592 research outputs found
Experimental Evidence for a Structural-Dynamical Transition in Trajectory Space
Among the key insights into the glass transition has been the identification
of a non-equilibrium phase transition in trajectory space which reveals phase
coexistence between the normal supercooled liquid (active phase) and a glassy
state (inactive phase). Here we present evidence that such a transition occurs
in experiment. In colloidal hard spheres we find a non-Gaussian distribution of
trajectories leaning towards those rich in locally favoured structures (LFS),
associated with the emergence of slow dynamics. This we interpret as evidence
for an non-equilibrium transition to an inactive LFS-rich phase. Reweighting
trajectories reveals a first-order phase transition in trajectory space between
a normal liquid and a LFS-rich phase. We further find evidence of a purely
dynamical transition in trajectory space.Comment: 5 page
Localization landscape theory of disorder in semiconductors. III. Application to carrier transport and recombination in light emitting diodes
This paper introduces a novel method to account for quantum disorder effects
into the classical drift-diffusion model of semiconductor transport through the
localization landscape theory. Quantum confinement and quantum tunneling in the
disordered system change dramatically the energy barriers acting on the
perpendicular transport of heterostructures. In addition they lead to
percolative transport through paths of minimal energy in the 2D landscape of
disordered energies of multiple 2D quantum wells. This model solves the carrier
dynamics with quantum effects self-consistently and provides a computationally
much faster solver when compared with the Schr\"odinger equation resolution.
The theory also provides a good approximation to the density of states for the
disordered system over the full range of energies required to account for
transport at room-temperature. The current-voltage characteristics modeled by
3-D simulation of a full nitride-based light-emitting diode (LED) structure
with compositional material fluctuations closely match the experimental
behavior of high quality blue LEDs. The model allows also a fine analysis of
the quantum effects involved in carrier transport through such complex
heterostructures. Finally, details of carrier population and recombination in
the different quantum wells are given.Comment: 14 pages, 16 figures, 6 table
Fabrication technology for high light-extraction ultraviolet thin-film flip-chip (UV TFFC) LEDs grown on SiC
The light output of deep ultraviolet (UV-C) AlGaN light-emitting diodes
(LEDs) is limited due to their poor light extraction efficiency (LEE). To
improve the LEE of AlGaN LEDs, we developed a fabrication technology to process
AlGaN LEDs grown on SiC into thin-film flip-chip LEDs (TFFC LEDs) with high
LEE. This process transfers the AlGaN LED epi onto a new substrate by
wafer-to-wafer bonding, and by removing the absorbing SiC substrate with a
highly selective SF6 plasma etch that stops at the AlN buffer layer. We
optimized the inductively coupled plasma (ICP) SF6 etch parameters to develop a
substrate-removal process with high reliability and precise epitaxial control,
without creating micromasking defects or degrading the health of the plasma
etching system. The SiC etch rate by SF6 plasma was ~46 \mu m/hr at a high RF
bias (400 W), and ~7 \mu m/hr at a low RF bias (49 W) with very high etch
selectivity between SiC and AlN. The high SF6 etch selectivity between SiC and
AlN was essential for removing the SiC substrate and exposing a pristine,
smooth AlN surface. We demonstrated the epi-transfer process by fabricating
high light extraction TFFC LEDs from AlGaN LEDs grown on SiC. To further
enhance the light extraction, the exposed N-face AlN was anisotropically etched
in dilute KOH. The LEE of the AlGaN LED improved by ~3X after KOH roughening at
room temperature. This AlGaN TFFC LED process establishes a viable path to high
external quantum efficiency (EQE) and power conversion efficiency (PCE) UV-C
LEDs.Comment: 22 pages, 6 figures. (accepted in Semiconductor Science and
Technology, SST-105156.R1 2018
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Measurement of Extraction and Absorption Parameters in GaN-based Photonic-crystal Light-emitting Diodes
The light extraction efficiency of photonic-crystal (PhC) light-emitting diodes (LEDs) relies on the competition between the PhC extraction and dissipation mechanisms of the guided light within the LED. This work presents the experimental determination of the PhC extraction length of each guided mode and the absorption coefficient of the active region (AR) and quantum wells (QWs) from the observation of the LED far-field emission using a high-resolution angle-spectrum-resolved measurement. The angular and spectral linewidths of the extracted guided modes reveal, depending on the spectral range, the modal extraction length of the PhCs, the AR absorption length, or a combination of both. Modes with a high confinement with the QWs presented a shorter absorption length compared with their extraction length by a shallow surface PhC (95-nm-deep), meaning that the AR absorption was a more efficient mechanism than the PhC extraction. The measured modal extraction length of the shallow surface PhC varied in the range of 55–120 μm, which determines the minimum dimensions of the device and the maximum acceptable dissipation length for an efficient extraction of the guided light by the PhCs. This paper presents also a discussion on the PhC designs that yield PhC extraction lengths shorter than other dissipation lengths, a fundamental requirement for high-efficiency PhC LEDs. The same technique was also applied to estimate the absorption coefficient of the InGaN-based QWs, and can be extended to experimentally determine losses by metallic layers from electrical contacts or other dissipation mechanisms, which are parameters of interest to a broader class of optoelectronic devices, not only PhC LEDs.Engineering and Applied Science
In vacancies in InN grown by plasma-assisted molecular beam epitaxy
The authors have applied positron annihilation spectroscopy to study the
effect of different growth conditions on vacancy formation in In- and N-polar
InN grown by plasma-assisted molecular beam epitaxy. The results suggest that
the structural quality of the material and limited diffusion of surface adatoms
during growth dictate the In vacancy formation in low electron-density undoped
epitaxial InN, while growth conditions and thermodynamics have a less important
role, contrary to what is observed in, e.g., GaN. Further, the results imply
that in high quality InN, the electron mobility is likely limited not by
ionized point defect scattering, but rather by threading dislocations.Comment: 15 pages, 2 figure
Feature interaction in composed systems. Proceedings. ECOOP 2001 Workshop #08 in association with the 15th European Conference on Object-Oriented Programming, Budapest, Hungary, June 18-22, 2001
Feature interaction is nothing new and not limited to computer science. The problem of undesirable feature interaction (feature interaction problem) has already been investigated in the telecommunication domain. Our goal is the investigation of feature interaction in componet-based systems beyond telecommunication. This Technical Report embraces all position papers accepted at the ECOOP 2001 workshop no. 08 on "Feature Interaction in Composed Systems". The workshop was held on June 18, 2001 at Budapest, Hungary
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TGF-β Signaling Initiated in Dendritic Cells Instructs Suppressive Effects on Th17 Differentiation at the Site of Neuroinflammation
While the role of Transforming Growth Factor β (TGF-β) as an intrinsic pathway has been well established in driving de novo differentiation of Th17 cells, no study has directly assessed the capacity of TGF-β signaling initiated within dendritic cells (DCs) to regulate Th17 differentiation. The central finding of this study is the demonstration that Th17 cell fate during autoimmune inflammation is shaped by TGF-β extrinsic pathway via DCs. First, we provide evidence that TGF-β limits at the site of inflammation the differentiation of highly mature DCs as a means of restricting Th17 cell differentiation and controlling autoimmunity. Second, we demonstrate that TGF-β controls DC differentiation in the inflammatory site but not in the priming site. Third, we show that TGF-β controls DC numbers at a precursor level but not at a mature stage. While it is undisputable that TGF-β intrinsic pathway drives Th17 differentiation, our data provide the first evidence that TGF-β can restrict Th17 differentiation via DC suppression but such a control occurs in the site of inflammation, not at the site of priming. Such a demarcation of the role of TGF-β in DC lineage is unprecedented and holds serious implications vis-à-vis future DC-based therapeutic targets
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