EC Top Publications
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December 13, 2012
Professor Emmanouil (Manos) Kioupakis, from the University of Michigan, visited Sandia for technical discussions and to give a seminar on “Auger Recombination in Nitride Light-Emitting Diodes from First Principles Calculations.” Professor Kioupakis just finished a post-doc with Professor Chris van de Walle at UC Santa Barbara, and has been doing first-principles calculations of Auger recombination as a possible (and perhaps likely) mechanism for efficiency droop in InGaN LEDs. More recently, Professor Kioupakis has been working on the scaling with polarization fields not only of Auger recombination, but of other recombination processes (Shockley-Read-Hall and radiative). This last topic is of indtence interest for predicting efficiency droop in novel architectures such as non-c-plane LEDs, and c-plane and non-c-plane laser diodes. At UCSB, Manos was supported in part by the Center for Energy Efficiency Materials (CEEM); now, at the University of Michigan, Manos is supported in part by the Center for Solar and Thermal Energy Conversion (CSTEC).
December 3, 2012
In this most recent publication titled “Red-Emitting Quantum Dots for Solid-State Lighting” published by the ECS Journal of Solid State Science and Technology, EFRC scientists, Lauren E. Shea-Rohwer, James E. Martin, Xichen Cai and David F. Kelly, discuss the addition of a red-emitting component to make the cold white emission from White LEDs warmer. However, red emitters that could be used in solid-state lighting applications are not yet available.
Abstract: Red emitters that can be excited with blue InGaN LEDs are essential for warm white LEDs for solid-state lighting (SSL). Current red phosphors do not satisfy all of the criteria for SSL. Red-emitting CdTe-based quantum dot (QD) heterostructures meet several of those criteria, such as narrowband emission, broad blue absorption, high quantum yield (QY), and high luminous efficacy of radiation. This paper describes the synthesis of CdTe, CdTe/2CdSe, CdTe/2CdSe/CdS, and CdTe/2CdSe/CdS/5ZnS QDs and their photoluminescence. The growth of two CdSe shells on the CdTe core increases the QY to 95.5%. The shells reduce the thermal quenching above room temperature. CdTe cores that were aged for one year, exhibited thermal quenching of 73% at 100⁰C, whereas the aged CdTe/2CdSe and CdTe/2CdSe/CdS QDs had thermal quenching of 39% and 38%, respectively at 100⁰C. After cooling to room temperature, the QDs retained ~92% of their initial QYs. CdTe-based QDs have substantially less thermal quenching than CdSe/ZnSe QDs, which exhibit an 87% reduction in the QY and thermal degradation at 100⁰C. Aging the various QDs for one year resulted in blue-shifts of the absorbance and PL emission by ~5-11 nm; broadening of the FWHM by ~0.7-4 nm; and an increase in the PL lifetimes.
November 4, 2012
Solid-State Lighting was well represented at the recent Girl Scout Science day, held on November 4th at Albuquerque’s National Museum of Nuclear Science and History. One of the demonstrators, EFRC member Mary Crawford, introduced LED-based lighting and led a hands-on demonstration to show how the color components of lighting affect how we see everyday objects. The event, which was open to Girl Scouts and their families, had a total attendance of nearly 400 and was deemed to be a big success based on evaluations from the participants.
August 10, 2012
Our SSLS EFRC brought in four high school interns this summer. All were between their junior and senior years of high school, and had strong backgrounds math and science.
Our interns participated in a wide range of activities: ES&H courses, intense solid-state lighting science and technology reading, experimental work in one of our optics laboratories, energy economics data collection and analysis projects, editing of draft manuscripts for submission to journals or books, helping with the logistics of our Webexed SSLS coffee/dessert hours, attending (and occasionally transcribing) lectures, and touring various Sandia laboratories. They each also had one open-ended mini-project that each was able to carry to a nice level of depth, and that with follow-on work over the coming year might develop into a contribution to the scientific community.
The first mini-project was “Projections of 2030 consumption of light assuming saturation.” Previous projections of 2030 consumption of light assumed either of two extremes: zero elasticity of consumption of light with respect to cost of light, or unity elasticity of consumption of light with respect to cost of light. The student developed the database and equations necessary to project 2030 consumption of light with an assumption intermediate between these two extremes: unity elasticity of consumption of light with respect to cost of light for developing countries, but zero elasticity of consumption of light with respect to cost of light for developed countries.
The second mini-project was “A model for goal-directed science when progress is random.” An often discussed trade-off in science is the one in which goal-directed scientific exploration is most successful when that exploration is somewhat unfettered (scientists can choose within some range the directions they consider most profitable) but not too unfettered. The student developed a computation simulation in which this trade-off could be quantified, a simulation which takes into account the random nature of progress in scientific exploration.
The third mini-project was “The rebound effect in consumption of transportation.” It has been found empirically that there has historically been a unity rebound effect in the consumption of lighting – that is, consumption of light increases inversely with cost of light. Building on the monumental work of Roger Fouquet and Peter Pearson on historical energy consumption in the United Kingdom, the student developed a database that can be used to test whether a similar rebound effect might hold for the consumption of transportation – both freight and passenger.
The fourth mini-project was “Wavelength downconversion routes to smart lighting.” Up until recently, smart lighting, in which light chromaticity is controlled in real-time during use, has been thought to require independent RGB (red-green-blue) or RYGB (red-yellow-green-blue) light sources. The use of wavelength downconverters, in which a fixed fraction of blue light is converted to red and/or green light, seems to preclude control of chromaticity in real-time. However, there is the possibility that electric fields (in the case of quantum dot wavelength downconverters) or duty-cycle variations (in the case of quantum dots or phosphors) could be used to control the fraction of blue light that is converted to red and/or green light. The student did simulations and experiments to explore these possibilities.
III-Nitride core-shell nanowire arrayed solar cells
April 27, 2012
In a new EFRC-supported publication titled “III-nitride core-shell nanowire arrayed solar cells” published in Nanotechnology, EFRC scientists Jonathan J. Wierer, Qiming Li, Daniel D. Koleske, and George T. Wang and Sandia National Laboratories scientist Stephen R. Lee present their work on a unique hybrid nanowire structure. The article established that this structure served a dual purpose, both in improving power conversion efficiency as well as being beneficial for other III-nitride devices such as light-emitting diodes (LEDs).
Abstract: A solar cell based on a hybrid nanowire–film architecture consisting of a vertically aligned array of InGaN/GaN multi-quantum well core–shell nanowires which are electrically connected by a coalesced p-InGaN canopy layer is demonstrated. This unique hybrid structure allows for standard planar device processing, solving a key challenge with nanowire device integration, while enabling various advantages by the nanowire absorbing region such as higher indium composition InGaN layers by elastic strain relief, more efficient carrier collection in thinner layers, and enhanced light trapping from nano-scale optical index changes. This hybrid structure is fabricated into working solar cells exhibiting photoresponse out to 2.1 eV and short-circuit current densities of ~1 mA cm−2 under 1 sun AM1.5G. This proof-of-concept nanowire-based device demonstrates a route forward for high-efficiency III-nitride solar cells.
Take Our Daughters and Sons to Work Day 2012
April 26, 2012
The Solid-State Lighting Science Energy Frontier Research Center (SSLS EFRC) participated in Sandia National Laboratories’ “Take our Daughters and Sons to Work Day” on April 26th. Both students and parents were actively engaged in a series of “Coloring Your World” exhibits. One such display included a hands-on exhibit on solid-state lighting that illustrated the differences in how the colors of typical objects in daily living look under these lights managed by the SSLS technical staff. Another demonstration was an invigorating electronic “LED Lighting 101” Jeopardy game with an emphasis on understanding LEDs and SSLS. As a unique way of educating the next generation about energy efficiency, the day was filled with edification as well as triumph upon a win in Jeopardy.
Solid State Lighting Celebration
March 31, 2012
The EFRC team and family members joined together for a celebration of the achievements in the Solid State Lighting Program at EFRC Chief Scientist Jeff Tsao’s home on Saturday, March 31. This social was complete with an assortment of delicious food, laughter, and an appreciation for the Solid State Lighting program at Sandia National Laboratories. The night ended with a cake mimicking an iPad with the congratulatory statement, “Best Wishes, SSL Team!!”. The food was great and a good time was had by all.
Assessment of deep level defects in m-plane GaN grown by metalorganic chemical vapor deposition
February 22, 2012
Through the publication titled, “Assessment of deep level defects in m-plane GaN grown by metalorganic chemical vapor deposition” published in the Applied Physics Letters, EFRC scientists examine how spontaneous polarization and strain-induced piezoelectric polarization generate strong internal electric fields in gallium nitride (GaN).
Abstract: We report on deep level defect incorporation in n-type m-plane (10 0) GaN grown by metalorganic chemical vapor deposition (MOCVD) on built m-plane GaN substrates. Deep levels were observed at 2.85 eV and 3.31 eV relative to the conduction band minimum. While the energetic distribution of defect states for m-plane GaN was similar to the previous reports of n-type c-plane GaN grown by MOCVD, the deep level densities of the m-plane GaN were significantly lower. The comparatively low defect density in homoepitaxially grown m-plane GaN is attributed to reduced point defect incorporation. In addition to the absence of polarization fields, the low deep level density achieved by homoepitaxial growth on high quality bulk GaN substrates makes m-plane GaN highly attractive for opto-electronic devices.
Materials for Energy Applications Conference
January 30 – February 1, 2012
On January 30-February 1, 2012 Lawrence Berkeley National Laboratory will host a conference on Materials for Energy Applications focusing on collaboration between the DOE Laboratories and US industry. EFRC Senior Leadership Council ChairJerry Simmons plans on attending the conference, and will represent the work being done on solid-state lighting at the EFRC and in other DOE Laboratories.
Using the kinetic Wulff plot to design and control nonpolar and semipolar GaN heteroepitaxy
January 19, 2012
The Solid-State Lighting Science (SSLS) Energy Frontier Research Center (EFRC) Director, Dr. Michael E. Coltrin, recently collaborated with colleagues from the Department of Electrical Engineering at Yale University on a publication in Semiconductor Science and Technology titled “Using the kinetic Wulff plot to design and control nonpolar and semipolar GaN heteroepitaxy”. The article investigates GaN faceted growth as a function of crystallographic orientation to understand and control growth for both nonpolar and semipolar GaN.
Abstract: For nonpolar and semipolar orientations of GaN heteroepitaxially grown on sapphire substrates, the development of growth procedures to improve surface morphology and microstructure has been driven in a largely empirical way. This work attempts to comprehensively link the intrinsic properties of GaN faceted growth, across orientations, in order to understand, design and control growth methods for nonpolar (1 1 2 0) GaN and semipolar (1 1 2 2) GaN on foreign substrates. This is done by constructing a comprehensive series of kinetic Wulff plots (or v-plots) by monitoring the advances of convex and concave facets in selective area growth. A methodology is developed to apply the experimentally determined v-plots to the interpretation and design of evolution dynamics in nucleation and island coalescence. This methodology offers a cohesive and rational model for GaN heteroepitaxy along polar, nonpolar and semipolar orientations, and is broadly extensible to the heteroepitaxy of other materials. We demonstrate furthermore that the control of morphological evolution, based on invoking a detailed knowledge of the v-plots, holds a key to the reduction of microstructural defects through effective bending of dislocations and blocking of stacking faults. The status and outlook of semipolar and nonpolar GaN growth on sapphire substrates will be presented.
Introduction of Prof. Jim Speck and UCSB to SSLS
January 11, 2012
We are pleased to announce the addition of Professor Jim Speck of the University of California at Santa Barbara (UCSB) as a member of the Solid-State Lighting Science EFRC. Jim has developed a collaboration with SSLS scientist Andy Armstrong using deep level optical spectroscopy to investigate defects in m-plane GaN.
Jim is a Professor in the Materials Science at UCSB, and serves as thrust leader in their Center for Energy Efficient Materials (CEEM) research on solid-state lighting.
Jim and Andy are working in our Research Challenge on Defect-Carrier Interactions investigating the role of defects in non-radiative recombination processes in light emitting diodes. In recognition of the importance of this collaboration, Professor Speck has been added as a Senior Investigator in the SSLS EFRC, and UCSB is now a SSLS participating institution.
January 11, 2012
It is our pleasure to announce the addition of Professor David Kelley as a Senior Investigator in the Solid-State Lighting Science EFRC. He is a Professor and Founding Member of the Chemistry and Chemical Biology Department at the University of California at Merced.
Professor Kelley and his research group are world leaders in synthesis and properties II-VI quantum dots (QDs). He will be collaborating with Jim Martin and Lauren Rohwer in our Quantum Dots and Phosphors Research Challenge. David will be guiding the synthesis of QD core / shell heterostructures, especially through developing simulation codes for computing the QD strain energy and electronic structure calculations.
Genetic Algorithm for Innovative Device Designs in High-Efficiency III-V Nitride Light-Emitting Diodes
January 5, 2012
EFRC scientists from Fred Schubert’s research group at RPI and at Sandia Labs have published a new paper titled “Genetic Algorithm for Innovative Device Designs in High-Efficiency III-V Nitride Light-Emitting Diodes” in the journal Applied Physics Express. The work is a starting point of applying artificial evolution to practical semiconductor devices, open new perspectives for complex semiconductor device optimization, and enable breakthroughs in high-performance LED design. According to the Applied Physics Express website, this publication was the their most downloaded article in January 2012.
Abstract: Light-emitting diodes are becoming the next-generation light source because of their prominent benefits in energy efficiency, versatility, and benign environmental impact. However, because of the unique polarization effects in III–V nitrides and the high complexity of light-emitting diodes, further breakthroughs towards truly optimized devices are required. Here we introduce the concept of artificial evolution into the device optimization process. Reproduction and selection are accomplished by means of an advanced genetic algorithm and device simulator, respectively. We demonstrate that this approach can lead to new device structures that go beyond conventional approaches. The innovative designs originating from the genetic algorithm and the demonstration of the predicted results by implementing structures suggested by the algorithm establish a new avenue for complex semiconductor device design and optimization.