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Purpose Glioma patients, especially recurrent glioma, suffer from a poor prognosis. While advances to classify glioma on a molecular level improved prognostication at initial diagnosis, markers to prognosticate survival in the recurrent situation are still needed. As 18 kDa translocator protein (TSPO) was previously reported to be associated with aggressive histopathological glioma features, we correlated the TSPO positron emission tomography (PET) signal using [18F]GE180 in a large cohort of recurrent glioma patients with their clinical outcome. Methods In patients with [18F]GE180 PET at glioma recurrence, [18F]GE180 PET parameters (e.g., SUVmax) as well as other imaging features (e.g., MRI volume, [18F]FET PET parameters when available) were evaluated together with patient characteristics (age, sex, Karnofsky-Performance score) and neuropathological features (e.g. WHO 2021 grade, IDH-mutation status). Uni- and multivariate Cox regression and Kaplan–Meier survival analyses were performed to identify prognostic factors for post-recurrence survival (PRS) and time to treatment failure (TTF). Results Eighty-eight consecutive patients were evaluated. TSPO tracer uptake correlated with tumor grade at recurrence (p IDH-wild-type versus IDH-mutant tumors. Within the subgroup of IDH-mutant glioma (n = 46), patients with low SUVmax (median split, ≤ 1.60) had a significantly longer PRS (median 41.6 vs. 25.3 months, p = 0.031) and TTF (32.2 vs 8.7 months, p = 0.001). Also among IDH-wild-type glioblastoma (n = 42), patients with low SUVmax (≤ 1.89) had a significantly longer PRS (median not reached vs 8.2 months, p = 0.002). SUVmax remained an independent prognostic factor for PRS in the multivariate analysis including CNS WHO 2021 grade, IDH status, and age. Tumor volume defined by [18F]FET PET or contrast-enhanced MRI correlated weakly with TSPO tracer uptake. Treatment regimen did not differ among the median split subgroups. Conclusion Our data suggest that TSPO PET using [18F]GE180 can help to prognosticate recurrent glioma patients even among homogeneous molecular subgroups and may therefore serve as valuable non-invasive biomarker for individualized patient management. Graphical abstract

β-amyloid (Aβ) and tau aggregation as well as neuronal injury and atrophy (ATN) are the major hallmarks of Alzheimer’s disease (AD), and biomarkers for these hallmarks have been linked to neuroinflammation. However, the detailed regional associations of these biomarkers with microglial activation in individual patients remain to be elucidated.We investigated a cohort of 55 patients with AD and primary tauopathies and 10 healthy controls that underwent TSPO-, A-, tau-, and perfusion-surrogate-PET, as well as structural MRI. Z-score deviations for 246 brain regions were calculated and biomarker contributions of Aβ (A), tau (T), perfusion (N1) and gray matter atrophy (N2) to microglial activation (TSPO, I) were calculated for each individual subject. Individual ATN-related microglial activation was correlated with clinical performance and CSF soluble TREM2 (sTREM2) concentrations.In typical and atypical AD, regional tau was stronger and more frequently associated with microglial activation when compared to regional A (AD: βT= 0.412±0.196 vs. βA= 0.142±0.123, p < 0.001; AD-CBS: βT= 0.385±0.176 vs. βA= 0.131±0.186, p = 0.031). The strong association between regional tau and microglia reproduced well in primary tauopathies (βT= 0.418±0.154). Stronger individual associations between tau and microglial activation were associated with poorer clinical performance. In patients with 4RT, sTREM2 levels showed a positive association with tau-related microglial activation.Tau pathology has strong regional associations with microglial activation in primary and secondary tauopathies. An index of tau- and Aβ-associated microglia activation accounts for regional heterogeneity and allows for clinical and biomarker correlations with ATN-specific neuroinflammation.

The purpose of this study was to determine whether different types of position-related cranial deformities show changes after completion of head orthosis therapy. We investigated how children's age at the begin of molding helmet therapy affects the duration and long-term stability of treatment. In addition, parental satisfaction with helmet therapy has been investigated.Between 2012 and 2019, 47 patients have been analyzed after undergoing helmet therapy. At the beginning of treatment different disease pattern were classified. Posterior deformational Plagiocephalus defined by a cranial vault index (CVA)1 cm and a cranial index (CI) ≤ 90%. Posterior deformational Brachycephalus defined by CVA ≤ 1 cm and CI90%. Posterior deformational combined Plagio- and Brachycephalus defined by CVA1 cm and CI90%. At the beginning of therapy, the end of therapy and within 5-year-follow-up CI and CVA was measured by three-dimensional photogrammetry for 2 age groups (4-6 and 7-12 months). Additionally, parents completed a standardized questionnaire to evaluate the personal assessment of treatment outcome.During treatment CI and CVA of all children decreased significantly (P 0.001). Furthermore, CI significantly decreased after ending helmet therapy (P 0.001). Cranial vault index decreased not significantly in the same period (P = 0.361). For the 4 to 6 months old group treatment time was significantly shorter than for the older group. Before starting helmet therapy nearly half of the parents graded the scull as moderate and one-third as severe deformed. After ending treatment, the majority of parents report satisfaction and compliance. However, one-third of parents noted a slight scull deformation and personal load during therapy.After completion of therapy an improvement of head shape can be expected for the majority of children. This is particularly evident for improvement of the CI and a shorter treatment time until the age of 6 months at the beginning of therapy. Parents reported satisfaction and child compliance. Almost all parents were convinced that treatment was useful and would repeat it again.

350 nm and 550 nm thick InGaN/GaN bilayers were irradiated with different energies (from ∼82 to ∼38 MeV) of xenon (129Xe) ions and different fluences of 1.2 GeV lead (208Pb) ions, respectively.

Purpose Glioma patients, especially recurrent glioma, suffer from a poor prognosis. While advances to classify glioma on a molecular level improved prognostication at initial diagnosis, markers to prognosticate survival in the recurrent situation are still needed. As 18 kDa translocator protein (TSPO) was previously reported to be associated with aggressive histopathological glioma features, we correlated the TSPO positron emission tomography (PET) signal in a large cohort of recurrent glioma patients with their clinical outcome. Methods In patients with TSPO PET at glioma recurrence, TSPO PET parameters (e.g. SUVmax) as well as other imaging features (e.g. MRI volume, [18F]FET PET parameters when available) were evaluated together with patient characteristics (age, sex, Karnofsky-Performance score) and neuropathological features (e.g. WHO grade, IDH-mutation status). Uni- and multivariate Cox regression and Kaplan-Meier survival analyses were performed to identify prognostic factors for post-recurrence survival (PRS) and time to treatment failure (TTF). Results 88 consecutive patients were evaluated. TSPO tracer uptake correlated with tumor grade at recurrence (p IDH-wildtype versus IDH-mutant tumors. Within the subgroup of IDH-mutant glioma (n = 46), patients with low SUVmax (median split, ≤ 1.60) had a significantly longer PRS (median 41.6 vs. 25.3 months, p = 0.031) and TTF (32.2 vs 8.7 months, p = 0.001). Also among IDH-wildtype tumors (n = 42), patients with low SUVmax (≤ 1.89) had a significantly longer PRS (median not reached vs 8.2 months, p = 0.002). SUVmax remained an independent prognostic factor for PRS in the multivariate analysis including CNS WHO grade, IDH status and age. Tumor volume defined by [18F]FET PET or contrast-enhanced MRI correlated weakly with TSPO tracer uptake. Treatment regimen did not differ among the median split subgroups. Conclusion Our data suggest that TSPO PET can help to prognosticate recurrent glioma patients even among homogeneous molecular subgroups and may therefore serve as valuable non-invasive biomarker for individualized patient management.

The effects of 129Xe swift-heavy ion (SHI) irradiation on the optical and structural properties of InGaN/GaN multiple quantum wells (MQWs) are studied using non-destructive optical spectroscopy techniques. Through excitation wavelength-dependent μ-Raman spectroscopy, it is possible to identify the damage profile and to notice the surface region (around the first 40 nm) as the most affected one. Independent Rutherford backscattering spectrometry in channelling mode experiments corroborate this. By using resonant excitation with InGaN electronic states, it is possible to observe an increase of the A1(LO)InGaN phonon intensity for higher SHI energies. This is attributed to intermixing at the MQWs’ interfaces for electronic energy losses above 20 keV nm−1. Photoluminescence studies reveal an intense yellow emission band (YB) with two distinct contributions: excitonic (InGaN) and deep-level defects (GaN). After irradiation, strong luminescence intensity quenching and a change in its excitation mechanisms occur. The YB is no longer excited with energy above the GaN bandgap, but instead through an excitation band (at about 2.75 eV) attributed to gallium vacancies introduced in the GaN barriers. The strong quenching of the luminescence reveals that SHI irradiation introduces detrimental defects in the structure, which may compromise its use (without additional post-implantation treatments) for obtaining more efficient green light emitters based on InGaN/GaN MQWs.

Corticobasal syndrome (CBS) is a rare neurodegenerative condition characterized by four-repeat tau aggregation in the cortical and subcortical brain regions and accompanied by severe atrophy. The aim of this study was to evaluate partial volume effect correction (PVEC) in patients with CBS compared to a control cohort imaged with the 18-kDa translocator protein (TSPO) positron emission tomography (PET) tracer [

To compare flow-regulated (FR) and differential pressure (DP) valves for treatment of patients with idiopathic normal hydrocephalus (iNPH) focusing on overdrainage and neurological outcome. This is a retrospective study of patients with iNPH treated with FR and DP valves at a single institution between 2008 and 2018. The neurological status was evaluated retrospectively with the Kiefer scale at baseline, after shunt placement and at the 6-month follow-up. Groups were compared using inverse probability of treatment weighting based on propensity scores. The study cohort consisted of 38 patients treated with FR valves and 49 with DP valves. The mean patient age was 72.0 ± 7.6 years. Based on the Kiefer scale score, neurological improvement at the 6-month follow-up was recorded in 79.6% in the DP group and 89.5% in the FR group (p = 0.252). The overdrainage rates were higher among DP valves (10.2%) than among FR valves (2.6%, adjusted p = 0.002). Valve malfunction occurred in 2.0% in the DP group and 5.3% in the FR group (adjusted p = 0.667). The current study demonstrates a comparable neurological improvement between DP and FR valves, with potentially lower overdrainage rates among FR valves. Long-term studies will be necessary to draw a definite conclusion on FR valves for treatment of iNPH patients.

The elastic properties of the cubic (c-) phase GaN confined in a nanoscale one-dimensional (1D) v-grooved Si(001) substrate are investigated. Along a ∼900 nm-wide v-groove formed with two facing Si(111)-type facets, sub-micrometer-wide c-GaN is achieved by the hexagonal- to c-phase (h–c) transition from the h-GaN which plays the role of an interlayer in its epitaxy on Si. The resulting nonplanar stack of c-GaN/h-GaN on Si has complicated stress distribution. This work focuses on the elastic properties the c-GaN, which are critically affected by its low dimensionality, and presents experimental evidence for it with an analytical stress modeling. A reciprocal lattice map reveals that the c-GaN in each groove consists of several micrometer-long single crystals which are microscopically tilted from each other in their serial coalescence, as its unit structures. The corresponding micrometer-scale lateral correlation length, dc, results from the h–c transition that is interrupted by the groove imperfections gen...

Overdrainage and frequent reprogramming are common issues with programmable valves after ventriculoperitoneal shunt surgery for idiopathic normal pressure hydrocephalus (iNPH). Flow-regulated valves may address these limitations, but data on their efficacy are sparse. We present our single-center experience with flow-regulated valves focusing on overdrainage and efficiency.Thirty-two patients with iNPH treated with the Integra® NPH Low Flow Valve were prospectively enrolled. Clinical evaluation was performed at baseline, postoperatively as well as 3 and 6 months after surgery. The outcome was assessed by employing the iNPH grading scale and the Mini Mental Status Examination (MMSE). Overdrainage was assessed clinically and radiologically by computed tomography.The mean patient age was 71 years. All patients presented with gait disturbance, 29 had cognitive impairment and 25 had urinary incontinence. The mean duration of symptoms was 22.9 months. At 3-month follow-up, 25/31 patients (80.6%) improved on the total iNPH score by at least 5 points (p 0.001). The mean MMSE score increased from 24.5 to 26.1 points (p = 0.013). After 6 months, the improvement rates of the iNPH (82.1%) and the MMSE scores (26.8 points) were stable. The rate of clinically significant overdrainage was 3.2%. One patient presented with subdural hygromas that necessitated evacuation. In the remaining patients, clinical and radiological signs of overdrainage were absent.The use of the Integra® NPH Low Flow Valve leads to a good neurological outcome and has low overdrainage rates without the need for reprogramming. These results are encouraging and justify further investigation of this valve.

InSe, a newly rediscovered two-dimensional (2D) semiconductor, possesses superior electrical and optical properties as a direct-band-gap semiconductor with high mobility from bulk to atomically thin layers and is drastically different from transition-metal dichalcogenides, in which the direct band gap only exists at the single-layer limit. However, absorption in InSe is mostly dominated by an out-of-plane dipole contribution, which results in the limited absorption of normally incident light that can only excite the in-plane dipole at resonance. To address this challenge, we have explored a unique geometric ridge state of the 2D flake without compromising the sample quality. We observed the enhanced absorption at the ridge over a broad range of excitation frequencies from photocurrent and photoluminescence (PL) measurements. In addition, we have discovered new PL peaks at low temperatures due to defect states on the ridge, which can be as much as ∼60 times stronger than the intrinsic PL peak of InSe. Interestingly, the PL of the defects is highly tunable through an external electrical field, which can be attributed to the Stark effect of the localized defects. InSe ridges thus provide new avenues for manipulating light-matter interactions and defect engineering that are vitally crucial for novel optoelectronic devices based on 2D semiconductors.

Providing concise proof of child abuse relies heavily on clinical findings, such as certain patterns of injury or otherwise not plausibly explainable trauma. Subdural hemorrhaging has been identified as a common occurrence in abused children whereas epidural hemorrhaging is related to accidents. In order to explore this correlation, we retrospectively analyzed clinical data of children under 19 years of age diagnosed with either injury. Reviewing 56 cases of epidural and 38 cases of subdural bleeding, it was shown that subdural bleeding is more common in young children and extremely often a result of suspected abuse in children under 2 years of age. Epidural hemorrhaging however never was found in the context of suspected abuse, was unrelated to other injuries typical for abuse, and did not see a statistically significant increase in any age group. In conformity with currently theorized mechanisms of injury for both types of bleeding, we found that subdural hemorrhaging in young children is closely associated with abuse whereas epidural bleeding is not.

Although the risk of aneurysm remnants after microsurgical clipping is generally low, complete aneurysm occlusion is not always guaranteed. We performed a morphometric analysis of intracranial aneurysms to identify predictors for aneurysm remnants and to propose a novel risk score.This is a retrospective, single-center analysis of consecutive patients with ruptured and unruptured aneurysms who underwent microsurgical clipping and postoperative digital subtraction angiography between 2010 and 2018. Based on preoperative rotational angiography, distinct morphologic aneurysm characteristics were determined and correlated with postoperative angiographic results. Factors predictive in the univariate and multivariate analyses were determined to establish a risk score for postoperative remnants after aneurysm clipping.Among 140 patients with 166 clipped aneurysms, aneurysm remnants were present in 19.9%. In the multivariate analysis, ruptured aneurysm status (odds ratio [OR], 7.8; 95% confidence interval [CI], 1.7-36; P0.01) and increased aspect ratio (OR, 1.9; 95% CI, 1.0-4.0; P = 0.07) were associated with postoperative aneurysm remnants. Anterior communicating artery location (P = 0.02), internal carotid artery location (P = 0.06), increased aneurysm inclination angle (P0.01), and irregular aneurysm shape (P = 0.07) were further predictors for aneurysm remnants in the univariate analysis. These factors were weighted and included into a risk sum score for postoperative aneurysm remnants (range, 0-8 points), which performed with good accuracy (area under the curve = 0.807).After external validation of the proposed risk score, it could help identify cases requiring angiographic control after aneurysm surgery.

For the sustainable redevelopment of existing properties,it is important thatthe measures taken pay off economically as well. With the energy-scoring method presented in this paper, portfolios can be classified according to their energy-saving potential. Besides the various factors affecting the reduction of primary energy demand, the 3-tier model analyzes and correlates the economic efficiency. The result is an objective ranking of each object, from which property owners can deduce where they should ideally invest in order to optimize the energy efficiency of their building stock sustainably.

An atomic-scale phase transition in heterophase epitaxy (HPE) of GaN on a 900 nm-wide v-grooved Si(001) substrate is reported. Two different incorporation mechanisms of adatoms sequentially occur for the hexagonal (h-) to cubic (c-) phase transition: orientation- and phase-dependent incorporation (ODI and PDI). Epitaxy begins with ODI that results in preferential growth of h-GaN individually aligned to opposing Si(111) facets inside a v-groove but incurs a structural instability by crystallographic mismatch at the groove bottom. This instability is relieved by an abrupt transition to c-phase, initiating from single or multiple atomic sites uniquely arranged atop the mismatch along the groove. Epitaxy proceeds with PDI that allows μm-scale c-GaN extended from these sites while suppressing growth of h-GaN. An important condition for HPE and the stability of c-GaN in further growth is derived from equilibrium crystal shape.

We present a simple and robust method to acquire quantitative maps of compositional fluctuations in nanostructures from low magnification high-angle annular dark field (HAADF) micrographs calibrated by energy-dispersive X-ray (EDX) spectroscopy in scanning transmission electron microscopy (STEM) mode. We show that a nonuniform background in HAADF-STEM micrographs can be eliminated, to a first approximation, by use of a suitable analytic function. The uncertainty in probe position when collecting an EDX spectrum renders the calibration of HAADF-STEM micrographs indirect, and a statistical approach has been developed to determine the position with confidence. Our analysis procedure, presented in a flowchart to facilitate the successful implementation of the method by users, was applied to discontinuous InGaN/GaN quantum wells in order to obtain quantitative determinations of compositional fluctuations on the nanoscale.

In this paper, we have investigated the high temperature characteristics of monolithically integrated LED and MOS-channel HEMT in GaN. The monolithically integrated LED/MOSC-HEMT was implemented by using a selective epi removal approach. The temperature dependence of optical and electrical characteristics of the integrated LED is found to be comparable to that of discrete GaN based LEDs. On-resistance of the MOSC-HEMT shows gradual increase with temperature (~1.6× increase from 25 °C to 225 °C) whereas LED LOP shows rapid decrease with temperature (~6× decrease from 25 °C to 225 °C). Light output of the integrated LED is modulated by the MOSC-HEMT gate bias up to 225 °C.

We report the successful epitaxial growth of biaxial GaN films using metal-organic vapor phase epitaxy on amorphous surfaces (native oxide covered Si wafer) through biaxial metal buffer layers of Mo and W. The biaxial metal films were grown using a dynamic oblique angle deposition technique. An epitaxial relationship of GaN(0002)//Mo(110) in the out-of-plane orientation and GaN[ 10 1 ¯ 0 ]//Mo[001] in the in-plane orientation was found by a detailed X-ray diffraction analysis. Similar epitaxial results have been found for GaN/W. These biaxial GaN films on biaxial Mo and W buffer layers were used as seeds to grow GaInN/GaN multiple quantum well structures (MQWs). A high intensity, narrow peak, and uniform luminance space distribution of photoluminescence signal was observed from these MQWs. This work opens up an alternative strategy for the growth of GaN-based films on amorphous substrates using biaxial metal buffer layers.

The advent of a wide-bandgap GaN p-n junction has enabled highly efficient blue light-emitting diodes (LEDs) in GaInN/GaN heteroepitaxy. The system also enables a much wider range of emission wavelengths. We summarize progress in epitaxial materials development of green, yellow, and orange direct-emitting LEDs that bypass the steps of external phosphor conversion to achieve higher stability, efficiency, and higher overall color rendering quality for wider adoption of one of the widest power savings resources so far identified.

We report that using a high/low p-type junction anode structure results in improved hole injection over a uniformly-doped p-anode layer in GaN pin junction diodes. Quasi-vertical diodes with a 20 nm thick, magnesium concentration of 2 × 1020 cm−3 layer on top of a 480 nm thick layer with a magnesium concentration of 1018 cm−3 show greatly increased forward current density – >100× higher – than those with a 500 nm thick uniformly 3 × 1019 cm−3 Mg-doped p-layer. Forward knee voltage and ideality factor are reduced by a factor of more than two, and reverse leakage current density is also reduced. Additionally, the specific differential series resistance is reduced significantly. With photoluminescence measurements, we found that these improvements are largely due to improved p-GaN material quality of the high/low junction sample with lower average Mg concentration.

Low-resistance Ohmic contacts are essential for the fabrication of electrical devices. While low contact resistance has been achieved to p -type layers or n -type layers separately, contacts are likely to degrade when both types need to be integrated into a single fabrication process, in particular when prior mesa etching is required. We present a solution to the problem, resulting in low-resistance Ohmic contacts on n -type GaN layers without post-deposition thermal anneal, while maintaining the quality of typical p -type contacts. We implement an integrated process for both, n - and p -contacts, involving an oxygen pretreatment to fabricate light emitting diodes with lower series resistance in the contacts and lower voltage drop at high current when compared to separately optimized contacts.

We demonstrate photocurrent (PC) spectroscopy of GaInN/GaN multiple quantum well structures with electroluminescence peak wavelengths near 550 nm. Two classes of closely related samples were grown by metal-organic vapor phase epitaxy on c -plane (0001) sapphire substrate. Although nominally identical, one set appears optically transparent while the other appears black to the eye. The PC spectrum of the black sample shows a strong enhancement of absorption at wavelengths longer than 450 nm. In UV microscopy clusters of dark areas are found in the black sample. Such findings are commonly ascribed to areas of Indium phase segregation but require further study. (© 2011 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We report the development of a novel ridge-type AlGaInN-based laser diode structure fabricated by a metal organic vapor phase epitaxial regrowth method. The selective-area regrowth of AlGaN was optimized and compared with regular planar growth. An AlN fraction of 13% in the regrown ridge was concluded from the cathodoluminescence spectrum. The laser diode structure showed superluminescence at 465 nm under optical excitation. Under electrical excitation a super-linear increase of the light output power was observed.

GaN, LED, nano-structure ** Corresponding author: e-mail starkc2@rpi.edu, Fax: +1 518 276 8042 ** e-mail wetzel@rpi.edu, Phone: +1 518 276 3755, Fax: +1 518 276 8042 The mesa width of green light-emitting diodes (LEDs) was scaled from the micrometer to the nanometer range. The devices were patterned by electron beam lithography and dry etching and contained up to 100 parallel nano-stripe LEDs connected to one common contact area. The mesa width was varied over 1 μm, 200 nm, and 50 nm. The LEDs were characterized electrically and optically, and the peak emission wavelength was found to depend on the lateral structure size. Light emission from the pat-terned region was found to be 4 times brighter than from the un-patterned regions.

This paper introduces a risk assessment procedure for the crosswind stability of ground vehicles. A random variable gust model is combined with constrained simulation techniques and variance reducing Monte Carlo methods for an efficient computation of failure probabilities. Based on the failure probability, sensitivities, the influence of additional effects such as turbulence, track irregularities or the friction coefficient between wheel and road can be investigated on an objective basis. Moreover, the procedure allows us to compute the failure probabilities for vehicle journeys or a whole line.

The initial stages of the nucleation of cubic (c-) GaN in heterophase epitaxy on a Si v-groove are investigated. Growth of GaN on a nanoscale {111}-faceted v-groove fabricated into a Si(001) substrate proceeds in the hexagonal (h-) phase that induces a secondary v-groove replicating the substrate topography with two opposing {0001} facets. The secondary v-groove is then orientationally mismatched at the junction of the h-GaN facets (h -h junction) resulting in structural instability. This instability is relieved either by the formation of voids that reduce the actual junction area or by the transition to c-phase (h-c transition) suppressing further extension of the h-h junction. The distribution of voids that is locally affected by the island growth mode of h-GaN on Si(111) and the imperfection in the groove geometry impacts the initial stage of heterophase epitaxy. Primarily, The h-c transition is observed as a non-local phenomenon; it occurs homogeneously and simultaneously along the bottom of the entire secondary groove and forms a one-dimensional (1D) seed layer except for some interruptions where the h-h junction is defected by gaps or incomplete voids. Between these interruptions, epitaxy retains a single crystal but results in a series of c-GaN nanodots on the seed layer with large fluctuation in size and spacing. The adatom incorporation observed in this heterophase epitaxy is a 1D analog to the wetting of a substrate followed by the self-assembly in conventional quantum dot epitaxy. The surface morphology of the c-GaN nanodots is governed by the faceting mostly composed of (001)- and (11n)-orientations and the roughening between these facets that ultimately affect the morphology of the final top surface of the c-III-N. The interruptions interfere with the homogeneity of the h-c transition and can cause antiphase defects and mosaicity. Based on experimental results, a solution to improve these issues is proposed.

The light output of 530 nm green GalnN/GaN light-emitting diodes on sapphire has been nearly doubled by the insertion of a 130-nm GalnN underlayer (UL) between the n-GaN electron injection layer and the quantum-well (QW) active region. Under variation of the alloy composition, best results were obtained for an x = 6.3% Ga1-xInxN UL. By low-temperature depth-resolved cathodoluminescence spectroscopy, an interplay of the impurity-related donor-acceptor pair recombination, the UL, and the QW emission has been observed. We propose that the resonance and level alignments between the defect and UL levels reroute excitation toward radiative recombination in the QWs.

A typology of buildings representative of the building stock for the EU-25 was developed characterizing 72 building types in terms of their representativity, geographical distribution, size, material composition, and thermal insulation. The life cycle impacts of the building types were calculated for different environmental impact categories both at building and EU-25 level. The use phase of buildings, dominated by the energy demand for heating is by far the most important life cycle phase for existing and new buildings. The environmental impacts were allocated to single building elements. Ventilation, heat losses through roofs and external walls are important for a majority of single- and multi-family houses. Three improvement options were identified: additional roof insulation, additional facade insulation and new sealings to reduce ventilation. The measures yield a significant environmental improvement potential, which, for a majority of the buildings types analyse represent at least 20% compared to the base case. The major improvement potentials at EU-level lie with single-family houses, followed by multi-family houses. Smaller reductions are expected for high-rise buildings due to the smaller share in the overall building stock. For both roof insulation and reduced ventilation, the measures were shown to be economically profitable in a majority of buildings.

We report the growth and structural characteristics of green and yellow (529-576 nm) GaInN/GaN light emitting diodes (LEDs) grown on two types of c-plane substrates - bulk GaN and sapphire. In this longer wavelength range, depending on the substrate, we find different strain relaxation mechanisms within the GaInN/GaN quantum well (QW) region. In optimized epitaxy, structures on sapphire that contain a low density of threading dislocations (TDs) within the n-GaN show virtually no generation of additional misfit dislocations (MDs) (

This paper deals with the probabilistic investigation of the crosswind stability of ground vehicles with a strong focus on railway vehicle dynamics. The turbulent crosswind excitation is reduced to an artificial gust model, which is based on the so-called constrained simulation concept and where the gust amplitude, the gust duration and the aerodynamic coefficients of the vehicle are considered as random variables. The railway vehicle is simulated as a nonlinear multi-body vehicle model. Because of the random variables, a probabilistic analysis of the system has to be performed, including a reliability analysis to compute the overturning probability and including a sensitivity analysis concerning the random variables and the parameters of the probability density functions of the random variables.

The junction temperature of homoepitaxial green and blue GaInN/GaN quantum well light emitting diode (LED) dies is analyzed by micro-Raman, photoluminescence, cathodoluminescence mapping, and forward-voltage methods and compared to finite element simulations. Dies on GaN substrate and sapphire were analyzed under variable drive current up to 200 mA (246 A/cm2). At 100 mA, dies on bulk GaN remain as cool as 355 K (83 °C) while dies on sapphire heat up to 477 K (204 °C). The efficiency droop and spectral line shift in green LEDs with increasing current density can now be separated into electrical and thermal contributions.

GaInN/GaN light emitting diodes are the primary choice for efficient green light emitters, yet their performance limiting factors have yet to be identified. Here we perform a low-temperature luminescence study in depth-resolved cathodoluminescence. A series of LEDs with a combination of blue and green light emitting quantum wells exhibiting low and high electroluminescence output power were compared. In all samples, a band of donor-acceptor pair recombination was identified at 77 K. In the samples with lowest performance, such transitions located within the active region of the quantum wells had a particular strong contribution to the spectrum. In LEDs of higher performance, such luminescence was substantially suppressed within those layers. We argue that the higher performance of LEDs without such donor acceptor transition bands may be associated with the absence of corresponding dopant impurities and indicate a higher epitaxial perfection within the active quantum well region.

An enhancement of radiative recombination in GaInN/GaN heterostructures is being pursued by a reduction of defects associated with threading dislocations and a structural control of piezoelectric polarization in the active light-emitting regions. First, in conventional heteroepitaxy on sapphire substrate along the polar c-axis of GaN, green and deep green emitting light-emitting diode (LED) wafers are being developed. By means of photoluminescence at variable low temperature and excitation density, internal quantum efficiencies of 0.18 for LEDs emitting at 530 nm and 0.08 for those emitting at 555 nm are determined. Those values hold for the high current density of 50 A/cm 2 of high-power LED lamps. In bare epi dies, we obtain efficacies of 16 lm/W. At 780 A/cm 2 we obtain 22 lm when measured through the substrate only. The 555 nm LED epi material under pulsed photoexcitation shows stimulated emission up to a wavelength of 485 nm. This strong blue shift of the emission wavelength can be avoided in homoepitaxial multiple quantum well (MQW) and LED structures grown along the non-polar a- and m-axes of low-dislocation-density bulk GaN. Here, wavelength-stable emission is obtained at 500 and 488 nm, respectively, independent on excitation power density opening perspectives for visible laser diodes.

We demonstrate homoepitaxial growth of GaInN/GaN-based green (500–560 nm) light emitting diodes (LEDs) on a-plane and m-plane quasi-bulk GaN prepared by hydride vapor phase epitaxy (HVPE). We find that in order to achieve an emission peak wavelength beyond 500 nm, a minimum InN-fraction of ∼14% is needed for both, a- and m-plane quantum wells (QWs), while ∼8% are enough for c-plane-oriented QWs. Besides increasing the InN-fraction in these non-polar QWs, widening the QW also proves to effectively shift the emission to longer wavelengths without loosing efficiency with the benefit of maintaining a low InN-fraction.

InGaN quantum well structures on GaN epilayers were exposed to 500 keV alpha particles to fluences above 1014 cm2 to probe the relative radiation tolerance of the epilayer and wells. Performance was estimated by the intensity of ion-beam induced luminescence. Two separate types of quantum well structures emitted at 470 and 510 nm prior to irradiation, and only small wavelength shifts were observed even with the highest alpha fluences. Complementary cathodoluminescence experiments showed that luminescence in the quantum wells is strongly influenced by charges injected deep into the GaN epilayer. The 500 keV alpha penetration depth was ~ 1 mum, so that defects were created at a faster rate in GaN compared to InGaN as alpha particles slowed and stopped within a minority carrier diffusion length of the quantum wells. However, the rate of luminescent decay was similar for both materials. Taken together with the cathodoluminescence data, this ion-beam induced luminescence comparison indicates that the quantum well luminescence decay rate is dominated by radiation-induced defects in the GaN epilayer. InGaN quantum wells are then demonstrated to be not more than a factor of ten more radiation sensitive than GaN, and may be substantially less sensitive than this upper bound.

GaInN/GaN multiple quantum well light-emitting diode structures in polar c -axis and non-polar m -axis growth have been compared in terms of luminescence properties. Grown under identical conditions, under low excitation density the c -axis structure has a luminescence maximum at 558 nm while the m -axis structure shows a maximum at 488 nm and shows superluminescence at 485 nm under high photoexcitation density. Under the same conditions, on increasing the excitation power, the peak intensity increases 40 fold in the m -axis structure without any variation of the emission wavelength. In similar but separately grown c -axis structures without a p-side, luminescence shifts from 555 nm at low excitation density to superluminescence at 485 nm under high excitation. The coincidence, of the superluminescence wavelength in the polar structure with the stable peak wavelength in the non-polar one, suggests that the wavelength shift in the polar structure is due to its piezoelectric polarization. The absence of such effects in the m -axis-grown structure therefore suggests a stronger dipole matrix element, potentially enabling higher quantum efficiencies and suitability for high efficiency light-emitting diode and laser diode designs in the green spectral region.

The efficiency of green GaInN/GaN light emitting diodes (LEDs) is known to drop under high current densities. To explore the reason, 535 nm emitting GaInN/GaN multiple quantum well (MQW) and GaN epilayers were characterized using Z-scan techniques under comparable excitation conditions with a continuous wave laser. Two wavelengths, 514 nm and 488 nm, were selected right below and above the apparent optical absorption edge. At 514 nm, a very large nonlinear absorption coefficient β = 2.6 cm/W was obtained. This leads to a 20% absorption enhancement at a photon flux of 21 kW/cm2. We attribute this to nonlinear free-carrier absorption. On the other hand, at 488 nm, a nonlinear saturable absorption with β = –1.7 cm/W was observed. This induced transparency indicates photon bleaching in the MQW. Apparently, free carrier dynamics strongly affects optical nonlinearity, while nonlinear absorption provides a small contribution to the limitations of current green LEDs. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

We perform a transmission electron microscopy (TEM) characterization of blue light-emitting diode epiwafers grown homoepitaxially on a c-plane GaN substrate and compare with such grown on sapphire. We find a threading dislocation (TD) density as low as 2 × 10 8 cm -2 in homoepitaxial growth, which is 1/30 of that in sapphire-based material. A unique type of inverted pyramid defect with diameter ∼650 nm was observed. It originates from the homo-interface in edge-type TDs. TDs were also found to be generated within the quantum wells without a precursor TD defect. Otherwise, high-resolution TEM suggests extremely homogeneous quantum wells and barriers in terms of composition and well width. Besides plan-view TEM, wet chemical etching on n-type GaN was carried out in hot phosphoric acid to evaluate the TD density more quickly. The two methods prove to be equivalent with respect of determined TD densities.

We present a junction temperature analysis of GaInN/GaN quantum well (QW) light-emitting diodes (LEDs) grown on sapphire and bulk GaN substrate by micro-Raman spectroscopy. The temperature was measured up to a drive current of 250 mA (357 A/cm2). We find better cooling efficiency in dies grown on GaN substrates with a thermal resistance of 75 K/W. For dies on sapphire substrates we find values as high as 425 K/W. Poor thermal performance in the latter is attributed to the low thermal conductivity of the sapphire. Three-dimensional finite-element simulations show good agreement with the experimental results, validating our thermal model for the design of better cooled structures.

Department of Electrical, Computer, and Systems Engineering, Rensselaer, Troy, NY 12180, USA Received 19 September 2006, accepted 29 November 2006 Published online 31 May 2007 PACS 73.40.Kp, 73.61.Ey, 78.60.Hk, 78.67.De, 85.60.Jb The electroluminescence of GaInN/GaN multiple quantum well light-emitting diode dies is analyzed at variable low temperatures. We compare the external quantum efficiency of three dies of nominally identi-cal structure but strongly different RT performance at 520 nm. For all dies, the external quantum efficien-cy increases as the temperature is lowered. A maximum is reached for all near 158 K while for lower temperatures as low as 7.7 K, the efficiency continuously drops. The low-temperature efficiency is the lowest for the highest performing die at RT. The peak energy exhibits a blue shift from RT to 158 K fol-lowed by a red shift for lower temperatures. In the same low-temperature range, a secondary emission peak appears near 390 nm that resembles a donor-acceptor pair transition. The pronounced efficiency ma-ximum is tentatively assigned to competition of carrier transport properties and the non-radiative lifetime in the active region.