High Performance Green LEDs by Homoepitaxial

This work's objective was the development of processes to double or triple the light output power from green and deep green (525 - 555 nm) AlGaInN light emitting diode (LED) dies within 3 years in reference to the Lumileds Luxeon II. The project paid particular effort to all aspects of the internal generation efficiency of light. LEDs in this spectral region show the highest potential for significant performance boosts and enable the realization of phosphor-free white LEDs comprised by red-green-blue LED modules. Such modules will perform at and outperform the efficacy target projections for white-light LED systems in the Department of Energy's accelerated roadmap of the SSL initiative

The Effect of Imbalanced Carrier Transport on the Efficiency Droop in GaInN-Based Blue and Green Light-Emitting Diodes

The effect of strongly-imbalanced carrier concentration and mobility on efficiency droop is studied by comparing the onset voltage of high injection, the onset current density of the droop, and the magnitude of the droop, as well as their temperature dependence, of GaInN-based blue and green light-emitting diodes (LEDs). An n-to-p asymmetry factor is defined as sigma(n)/sigma(p), and was found to be 17.1 for blue LEDs and 50.1 for green LEDs. Green LEDs, when compared to blue LEDs, were shown to enter the high-injection regime at a lower voltage, which is attributed to their less favorable p-type transport characteristics. Green LEDs, with lower hole concentration and mobility, have a lower onset current density of the efficiency droop and a higher magnitude of the efficiency droop when compared to blue LEDs. The experimental results are in quantitative agreement with the imbalanced carrier transport causing the efficiency droop, thus providing guidance for alleviating the phenomenon of efficiency droop.114sciescopu

Influence of electron injection on performance of GaN photodetectors

It is demonstrated that short-time (up to 1200 s) electron injection into the p-region of GaN p-n junction, as a result of forward bias application, leads to a long-term multifold enhancement of the device peak responsivity as well as to a spectral broadening of the photoresponse. The effect is found to persist for several days and is related to an increased minority carrier diffusion length in the p region, due to an injected electron trapping on deep levels associated with Mg acceptors

Enhanced overall efficiency of GaInN-based light-emitting diodes with reduced efficiency droop by Al-composition-graded AlGaN/GaN superlattice electron blocking layer

AlxGa1-xN/GaN superlattice electron blocking layers (EBLs) with gradually decreasing Al composition toward the p-type GaN layer are introduced to GaInN-based high-power light-emitting diodes (LEDs). GaInN/GaN multiple quantum well LEDs with 5- and 9-period Al-composition-graded AlxGa1-xN/GaN EBL show comparable operating voltage, higher efficiency as well as less efficiency droop than LEDs having conventional bulk AlGaN EBL, which is attributed to the superlattice doping effect, enhanced hole injection into the active region, and reduced potential drop in the EBL by grading Al compositions. Simulation results reveal a reduction in electron leakage for the superlattice EBL, in agreement with experimental results. (C) 2013 AIP Publishing LLC.open1133sciescopu

The electrification of energy: long-term trends and opportunities

We present and analyze three powerful long-term historical trends in the electrification of energy by free-fuel sources. These trends point toward a future in which energy is affordable, abundant, and efficiently deployed; with major economic, geo-political, and environmental benefits to humanity.We present and analyze three powerful long-term historical trends in energy, particularly electrical energy, as well as the opportunities and challenges associated with these trends. The first trend is from a world containing a diversity of energy currencies to one whose predominant currency is electricity, driven by electricity’s transportability, exchangeability, and steadily decreasing cost. The second trend is from electricity generated from a diversity of sources to electricity generated predominantly by free-fuel sources, driven by their steadily decreasing cost and long-term abundance. These trends necessitate a just-emerging third trend: from a grid in which electricity is transported unidirectionally, traded at near-static prices, and consumed under direct human control; to a grid in which electricity is transported bidirectionally, traded at dynamic prices, and consumed under human-tailored artificial agential control. These trends point toward a future in which energy is not costly, scarce, or inefficiently deployed but instead is affordable, abundant, and efficiently deployed; with major economic, geo-political, and environmental benefits to humanity

Energy bandgap variation in oblique angle-deposited indium tin oxide

Indium tin oxide (ITO) thin films deposited using the oblique angle deposition (OAD) technique exhibit a strong correlation between structural and optical properties, especially the optical bandgap energy. The microstructural properties of ITO thin films are strongly influenced by the tilt angle used during the OAD process. When changing the tilt angle, the refractive index, porosity, and optical bandgap energy of ITO films also change due to the existence of a preferential growth direction at the interface between ITO and the substrate. Experiments reveal that the ITO film's optical bandgap varies from 3.98 eV (at normal incident deposition) to 3.87 eV (at a 60 tilt angle). 2-10 OAD is generally associated with physical vapor deposition of thin films (prepared through, e.g., electron-beam or thermal evaporation), in which the material vapor flux arrives at the substrate surface at an oblique angle. Tilted and columnar nanostructures are the most typical morphological characteristics of OAD thin films. 1, Deposition of ITO films on soda lime glass and silicon substrates was conducted with an E-beam evaporation system using an ITO source composed of 90 wt. % In 2 O 3 and 10 wt. % SnO 2 . Prior to the deposition, all substrates were sequentially cleaned in acetone, isopropyl alcohol, and deionized water and dried under nitrogen flow. The apparatus used in our OAD process has a sample stage (onto which the substrate was loaded), allowing for control of the polarangle as well as azimuthal rotation. The distance between the substrate and evaporation source material was approximately 50 cm. The sample stage was positioned at a fixed polar angle so that the substrate had a certain tilt angle (deposition angle) with respect to the vapor-flux direction. The chamber was evacuated to a pressure less than 1.0 Â 10 À6 Torr, and substrates were held at room temperature. During the deposition, the deposition rate was held steady at 0.2 nm/s, as measured by a quartz crystal monitor inside the chamber. The low growth rate on the substrate at higher deposition angles 11 was compensated by increasing the deposition time to keep the same film thickness for all samples. The ITO films unloaded from the E-beam evaporation system were annealed in O 2 at 550 C for 1 min in a rapid thermal annealing system in order to compensate a deficiency of oxygen in the films. Optical transmittance measurements of the ITO thin films were performed using non-polarized light at normal incidence in the wavelength range of 280-780 nm using a JASCO UV-VIS spectrophotometer. The refractive index (n) was measured using a spectroscopic ellipsometry system, in which absorption is neglected during fitting. A film thickness was measured first by ellipsometry, and then confirmed by a scanning electron microscope (SEM, Hitachi S-4300). The porosity and optical bandgap energy values of the ITO thin films were calculated using theoretical models developed by Poxson et al. 11 and Tauc, 13 respectively. The morphological features of the films were examined using an optical microscope and SEM. Several regions on the sample surface were investigated in order to produce representative images. Furthermore, the crystallographic structure of the ITO thin films was examined by X-ray diffraction (XRD) using the nickel-filtered Ka emission of copper. Figures 1(a) and 1(b) show optical microscope images of the ITO films deposited on (a) silicon (Si) and (b) glass substrates at tilt angles ranging from 0 to 60 . A color difference of the ITO thin films is clearly evident, which can result from a variation in thickness and/or refractive index. In this case, because the thicknesses of all films are the same (about 210 nm), the variation in refractive index is the cause for the color variation. It is well known that the refractive index of a a

Polarization-engineered high efficiency GaInN light-emitting diodes optimized by genetic algorithm

A genetic algorithm is employed to find an optimum epitaxial structure of multiple quantum wells (MQWs) and electron-blocking layer (EBL) for a GaInN-based light-emitting diode (LED). The optimized LED is composed of locally Si-doped quantum barriers (QBs) in the MQWs and a quaternary heterostructured AlGaInN EBL having a polarization-induced electric field directed oppositely to that of a conventional AlGaN EBL. The optimized LED shows 15.6% higher internal quantum efficiency, 24.6% smaller efficiency droop, and 0.21 V lower forward voltage at 200 A/cm(2) comparing to the reference LED, which has fully Si-doped QB and 20-nm-thick Al0.19Ga0.81N EBL. We find that local Si doping near the QB/QW interface compensates the negative polarization-induced sheet charge at the interface and reduces electric field in the QWs, thereby enhancing electron-hole wave function overlap. In addition, the inverted polarization field in the quaternary EBL provides a high barrier for electrons but a low barrier for holes, resulting in enhanced electron-blocking and hole-injection characteristics.open1113sciescopu

Dynamic Emotional and Neural Responses to Music Depend on Performance Expression and Listener Experience

Apart from its natural relevance to cognition, music provides a window into the intimate relationships between production, perception, experience, and emotion. Here, emotional responses and neural activity were observed as they evolved together with stimulus parameters over several minutes. Participants listened to a skilled music performance that included the natural fluctuations in timing and sound intensity that musicians use to evoke emotional responses. A mechanical performance of the same piece served as a control. Before and after fMRI scanning, participants reported real-time emotional responses on a 2-dimensional rating scale (arousal and valence) as they listened to each performance. During fMRI scanning, participants listened without reporting emotional responses. Limbic and paralimbic brain areas responded to the expressive dynamics of human music performance, and both emotion and reward related activations during music listening were dependent upon musical training. Moreover, dynamic changes in timing predicted ratings of emotional arousal, as well as real-time changes in neural activity. BOLD signal changes correlated with expressive timing fluctuations in cortical and subcortical motor areas consistent with pulse perception, and in a network consistent with the human mirror neuron system. These findings show that expressive music performance evokes emotion and reward related neural activations, and that music's affective impact on the brains of listeners is altered by musical training. Our observations are consistent with the idea that music performance evokes an emotional response through a form of empathy that is based, at least in part, on the perception of movement and on violations of pulse-based temporal expectancies

Energy Frontier Research Center for Solid-State Lighting Science: Exploring New Materials Architectures and Light Emission Phenomena

The Energy Frontier Research Center (EFRC) for Solid-State Lighting Science (SSLS) is one of 46 EFRCs initiated in 2009 to conduct basic and use-inspired research relevant to energy technologies. The overarching theme of the SSLS EFRC is the exploration of energy conversion in tailored photonic structures. In this article we review highlights from the research of the SSLS EFRC. Major research themes include: studies of the materials properties and emission characteristics of III-nitride semiconductor nanowires; development of new phosphors and II−VI quantum dots for use as wavelength downconverters; fundamental understanding of competing radiative and nonradiative processes in current-generation, planar light-emitting diode architectures; understanding of the electrical, optical, and structural properties of defects in InGaN materials and heterostructures; exploring ways to enhance spontaneous emission through modification of the environment in which the emission takes place; and investigating routes such as stimulated emission that might outcompete nonradiative processes

Empirical Legal Studies Before 1940: A Bibliographic Essay

The modern empirical legal studies movement has well-known antecedents in the law and society and law and economics traditions of the latter half of the 20th century. Less well known is the body of empirical research on legal phenomena from the period prior to World War II. This paper is an extensive bibliographic essay that surveys the English language empirical legal research from approximately 1940 and earlier. The essay is arranged around the themes in the research: criminal justice, civil justice (general studies of civil litigation, auto accident litigation and compensation, divorce, small claims, jurisdiction and procedure, civil juries), debt and bankruptcy, banking, appellate courts, legal needs, legal profession (including legal education), and judicial staffing and selection. Accompanying the essay is an extensive bibliography of research articles, books, and reports