in their natural habitat suggests
that visual cues from conspecifics contribute significantly to regulation of
social behavior. Using a novel paradigm, we asked whether visual cues alone from
a larger conspecific male could influence behavior, reproductive physiology and
the physiological stress response of a smaller male. Here we show that just
seeing a larger, threatening male through a clear barrier can suppress dominant
behavior of a smaller male for up to 7 days. Smaller dominant males being
βattackedβ visually by larger dominant males through a clear barrier
also showed physiological changes for up to 3 days, including up-regulation of
reproductive- and stress-related gene expression levels and lowered plasma
11-ketotestesterone concentrations as compared to control animals. The smaller
males modified their appearance to match that of non-dominant males when exposed
to a larger male but they maintained a physiological phenotype similar to that
of a dominant male. After 7 days, reproductive- and stress- related gene
expression, circulating hormone levels, and gonad size in the smaller males
showed no difference from the control group suggesting that the smaller male
habituated to the visual intruder. However, the smaller male continued to
display subordinate behaviors and assumed the appearance of a subordinate male
for a full week despite his dominant male physiology. These data suggest that
seeing a larger male alone can regulate the behavior of a smaller male but that
ongoing reproductive inhibition depends on additional sensory cues. Perhaps,
while experiencing visual social stressors, the smaller male uses an
opportunistic strategy, acting like a subordinate male while maintaining the
physiology of a dominant male
Chemical diplomacy in male tilapia: urinary signal increases sex hormone and decreases aggression
Androgens, namely 11-ketotestosterone (11KT), have a central role in male fish reproductive physiology and are thought to be involved in both aggression and social signalling. Aggressive encounters occur frequently in social species, and fights may cause energy depletion, injury and loss of social status. Signalling for social dominance and fighting ability in an agonistic context can minimize these costs. Here, we test the hypothesis of a 'chemical diplomacy' mechanism through urinary signals that avoids aggression and evokes an androgen response in receiver males of Mozambique tilapia (Oreochromis mossambicus). We show a decoupling between aggression and the androgen response; males fighting their mirror image experience an unresolved interaction and a severe drop in urinary 11KT. However, if concurrently exposed to dominant male urine, aggression drops but urinary 11KT levels remain high. Furthermore, 11KT increases in males exposed to dominant male urine in the absence of a visual stimulus. The use of a urinary signal to lower aggression may be an adaptive mechanism to resolve disputes and avoid the costs of fighting. As dominance is linked to nest building and mating with females, the 11KT response of subordinate males suggests chemical eavesdropping, possibly in preparation for parasitic fertilizations.info:eu-repo/semantics/publishedVersio
Short-term buserelin administration induces apoptosis and morphological changes in adult rat testes
Electroreceptive and Mechanoreceptive Anatomical Specialisations in the Epaulette Shark (Hemiscyllium ocellatum)
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