Epitaxy and Device Design for High Efficiency Blue LEDs and Laser Diodes

The (Al,Ga,In)N materials system has impacted energy efficiency on the world-wide scale through its application to blue light-emitting diodes (LEDs), which were invented and developed in the 1990s. Since then, cost reductions and performance improvements have brought GaN-based LEDs into the mainstream, supplanting outdated lighting technology and improving energy efficiency.One of the main challenges that still limits commercial LEDs, however, is “efficiency droop,” which refers to the reduction in efficiency as the input current density (and with it, the carrier density) increases. This phenomenon especially plagues high power LEDs, which operate in the current density range of 100-1000 A/cm2.Few practical options exist to directly eliminate efficiency droop, however we investigated two complementary approaches to circumvent the phenomenon. The first “high power solution” would employ blue laser diodes as the engine of solid state white lighting in lieu of LEDs. When laser diodes reach the threshold current density for stimulated emission, the carrier density in the active region clamps, simultaneously clamping droop. The wall plug efficiency of the laser diodes can then continue to rise as input current density increases until another effect (usually thermal) overrides it. The second “low power solution” maintains the blue LED as the solid state lighting engine, but shifts the operation point to low current density (and low carrier density) where efficiency droop effects are negligible and other thermal and electrical constraints in the device design are alleviated, enabling designs for high wall-plug efficiency. Both approaches to circumventing efficiency droop are likely to find a home in diverse future technologies and applications for lighting and displays.The challenge to produce high performance blue laser diodes was approached from an m-plane epitaxy platform. m-Plane is a non-polar orientation of the wurtzite (Al,Ga,In)N, which is free from deleterious polarization-related electric fields in the growth direction. m Plane is a naturally occurring crystal plane with high material gain due to its non-degenerate valence band structure, and thus should be well-suited for laser diode applications. However, m plane blue emission suffers from low indium uptake and broad spontaneous emission linewidth. The use of surface “double miscut” was investigated to improve the local step structure and morphology, resulting in higher indium uptake, narrower linewidth and higher peak power in the blue spectrum.The complementary challenge to improve the wall-plug efficiency for LEDs at low power operation focuses primarily on improved light extraction efficiency and low voltage operation. The main sources of extraction efficiency losses in typical c-plane blue LEDs on patterned sapphire substrates are absorption on the metal contacts, in the current spreading layer and on the metallic reflector, which also doubles as the heat sink. With the relaxed constraints at low power operation, new designs become possible. High light extraction designs were vetted with ray tracing software prior to experimental implementation. The highest demonstrated wall-plug efficiency resulting from these designs was 78.2%, and was accompanied by a greater than unity electrical efficiency (1.03) resulting from thermoelectric pumping, suggesting a pathway for 100% or greater wall-plug efficiency

Making More Light with Less Energy

Representing the Center for Energy Efficient Materials (CEEM), this document is one of the entries in the Ten Hundred and One Word Challenge. As part of the challenge, the 46 Energy Frontier Research Centers were invited to represent their science in images, cartoons, photos, words and original paintings, but any descriptions or words could only use the 1000 most commonly used words in the English language, with the addition of one word important to each of the EFRCs and the mission of DOE: energy. The mission of the CEEM is to discover and develop materials that control the interactions among light, electricity, and heat at the nanoscale for improved solar energy conversion, solid-state lighting, and conversion of heat into electricity

Polarization-Resolved Near-Field Spectroscopy of Localized States in m-Plane InxGa1−xN/GaN Quantum Wells

Producción CientíficaWe present a polarization, spectrally, and spatially resolved near-field photoluminescence (PL) measurement technique and apply it to the study of wide m-plane InxGa1−xN/GaN quantum wells grown on on-axis and miscut GaN substrates. It is found that PL originates from localized states; nevertheless, its degree of linear polarization (DLP) is high with little spatial variation. This allows an unambiguous assignment of the localized states to InxGa1−xN composition-related band potential fluctuations. Spatial PL variations, occurring due to morphology features of the on-axis samples, play a secondary role compared to the variations of the alloy composition. The large PL peak wavelength difference for polarizations parallel and perpendicular to the c axis, the weak correlation between the peak PL wavelength and the DLP, and the temperature dependence of the DLP suggest that effective potential variations and the hole mass in the second valence-band level are considerably smaller than that for the first level. DLP maps for the long wavelength PL tails have revealed well-defined regions with a small DLP, which have been attributed to a partial strain relaxation around dislocations.Swedish Energy Agency (Contract No. 36652-1)Swedish Research Council (Contract No. 621-2013- 4096

Chemical Defense by the Native Winter Ant (Prenolepis imparis) against the Invasive Argentine Ant (Linepithema humile)

The invasive Argentine ant (Linepithema humile) is established worldwide and displaces native ant species. In northern California, however, the native winter ant (Prenolepis imparis) persists in invaded areas. We found that in aggressive interactions between the two species, P. imparis employs a potent defensive secretion. Field observations were conducted at P. imparis nest sites both in the presence and absence of L. humile. These observations suggested and laboratory assays confirmed that P. imparis workers are more likely to secrete when outnumbered by L. humile. Workers of P. imparis were also more likely to secrete near their nest entrances than when foraging on trees. One-on-one laboratory trials showed that the P. imparis secretion is highly lethal to L. humile, causing 79% mortality. The nonpolar fraction of the secretion was chemically analyzed with gas chromatography/mass spectrometry, and found to be composed of long-chain and cyclic hydrocarbons. Chemical analysis of dissected P. imparis workers showed that the nonpolar fraction is derived from the Dufour's gland. Based on these conclusions, we hypothesize that this chemical defense may help P. imparis to resist displacement by L. humile

Impact of Alloy-Disorder-Induced Localization on Hole Diffusion in Highly Excited c-Plane and m-Plane (In,Ga)N Quantum Wells

The diffusion coefficient of holes can provide knowledge about carrier localization in (In,Ga)N, where the carrier dynamics are altered by randomly fluctuating potential landscape. In group-III nitrides, the diffusivity of holes is difficult to measure by electrical methods but it can be studied using optical techniques. Here, we investigate the dependence of the hole diffusion coefficient on direction and carrier density in c-plane and m-plane (In,Ga)N structures by employing the light-induced transient-grating technique. We show that the hole diffusion coefficient is anisotropic in the m-plane structure, where it is several times larger along the a crystallographic direction than along the c direction. Such anisotropy is observed within the broad range of carrier densities from 10 18 to 10 20 cm-3. The diffusivity changes nonmonotonously with increasing photoexcitation, this dependence being different in thick and thin layers. We argue that an unexpectedly high diffusion coefficient at low carrier densities in thick quantum wells can be a signature of efficient hole transport via percolative paths occurring due to compositional disorder. In turn, a decrease of diffusivity with the excitation can reflect the effect of Coulomb blockade of these paths. Finally, we demonstrate that disorder impacts carrier diffusivity even at carrier densities above 10 19 cm-3 , where the overflow of localized states must be included to explain the observed increase of the diffusion coefficient with the carrier density

Amorphous Molybdenum Sulfide Catalysts for Electrochemical Hydrogen Production: Insights into the Origin of their Catalytic Activity

We present a scalable wet chemical synthesis for a catalytically active nanostructured amorphous molybdenum sulfide material. The catalyst film is one of the most active nonprecious metal materials for electrochemical hydrogen evolution, drawing 10 mA/cm² at ∼200 mV overpotential. To identify the active phase of the material, we perform X-ray photoelectron spectroscopy after testing under a variety of conditions. As deposited, the catalyst resembles amorphous MoS₃, but domains resembling MoS₂ in composition and chemical state are created under reaction conditions and may contribute to this material’s high electrochemical activity. The activity scales with electrochemically active surface area, suggesting that the rough, nanostructured catalyst morphology also contributes substantially to the film’s high activity. Electrochemical stability tests indicate that the catalyst remains highly active throughout prolonged operation. The overpotential required to attain a current density of 10 mA/cm² increases by only 57 mV after 10 000 reductive potential cycles. Our enhanced understanding of this highly active amorphous molybdenum sulfide hydrogen evolution catalyst may facilitate the development of economical electrochemical hydrogen production systems

Effect of chemical secretion on <i>L. humile</i>.

<p>Proportion of <i>L. humile</i> workers that demonstrated specific behaviors after contact with the <i>P. imparis</i> secretion. There were <i>N</i> = 14 trials in which the <i>P. imparis</i> ant secreted on the <i>L. humile</i> ant.</p

The <i>P. imparis</i> secretion.

<p>A single <i>P. imparis</i> worker is shown with a liquid droplet containing bubbles at the tip of its raised abdomen. The secretion is then applied to the body of the <i>L. humile</i> ant.</p

Effect of the relative numbers of <i>L. humile</i> and <i>P. imparis</i> on aggressive behavior.

<p>Shown are the mean number of observations per trial of secretion (filled bars), gaster-flagging (hatched bars), and fighting (open bars) in relation to the proportion of <i>P. imparis</i> workers. Each assay was performed with 20 total <i>P. imparis</i> and <i>L. humile</i> workers and <i>N</i> = 15 trials for each proportion. Error bars show standard error of the mean.</p