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We analyze the dust emission features seen in Spitzer Space Telescope Infrared Spectrograph (IRS) spectra of red supergiant (RSG) and oxygen-rich asymptotic giant branch (AGB) stars in the Large Magellanic Cloud and Small Magellanic Cloud galaxies and in various Milky Way globular clusters. The spectra come from the Spitzer Legacy program SAGE-Spectroscopy (PI: F. Kemper), the Spitzer program SMC-Spec (PI: G. Sloan), and other archival Spitzer-IRS programs. The broad 10 and 20 micron emission features attributed to amorphous dust of silicate composition seen in the spectra show evidence for systematic differences in the centroid of both emission features between O-rich AGB and RSG populations. Radiative transfer modeling using the GRAMS grid of models of AGB and RSG stars suggests that the centroid differences are due to differences in dust properties. We investigate differences in dust composition, size, shape, etc that might be responsible for these spectral differences. We explore how these differences may arise from the different circumstellar environments around RSG and O-rich AGB stars and assess effects of varying metallicity (LMC versus SMC versus Milky Way globular cluster) and other properties (mass-loss rate, luminosity, etc.) on the dust originating from these stars. BAS acknowledges funding from NASA ADAP grant NNX13AD54G.
The physical mechanisms responsible for electrically-induced parametric degradation in GaN-based high electron mobility transistors are examined using a combination of experiments, device simulation, and first-principles defect analysis. A relatively simple formulation is developed under the assumption that the hot-electron scattering cross-section is independent of the electron energy. In this case, one can relate the change in defect concentration to the operational characteristics of a device, such as the spatial and energy distribution of electrons (electron temperature), electric field distribution, and electron energy loss to the lattice.
To evaluate the impact of postprescription review of broad-spectrum antimicrobial (study-ABX) agents on rates of antimicrobial use.
Quasi-experimental before-after study.
Five academic medical centers.
Adults receiving at least 48 hours of study-ABX.
The baseline, intervention, and follow-up periods were 6 months each in 2 units at each of 5 sites. Adults receiving at least 48 hours of study-ABX entered the cohort as case-patients. During the intervention, infectious-diseases physicians reviewed the cases after 48 hours of study-ABX. The provider was contacted with alternative recommendations if antimicrobial use was considered to be unjustified on the basis of predetermined criteria. Acceptance rates were assessed 48 hours later. The primary outcome measure was days of study-ABX per 1,000 study-patient-days in the baseline and intervention periods.
There were 1,265 patients in the baseline period and 1,163 patients in the intervention period. Study-ABX use decreased significantly during the intervention period at 2 sites: from 574.4 to 533.8 study-ABX days/1,000 patient-days (incidence rate ratio [IRR], 0.93; 95% confidence interval [CI], 0.88-0.97; P = .002) at hospital В and from 615.6 to 514.4 study-ABX days/1,000 patient-days (IRR, 0.83; 95% CI, 0.79-0.88; P < .001) at hospital D. Both had established antimicrobial stewardship programs (ASP). Study-ABX use increased at 2 sites and stayed the same at 1 site. At all institutions combined, 390 of 1,429 (27.3%) study-ABX courses were assessed as unjustified; recommendations to modify or stop therapy were accepted for 260 (66.7%) of these courses.
Postprescription review of study-ABX decreased antimicrobial utilization in some of the study hospitals and may be more effective when performed as part of an established ASP.
Cognitive behavioral therapy (CBT) with exposure and response prevention (ERP) is the psychotherapeutic treatment of choice for obsessive–compulsive disorder (OCD). However, little is known about the impact of CBT on frontostriatal dysfunctioning, known to be the neuronal correlate of OCD.
A probabilistic reversal learning (RL) task probing adaptive strategy switching capabilities was used in 10 unmedicated patients with OCD and 10 healthy controls during an event-related functional magnetic resonance imaging (fMRI) experiment. Patients were scanned before and after intensive CBT, controls twice at comparable intervals.
Strategy change within the RL task involved activity in a broad frontal network in patients and controls. No significant differences between the groups or in group by time interactions were detected in a whole-brain analysis corrected for multiple comparisons. However, a reanalysis with a more lenient threshold revealed decreased responsiveness of the orbitofrontal cortex and right putamen during strategy change before treatment in patients compared with healthy subjects. A group by time effect was found in the caudate nucleus, demonstrating increased activity for patients over the course of time. Patients with greater clinical improvement, reflected by greater reductions in Yale–Brown Obsessive Compulsive Scale (YBOCS) scores, showed more stable activation in the pallidum.
Although these findings are preliminary and need to be replicated in larger samples, they indicate a possible influence of psychotherapy on brain activity in core regions that have been shown to be directly involved both in acquisition of behavioral rules and stereotypes and in the pathophysiology of OCD, the caudate nucleus and the pallidum.
We studied the effect of extended defects on electrical characteristics of Si doped n-type nonpolar a-plane GaN films. The n-type GaN layers were grown on co-loaded reduced defect density sidewall lateral epitaxial overgrowth (SLEO) a-plane GaN templates and high defect density planar a-plane GaN templates by metalorganic chemical vapor deposition (MOCVD). The highest conductivity value was observed at the carrier concentration of 1.05 × 1019 cm−3 as 261.12 cm2/Vs for SLEO a-GaN and 106.77 cm2/Vs for the planar a-plane GaN samples. At the same doping level, the carrier compensation for SLEO samples was ∼12% less than planar samples.
The physical origin of the yellow luminescence in MBE-grown GaN co-doped with C and Si was investigated. Deep level optical spectroscopy (DLOS), deep level transient spectroscopy (DLTS), and photoluminescence (PL) were used to study the deep level spectrum as a function of C incorporation. In the absence of C co-doping, samples were n-type and demonstrated a weak yellow luminescence band, likely related to VGa. For increasing C co-doping, samples became semi-insulating concurrent with increased intensity of the yellow luminescence and the concentration of C-related deep acceptors. The DLOS results were used to develop a configuration-coordinate model for a C-related deep level with optical ionization energy of 3.0 eV and Franck-Condon shift of 0.4 eV that is consistent with the observed yellow luminescence and DLTS results. From these findings, a general model for independent mechanisms of the yellow luminescence related to VGa for n-type GaN and C for n-type and semi-insulating GaN:C:Si is discussed.
Some proto-planetary nebulae (PPNe) exhibit an enigmatic feature in their infrared (IR) spectra at ~21 μm. PPNe which display this feature are all C-rich and all show evidence for s-process enhancements in their photospheres, indicative of efficient dredge-up during the ascent of the asymptotic giant branch (AGB). Furthermore, this 21 μm feature is not seen in the spectra of either the precursors to PPNe, the AGB stars, or the successors of PPNe, planetary nebulae (PNe). However the 21 μm feature has been seen in the spectra of PNe with Wolf-Rayet central stars. Therefore the carrier of this feature is unlikely to be a transient species that only exists in the PPNe phase. It is more likely that the physical conditions in the AGB stars and PNe conspire against the observation of an IR feature at 21 μm. This feature has been attributed to various molecular and solid state species, none of which satisfy all constraints, although TiC and PAHs have seemed the most viable.
We have presented new observations of the ionized gas, molecular gas and cool dust in the Helix nebula (NGC 7293). The ionized gas is observed in the form of a Hα image, which is constructed using images from the Southern H-Alpha Sky Survey Atlas (SHASSA). The molecular emission was mapped using the H2v = 1 → 0 s(1) line at 2.122μm. The far-infrared (FIR) observations were obtained using ISOPHOT on the Infrared Space Observatory (ISO).
The relative morphologies and structures of molecular and ionized gas emission from planetary nebulae (PNe) allow a better understanding of the nature and evolution of these objects. The classical paradigm for the structure of PNe is that of an ionized gas bubble bounded by neutral gas and molecules. However, it has been shown that molecular gas exists within ionized regions, leading to a re-evaluation of the classic structure. In the Helix Nebula (NGC 7293) dense condensations known as cometary knots are known to exist in the main ionized nebula. The molecules in these knots are shielded from the ionizing radiation and thus survive within the ionized zone. Another PN in which H2 emission is seen to originate from within the ionized nebula in NGC 6720 (the Ring Nebula).
Carbon doping of GaN is of great interest in part because it has been shown to result in semi-insulating (SI) behavior. However, determination of the bandgap states and hence the exact mechanism responsible for the SI behavior is, to date, an unresolved issue. A key impediment is that the presumed C acceptor levels are likely near the minority carrier (valence) bandedge of otherwise background n-type GaN, and hence their precise detection by usual methods is difficult. In this paper, we exploit the inherent ability of deep level optical spectroscopy (DLOS) to detect states near the minority carrier band edge, as well as potentially deep states associated with C in background n-type GaN. This is accomplished by comparing unintentionally doped (uid) GaN grown by atmospheric pressure (AP) MOCVD, which has residual n-type conductivity, with LP MOCVD GaN that incorporates a large concentration of C for both uid and intentionally Si co-doped conditions. The results show the emergence of a shallow state at Ec - 3.28 eV (Ev + 0.16 eV) for the LP samples with a minimum concentration of 3.6 × 1016 cm-3 that efficiently compensates Si donors for the co-doped sample, and results in semi-insulating behavior for the uid-LP sample. In contrast, this state is not observed for the AP GaN material, which incorporates a factor of ∼10 times less C, and instead only the expected residual Mg acceptor level at Ec - 3.22 eV is observed. Additionally, a state at Ec - 1.35 eV, near the theoretically expected C split-interstitial level in n-type GaN, is observed to increase significantly in concentration with increased C concentration.
The impact of growth temperature and Ga/N flux ratio on deep levels in GaN grown by molecular beam epitaxy (MBE) is systematically investigated using both deep level optical spectroscopy (DLOS) and deep level transient spectroscopy (DLTS) in a study designed to map out the presence and concentration of defects over a defined region of the MBE GaN growth phase diagram. A series of Si-doped GaN films were grown to cover a substrate temperature range and a Ga/N flux ratio range that spans from the N stable to the Ga droplet regimes along both variables. Identical growth templates were used to eliminate variations in dislocations between samples so that point defect variations could be tracked. For these samples, traps are detected at EC-Et=0.25, 0.60, 0.90, 1.35, 2.40, 3.04, and 3.28 eV. The near valence bands states at EC–3.04 and EC–3.28 eV are found to be strongly dependent on Ga/N flux with decreased concentrations as a function of increasing Ga flux toward the Ga droplet regime, but with little effect from growth temperature. The EC-1.35 eV level shows a strong dependence on growth temperature and only slight dependence on Ga/N flux ratio. In contrast, the concentration of the EC-Et=0.25, 0.90 eV levels increased with increasing Ga flux toward the Ga droplet regime, while the EC-Et=0.60 shows no dependence. The variation in concentration of the EC-2.40 eV level that has been related to VGa was difficult to quantify, but tends to increase towards nitrogen rich growth. The dependencies for the detected states with respect to growth temperature and Ga/N flux ratio suggest different physical point defect sources.
This paper describes an optical investigation technique for visualizing the cross
section of a vertical air stream flowing downward on a wet horizontal tube with moving liquid
film. This method which associates the laser induced fluorescence and laser tomography
techniques, uses Mie scattering tracers and fluorescent dye for marking the air and liquid flows,
respectively. It allows us to visualize the air and liquid flows independently or simultaneously,
and to measure the air flow separation angle. Results are compared with values obtained for a
dry tube by the present and of other authors.
AuTiAlTi, AuPdAlTi and AuAlTi ohmic contacts to AlGaN/GaN layers rapid thermal annealed at temperatures up to 950°C have been characterised using conventional and chemical transmission electron microscopy techniques. The relationship between the as-deposited metallic structure, annealing temperature, post-anneal interfacial microstructure and contact resistance is examined.
The presence of a TiN interfacial layer is found to correlate with the onset of ohmic behaviour. Ti and Pd barrier layers are found to be ineffective at stopping the diffusion of Au to the interface. Au is implicated in the development of the inclusions, which are associated with threading dislocations. Once activated, the presence of the inclusions has little influence on the ohmic behaviour of the sample.
We have found that InxGa(1-x)N/GaN multi-quantum-well (MQW) light emitting diodes (LEDs) having periodic thickness variation (TV) in InxGa(1-x)N active layers exhibit substantially higher optical efficiency than LEDs with uniform InxGa(1-x)N layers. In these nano-structured LEDs, the thickness variation of the active layers is shown to be more important than In composition fluctuation in quantum confinement of excitons (carriers). Detailed STEM-Z contrast analysis, where image contrast is proportional to Z2 (atomic number)2, was carried out to investigate the thickness variation as well as the spatial distribution of In. In the nanostructured LEDs, there are short-range (SR-TV, 3 to 4 nm) and long-range thickness variations (LR-TV, 50 to 100 nm) in InxGa(1-x)N layers. It is envisaged that LR-TV is the key to quantum confinement of the carriers and enhancing the optical efficiency. We propose that the LR-TV thickness variation is caused by two-dimensional strain in the InxGa(1-x)N layer below its critical thickness. The SR-TV may be caused by In composition fluctuation. The observations on thickness variation are in good agreement with model calculations based upon strain effects.
Waveguide prism-coupling methods were used to measure the ordinary and extraordinary refractive indices of AlxGa1-xN films grown on sapphire substrates by HVPE and MOCVD. Several discrete wavelengths ranging from 442 nm to 1064 nm were used and the results were fit to one-term Sellmeier equations. The maximum standard uncertainty in the refractive index measurements was ± 0.005 and the maximum standard uncertainty in the self-consistent calculation for film thickness was ± 15 nm. Analysis of normal-incidence spectroscopic transmittance and reflectance measurements, correlated with the prism-coupling results, was used to determine the ordinary refractive index as a continuous function of wavelength from the band gap wavelength of each sample (between 252 nm and 364 nm) to 2500 nm. The Al compositions of the samples were determined using energy-dispersive X-ray spectroscopy analysis (EDS). HVPE grown samples had compositions x = 0.279, 0.363, 0.593, and 0.657. MOCVD samples had x = 0.00, 0.419, 0.507, 0.618, 0.660, and 0.666. The maximum standard uncertainty in the absolute EDS-determined value for x was ± 0.02.
Rare-earth doped GaN structures offer potential for optical devices emitting in the visible region [1,2]. We describe a study of MOVPE grown GaN-on-sapphire epilayers implanted with Europium ions, producing characteristic red emission lines between 540 and 680 nm due to intra-4f(n) electron transitions. As-implanted and subsequently annealed samples are investigated using a combination of wavelength dispersive x-ray analysis (WDX), electron microscopy, cathodoluminescence (CL) and photoluminescence (PL). WDX is shown to be a powerful technique for quantifying rare-earth concentrations in GaN, with varying electron beam voltages allowing a degree of depth profiling, further enhanced by the simultaneous collection of room temperature luminescence (CL) from the analysed region . The intensities of the sharp lines observed in the luminescence spectrum are compared to the doping density (between 1014 – 1015 cm−2) and the Eu content measured by WDX, using a Eu-doped glass standard. Differences observed in the luminescence spectra produced by laser and electron beam excitation will be discussed along with the importance of the annealing conditions, which “heal” defects visible in the electron micrographs.
GaN layers were grown on a (001) rutile TiO2 substrate by electron cyclotron resonance plasma-excited molecular beam epitaxy. For the first time, c-GaN with a preferential growth orientation was obtained. Based on the results from electron diffraction and X-ray diffraction analysis, we found that c-GaN with the growth direction of  was grown on the TiO2 substrate. The formation of c-GaN was also confirmed by cathodoluminescence, in which a luminescence peak was observed at 3.24eV.
Single crystalline InN films were grown on Si substrates by radio-frequency plasma-excited molecular beam epitaxy. Electrical property of InN/Si heterojunction was investigated. We obtained rectifying characteristics in n-InN/p-Si heterostructure for the first time. Forward I-V characteristics were affected by both the buffer layer deposition and the nitridation process. Strong photoluminescence peaks for both single crystalline and polycrystalline InN films grown on the Si substrates were observed at around 0.8 eV, which were smaller than the previous reported PL emission peak of around 1.9 eV.
Electrical contacts to both n and p-type GaN films have been investigated using electron-beam evaporated and sputtered films of metals such as Al, Au, Cr, Cu, Ni, Pt, and Ti. Films deposited by electron-beam evaporation for the n-type films with doping levels of 1 × 1018/cm3 and lower showed rectifying characteristics with all the metals studied with the exception of Al. Aluminum contact diodes were ohmic in the as-deposited state. The Pt rectifying contact was near-ideal with an ideality factor close to 1.0. Ideality factors for the other metals were much greater than 1. This deviation from thermionic behavior was interpreted as space charge limited current conduction in the presence of deep-level states. Sputtered films showed very similar characteristics to electron-beam deposited films, with the exception of Ti. The Ti contact was ohmic in the as-deposited state. Non-linear Cu contacts to n-type films became ohmic on annealing. However, for p-type films, Ar ion sputter-cleaning prior to metal deposition by sputtering created ohmic contacts with Cu and Pt. Low resistance ohmic contacts were achieved by ion implantation and anneal of Si in n-type and Mg in p-type films, prior to metallization. The implant parameters and anneal temperatures are currently being optimized.